AutoX sidewall flex video, a.k.a. one reason why the Type R has 30-series tires

baldheadracing

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There was a lot of comment a few months ago about Honda, via suspension engineer Hideki Kakinuma, putting 20" 30-series tires on the FK8. What I didn't realize is how much sidewalls flex, even on stiff tires.

The tire in the video is a 205/50R15 RE-71R - and RE-71R's have pretty freakin' stiff sidewalls.
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Yep, that's why you need some neg camber. The tire with the stiffest sidewall is the BS RE-55S. Afiak.
 

THE! CrazyKiwi

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Yep, that's why you need some neg camber. The tire with the stiffest sidewall is the BS RE-55S. Afiak.
There is also a lot of camber change going on in that video, it isn't all sidewall flex.
 


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baldheadracing

baldheadracing

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There is also a lot of camber change going on in that video, it isn't all sidewall flex.
True, it isn't all sidewall flex, but I was surprised by how much flex there was in what is essentially a tire designed to the SCCA Solo2 Street-class rulebook. (As an aside, the new BFG Rival S 1.5 may have the edge over the RE-71R.)
 

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race car, race tire. they are meant to do that. too ridgid and they would have less mechanical grip.
 

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race car, race tire. they are meant to do that. too rigid and they would have less mechanical grip.
It isn't that they are meant to do that it is just a product of the conditions, there is always sidewall flex, it's just the degree that matters. Of course a sidewall made of concrete would be a disaster but the tires in your example are made specifically for the vehicles that are using them as are many race tires. The general public get to choose from generic tires, not those that are optimized for specific series/cars/conditions so whatever setup you have has to take the spring rate of the tire into account. Tire spring rate, road spring rate, wheel rate, they are all interrelated.

My comment about the first video was that the camber change is excessive which makes it very difficult to determine your optimum static camber setting. Keep camber change to a minimum and always consider the effect of camber loading, a little more static negative is best, the inside of the tire should always be hotter than the outside by a small amount. Setups are a dance where the music often changes tempo. It can be viewed as a black art but I always tell people that you can't change physics..

My advice. Work backwards from the tires you have and optimize your setup around them not vice versa. When moving from one wheel diameter to another you need to match overall diameter so as to not effect the gear ratios among other things. The problem there is that you need to change aspect ratios which, in turn, generally leads to taller sidewalls which are more prone to flex. Tire pressures are absolutely critical as is the control of them.
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