Yoshi

How much do you hardcore weekend racer guys know about tire contact patches and sidewall effects on cornering?

Sooner than expected, my other pair of rims arrived today. Finally i have a matching set! yay! So now I'm running 17x9 in the rear with 215/40... it occurred to me that I have several half used up 215/40 pairs sitting in my garage... so on they went to the new rims (front) 17x8. Here's the question:

If I have a 17x8 w/215/40 and it's a slight stretch, and 17x9 w/215/40 which is a pretty big stretch. Normal logic would suggest that even tho the rims are "staggered" there should be an extremely minimal effect on handling differences between running the same size rims all around... but the tires are not solid rubber, but filled with air, and will contort... so where does that leave the affect on cornering?

This is beyond my basic knowledge of physics

I'm assuming that the rather aggressive stretch on the 215s on a 9" rim would yield a slightly larger contact patch, and depending on tire pressure, could equivocate more or less traction vs. the 8" wide 215. THUS if the front tires have more give due to sidewall flex vs. the 9" 215, which is practically incapable of flexing due to the stretch; then oversteer (vs. say 17x8 215 all around) would be maintained correct? Am I making any sense? That was a lot of babble

This obviously isn't taking account other suspension goodies, just tire tech alone in a speculative enviornment.

Cliff Notes:
same size tires on different width rims. The stretch will minimize the expected affect of understeer inherent in a staggered wheel width setup, as long as the wider/stretched tires remain in the rear.
True or False?

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DoriftoSlut

Basically, you have the right principle. A certain degree of stretching increases your contact patch because it flattens out the shoulder of the tyre whereas a non-stretched scenario, the shoulders would be off the pavement. However, excessive stretching (215 on a 10, for instance) actually decreases the contact patch because the shoulder is so layed over that the tyre sits on the 2 shoulders instead of the middle, effectively elliminating your normal contact patch to a certain degree. (this is all talking theory and technical this and that).

In real life, stretching tyres means: more response. In drifting, the car will transition quicker, break traction easier, and respond faster. In grip driving: sudden loss of grip at the limit, no warning "squeel" etc. I used to drive 215/40/17 on 17x9 up front in the mountains. Yeaaaa. Its kinda scary, b/c the car reacts so quickly that small bumps throws it off and trail braking locks the front wheels. (this is with full suspension (coilovers and arms).

In short: You have the right idea. Just drive.

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DJPimpFlex

well the streched tire does give the feel of a stiffer sidewall because it can transfer more enegry into the ground. This will make the 215's on the 10" rim feel like stiffer tires. Also by stiffining the sidewall (effictively) you can run lower tire pressures and get a larger contact patch. The contact patch is unaffected if you only change tire size. If you upgrade from a 205 to a 235, your contact patch will be identical assumming you dont change your tire pressure. Contact patch=load+PSI. It does however change the shape of the contact patch which matters greatly. Width doesnt matter for contact patch. With overall lateral grip it does matter though. With a wider tire your spreading the load over a larger width and thus are able to run lower tire pressure and that does increase your contact patch. Running 215's on 9" wheels vs 215 on 10" wheels makes no difference in the contact patch, but it does change the overall feeling of the tire. It should give you slightly more oversteer tendencies vs having a non-streached tire. I think that all makes sense. Go here and read up if I can explain it correctally.


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axiomatik

The end result will be more oversteer. Any change in the contact patch will be minimal, but the increased sidewall stiffness will increase oversteer

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sideview_180sx

If you are gripping hard, then a stretch like that, you prolly could unseat the bead slightly and cause air to escape just slightly tho. something to consider in regards of gripping. Check out bimmer forums in regards to tire sizes and width if you are going to pursue it, but the seat time running on those tires will help in both grip and slip.

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TheSquidd

We need a dedicated "best of tech" section for threads like these.

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DJPimpFlex

yea fresh alloy has a best of section, but they put a lot of crap into it. Some useful stuff too. I found out about the suzuki ignitor for SR20 on there. Pretty interesting. . .

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EchoOfSilence

That would be the Archive Section. Granted, there should be sections for Tech, Chat, etc., but yeah.

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