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PostPosted: Fri Dec 30, 2016 11:14 pm 
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its my understanding that it is actually -0.3 on the <275mm tire rule.. the verbiage used in the rules seems to be the exact opposite of what makes sense - it would be a higher weight-to-power ratio for running narrower tires if you read it the way its written, I'm not really sure what is going on there but I have to assume its the opposite of what they're saying. Everyone in TT4 is going to have an initial base of 11.7:1 because we're not allowed to run larger than 275's, so we all get the -0.3 break... which also seems asinine to me, but what can you do.

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PostPosted: Fri Dec 30, 2016 11:46 pm 
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Why?

From the rules:
Code:
Tire Size: Size 245 or smaller (DOT-approved) = +0.7
Size 275 to 250 (DOT- approved) = +0.3


This makes sense. That doesn't look the wrong way around at all?


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PostPosted: Sat Dec 31, 2016 10:21 am 
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int2str wrote:
Why?

From the rules:
Code:
Tire Size: Size 245 or smaller (DOT-approved) = +0.7
Size 275 to 250 (DOT- approved) = +0.3


This makes sense. That doesn't look the wrong way around at all?



So you're at 11.58:1 right now - if you run 275's you add +0.3 so you now have to be at 11.88:1 weight/power (ignoring the weight points for simplicity).. but if you run a narrower tire you have to add 0.7, bringing your weight/power ratio to 12.28:1 right? How does that make sense? Being penalized and forced to run a higher weight/power ratio for running a narrower tire. Unless I'm missing something glaring, I feel like I'm living on Bizarro.

Also, you don't start with your cars power to weight ratio before adding modification factors - you start with the base class ratio of 12:1 then add/subtract the modification factors - once you figure out that ratio, you have to get your car at or above it. It's been discussed and confirmed that everyone is starting at 11.7:1 in TT4 because we're all getting the 0.3 break for running 275 or smaller, since tires larger than 275 are banned.

They used to have a TT1-3 class calculator in the form of an excel spreadsheet posted on the NASA site but I can't find it now :/

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PostPosted: Sat Dec 31, 2016 11:16 am 
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JCathers wrote:
int2str wrote:
Why?

From the rules:
Code:
Tire Size: Size 245 or smaller (DOT-approved) = +0.7
Size 275 to 250 (DOT- approved) = +0.3


This makes sense. That doesn't look the wrong way around at all?



So you're at 11.58:1 right now - if you run 275's you add +0.3 so you now have to be at 11.88:1 weight/power (ignoring the weight points for simplicity).. but if you run a narrower tire you have to add 0.7, bringing your weight/power ratio to 12.28:1 right? How does that make sense? Being penalized and forced to run a higher weight/power ratio for running a narrower tire. Unless I'm missing something glaring, I feel like I'm living on Bizarro.

Also, you don't start with your cars power to weight ratio before adding modification factors - you start with the base class ratio of 12:1 then add/subtract the modification factors - once you figure out that ratio, you have to get your car at or above it. It's been discussed and confirmed that everyone is starting at 11.7:1 in TT4 because we're all getting the 0.3 break for running 275 or smaller, since tires larger than 275 are banned.

They used to have a TT1-3 class calculator in the form of an excel spreadsheet posted on the NASA site but I can't find it now :/


I think you're misunderstanding how to do factors.

To run in TT4, your final adjusted wt/hp has to be above 12:1.

So let's just take tires as only modification that your car has for simplicity.

First you start with your unadjusted wt/hp. Average hp divided by actual wt. Before all adjustments/modifiers.

If your unadjusted wt/hp is 11.31, then you would have to run 245's DOT or skinnier (+0.7) to have a final adjusted wt/hp of 12:01.

If your unadjusted wt/hp is 11.71, then you would have to run 275's DOT (+0.3) to have a final adjusted wt/hp of 12:01.

So, basically running narrower tires allow you to run a lower unadjusted wt/hp which basically means either more starting power or less starting weight.

I'm in TT3. My average hp is 338. My weight is 3145. This gives me an unadjusted wt/hp of 9.305. I run 245 Hoosiers (+0.7) and non-OEM aero (+0.0) which puts me at 10.005 final adjusted wt/hp. If I tried to run 275's, I would be at 9.605 and would have to either add weight, cut power, or move to TT2.

Hopefully this explanation helps.

Denis

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2017 NASA TX TT3 Champion
2014-2016 NASA TX TT3 Runner-up


Last edited by Clavette on Sat Dec 31, 2016 7:22 pm, edited 1 time in total.

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PostPosted: Sat Dec 31, 2016 2:13 pm 
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Yes, Denis has explained it well.

I'll repeat the calculation for my car as well here:

3406 lbs / 294 avg.hp. = 11.58 : 1
+0.2 Vehicle weight >3400lbs
+0.3 Tire size 255
=
12.08 : 1 = TT4


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PostPosted: Sat Dec 31, 2016 3:52 pm 
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Clavette wrote:
JCathers wrote:
int2str wrote:
Why?

From the rules:
Code:
Tire Size: Size 245 or smaller (DOT-approved) = +0.7
Size 275 to 250 (DOT- approved) = +0.3


This makes sense. That doesn't look the wrong way around at all?



So you're at 11.58:1 right now - if you run 275's you add +0.3 so you now have to be at 11.88:1 weight/power (ignoring the weight points for simplicity).. but if you run a narrower tire you have to add 0.7, bringing your weight/power ratio to 12.28:1 right? How does that make sense? Being penalized and forced to run a higher weight/power ratio for running a narrower tire. Unless I'm missing something glaring, I feel like I'm living on Bizarro.

Also, you don't start with your cars power to weight ratio before adding modification factors - you start with the base class ratio of 12:1 then add/subtract the modification factors - once you figure out that ratio, you have to get your car at or above it. It's been discussed and confirmed that everyone is starting at 11.7:1 in TT4 because we're all getting the 0.3 break for running 275 or smaller, since tires larger than 275 are banned.

They used to have a TT1-3 class calculator in the form of an excel spreadsheet posted on the NASA site but I can't find it now :/


I think you're misunderstanding how to do factors.

To run in TT4, you're final adjusted wt/hp has to be above 12:1.

So let's just take tires as only modification that your car has for simplicity.

First you start with your unadjusted wt/hp. Average hp divided by actual wt. Before all adjustments/modifiers.

If your unadjusted wt/hp is 11.31, then you would have to run 245's DOT or skinnier (+0.7) to have a final adjusted wt/hp of 12:01.

If your unadjusted wt/hp is 11.71, then you would have to run 275's DOT (+0.3) to have a final adjusted wt/hp of 12:01.

So, basically running narrower tires allow you to run a lower unadjusted wt/hp which basically means either more starting power or less starting weight.

I'm in TT3. My average hp is 338. My weight is 3145. This gives me an unadjusted wt/hp of 9.305. I run 245 Hoosiers (+0.7) and non-OEM aero (+0.0) which puts me at 10.005 final adjusted wt/hp. If I tried to run 275's, I would be at 9.605 and would have to either add weight, cut power, or move to TT2.

Hopefully this explanation helps.

Denis


That makes sense, I guess I've been looking at it from the flip side since I'm well above 12:1 in B trim so I've been working toward getting it to the actual 11.8:1 I'll be at with the "+0.3" I'll get for <275's and the "-0.1" for being <3000lbs - I misunderstood the OP and thought he was under the impression that he had ballast/detune up to 12.08:1

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PostPosted: Sat Dec 31, 2016 7:29 pm 
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int2str wrote:
Yes, Denis has explained it well.

I'll repeat the calculation for my car as well here:

3406 lbs / 294 avg.hp. = 11.58 : 1
+0.2 Vehicle weight >3400lbs
+0.3 Tire size 255
=
12.08 : 1 = TT4


If I were you, I'd drop weight back as low as you can and switch to 245's. Hoosier 245's are plenty wide.

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2017 NASA TX TT3 Champion
2014-2016 NASA TX TT3 Runner-up


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PostPosted: Sun Jan 01, 2017 8:26 am 
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Leaving the horsepower where it currently is, I would be able to drop weight to 3292.8 lbs. Or, about 100lbs less than it is currently. Then the million $$$ question becomes whether that makes a big enough difference to make up for the downsides.

Loosing 100lbs at this point would be massively reducing streetability and the car is already not that great to drive on the road :D

If I ever decide to trailer it, then of course, all bets are off...


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PostPosted: Tue Jan 03, 2017 3:52 pm 
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int2str wrote:
Then the million $$$ question becomes whether that makes a big enough difference to make up for the downsides.

Yes, weight loss will help.


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