Taller tires in rear, handling/braking issues? - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 7 Old 09-30-2009, 02:24 PM Thread Starter
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Taller tires in rear, handling/braking issues?

I have 28" tall tires (255/55/17) in the rear, and 26" tires (245/45/17) up front. How will this effect how the car turns, brakes or handles when taken to the limit? Thanks


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post #2 of 7 Old 09-30-2009, 07:23 PM
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The car would probably handle and brake better with the bigger tires up front and the smaller tires in the rear. With 518 rwhp, it ain't going to matter which tires you have on the rear for grip purposes.

You are going to need to either lurk on Corner-Carvers and use their search function for a month OR buy a copy of Race Car Vehicle Dynamics and read it to learn about roll centers to get a good answer to this question. Playing around with Bill Shope's suspension website is also a good idea (google it).

Reader's digest version: It's not optimal. 245/45-17s are 25.7 inches tall. 255/55-17s are 28.0 inches tall. That's a difference of 2.3 inches, which means that the rear axles are 1.15 inches higher than the front spindles. On a tiny level, this actually shortens your effective wheelbase (think about what having 5' tall rear tires would do to the car. As the ass end lifts up, the horizontal distance from front contact patch to rear contact patch lessens AND you have more weight on the front wheels of the car). Is this a huge deal? No. Is it optimal for handling? Nope. Unlike Porsches, Vettes, and other cars with different stock tire sizes front/rear, stock Mustangs, imo, were designed to run the same wheel/tire size front and rear (although some newer special package Mustangs may not be like this anymore, I dunno).

If you raise the ass end on an already nose heavy car, do not expect it to handle well. It will likely turn-in harder but the car will pivot around the front nose more, causing the unloaded ass end to want to keep pivoting so much that the car oversteers even more wildly than normal. Also, it won't put down any power coming out of a turn (especially if you still have rear UCAs) because your rear geometry will be even more messed up.

If you have rear coil overs, you can lower the rear spring height to help move some of the weight back to the rear of the car back, thereby relieving some of the problems of taller rear tires (or, with traditional springs, just cut them). You can also try tuning with sway bars, shocks, spring rates, etc..

Are there fast track cars out there with mismatched front/rear tire sizes? Without a doubt. Would they go same sizes front/rear if they could? Probably, imo, unless they've got some trick front spindles/a-arm/k-member setup which addresses the problem.


Last edited by aurdraco; 09-30-2009 at 07:36 PM.
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post #3 of 7 Old 10-01-2009, 09:04 AM
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For one thing, you may have issues with the ABS...
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post #4 of 7 Old 10-04-2009, 07:31 PM Thread Starter
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Quote:
Originally Posted by aurdraco View Post
The car would probably handle and brake better with the bigger tires up front and the smaller tires in the rear. With 518 rwhp, it ain't going to matter which tires you have on the rear for grip purposes.

You are going to need to either lurk on Corner-Carvers and use their search function for a month OR buy a copy of Race Car Vehicle Dynamics and read it to learn about roll centers to get a good answer to this question. Playing around with Bill Shope's suspension website is also a good idea (google it).

Reader's digest version: It's not optimal. 245/45-17s are 25.7 inches tall. 255/55-17s are 28.0 inches tall. That's a difference of 2.3 inches, which means that the rear axles are 1.15 inches higher than the front spindles. On a tiny level, this actually shortens your effective wheelbase (think about what having 5' tall rear tires would do to the car. As the ass end lifts up, the horizontal distance from front contact patch to rear contact patch lessens AND you have more weight on the front wheels of the car). Is this a huge deal? No. Is it optimal for handling? Nope. Unlike Porsches, Vettes, and other cars with different stock tire sizes front/rear, stock Mustangs, imo, were designed to run the same wheel/tire size front and rear (although some newer special package Mustangs may not be like this anymore, I dunno).

If you raise the ass end on an already nose heavy car, do not expect it to handle well. It will likely turn-in harder but the car will pivot around the front nose more, causing the unloaded ass end to want to keep pivoting so much that the car oversteers even more wildly than normal. Also, it won't put down any power coming out of a turn (especially if you still have rear UCAs) because your rear geometry will be even more messed up.

If you have rear coil overs, you can lower the rear spring height to help move some of the weight back to the rear of the car back, thereby relieving some of the problems of taller rear tires (or, with traditional springs, just cut them). You can also try tuning with sway bars, shocks, spring rates, etc..

Are there fast track cars out there with mismatched front/rear tire sizes? Without a doubt. Would they go same sizes front/rear if they could? Probably, imo, unless they've got some trick front spindles/a-arm/k-member setup which addresses the problem.
Thanks aurdraco, nice analysis.

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post #5 of 7 Old 10-04-2009, 09:34 PM
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the other problem is oversteer. It's a rule of thumb to have an equal or lower aspect ratio tire in the rear. The aspect ratio is the second number in the tire size.
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post #6 of 7 Old 10-04-2009, 09:48 PM Thread Starter
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I was actually worried about the front/rear tires rotating at different rates, particularly in turns and during braking.

T-trimmed black 99 GT
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RR intake, faceplated T56
20+ psi @ 7k, 10.327/136.14/1.417
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post #7 of 7 Old 10-05-2009, 01:25 PM
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I wouldn't recommend it. You've thrown a huge variable into a suspension design. Height, angles have changed, not to mention the confusion the ABS will be looking at given the different diameters of the tires.

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