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PostPosted: Tue Jan 06, 2015 7:21 am 
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This car versus the others you typically see in the class are going to have some different strengths and weaknesses. Going to be some "horses for courses" sorta phenomenons going on as certain tracks are going to highlight some sets of strengths and punish others differently. Then there's that whole driver thing to throw into the mix as well :)

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PostPosted: Tue Jan 06, 2015 2:45 pm 
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Fair wrote:
I owned a nearly identical 1994 Corvette that I had dyno'd on a modern DynoJet, where it made 277 whp in bone stock form, back in 1996 (these cars were a hair under-rated at "300 hp" crank).


Fair wrote:
We will run this car at the 3203 pound stated minimum weight and NOT burn points on running a lower weight. Running lighter might seem advantageous but we would have to lower the power output as well (the 12:1 ratio doesn't change for TTC class even if you burn points to "run lighter than minimum").


Terry,
When I first looked at this, I wondered how you were staying at 12:1. Then I noticed you said you were running a 30mm smaller than stock tire. Stock is a 275, so you are running a 245. This means you can take a power modification +0.8. Thus your raw p/w can be be 11.2:1 and still be legal.

Any competitor that is following this thread take note: This is how you are competitive without over spending--look for things that give you an advantage and save money. A 245 tire costs less than a 275. You might give up a little mid-corner grip, but you can offset that by running more power down the straights.

Again, great job of maximizing the rules! I'm really starting to get excited about running TTC this year!

-bj

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PostPosted: Fri Jan 09, 2015 9:55 am 
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loftygoals wrote:
I'm going to be debuting my 2003 MINI Cooper S in TTC at MSR-H as well. I was hoping that Terry got something wrong in his interpretation of the rules, but he is spot on. This is a legal upgrade specific to the 92-96 LT1 powered base Corvette. The 1996 model came with the "Heavy Duty Brakes" as standard on the base model. So although they were an optional upgrade in 1992, they are legal under the updating/backdating rule because they were the standard brakes in a model of the model group listing and in the same generation. Great work maximizing your no point modifications, Terry!


Yep, you know your Corvettes, my friend! That's the exact loophole we found - all 1996 models came with 13" brakes, even the BTM. And the listing is for non-LT4 "1992-1996 Corvettes". :D

Hopefully the brake parts will get here in time, otherwise we'll do the first event on the wimpy 12" stuff. Yuck!

loftygoals wrote:
I don't know how well my 1.6L, FWD, McPherson strut, brick of a car will do up against a V8 Corvette. Could there be any two cars as vastly different competing? I kinda feel like I'm bringing a knife to a gun fight. It should make for an interesting David vs. Goliath battle!

-bj

For sure, very different platforms, but very different weights, too. Should be a fun battle, as long as this 24 year old jalopy holds together! :mrgreen:

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We're thrashing on DANGER ZONE right now and I will update the Build Thread later today. Lots to do!

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PostPosted: Fri Jan 09, 2015 12:44 pm 
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Project Update for January 9th, 2015: The first stage of "initial race prep" for this project is getting down to the wire with the NASA race debut only ONE week away! Let's get caught up on the work we've been knocking on Project DANGER ZONE, our TTC class prepped 1992 Corvette. I had intended this to be a quick update, showing the initial stages of race prep, but I started writing and somehow this post spiraled out of control. It now includes some aero/drag reduction theory, the History of Iron and Steel, a good bit on roll cage tubing and design, some tire analysis, and other random tidbits of tech. If you get bored easily just skip down to the pictures and enjoy.

Safety First, Kids!

There are many Safety Upgrades we want to add to (or repair on) this Corvette during the 2015 season, which are the same for virtually any dedicated road race build. Unfortunately we were pressed for time and won't get them all done before the first race. The Safety list includes: a full roll cage, racing seat install, 6-point racing harness, window nets, full fire extinguisher system, secondary 2.5 pound fire bottle, tow hooks at both ends, tie-down hooks at both ends (for towing), replace the broken windshield, replace the rear hatch glass with Lexan or Plexiglass, and more. That's a lot of parts and work on the "Punch List", but as far as what's "required" for NASA Time Trial, that is much less. We also have some repairs and performance upgrades to tackle, too. We got almost nothing done over the holidays (short weeks, busy on customer cars) so let's see what we can get done in two weeks.

Windshield + Small Aero Improvements

This repair is really a safety upgrade, because you cannot race with a busted windshield. Before I bought the car from Matteucci he had warned me about this problem - it had a lot of big cracks on the passenger side and chunk missing where the OEM rear view mirror attached, with the cracks propagating into the driver's view. The upper right corner of the windshield trim and weatherstrip took a hit when a former owner (ie: The Crackhead) drove it through a barbed wire fence. Matteucci literally found a crack pipe (maybe a meth pipe) in the car after he bought it. So yea, now you understand part of why this was a $3000 running and driving car purchase.

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Nasty, broken windshield and trashed weatherstripping has to be replaced

We discussed delaying the windshield replacement because when you install a roll cage it is ALWAYS easier (and often required) to get the front windshield out of the way to access and weld tubes along the front of the cage. In some cars with a fixed rear glass window, that is often removed as well. The cages we built in the two cars below required the windshield to be out. At left is an SCTA legal cage for 200+ mph use on the salt flats, which has more of a "Funny Car" drag racing cage (built per the rules). To weld in the the front tubing gussets requires windshield access. The one at right is a NASA ST3 legal Mustang the the dimple-die gussets and front corner tubes need windshield access - as does a proper paint job on any cage.

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We knew we wanted a roll cage in DANGER ZONE (to reduce the "zone of danger"), but with only 2 weeks of time build available after the short holiday weeks between Christmas and New years, and many other items that needed attention, meant only building a 4-point roll bar for this first race. Which meant the windshield didn't need to come out. But it was so cracked that it would never pass tech, so it had to be replaced. And when we go back and finish the full cage, out it will come again!

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We called our buddies at Titan Auto Glass and they extracted the old and busted and installed the new hotness. There are normally several windshield choices for most cars, varying in price, but for 24 year old Corvettes there was one - and it was a tick pricy at $260 installed. But hey, can't race with a busted windshield. Going to Lexan is an option but there are more downsides (more costly, harder to install, easily scratched, difficult to use wipers with them, more easily nicked by rocks or tire klag) than upsides (slight weight savings). A two layer, laminated, glass OEM style windshield is preferred by many racers when they have a choice.

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The OEM windshield surround rubber weather stripping was a total mess (above right). Matteucci had cut away the bits around the A-pillars when he gutted the interior and the top bit was destroyed by the barbed wire. The crackhead former owner had filled in the missing chunk in the rubber seal with SHOE GOO, and that had to be chiseled away (thanks Titan!).

I wasn't about to put this mangled mess of rubber seals back on, but we needed the top bit of rubber to seal the targa roof panel smoothly along the top of the windshield and I also wanted the seals back in place along the edge of the windshield at the A-pillar. This should help smooth the airflow in a high payoff "Green Zone" of potential drag reduction - the edges of the windshield. Read this NASA Speed News article called "Getting Into The Zones" (page 60) written by aero guru Neil Roberts (also read his ThinkFAST Engineering blog for more great articles!) and that will make more sense.

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A smooth, new set of weather stripping should help aid the transition from the edge of the windshield (sides and top), reducing drag. We are looking to reduce drag in ALL of the Green areas (again, read Neil's article) on this car, and do so legally. We cannot run NASA events with the windows up, so the door window openings have to stay. We have tried to read the rules to say otherwise, but rule 7.2 of the NASA TT rules is pretty clear:

7.2 Front driver and passenger side fixed/Lexan windows are specifically not permitted unless they are factory installed during the manufacturing of the vehicle. Both front side windows must otherwise be in the down position while on track."

Running the windows UP would be a decrease in drag but it is not allowed in virtually any form of road racing, for safety reasons (easier extraction after a crash). Some drag racing classes and high speed events like Bonneville do allow for side windows, so the silver Subaru we're building the cage for above is getting a full Lexan window package (4 side windows + front and rear windscreens).

Why Terry Needs A Roll Cage

There isn't any additional safety requirements in NASA Time Trial groups than what is called for in HPDE run groups: a Snell SA2010 rated helmet and OEM seat belt, plus a roll bar for convertibles. There isn't supposed to be wheel to wheel contact in TT, but we are running for times and competing for contingency prizes, and many TT racers take it pretty seriously. I won six sets of Hoosier race tires racing in NASA TT3 class in 2014, and these were BIG tires that cost $1710 a set, so there is some decent swag on the line. When you are chasing a TT win you often push the limits and do stupid stuff...

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Crap like this

My personal safety record, for the amount of laps I've driven on track, was pretty damned good up until 2014. In 27 years of running on road courses I only had a couple of "offs" that were worth mentioning. I de-beaded a couple of tires in a high speed off at TWS in the late 1980s that curled my hair a bit. A number of times I've had a quick "off and on" that bent a splitter or packed a grill with grass, sure. The stock brake pads came apart and I left Turn 7 at ECR at 90+ mph in 2013 in a stock '13 Mustang. But by far my most memorable off-track experience happened in 2014 (shown above).

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After the crash I began wearing a HANS device and fire suit to complement the FIA halo seat, 6-poiont harnesses and roll bar in our TT3 Mustang

I briefly mentioned this in my first post, but it was a pretty spooky incident and I figured it might explain my "overkill" safety requirements for this TT build. After losing brakes at at Road Atlanta at a Global Time Attack event in May 2014, I went off the end of Turn 10A at 150mph, through the gravel trap, and took a big vertical hit coming out of a trap. I got hurt but the car barely took a scratch (splitter came off, was repaired and reinstalled and back on track 2 weeks later, but not with me driving). Even though I had a proper FIA halo seat, good harnesses, and a good roll bar, I wasn't wearing a HANS device. We think this might be why I fractured a vertebrae in my back and broke a rib. After this incident I was in a lot of pain, wearing a back brace for 2 months, and not racing.

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Having gone through the crash scenario many times and analyzing frame-by-frame pictures of the crash since, the injury seems to come down to too much "arching" of my back in the impact that broke these bones. A properly worn HANS device would have likely prevented this injury. The "off" happened because I ran out of brake pads, and had been ignoring measured brake caliper temp data of 490°F+ for months. I'm not going to make those series of mistakes again, and I also vowed in 2014 to start racing with better personal safety gear. I was setting a bad example and I needed to do better.

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So obviously, after this back-breaking scare I'm taking my safety on track a lot more seriously. I've starting using a HANS device (still haven't picked my favorite model after trying 4 different brands - and I'm about to try the brand new Schroth HANS design, since we are a dealer) and an FIA 3-layer driving suit in all TT events. I am also wearing my harnesses TIGHTER and keeping a much closer eye on things like brake fluid temps and brake pad material depth, so this scenario of failures never happens again.

Roll Cage Is Safety More Than Performance

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The steel frame structure of the C4 Corvette. This is a 1984 model

After the personal safety gear, the next most important safety aspect of any race car is the roll cage. This structure is helpful to make the chassis more rigid, sure, but it is there mostly to prevent bodily injury in many types of crashes and "offs". These include single car off track frontal collisions with a barrier (somewhat common), a roll-over crash (very rare), or any car-to-car contact (more common). While a roll cage wouldn't have helped me in my gravel trap "jump" incident at all, there are other types of crashes where it could save your life - and do so more effectively than the basic 4-point roll bars we have used in my last 3 personal race car builds. I own a shop that builds roll cages and haven't had one in my own cars in 6 years... that's crazy.

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Corvette roll cages are tricky. All Corvette chassis generations (C1-C7) have a strong metal frame (steel frames through the C5, with aluminum frames for C6 Z06 and all C7s) with a composite body attached to it. Adding a roll cage to these cars has some extra challenges, since you need to cut away fiberglass to access the metal frames, but its nothing we haven't done before.

As I spoke about above, TIME is not on our side for this first NASA event, so we had to cut back on the roll cage plans for the maiden voyage. We could have bought a second-hand 4-point roll bar but it would never fit as tight to the roof structure in this gutted car (they are usually made for full interior cars).

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Frame differences on the 1984-1991 (left) and 1992-1996 (right) C4 Corvettes

Quick sidebar: the C4 Corvette had major changes to the suspension (1989) and even to the frame (1992), along with major changes to the drivetrain (1989 for ZF and 1992 for LT1), front crossmember, and transmission tunnel over the 13 year long model run. The two "body shop spec" images above show the changes to the frame at the 1992 model year, and we've noticed a lot of other differences to the interior fiberglass structure.

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Last year we caged and safety prepped a 1987 Corvette convertible (shown above) and after looking at pictures of both that and our 1992, it had a lot of structural differences at the tunnel, firewall, and rear bulkhead (behind the front seats), not to mention the dash, body, and other obvious differences. The frame was also different in some key areas. We are using the NASA CCR for cage design specifications on our car, but the black 87 used a mix of NASA, WRL, Lemons and ChumpCar series cage rules.

A Brief History of Iron and Steel

OK, this is a big tangent. It might not be boring to a racer - unless you are a metallurgical engineer. Modern race car roll cages are made from steel tubing (aluminum is not allowed) and picking the right alloy and type of tubing for the job involves both a rule book (General Competition Rules or GCR/CCR) and some engineering knowledge. There are generally TWO accepted types of tubing allowed for roll cages in SCCA and NASA: 4130 Alloy steel and 1018/1020 Low Carbon (aka: Mild steel) Steel DOM tubing.

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Working as a Mechanical Engineer at a foundry in my first job out of college exposed me to a lot of practical metallurgical design and lots of different steel alloys. I also took a welding class at college, where we talked a lot about steel and iron and the changes welding can do to metal's molecular structures. That's where I learned that steel is the best metal on planet earth, and in many ways unique among all metals.

Iron is is, by mass, the most common element on Earth, forming much of Earth's outer and inner core (same scenario in almost any rocky planet). It is also the fourth most common element in the Earth's crust, which means it is relatively easy for humans to get at and mine. The production of iron by humans began sometime around 2000 BC and was so significant it began what is now called the Iron Age - when iron replaced bronze in implements and weapons. This shift occurred because iron, when alloyed with a bit of carbon, is harder, more durable, and holds a sharper edge than bronze. For nearly four thousand years, until replaced by steel after ~1870, iron formed the material basis of human civilization in Europe, Asia, and Africa. Iron has shaped human history for the past four thousand years, and it's use accelerated technological growth.

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Natural "iron ore" has a lot of oxygen in it, so it is smelted at high temperatures to extract a more pure mass of iron. Carbon naturally gets mixed in at these high temperatures (along with 2-3 other elements) which means cast iron has a relatively high proportion of carbon (3-4.5%). This makes cast iron hard and brittle; it is liable to crack or shatter under a heavy blow, and it cannot be forged.

Blacksmiths learned to work iron - after heating it in a furnace at high temps they removed a pasty mass and hammered it on an anvil to drive out the cinders and slag and to compact the metallic particles. This Wrought iron (“wrought” means “worked” or hammered) contained generally from 0.02 to 0.08% percent of carbon (absorbed from the charcoal), just enough to make the metal both tough and malleable. Wrought iron was the most commonly produced metal through most of the Iron Age.

Steel alloys have a little bit of carbon in them (0.2 to 1.5%), enough to make them harder than wrought iron, but not so much as to make it as brittle as cast iron.

continued below


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PostPosted: Fri Jan 09, 2015 2:22 pm 
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continued from above

Its hardness combined with its flexibility (from some other alloying elements) and high tensile strength make steel far more useful than either type of iron: it is more durable and holds a sharp edge better than the softer wrought iron, but it resists shock and tension better than the more brittle cast iron. After about 1856 (the invention of the Bessemer converter) and into the 1870's (Andrew Carnegie's grasp of the vital importance of chemistry in steel making) steel alloys became cheap to manufacture and exploded in use, replacing wrought iron rails in railroad tracks and other uses.

Some of the sources I used, other than past knowledge: This and this and that, and more.

Modern Roll Cage Steel Choices - 4130 vs 1020/1080 Alloys

There are two basic steel alloys used in roll cage structures and we will start with the "stronger" and more expensive alloy allowed: 4130. AISI 4130 alloy steel is about 97% Iron and has 6 other alloying agents that make up the last 3%. Chromium (0.80 – 1.10%) Molybdenum (0.15 – 0.25%) are the two key elements added that give this metal its higher tensile and yield strengths, and are the two most expensive elements in the alloy as well - hence the nickname "Chromoly Steel". These alloys are harder to weld properly (generally they are only TIG welded) when compared to Low Carbon/Mild steels. The yield strength of 4130 is 66,700 psi (67 KSI) and when this metal is used, a little LESS of this alloy is needed to achieve the same total assembly strength as Mild steel. It has a good strength to weight ratio, but the same density as all steels (all steel alloys and iron have nearly the exact same density, .284 lbs/cubic inch, due to the fact that all steel alloys are still almost entirely made of iron). In the past, roll cage rules allowed for thinner 4130 tubing to be used relative to Mild Steel, but that is no longer the case for most road racing bodies.

I used to use a lot of "A36" mild steel 15-20 years ago when I designed oilfield equipment, which had a minimum yield strength of 36KSI, which is relatively soft and very cheap. The modern 10XX series steels have gotten better and a lot stronger - closer in strength to 41XX Chromoly steels, but without the negatives. AISI 1018 and 1020 "Low Carbon" or "Mild" Steel alloys (also known simply as Carbon Steel) are lower cost and slightly weaker than 4130, but these 10XX series alloys have excellent weldability and offer a good balance of toughness, strength and ductility. Once cold worked (via the DOM or CDS process) these Mild steels become even stronger and stiffer.

1018 steel (0.14 - 0.20% Carbon) has a yield of 54KSI and 1020 steel (0.18-0.23% Carbon) has a 51 KSI yield - which isn't that far off of 4130 (67 KSI). Ultimately 4130 is about 20% stronger than Mild steel. But 1018 tubing that is DOM cold worked gets stronger, and is rated at 70KSI, and 1020 DOM tubing is rated is 65 KSI. Cold working 4130 tubing via the DOM process turns it up to 90 KSI yield... roughly 22% stronger.

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The key benefit to racers building roll cages out of Mild steel over Chromoly is that 10XX alloy steel is much more forgiving with respect to weld embrittlement and tends to "crash better" than the harder "alloy" steels. When you weld the 41XX series alloys the molecular structure of the alloy changes near the heat affected zone, especially if you put too much heat into the weld (and some welders like to "weld hot", which can really make the weld area change), so 4130 cages are almost exclusively welded with the trickier TIG welding process (a Tungsten tipped torch with a shielding gas and a separate metal rod, with a variable control on the welding arc).

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Mild steel isn't nearly as susceptible to this issue and can be TIG or MIG welded and generally does not lose much strength at the welded joints. 10-20 years ago 4130 was all the rage for roll cages but lately 1018 or 1020 Mild steel is the norm, as long as they are DOM. To me nothing beats a properly designed, TIG welded, Mild Steel DOM tubing roll cage. This has the best combination of variables and the least number of compromises.

DOM vs ERW Tubing?

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ERW or "Electro Resistance Welded" tubing is how steel tubing and pipe is made (at least initially) - where a continuous, flat roll hot rolled steel is bent around round (or square or rectangular) dies and welded at a seam (see image above). That's how lower cost pipe and tubing is left - with this visible welded seam on the outside and often a physical "ridge" on the inside of the tube (see below). This problem is - seam ultimately becomes the weakest point in this type of tubing. And the hot roll plate material isn't ever as strong as cold worked steel.

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A visible seam and often a raised ridge is the result of the welding process from ERW tubing. All tubing starts as ERW...

The DOM process (Drawn Over Mandrel) takes ERW tubing and "cold works" it by drawing over a round mandrel and through round dies, inside and out. This makes the now DOM tubing "seamless" (its really hard to find the seam with your eyeball) and work hardens the steel structure - adding strength and removing the stress riser at the seam. ERW tubing was previously allowed in roll cages (up until just a few years ago) and the various CCR/GCR rules sometimes still reference ERW for "grandfathered" cages built before it was outlawed, but nowadays all roll cages are spec'd as seamless tubing - either 1018/1020 Mild Steel (DOM or CDS) or 4130 Alloy steel (DOM or CDS). There's another cold working seamless tubing process nowadays called CDS (Cold Drawn Seamless), but I can't seem to find any CDS tubing in common roll cage sizes - yet. The CDS specification seems to be more common in Europe. It could be the exact same process as DOM, but some U.S. tubing companies specify them separately, so I don't know.

The stiffness difference between ERW and DOM is shown in this video, with the same diameter and wall thickness tubes of both types in a side-by-side bending test. The lower strength of ERW + the stress riser of the seam are why it isn't specified in roll cages any longer, but it was a pretty recent deletion from roll cage specifications.

The FIA has updated their specified tubing to 350 N/mm2 (50.76 KSI) tensile strength (see page 46, rule 8.3 of Appendix K here), and material is simply listed as "Cold drawn seamless carbon steel". They used to only spec 4130 alloy tubing (or the European equivalent) but even the French have seen the benefits of using Mild Steel DOM/CDS tubing. As we have seen with changes to rules specs, ERW is no longer allowed and the advantages in welding Mild Steel outweigh the weight savings or 20% strength benefits of 4130 Chromoly.

Picking the Tubing Material, Tubing Sizes and Cage Design Layout

OK, that got a bit long, but it was hopefully worthwhile tech. Now that we know why we use steel, know more about the alloys, understand the benefits of the cold working and seamless processes that are required in the steel tubing specified, and why more cages are using mild steel DOM - let's pick the cage tubing size for this build and show some Corvette cage pictures already!

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Many of the cars we work on at Vorshlag lately, that are built around NASA specs, weigh over 3000 pounds so we're often using 1.75" diameter x .120" wall thickness DOM Mild steel tubing. And since the 1992-96 C4 Corvette is listed as a base class of TTC and a Minimum Competition Weight of 3203 pounds, I assumed that we had to use this tubing size. This is nearly the heaviest cage tubing in all of the NASA CCR, but lower weight cars can use thinner tubing diameters and wall thicknesses, as shown below (copied from the 2015 NASA CCR).

NASA 15.6.18 - Roll Cage Tubing Sizes

For the purposes of determining roll bar tubing sizes, vehicle weight is as raced, but [I]without fuel and driver. Minimum tubing size for the roll
cage is:[/I]

Up to 1500 lbs:
  • 1.375” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM
  • 1.500” x 0.080” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

1501 - 2500 lbs:
  • 1.500” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

2501 - 3000 lbs:
  • 1.500” x 0.120” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM
  • 1.750” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

3001 - 4000 lbs:
  • 1.750” x .120” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

Over 4000 lbs:
  • 2.000” x 0.120” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

Since we do not need to include the weight of the driver (200 pounds) or fuel (20 gal x 6 pounds/gallon = 120 pounds), that means our goal weight of 3203 really translates to a caged race car weight of about 2900 pounds. So we can use the lighter 1.75" x .095" wall DOM Mild Steel tubing. I like this for two reasons. First, we have a bunch of this tubing already in stock at the shop. And two, we typically bend 1.75" tubing, so our tubing bender has this set of dies already installed. Right now we're building two cages at once, both with 1.75" diameter tubing (one is .095 and the other is .120" wall), so we won't have to keep switching the dies. This thinner wall tubing is also easier to bend.

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We use a JD2 Model 32 manual tubing bender and dies

The weight is not insignificant: 1.75" x 120" wall DOM weighs 2.089 pounds/foot of tube length. The 1.75" x .095" wall DOM weighs 1.679 pounds/foot (19.6% lighter and about the same amount cheaper). The other choice for this weight is 1.5" x .120" wall DOM, which weighs more at 1.769 pounds/foot. Sure, we could have stepped up to a larger tubing size, but those CCR minimums are there for a good reason... mega-sized tube in a smaller/lighter car makes for less room to the driver and less energy absorption in a crash, so we're going with the recommended tubing range for a 2900 pound car, then picking the larger tubing diameter of the two options given there, which is slightly lighter.

Cage Layout and Design

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There are three cage design options we can choose form the NASA CCR, shown in 15.6.8, -.9 and -.10. We're going with the "Forward Hoops" version from 15.6.8, shown above. This is the most common of the 3 methods (another is the "Halo style") and makes for the most room for the driver's head in a car like this. So about halfway through the day on Friday the 2nd, our fabricator Olof stared work on the cage install. He will build a majority of this cage while Ryan finishes a cage on another car at the same time.

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Before you can start bending any tubes you have to clear out the interior. This car was already gutted, which saved us 15-20 hours of labor. That work is never included in the "cage" price, which some people don't always understand. If you want to save some money, bring in a NAKED car with zero interior bits, like this. I came in early that Friday and removed the driver's seat and targa top, then the guys pulled the rear hatch glass off.

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The driver's seat normally weighs more than this, but it was alreay partially gutted by Matteucci and only tipped the scales at 34 pounds - I've weighed a lot of modern power front seats in the 60-75 pound range. The targa top weighed less than I had thought at 22 pounds. This is the plexiglass "See through" version, but I'm looking for a fiberglass version (both were offered from the factory) which we could paint white to match the car, but they sell for $$$ used. The weight is mostly in the metal frame structure, so the Plexiglass vs Fiberglass is probably a wash - except the fiberglass OEM version is likely stiffer. We might replace the Plexiglass with a custom Carbon Fiber skin (stiffer than Fiberglass). Is it legal? Well since we can run with the targa top removed (wouldn't that make its construction insignificant?) and as I read TT rule 8.3.B, we can lighten the "roof, hood, body panels and doors" as long as they "maintain their BTM (Base Trim Model) size and shape". The "no points" listing for I.h.20 says the same thing, with more details with respect to carbon/fiberglass doors being legal as long as the BTM body lines, hinges and handles are still operational. And an in-house built Carbon Fiber roof would be, you know, cool...

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The rear glass was much heavier at 46 pounds. That bit will likely never go back onto this car, as we have a formed, 3/16", trimmed Plexiglass rear hatch replacement inbound that should save 30+ pounds. I will show that in my next post, if it gets here before the NASA race Jan 17th. This is legal per the "No Points Modifications" rule I.b.8, as long as it has the factory BTM shape and no uncovered holes.

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Once the interior was cleared out enough to start Olof began cutting bits of fiberglass out of the way. See why I had the pictures of the C4 frame structures up above? That was to help us find where the frame is - which isn't obvious in some areas as there are big gaps between the shape of the interior fiberglass structure and the metal underneath.

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If you ever get a roll cage quote on a Corvette, now you know why it costs more than a traditional steel unibody car - because you have to cut access holes to get to the frame. And they need to be fairly big holes, to give the fabricator access to weld a reinforcement plate to the frame. Then you have to close up the holes in the fiberglass later... all of that is extra work.

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Once you have access to the frame structure it has to be cleaned of all paint (we use a pneumatic wire brush tool called a "Crud Buster" along with a flap disc on an electric angle grinder). Then the plates are drawn in cardboard and transferred to steel, in this case 1/8" thick hot rolled plate (minimum thickness is .080" for these plates, but we tend to use .125", since it is stronger and easier to weld).

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Olof cut the plates and tack welded them to the frame at the main hoop, which is in an unusual spot. Normally the main hoop mounts to the floor behind the driver's seat - often 6-8+ inches behind the back of the seat. But in a Corvette, for tall-ish folks, the back of the seat ends up right at the rear bulkhead, so the main hoop has to go up on the rear deck area. The frame extends up here and we've checked with NASA inspectors on Corvette cage hoop placements and have also built cages in Corvettes like this. Miata cage main hoops are done the same way - just the nature of these 2 seat cars and their compact interiors.

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On Monday the 5th, Olof designed and bent the main hoop, with help from our head fabricator Ryan. They got the hoop TIGHT up against the high strength steel roof structure, and placed it back the correct distance from my head. We had already done a number of seat mock-ups at this point and we knew where I needed to sit - with the seat almost touching the rear bulkhead. This put the main hoop where it is above.

continued below


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PostPosted: Fri Jan 09, 2015 2:25 pm 
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continued from above

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By this point we had switched our focus from the larger Kirkey aluminum seat we had in stock, to a PORNO RED! Cobra Suzuka Kevlar FIA seat we "horse traded" with a friend for. My buddy Jason McCall had ordered this seat from us last year for his 1989 Corvette but it wasn't fitting with the electric seat adjuster he wanted to use (for better fit with his shorter co-driver - his wife). It is brand new and still good through 2019 on the FIA certification.

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This seat happened at the 11th hour - the day Olof needed to start on the seat mounting and to lay out the harness bar. It turns out our aluminum seat fits better in his full interior C4 and his composite Cobra seat fits better in my gutted C4 with no slider. So we made a seat swaperoo!

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Once more access holes were cut in the rear fiberglass (shown below left) the rear downbars could be cut, notched and built. These will land on 1/8" thick pads on top of the frame, as shown. Two thickness of pad, actually...

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One of the compromises made from our reduced timeline was that the cage became a weld-in 4-point roll bar, and then when we looked at the next step, it became a bolt-in roll bar. Now before you hurl insults, you have to realize that this is going to be a VERY beefy design that can still become a proper weld-in roll cage shortly after the first race. Weld-on "nut plates" (see above right) were created and access holes for the nuts were cut in the frame. This is because the frame is fully boxed and we couldn't bolt into the frame otherwise. These plates have nuts welded to the back side and will be seam welded to the frame, then a matching "footer" plate from the 4 main tubes will land onto these and bolt in place.

All this bolt-in nonsense was done for future access. After our first TT event we have a month off before the SCCA Club Trials event at TWS. During this break we can take the time to turn the 4-point roll bar into a fully welded in 6-point roll cage. The front cage section and door bars take the most time to fit, and we ran out of time. But to do the final welding on the door bars and A-pillar tubes, the cage has to be rotated forward and down, and this bolt-in rear layout will allow for this rear section to be moved for that access. Once the final welding is done up front the four "footers" of the roll bar portion will be welded to the frame plates, and the bolts can be removed. Make sense?

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Yes, that's a little crazy, but our 2 week timeline was just too tight to fully cage the car AND do all of the other performance, safety and maintenance work needed. Next up in the roll bar design is the main harness bar (which the shoulder harness straps will wrap around), then the main hoop diagonal. This is a horizontal bar that is kicked back from the main hoop about 5 inches, to allow for the shoulder harness adjusters to loop around the bar.

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The diagonal bar was cut and being tack welded in right before I made this post on Friday Jan 9th. One more tube is needed for the roll bar (a short tube connecting the harness bar and diagonal) and then it will come out for TIG welding. All of the pictures shown were just tack welds, which were done with the MIG. I'll show the rest of the roll bar and all of the other work happening next week in my follow-up "initial race prep" post. Gotta wrap it up!

Seat Mounting

Mounting a racing seat into a car is NEVER a fun job - installing a real racing seat is always a LOT more work than you might think. Ask any race car fabricator and they will tell you that this type of job sucks. We've installed a lot of racing seats over the years and it is never an easy "bolt-in". Any off-the-shelf seat bracket we've ever seen usually needs massive modification, and some of them raise the seat height by 2-5 inches. They only seem to work for little tiny short European children. Its a dirty little secret in motorsports - bolt-in seat brackets for fixed-back seats almost never fit.

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And this only gets worse with drivers over 6 feet tall with racing helmets adding another 2-3" to their torso height. At 6'3", I'm not a good fit in many OEM seats much less with a racing helmet added. Here at Vorshlag there are 5 people that are 6'2" tall or taller, so we're all used to these seat mounting headaches. The Corvettes from C4-C7 are all pretty cramped inside as well, and we've had to really fight to make racing seats fit in these cars.

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In the most difficult situations (cramped cabin + tall driver) it is not uncommon to spend 6-8 hours fabricating mounts for one seat. Adding in a slider makes this take even longer, but we were out of room here and just mounted the seats directly to the floor (my co-driver Matteucci is almost the same height, luckily).

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Olof took most of a day to test fit the seat (with me sitting in it in a helmet), mock-up the angles and height, reinforce the floor, then modify the OMP side brackets (see image in this section) to get the seat bolted in where I had enough head room to the targa roof with a helmet on. It was tricky and he lowered the "lowest" mounting holes in the OMP brackets by about 5 inches. The original OMP seat mounting holes are crazy tall - doesn't matter what brand of brackets, this always happens.

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I don't have good pictures of the seat mounting from underneath, with the car in the air, but we have beefed up all of the seat mounting points to the chassis. The rear studs were removed and an 1/8" thick doubler runs across the entire width under the steel floor pan section. The front studs were also reinforced. We are adding clip-in harnesses so eyelets with reinforcement plates will go in for lap belt anchors as well as a solid mount for the anti-sub belt under the seat. Will show all of this next time.

Tires Are Everything

Its time to talk about the single most important aspect of this TTC build - the wheels and the TIRES.

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Tires are the most important aspect of a road race car. Let me repeat that for emphasis: Tires Are The Most Important Thing In Racing. The four tire contact patches are the only things connecting your car to the race track. Through these four little patches all of your forward accelerations, braking and cornering loads are generated. All of the work we do on the suspension is just to make sure the tires are happy - to make them stay flat, to always keep them in contact with the road, and to make sure loads are distributed as evenly to all 4 patches as possible.

So with this car being based in TTC class with a 7 point penalty, that leaves us with only 12 points to work with (19 class points - 7 penalty). And while that gives us some options for lots of different mods (upgrading power, brakes, suspension, lightening the weight, aero and tires), we're going to burn almost all of our points on the tires. This is a very critical decision, so let me explain what we're doing. This decision was made after hours of internal debate, hundreds of permutations of width + compound (+ other non-tire mods), but mostly comes from years of racing experience and knowledge: Knowing that the tires are almost all that matter.

TIRE WIDTH - As I pointed out in a previous post, everything you modify in the TT letter classes is either listed as a No Points Modification (which we are using every one we can!) or is assigned a number of points. It is all clearly stated in the TT rules. Tire width changes are "expensive", and the points in sizes increases above the "base class size" (TTC = 255mm) are shown below.

  • Equal to or greater than: 10mm +1, 20mm +4, 30mm +7, 40mm +10, 50mm +13, 60mm +16, 70mm +19, 80mm +22, 90mm +25, 100mm +28, 110mm +31, 120mm +34, etc.
  • Equal to or less than: -10mm -1, -20mm -4, -30mm -7, -40mm -10,

Big Wheels Keep On Turning!

As you can see you can get points BACK by going to a SMALLER tire as well. There is no other way in TT-Letter classes to gain points back, so this a big deal - and something we are going to do. Many will be surprised by this, as I've preached "BIGGER IS BETTER" for so many years. And while that is still true, we just don't have the points to go bigger, and feel that burning the points ALL on the compound makes more sense. Here's a comment from a corner-carvers reader and my reply:

Nick C wrote:
Will the rules let you put 335's on? 17x12" rims are a bolt on affair.


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Yes, it would is technically "legal" to run 335mm tires on a C4, but unfortunately the points just aren't there to do this and stay in TTC class. We're going to be running much narrower tires than that, but with what we feel is the right compound.

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These pictures are of Jason McCall's 1989 Corvette that is prepped for SCCA BSP class (and was the National Championship winning car in 2005). It runs 17x11" CCW wheels in front and 17x12" wheels out back with Hoosier A6s in 315mm up front and 335mm out back. The fit is pretty tight - it has custom flared front "fenders" (the hood) and has the little 1996 Grand Sport "export" flares out back to make these fit - and we can legally add flares for zero points.

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I've driven and ridden in this car and it is a GRIP MACHINE, just a big go-kart. Very fun, and the wide, sticky autocross compound tires he runs are why its so fast. And while I'd love to do this on our C4, the points for the compound (Hoosier A6 = +17 points and A7 = +22 points!) plus the increase in tire width (255->335 = +80mm = +22 points) would cost a whopping +44 points for just this tire upgrade. Using all of the points we have in TTC (19 - 7 = 12) and then even moving up to TTB (+20 more) we're still short by 12 points for a 335mm A7, so that tire choice would be a move straight to TT3. This is why we cannot use the tires we'd LIKE to use (I'd slap 335mm Hoosier A7 tires on this in an instant if the points allowed it!) but the compromise we have chosen will still work well enough - we suspect. Remember: Everything in racing is a compromise... and everything depends on everything else.

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Many of you that have experience with the C4 Corvette know that most of the later C4s came with a 275/40/17 tire on 17x9.5" wheels at all four corners, as did our 1992 Base Trim Model Corvette. But the TTC class "base tire" is 255mm, no matter what the OEMs put on the car. Wheel width is unrestricted, other than a track width change limit of +4 inches. Beyond that you take points. Our car has 285/40/17 old and crusty street tires on it right now, which would cost us (+30mm over 255) +7 points to use, but they are a joke. So hard they can spin freely through the first 3 gears. I won't be caught dead on a road course with old street tires, not even brand new 120-200 treadwear street tires (which are worth +2 points), unless the rules require that for everyone.

After racing our TT3 car in various "street tire" events/series last year, and at some tracks we also ran with R-compound Hoosier A6s in other series, I know the true lap time value of sticky R-compound tires. Going from a 335mm BFGoodrich Rival to a 345mm Hoosier A6 is worth a MASSIVE amount of time. On a typical 2 minute road course that difference is 5-7+ seconds per lap with the Hoosier over a 200 treadwear tire, and the Hoosier is MUCH easier to drive. So we're gonna stick with what we KNOW works and that has a great NASA TT contingency program: Hoosier.

TIRE COMPOUND - The compound of the tire is as important than width in Time Trial. Maybe even more important. Why? Because every TT lap is essentially run at a Qualifying lap pace, where you need to be pushing 10/10ths. To win you just need to set ONE fast lap per day (each day is a new competition), and waiting around for 3-4 laps for your "tires to warm up" will only get you mired up in traffic, as the front of the field catches the back end. There are a LOT of tire compounds listed and points assigned for each. The only "free" tire compound in TT-letter are those over 200 UTQG treadwear numbers. The tire models are grouped together with compound and performance parity, and the points given look to be pretty fair. There were massive adjustments made to these points for 2015, which was long overdue.
  • DOT-approved R-compound tires: BFG R1S, Goodyear Eagle RS AC (autocross), Hankook Z214 (C90 & C91 compound only), Hoosier A7, Hoosier Wet DOT (if used in dry conditions—see section 5.6) +22
  • DOT-approved R-compound tires: Hoosier A6 +17
  • DOT-approved R-compound tires and those with a UTQG treadwear rating of 40 or less not listed otherwise in these rules: BFG R1, Goodyear Eagle RS, Hankook Z214 (C71, C70, C51, C50), Hoosier R6 & R7 & SM7, Kumho V710 (note: Continental Tire Sportscar Challenge EC-Dry tires OK (225, 245, 275) +10
  • DOT-approved R-compound tires: Toyo Proxes RR, Hankook TD +7
  • DOT-approved R-compound tires and those with a UTQG treadwear rating of 50 to 130: Maxxis RC-1 (ex. Kumho V700, Michelin Pilot Sport Cup, Nitto NT01, Pirelli PZero Corsa, Toyo R888, Toyo RA-1, Yokahama A048, etc.) +6
  • (non-R-compound) tires with a UTQG treadwear rating of 120-200 (examples: BFG g-Force Rival, Bridgestone Potenza RE070, Dunlop Direzza Sport Z1 Star Spec, Hankook R-S3, Kumho Ecsta XS, Toyo R1R, Yokohama Advan A046 & Neova AD08,) +2
  • Non-DOT-approved racing slicks +30

That is a dizzying array of compound choices and, when combined with the size choices, it makes for a lot of possibilities. But we've run the numbers using these compounds + various widths and have settled upon: 245/40/17 Hoosier R7. Not the softest tire but damned close. Not the widest tire but "wide enough" (and it gives us a point back). The tire choices are still very limited in the brand new Hoosier A7/R7 compounds, but this seems to fit the bill. The spec's on this particular 245mm tire look pretty dang good, and I'm hearing good things about the R7 compound in tests. Our first event will teach us a lot... either we guessed right or made a big mistake!

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These wheels are stupid light! We will weigh the wheels alone once the old 275 Hoosiers that came on them are dismounted

WHEELS - We will run these Hoosiers initially on some 17x9.5" SSR wheels, which are both light and strong. Very light, in fact... around 15 pounds. Getting a set of these Corvette sized SSRs is like finding a wild unicorn - very rare and no longer made. SSR went out of business after the 2008 recession but it seems that they have reformed and are back - but not making a lot of the "big" sizes that fit Corvettes any longer. This set came from our shop manager Brad's former Super Stock 1994 Corvette, and he has two identical sets in perfect shape. I have dibs one set but the other is available. The Hoosier A6s on these wheels are DOT stamped from 2008! These wheels are perfect and have been sitting in his attic for almost 7 years.

What's Next

I could go on. And on. But I have probably bored you enough! Our crew is still busy at work finishing the prep on the Corvette for the first race and I'll try to do a quick update next week, right before we head down to MSR Houston Jan 16th. There's still a lot to do and not much time left...

Cheers,

Terry Fair - http://www.vorshlag.com

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PostPosted: Fri Jan 09, 2015 3:45 pm 
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Fair wrote:
But we've run the numbers using these compounds + various widths and have settled upon: 245/40/17 Hoosier R7.


loftygoals wrote:
When I first looked at this, I wondered how you were staying at 12:1. Then I noticed you said you were running a 30mm smaller than stock tire. Stock is a 275, so you are running a 245. This means you can take a power modification +0.8. Thus your raw p/w can be be 11.2:1 and still be legal.



Looks like I nailed your tire size too. I would have put money on a R7 in 245. It is the only thing that makes sense, because you need the points for suspension.

Tire widths are interesting. I agree that in general "BIGGER IS BETTER", but it depends on a lot of things. Overall, the difference in raw grip between a 245 and a 295 is next to nothing. This is physics. Tire compounds have some interesting characteristics that makes it slightly advantageous to be wider. But, the penalty for going wider is weight. It is unsprung rotating weight, so you take a huge performance penalty for tire weight. The real reason for going wide is to manage heat. All tires have an optimal operating temperature range. If you are overheating tires, going wider gives you more surface area reducing heat and letting it cooler more efficiently. Also the wider the tire the stiffer the sidewall. Sidewall flex actually generates a lot of tire heat. If you reduce flex you reduce heat.

Now as you said, in TT the goal is 1 flying lap. Thus, as long as you can do that early in a session, heat isn't an issue. This makes a narrower tire a good choice in TT. This is exactly why I'm only running a 205 on my MINI Cooper. I don't need a wider tire and a narrower tire weighs less and gives me points back.

Good work, Terry!

-bj

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PostPosted: Sun Jan 11, 2015 7:06 pm 
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Read the whole thing - interesting, and very well thought out.

I am debating tire size for my car as well. In ST3 I don't get points, but I do get HP, and my car will be pretty light - around 2600lbs as raced. I could run a 245 R7, which would give me around 24 HP over a 275 tire, which is about as large as I can comfortably go in the stock bodywork (maybe a 285, but it's not worth it to just break into the next HP adjustment range).


BJ, you should post up some details on your build as well!

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PostPosted: Sun Jan 11, 2015 8:32 pm 
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Sterling Doc wrote:
I am debating tire size for my car as well. In ST3 I don't get points, but I do get HP, and my car will be pretty light - around 2600lbs as raced. I could run a 245 R7, which would give me around 24 HP over a 275 tire, which is about as large as I can comfortably go in the stock bodywork (maybe a 285, but it's not worth it to just break into the next HP adjustment range).


Hey Eric!

You were the first to turn me on to the idea that tire width isn't everything when you were testing the 225/50R15 vs. the 205/50R15 for the 944 spec cars. I think 245 vs. 275 would come down to how the 245 would hold up over the course of an entire race. Testing different tires is expensive, but if you can run the same laps on the 245 vs. the 275, without more falloff on the 245, then the 245 would be fast because of the HP allowance.


Sterling Doc wrote:
BJ, you should post up some details on your build as well!


I will as soon as I have time to finish writing it. I've been so busy building the car that I haven't had much time for anything else. It will be down to the wire to get it ready for this coming weekend.

When are you going to come down here to Texas for a race? We have a growing ST field...

-bj

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PostPosted: Mon Jan 12, 2015 5:36 am 
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Yeah interesting stuff. We could turn pretty similar lap time in a sprint race with a durable R-compound tire like the RA-1. But when we tries to do it with same size SM6 in a shorter enduro, we destroyed the tire to the point of it being non-drivable. Context means a lot. On the face of it, it seems like a 245 would be a lot of tire compared to the 225's I'm used to driving, and could keep under me for a whole 4 hour enduro, but then I wasn't putting much heat into the tires from acceleration, and braking.

I would love to get down, and see the shop some day.

Now, back on topic - metallurgy and 'Vette's - I'm learning a lot!

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