Death of the 17" tyre?

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AngusF1
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Joined: Sun Aug 13, 2017 9:54 am

Re: Death of the 17" tyre?

Post by AngusF1 » Sun Nov 04, 2018 12:48 am

Thanks Strad, definitely many more tyres available when the fronts and rears are searched separately. (??)

To figure out why I ran a cross check between the lists of Y rated fronts and Y rated rears - there were only three common models! (The ones I listed above.)

So it seems like there are plenty of Y rated tyres around, just very few models which are actually produced in both a 245 and 205.

The motivation for the different sizes is that this hypothetical car would have a weight distribution of around 40-60 front to rear.



Edit: Increasing the fronts to 215 provides seven results, which is much better than three. It looks like 215 may be a more prudent choice.

Greg Locock
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Joined: Fri Jun 29, 2012 11:48 pm

Re: Death of the 17" tyre?

Post by Greg Locock » Sun Nov 04, 2018 4:26 am

Given that braking is very important I suspect that forward biasing your tire sizes is not a bad idea.

AngusF1
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Joined: Sun Aug 13, 2017 9:54 am

Re: Death of the 17" tyre?

Post by AngusF1 » Sun Nov 04, 2018 12:00 pm

Greg Locock wrote:
Sun Nov 04, 2018 4:26 am
Given that braking is very important I suspect that forward biasing your tire sizes is not a bad idea.
Hi Greg, yes, agreed that may be necessary. So far I've just been looking at traction requirements and weight distributions, but at some point I'll have to look at braking requirements. It may turn out that braking from 300km/h is the limiting factor on front tyre width... The McLaren F1 used 235 but some owners complained that the brakes were no good.

Surely though, if braking requires wider fronts then it would be necessary to increase the width of the rears as well? Otherwise wouldn't the car tend to oversteer, or require some bizarre suspension geometry to reduce front cornering force?

e36jon
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Joined: Mon Apr 25, 2016 1:22 am
Location: California, USA

Re: Death of the 17" tyre?

Post by e36jon » Sun Nov 04, 2018 4:48 pm

Dynamic suspension behavior is not an easy thing to get right. You have all of these different states to consider, plus combinations thereof: acceleration, braking, turn-in, mid-corner, corner exit + acceleration, heave, roll, etc... Depending on your suspension geometry and construction you can have all sorts of different behaviors during these different handling states: Rear wheel steering? Bushings or other elements that deform in some instances (cornering, say.) but not in others (braking, maybe?)? Camber gain or loss during cornering? Yadda yadda yadda. You can read the press releases for every new model year of the enthusiast grade cars and there is always some new bit of wizardry that makes for better performance across the board even though the car has inevitably gained weight.

I am not mentioning electronic stability control systems because I am assuming that's beyond reach for this project.

Maybe lifting the entire suspension off of something that's in the window for the specs you're after is a solution? If you went after a Porsche chassis, for instance, you could even go fully aftermarket for the pieces due to the support they have. Those aftermarket pieces also address any shortcomings the stock parts had. And in Porsches case they also make a full line of 'motorsport' bits themselves. Ditto for Subaru, Toyota GT86, some BMW's, Corvette, etc... (Apologies for not including YOUR particular hotness.)

Cheers,

Jon

strad
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Re: Death of the 17" tyre?

Post by strad » Mon Nov 05, 2018 12:35 am

Many performance vehicles use the same size all around. The Cobra is the same on all four corners.
It's not a given that if you use wider fronts you will need wider rears.
Motorsport without danger is like cooking without salt
Sir Stirling Moss

AngusF1
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Joined: Sun Aug 13, 2017 9:54 am

Re: Death of the 17" tyre?

Post by AngusF1 » Tue Nov 06, 2018 4:44 am

e36jon wrote:
Sun Nov 04, 2018 4:48 pm
Dynamic suspension behavior is not an easy thing to get right.
Thanks Jon, yes, completely agreed on that point! I've read several of the classic texts on suspension design and there is rather a lot to it, and many conditions under which the car must exhibit acceptable behaviour - even more on a road car than on a track car.
strad wrote:
Mon Nov 05, 2018 12:35 am
Many performance vehicles use the same size all around.
Strad, yes, but I believe those cars are typically front engined. Consequently the mass balance is forward rather than rearward, providing naturally understeering behaviour. A rear-balanced car with equal wheel sizes will naturally tend towards oversteer and require specific suspension design attention in order to provide understeer. At least, as far as I can tell...