Why wider tires on the rear?

[keitho64]

I have searched this question out and cannot seem to find much of an answer. I have a 911T that has been converted to an RS30 / IROC clone. Right now I have turbo twists with 265/35-18 in the rear and 225/40-18 in the front.






I really prefer the looks of the Fuchs but due to tire size issues I am not sure what to do. Here is the car with 4 15x8 Fuchs, 245/50-15 and 225/50-15. Yes this picture was taken before the project was complete so the Carrera script was not installed yet.




I cannot find any 245/50-15 tires but can find 225/50-15. What would be the down side of installing all 4 tires of 225/50-15? Does this size difference really change the handling? Or should I stay with the turbo twists?

[Bill Verburg]

absolute tire size and the ratio of f/r tire size is a major determinant of the handling characteristice of a car. The more hp & torque that are available the more critical these issues are.

rear tire height also affects gearing, which affects the acceleration of the vehicle.

the bigger the rear is relative to the front the less understeer will be present

the taller the rear tire the slower the acceleration through any gear

[tcar]

I like the Fuchs better... a lot better. 18's look WAY too big on that car.

Wider rear tires help the rear wheels follow the fronts on hard accelleration in a curve instead of vice/versa.

Help reduce the oversteer tendency with the rear weight bias.

[red 69]

you can find 245- 50- 15 in Dunlops but you will have to pay.... I have 205 - 55 in the frt and 245 - 50 in the rear and it works .

[partrin]

One solution I considered is having the rims converted to 17s....Lindsey Racing in OK is one place that does this, but there are others. IMO these wheels look great, and you can use centers from 6 inch rims - less expensive - since they cut the original rim off.

http://www.lindseyracing.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LR&Category_Code=WHEELSLINDSEY

[dentist90]

Originally Posted by Bill Verburg

.
the bigger the rear is relative to the front the less understeer will be present

May be going slightly OT here.

By bigger, do you mean wider Bill? I understand that engineers might use wider tires on the rear partly due to the preponderance of weight back there, ie. to prevent tire overheating and premature wear due to the extra load carried by the rear tires.
Wider tires would also be less likely to lose traction under acceleration (on dry pavement) wouldn't they? I would think that this would help prevent oversteer, or conversely, increase understeering tendencies under accel.
For safety most (all?) car manufacturers try to design understeer into their cars, as many drivers don't like the feel of their rear ends exiting the corner first.
Am I confused?

[Bill Verburg]

Originally Posted by dentist90

May be going slightly OT here.

By bigger, do you mean wider Bill? I understand that engineers might use wider tires on the rear partly due to the preponderance of weight back there, ie. to prevent tire overheating and premature wear due to the extra load carried by the rear tires.
Wider tires would also be less likely to lose traction under acceleration (on dry pavement) wouldn't they? I would think that this would help prevent oversteer, or conversely, increase understeering tendencies under accel.
For safety most (all?) car manufacturers try to design understeer into their cars, as many drivers don't like the feel of their rear ends exiting the corner first.
Am I confused?

No, I meant less oversteer

[dentist90]

All is good now. Thanks.

So, for Keitho64, by installing 4 tires of the same width/aspect ratio you will probably find that your Porsche is too eager to come around (oversteer) in corners, both on the gas and even more so on lift off.

[barney911rs]

For that bodywork, you really need a 9" in the rear. Not cheap in 15 or 16 for real Fuchs, but would be the optimal solution. More tire choices in 16" so that may be your better solution. The Lindsey option could be your best bet.

[Turbo Den]

simple answer: Because the rear fender can fit wider tires.
i believe more tires are always better because greater traction.
if i want more or less oversteer/understeer, i can always adjust with shocks, sway bar, and tire pressure.
but the fender limits the max tire size.
so, my SC can fit max 265 in rear, 235 in front.
but i would fit 265 all around if possible LOL.

[KTL]

Keith,

Stefan and I sort of go against the grain here and run 225-50/16 all around on our cars. The advantage is tire rotation and maximizing the even wear of the tires. If you use the same size & offset at all four corners it's quite easy to juggle them around the car. It's likely not THE fastest way around the track. But it's still manageable. As you can see the car exhibits a bit of oversteer at the limit. But it's tuneable with an adjustable rear sway bar. See my racing "expertise" below. Please ignore the sissy-boy early apexing.......


This is running 16x8's all around, Victoracer front, Yoko A032R rear

http://www.youtube.com/watch?v=B-zgDmH2JTQ

This is running 16x7's all around, all Hankook R Z211

http://www.youtube.com/watch?v=QHYwvfPwxFk

Tristan can confirm there's a guy running 225-15's all around in PCA club racing and mopping the floor with everyone.

[mca]

Originally Posted by tcar

I like the Fuchs better... a lot better. 18's look WAY too big on that car.

+1