Tire + Rim size & understeer

[eho3]

Drove my buddies' z4 recently, both of them running the same Goodyear F1 tires.

Tire size as follow:

Front 225/40/18X8, Rear 255/35/18X8.5 .......z4 2.5
F&R 225/35/19X8.5....................................z4 3.0

tire pressure is 30psi front and 33psi rear for both cars.

I found that the 18' setup is a lot better around the corner, the 19' understeer big time and provides not enough road feel. I feel not comfortable enough to push the limit and seems like I need to back off the brake sooner or before I start to turn the steering wheel.

Is that because of the width of the rims, for 8.5 width, 225 stretch too much and have less contact patch through cornering?

Is there anyway to improve the cornering for the 19' setup?

Thanks....

[Yorgi]

Those are very strange results. The 19” wheels should provide a lot more feedback than the 18” wheels. The thinner sidewalls should be stiffer giving you sharper turn in and a harsher, direct feel through the steering wheel.

Also, the non-staggered setup on the 19” wheeled car should result in way less understeer, especially considering the very large stagger on the 2.5 Z4 which would cause massive understeer.

The only things I can think of:
- Tire pressure should be increased on the front of the 19” wheel car. 30psi seems way too low for such a low profile tire. I never go below 34psi on 19” wheels.
- The 3.0 Z4 was overpowering the rears and causing some oversteer which made the car feel less stable.
- Maybe the 2.5 Z4 had a sport package vs a non-sport setup on the 3.0?

The specs for the Eagle F1 GS-D3 say for a 225/35-19 tire the acceptable rim width range is 7.5-9.0”, with 8.0” being optimal. So I don’t think the tire was severely overstretched.

I would start by trying 36psi all around on the 19” car and adjust pressure from there to balance the car how the owner likes it. Increasing pressure will increase grip on most Z-rated low profile tires (up to a point, where you start to lose grip as the tire rounds due to overpressure). So if you want to reduce understeer add air to the front tires.

[eho3]

Thanks, will give it a try. The BMW recommended pressure for Z4 is 30/33 for 16' - 18' tires. That's why both cars are set at 30/33.

For the one with 18' setup, would 30/33 be too low as well?

[Blades]

i find that if the rim width is larger i go into a more controlled drift. Like i feel safer to go into a drift with a corvette then with my bmw (maybe its the awd i dunno).

[eho3]

Problem solved, thanks Yorgi...! We used 30/33 as stated, I figure it's for run flat only. My buddy changed to Goodyear F1... and he use 36 all around. Day and night difference....

[STALKER]

I dont really run staggered set up anymore, keep the front are rear pressures the same or very close. 35/36 is what IM using now. Car responds better, and feels more balanced.
Also, the weight of the rim might place a part too, heavy rims will tend to throw off things I bit.

[eho3]

I dont really run staggered set up anymore, keep the front are rear pressures the same or very close. 35/36 is what IM using now. Car responds better, and feels more balanced.
Also, the weight of the rim might place a part too, heavy rims will tend to throw off things I bit.

For staggered setup, would 36/35 reduce understeer compare to 35/36?

[Yorgi]

For staggered setup, would 36/35 reduce understeer compare to 35/36?Yes, running a little more pressure up front will help offset the staggered setup.