THSCC Forums

Hi All,

I'm not smart enough to figure this out, but figure some of you mechanically inclined folks may be able to help me out. I'm trying to get an idea of the impact of running a taller tire on my acceleration...for lack of a better benchmark I'll use 0 - 60 times. Ideally, I'd like to know it for 30 - 50 times.

If I currently run a tire that is 22.8 inches tall on the rear of a Toyota Spyder that does 0 - 60 in 6.8 seconds. What effect would a 24.4 inch tire have on these times. The difference in gearing difference is 7.0% according to the tire/gear calculators. It changes the top speed of 2nd gear from 55 to 59 MPH. I'm assuming it would be in correct to add 7.0% to whatever time I was calculating.

I guess, I'm generally looking for some feedback like...it will cost you half a second, no time, an assload of time, etc.

Thanks,
Eric
Any feedback would help. Also, I guess I can just

You really need Cartest v4.5. It doesn't have the Spyder in the built-in database, but if you have the relevant specs you can create a new car (or easier, duplicate an older MR-2 and modify so it looks like the spyder).

http://www.cartestsoftware.com/cartest4.5/index.html

This program is actually a little "too" powerful for your simple comparison, but once you get the details worked out it is a fantastic tool for doing hypothetical "what-if" comparisons. I use it a few times a year.

Short answer is - tire size changes like FD changes don't actually change acceleration times much unless there is a change in necessary gear changes to achive the target speed. Think about it this way - with shorter gearing, you get better acceleration in low gears, but you spend less time there before a taller and slower gear is needed.

IMO, for autocross, it comes down to minimizing required 2-3 shifts. 59 MPH sounds about right. 55 Mph would get annoying. On track, in most situations, gearing should be selected to be at redline in top gear on the fastest straight. But don't listen to me, what do I know? Anyone recommend a good text for gearing issues? I have a stack of books that I haven't even read...

_________________
Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

There is the gearing issue (which is part of teh equation), but it's offset by the shifting issue.

A larger tire does have more rotational inertia, so it will hurt acceleration times in that regard as well (and make it harder to stop the car).

Scott

Eric,

I agree with the previous comments. I tried second gear changes up to 10 percent in my FF with no acceleration "problem" with the higher gearing. Since I assume this is for an autocrossing Tour/Pro Solo application I think the biggest potential problem may be Pro Solo starts. If the combination of taller tires and, hopefully, more grip cause the engine to bog at the start, your 0 - 60 times will suffer. Another potential problem could be exit from slower corners. Two possible problems: Higher grip in the rear may tighten the car causing understeer. Also, if the higher grip causes corner exit bog it may slow your elapsed time to the next corner. Of course, if the car is way too loose in the rear and you need more traction for both cornering and acceleration, then bigger tires should help. In fact, if you are getting too much wheelspin out of a lot of corners, the shorter tires are no advantage then anyway.

If the taller tire is also wider, it may be more sensitive to normal camber changes . . . unless the taller sidewall makes it less sensitive.

Note that the extra 0.8 inch of rear ride height and resulting rake of the car and higher rear center of gravity may have more impact than any of the other factors.

Testing under representative conditions is the only way to find out (other than seeing what the other fast guys in the same car are doing).

_________________
Dick Rasmussen

FS 50 2018 Mustang GT

Mike, Scott, & Dick,

Thanks for the feedback! We had to choose between running 215/40-16s or 245/45-16s on the rear of Donnie's Spyder at Ft. Myers. We decided to run the 245s after some testing on the Mini-course. I usually avoid the Mini-Course since it's almost always very different than the competiton course. Anyway, the new 16 x 7 rear wheel for the 2003 and 2004 Spyders is problematic in that there is not a good tire size available compared to the 225/45-15s I used to run on my black Spyder. The 225s put plenty of rubber down and don't penalize me with the gearing issues. If they could have left the rear wheels at 15 inches on the new 2004 I would have been in great great shape. Unfortunately, the next smallest available tire in the S04 is a 205, which is 8/10ths inch less rubber than the 225 (what the Miata folks are running).

Chris and I will probably run some different tire combos at Laurinburg in March to see the difference.

Thanks for the help!

Eric

Here are some formulas to help out.

Mph = (rpm x tire diameter)/(gear ratio x 336)
Tire Diameter = (mph x gear ratio x 336)/rpm
Gear Ratio = (rpm x tire dismeter)/(mph x 336)
Rpm = (mph x gear ratio x 336)/tire diameter

So you can get your new final ratios and go from there.