Hoosier sidewall spring rates

[Maurini]

I was looking through the Hoosier website, and the information provided for equivalent sidewall spring rates is interesting. With the possibility of both the 20.5x6.0-13 and 20.5x7.0-13 being mounted on a 6" rim, I would have expected the sidewall spring rates to have changed far more dramatically.

Have any teams done track testing between the 20.5x6 and 20.5x7 both on 6" wheels? I imagine the 7 incher was more progressive/drivable when exploring the limit of the tires, and also ultimately provided more lateral grip?

If anyone can set me straight with my logic or provide me a little reassurance I would really appreciate it.

Marc Maurini
University of Windsor
Formula SAE 2007
Suspension Team

[Maurini]

I was looking through the Hoosier website, and the information provided for equivalent sidewall spring rates is interesting. With the possibility of both the 20.5x6.0-13 and 20.5x7.0-13 being mounted on a 6" rim, I would have expected the sidewall spring rates to have changed far more dramatically.

Have any teams done track testing between the 20.5x6 and 20.5x7 both on 6" wheels? I imagine the 7 incher was more progressive/drivable when exploring the limit of the tires, and also ultimately provided more lateral grip?

If anyone can set me straight with my logic or provide me a little reassurance I would really appreciate it.

Marc Maurini
University of Windsor
Formula SAE 2007
Suspension Team

[mtg]

If you can't find this information, my last project at OptimumG was to build a simple tire testing machine that measures vertical tire spring rate and damping ratio. Call Claude and ask about getting a couple FSAE tires on there.

www.optimumg.com (http://www.optimumg.com)

[KU_Racing]

Why exactly would the springrates change on different width rims? The geometry change is very small. dont forget that any traction limit characteristics or ultimate grip comparisons depend heavily on variations in weight and suspension geometry, so just because another team sees the most grip with 7 inch wide tires does not mean that you will.

[Maurini]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Why exactly would the springrates change on different width rims? The geometry change is very small. </div></BLOCKQUOTE>


If you were to run a 7" tire on a 6" wheel in comparison to a 6" wheel the sidewalls would change very little. In comparison, on the Hoosier website they have FSAE tires that they claim will fit 5.5 - 7" wheels. That would pose a far greater problem.

The point I am getting at is has anyone in the past noticed a drive ability difference between a 7 and 6.5" tire on the same wheel? If given the option which tire would be recommended? At the same point in time I understand that this is only a VERY small piece of the total V.D equation.

Thanks for the input.

Marc

[Charlie]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by KU_Racing:
Why exactly would the springrates change on different width rims? </div></BLOCKQUOTE>

Why wouldn't it?

It does. When I was at Auburn we experimented with a lot of different tires and I put them all on a Hunter GSP9000 machine. The machine is meant to test 'rolling runout' of a tire/wheel assembly by pressing a roller on a tire, by running it in diagnostic mode I was able to get accurate spring rate data at different displacements.

Naturally I ran different wheels with the same tire. The result was as the wheel got wider the compliance went up (spring rate went down). This makes sense- as the sidewall becomes less vertical it can resist less. I did not try a wheel more narrow than the tire patch so it may not work past that threshold.

Anyway my theory is that, if normal force goes down lateral stiffness would go up, leading to a stiffer tire laterally with a higher peak grip. However with increasing lateral stiffness the tire should become more difficult to drive, not easier as the original poster is implying.

In the end we never did a proper back to back test of this when I was at Auburn, so the theories remain theories.