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Years ago I was running a low hp 4 cyl (about 170 hp) in E Prepared on magnesium wheels that were very, very light. Wanted a wider rim, so went to wheels that weighed 19 pounds compared to 11 pounds. While the car looked better, its performance dropped off noticeably. Braking became very dicey, acceleration was hurt, and probably cornering, too.

Given that background, I've been leery of really big wheels and tires. Currently running Hoosier 225/45x15 on 15x7 rims. They are definitely wider by nearly an inch than the Kumho Victoracer 225/50s they replaced.

My RWD Street Modified Alfa sedan weighs 2800 pounds and has 240 hp at flywheel. Can't help but notice the M3s at the top of the class running 285 to 315s on 10 to 12 inch rims, and they weigh about the same(but have about 100 more hp!). Before I start cutting the fenders and fitting flares, I'd be interested in thoughts/ideas/experiences from the group.

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Bill Sims
2001 Corvette Z06
SS #10

I think you have answered your own question. You need more rim, tire and power.

Ever thought of going the other way... like maybe to a 13"x8" or 13"x9"? Stretch some 225/45-13 Hossiers on a 9" wheel, and you'll be suprised at how much wider they are than a similarly-sized tire on a 7" wide wheel.

You'll lower the car's static CG without losing suspension travel, you'll get a wider contact patch, you'll drop unsprung weight with a lighter overall wheel/tire setup, and you'll improve your acceleration by increasing your effective gearing. The decrease in unsprung weight will also help the car brake and track better over uneven surfaces.

Lastly, 13" tires are cheaper then 16" or 17" tires, and custom 13" wheels are much cheaper than custom 16" or 17" wheels. ;)

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Iain

2004 Mazda RX-8 - torqueless track toy and wannabe STX car
2005 Honda CR-V EX AWD - alphabet soup family truckster

If you go smaller, confirm that the wheels will clear your brakes. If your gearing is already short, 13s may not be a reasonable option.

Maybe this is why biiig tires are common on M3s?
http://bimmerhaus.com/raceprep/M3facts.html#anchor1535773
http://www.siastuning.com/YelloSpecs.htm

Not a street mod car but my ESP camaro really picked up performance in every category when I went from 16X9.5 heavy wheels to 17X11 lightweight wheels.

The drop in acceleration performance with the heavier wheels on your EP car is rather predictable with the heavy wheels, with a lower hp car the parasitic loss of spinning those wheels is noticable.

The decline in braking performance was probably attributable to the same phenomenon. Decelerating those heavier wheels probably added more strain to the brake system which then had to perform closer to it's limit. Therefore your ability to modulate and control the brakes degraded. This is a guess based on what you said so I could be off base.

Pretty cool to see someone running an Alfa in SM. I like seeing unusual cars coming out and running. Now if I could just win the lotto so I could build my Maserati Biturbo project! Perfect car for autocross since it is difficult to make run for more than 60 seconds at a time =)

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Brian "Big Enos" Burdette

those bimmers use higher final drive ratio

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Ed
SM2 82

Originally posted by altiain: You'll lower the car's static CG without losing suspension travel, you'll get a wider contact patch,

This is something that I have not been able to figure out for years ...

the contact patch should be, the weight of the car divided by 4 tires, and then the number of square inches at the PSI you pick. If you want more contact patch, add more weight to the car (! not a good idea), or lower the PSI in the tire. It would seem that wider tires likely have a wider contact patch, but then it would be thinner (front to back), whereas a thinner tire, would have a thinner contact patch, but the contact patch would be wider (front to back). So all things being equal, aren't you just changing the shape of the contact patch?

Looking at the history of racing, there must be something wrong in my logic/math - or else changing the shape of the contact patch is a big deal.

Can anyone explain why the contact patch isn't the number of square inchers of tread times the PSI of the tire times 4 tires == the weight of the car??

thanks

Kurt
:burn:

I don't think tires conform to a model that is that "simple". Even radial vs. bias the effect that PSI and weight have on contact patch is radically different I think. I don't have anywhere near the amount of education necessary to explain it, though.

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Brian "Big Enos" Burdette

I would say that Kurt is right, you can use his calculation to estimate the area of rubber on the road. However, there are 2 advantages to a wider tire that can improve cornering:

1) I assume you already understand the concept of slip angle. Slip angle is caused by the deflection of the tire at the point where it touches the ground. We commonly refer to this as rollover and use a chalk line on the side of the tire to measure it. But, in fact, the entire tread must bend when a tire rolls over. A wider tire tread will be more resistant to bending. Therefore, with a wider tire the slip angle is reduced.

2) The wider tire will be able to handle more heat because each rubber particle will be touching the pavement for a shorter length of time. This can reduce the melting of rubber at the contact patch and provide an increased friction coefficient. (This could be a detriment in autocross because the length of the race is so short that the tire might not reach it's correct temperature for optimum friction coefficient.)

Jared

Originally posted by kurt friedrich: the contact patch should be, the weight of the car divided by 4 tires, and then the number of square inches at the PSI you pick.

If tires were tori that expanded in all directions equally when inflated, then, sure, this is correct. But tires are not only not tori, but some parts (e.g., the tread) are stiffer than others (e.g., the sidewall). So simplistic models go out the window.

- Jtoby

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"All your boost are belong to us"

If tires were tori that expanded in all directions equally when inflated, then, sure, this is correct. But tires are not only not tori, but some parts (e.g., the tread) are stiffer than others (e.g., the sidewall). So simplistic models go out the window. - Jtoby

I agree with you, and as I said up front, I didn't belive my own equation ... but still, I haven't heard a good explanation about wider tires do help? And what does happen to the contact patch?

Kurt