Ideal tire height for roll race -- go taller?

[WildHorse]

ould you send me the tool you used? Thanks!

S550 Shift Points - aaron rivero | Tableau Public

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[NGOT8R]

Have you considered taking the car to the dyno and seeing what you get for MPH? You can also use a track outing to test MPH and see if it coincides with your gear/tire sizes. Once you’ve used a few methods to crosscheck your data, you should be gtg.

 

[2019GT]

Have you considered taking the car to the dyno and seeing what you get for MPH? You can also use a track outing to test MPH and see if it coincides with your gear/tire sizes. Once you’ve used a few methods to crosscheck your data, you should be gtg.

Wait a second -- I do have my dyno printout l. I'll take a look at that and see!

Also I think I see the speedo problem actually! Going to try correcting it with the whipple tomahawk.

So basically, the nGauge only let's you enter tire diameter, which likely just calculates the revs per mile. For me, I get about 755. But Michelin says the actual revs per mile is 778!

So this should explain why I'm reading low.

 

[TonyT930]

Now that I got the traction issues sorted out from my other thread ..

I compared datalogs from my 28.7" tall tires to 26.7" tires, and the difference is bigger than I realized.

On the 26.7" tires, I'm already in 6th gear at around 121mph, but the 28.7" tires can hold on until about 133mph! I know there's no escaping 6th gear anyway lol, and I'd like to think the shorter tires will hold the advantage *eventually*, but not until much higher speeds where I'm probably not going to be.

I don't have actual dragy pulls, but I have datalogs and at -800ft DA on the 28.7" tall I did a 5.7s 60-130, and the same strip at +700ft DA nailed a 6.1 on the shorter tires. PCM is updated for tires, and Ive done quite a few dragy runs in the past w/ datalogs and the datalogs were always slower than what dragy reported. So I trust these times.

Also I compared spark timing between the two runs just to be sure and my timing was actually even better on the slower run. And I don't think the DA can account for half a second.

So I'm just curious if I'm actually slowing myself down with the shorter tires!

Maybe I just need to do a few more runs and this was just a fluke, but something tells me if I care about 60-130 then having to hit 6th gear is always going to be disadvantageous.

Get a tire with better traction. Taller is slower

 

[2019GT]

Get a tire with better traction. Taller is slower

Yeah I'm trying to wrap my head around it. I know taller tires will have a larger contact patch and from a dig that's where they offer the biggest advantage, but they are also heavier (all optimizations equal) with greater inertia.

I actually just tried to solve this with basic calculus, comparing a 26.5" tire against a 28.5" tire, and if all of my math checks out, then *IGNORING* shift times, the shorter tire actually spends more *OVERALL* under a higher ratio.

Somebody who cares could review and see if what I did makes sense LOL

---------------------------------------

Transmission Ratios:

1st: 4.696
2nd: 2.985
3rd: 2.146
4th: 1.769
5th: 1.520
6th: 1.275
7th: 1.000

Final Ratio: 3.15

Stock Tire: 27.7"

Area Under Curve Formula:

f(x) = x*g = MPH*TransRatio*FinalRatio*TireRatioOffset
Integral f(x) = ((x^2)*g)/2 = area under curve
area = (b^2)*g)/2 - (a^2)*g)/2


26.5", 7800rpm, TireRatioOffset = 1.045283

3rd: [60-91], 2.146*3.15 (6.7599)
4th: [91-110], 1.769*3.15 (5.57235)
5th: [110-128], 1.520*3.15 (4.788)
6th: [128-153], 1.275*3.15 (4.01625)

Area for 60-130 = ...

(91^2)*(6.7599*1.045283))/2 - (60^2)*(6.7599*1.045283))/2 +
(110^2)*(5.57235*1.045283))/2 - (91^2)*(5.57235*1.045283))/2 +
(128^2)*(4.788*1.045283))/2 - (110^2)*(4.788*1.045283))/2 +
(130^2)*(4.01625*1.045283))/2 - (128^2)*(4.01625*1.045283))/2

39463.652822282325

Area for 100-150 = ...

(110^2)*(5.57235*1.045283))/2 - (100^2)*(5.57235*1.045283))/2 +
(128^2)*(4.788*1.045283))/2 - (110^2)*(4.788*1.045283))/2 +
(150^2)*(4.01625*1.045283))/2 - (128^2)*(4.01625*1.045283))/2

29674.074981348

:::::::::::::::::::::::::::::::::::::::::::::::::

28.5", 7800rpm, TireRatioOffset = 0.97192982


3rd: [60-98], 2.146*3.15 (6.7599)
4th: [98-119], 1.769*3.15 (5.57235)
5th: [119-138], 1.520*3.15 (4.788)
6th: [138-164], 1.275*3.15 (4.01625)

Area for 60-130 = ...

(98^2)*(6.7599*0.97192982))/2 - (60^2)*(6.7599*0.97192982))/2 +
(119^2)*(5.57235*0.97192982))/2 - (98^2)*(5.57235*0.97192982))/2 +
(130^2)*(4.788*0.97192982))/2 - (119^2)*(4.788*0.97192982))/2

38436.8942798734005

Area for 100-150 = ...

(119^2)*(5.57235*0.97192982))/2 - (100^2)*(5.57235*0.97192982))/2 +
(138^2)*(4.788*0.97192982))/2 - (119^2)*(4.788*0.97192982))/2 +
(150^2)*(4.01625*0.97192982))/2 - (138^2)*(4.01625*0.97192982))/2

29374.8839339816385

::::::::::::::::::::::::::::::::

Yes I am crazy

So ignoring the worse inertia of the taller tires, and ignoring the extra shift on the 60-130 with the shorter tires ...

The shorter tires should hold the advantage in both the 60-130 and 100-150, but the advantage is extremely slim in the 100-150, I guess because both runs would be using the same gears.

My math could be wrong, and I understand nobody probably cares about the math and would rather just do real world runs. I'm just trying to see if I should spend money or not first

Anyway, I asked my tuner about ensuring the shift points are bumped a bit, and I've taken the point about shorter is faster (which seems to agree with the numbers I made).

 

[K4fxd]

It takes about 1/2 second for the trans to shift.

 

[2019GT]

It takes about 1/2 second for the trans to shift.

If it's actually that slow, then with the numbers being so close in the "ratio curve area" above ... the tire that minimizes the number of shifts might hold the advantage then

So far my best 60-130 with an extra 1400ft DA is about 0.4s slower than my best runs on the taller tires.

I dunno, I just find it all interesting to experiment lol.

 

[K4fxd]

If it's actually that slow

It's not a linear hit because you are moving. The shifts cost you more at the beginning of a run than at the end.

If I had the room I'd raise the shift RPM and use the shorter tire for the better overall gear ratio. If I'm out of RPM I'd use the taller tire. A shift just before the end of a run is always going to cost you time.

 

[qwiknotch]

When my car made 600rwhp, I ran it at Coalinga for a 1/2 mile event. I was on a 305/35/20 Et Street SS. 28.4 tall
if I let the trans shift itself, it would go into 7th about 100ft before the 1/2. and it ran 156-158 5 passes in a row. commanded rpm was 7700 and it was shifting at 7400-7500. The last pass I decided to put the trans in sport mode and shift it with the paddles. I shifted it at 7600 and the delay with the paddles was around 200rpm.
I held 6th out and crossed at 7800 and went 160.71. in all the passes, it shifted out of 5th around 129, except the 160.71 pass, it shifted into 6th at 133. At the time, my car was running 6.0 in the 60-130 thru the factory exhaust with the Vortech smog tune. No idea what it will do now with 400 more rwhp.

if you hook no matter what, in theory, the shorter tire will accelerate thru the rpm quicker, but will have less mph at the same rpm as the taller tire. With a 10r and a whipple, it wont give 2 shits less about the taller tire.
For roll racing, I'd run the taller tire, in my opinion.

 

[robvas]

This is exactly the same as 3.55 vs 3.31

26.5" vs 28.5"

 

[2019GT]

When my car made 600rwhp, I ran it at Coalinga for a 1/2 mile event. I was on a 305/35/20 Et Street SS. 28.4 tall
if I let the trans shift itself, it would go into 7th about 100ft before the 1/2. and it ran 156-158 5 passes in a row. commanded rpm was 7700 and it was shifting at 7400-7500. The last pass I decided to put the trans in sport mode and shift it with the paddles. I shifted it at 7600 and the delay with the paddles was around 200rpm.
I held 6th out and crossed at 7800 and went 160.71. in all the passes, it shifted out of 5th around 129, except the 160.71 pass, it shifted into 6th at 133. At the time, my car was running 6.0 in the 60-130 thru the factory exhaust with the Vortech smog tune. No idea what it will do now with 400 more rwhp.

if you hook no matter what, in theory, the shorter tire will accelerate thru the rpm quicker, but will have less mph at the same rpm as the taller tire. With a 10r and a whipple, it wont give 2 shits less about the taller tire.
For roll racing, I'd run the taller tire, in my opinion.

Thanks for your insight, so it seems holding out the gear for just a bit longer made quite a difference for you.

Carrying on with your last point, I am almost considering to upsize to the taller tire just for the consistent peace of mind in traction.

I was able to get my shifts updated to 7800 and I did some testing finally today. Once again I seem to be running into traction problems. I was able to clear the 3-4 and 4-5 shifts but traction felt a little off so I didn't carry on to the 5-6.

So I don't know if the higher shift points with the whipple are pushing me too close to my tires limits, or if I just need to wait for an even warmer day. We had some fog overnight and temps are low 70s. So could just be that and not the RPMs.

But honestly I'm getting tired of having to be unsure about traction lol.

 

[qwiknotch]

Thanks for your insight, so it seems holding out the gear for just a bit longer made quite a difference for you.

Carrying on with your last point, I am almost considering to upsize to the taller tire just for the consistent peace of mind in traction.

I was able to get my shifts updated to 7800 and I did some testing finally today. Once again I seem to be running into traction problems. I was able to clear the 3-4 and 4-5 shifts but traction felt a little off so I didn't carry on to the 5-6.

So I don't know if the higher shift points with the whipple are pushing me too close to my tires limits, or if I just need to wait for an even warmer day. We had some fog overnight and temps are low 70s. So could just be that and not the RPMs.

But honestly I'm getting tired of having to be unsure about traction lol.

With my Vortech on E85 and 10speed, the Mickey et street SS 305/35/20 works great. I also use the 28x10.5-17 Hoosier at the track, but prefer to just leave the 20" wheels on

 

[2019GT]

With my Vortech on E85 and 10speed, the Mickey et street SS 305/35/20 works great. I also use the 28x10.5-17 Hoosier at the track, but prefer to just leave the 20" wheels on

At my power level (~700whp) I was completely confident in my AS4s for 3rd gear pulls, but those were sized similarly to your Mickeys at 28.7" tall.

I think I'll wait for temps to rise a bit more, 70s is just getting started here. If I seriously still feel sketched in the 80s and up I'm going to upsize.

 

[cbrtrx]

Lots of info here, some better then others. Without trying to explain everything I'd use the taller tire for your particular setup. This comes from my experience.

 

[2019GT]

Lots of info here, some better then others. Without trying to explain everything I'd use the taller tire for your particular setup. This comes from my experience.

Especially since I'm not running an R compound I'm extremely tended to agree with you.

If you wanted to expand on your reasoning I'm always willing to learn more!

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