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Closer than you think
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Discussion Starter #1
I recently bought new tires for my 2005 Elise Sport on LSS wheels. Because I liked my old Toyo Proxes 4's, I bought the same tires but switched from 195 to 205 width on the fronts and from 225 to 235 on the rears. (The shims are still in place.)

205/50/16 fronts
235/45/17 rears

I've noticed a difference in steering/suspension feel, but instead of describing it right away, I'd like to see what the "experts" here would predict based on knowledge or experience...

(note: Please no posts yet from Me!... That's not fair...:D)
 

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Gamera The Atomic Turtle
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and those should also fit base wheels as they are the same sizes as the star specs we are running.
 

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Banned
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I’m not an expert but I would like to test my knowledge of suspension dynamics…and I’ll stick my neck out and do it in front of everybody.

But I have a couple questions first;

Has the rolling diameter changed going from the smaller (narrower) to the larger?

And…did you use the same wheel…ie, did you take off the old (smaller tire) and install the wider tire on the same wheels?
 

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;1717849 said:
Has the rolling diameter changed going from the smaller (narrower) to the larger?

And…did you use the same wheel…ie, did you take off the old (smaller tire) and install the wider tire on the same wheels?
Answer is yes to both
 

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This makes it much more difficult to predict because you have pluses and minuses that are next to impossible to predict which will have the greater influence…to add up to an improvement or a negative…this is what I think will happen;

1. Larger diameter – slightly slower acceleration and slightly more brake pedal required to slow down / stop the car.
2. Heavier wheel/tire – will make the car handle more sluggish, not as crisp.
3. Sidewall flex – since the tire is wider than optimum for the wheel, the sidewall will flex more and give you a float feel in the transition from straight ahead to corner. The car will feel like it is floating in the transitions and not solidly planted.
4. In a smooth, long, high-speed corner you should see higher peek corner speeds and higher lateral g’s, but when the car reaches the limit, it should break sharper with less warning.
5. In tighter, rougher corners, I think you will lose a little time.

On a long, high-speed track, I think you will see an improvement in lap times. On a short track, I don’t think your times will change. And on a short rough track, I think you will lose time…
 

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Closer than you think
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Discussion Starter #7
…this is what I think will happen;

3. Sidewall flex – since the tire is wider than optimum for the wheel, the sidewall will flex more and give you a float feel in the transition from straight ahead to corner. The car will feel like it is floating in the transitions and not solidly planted.
Well, this is a pretty good description of the change I notice. The front end feels lighter and requires less input to turn-in.

I'd not heard that 205cm is "wider than optimum" for the LSS wheel though, as so many people seem to have this combination...

The Proxes 4's wear very well, are great in the rain, look good, and are grippy enough for a little track action, but If I was serious about tracking I'd have true R-compound tires.
 

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More rubber is usually good (overall), provided you have wider wheels to support the tire. I run 7 inch in the front, 8 inch in the rear; overall grip and handling is improved with 205 width tire front and 245 width rear. Pinching a wider tire on a narrow wheel just encourages sidewall flex and muddies transitions.
 

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Well, this is a pretty good description of the change I notice. The front end feels lighter and requires less input to turn-in.

I'd not heard that 205cm is "wider than optimum" for the LSS wheel though, as so many people seem to have this combination...

The Proxes 4's wear very well, are great in the rain, look good, and are grippy enough for a little track action, but If I was serious about tracking I'd have true R-compound tires.
But that quality was not what I was describing…what you are describing comes from the taller tires changing the contact patch centerline in relation to the imaginary line connecting the top king-pin point to the bottom king-pin point on your front steering knuckle. That line slopes outward toward the ground. The centerline of the tire also intersects that line. You design a car for those two lines to intersect at ground level (center of the contact patch). If the tire centerline moves inboard of the king-pin line, you get a lighter feeling steering wheel…to the point of instability in extremes. A larger rolling diameter will get you that. The opposite is also true. Just for information purposes, this is exactly opposite on a FWD car.

Like I said, so many variables changed, I just took a stab at what I thought you would feel the most. I was wrong.
 

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!!! Great stuff, I just learned a lot! Very nicely explained
 

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I put the old tires on the new (wider) rims, and the change was shocking good.
much less understeer.
 
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