Pelican Parts Forums - 15" vs 16" Fuchs for track use

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- - 15" vs 16" Fuchs for track use (http://forums.pelicanparts.com/porsche-911-technical-forum/1051025-15-vs-16-fuchs-track-use.html)

Quote:

Originally Posted by 3rd_gear_Ted (Post 10734434)

For the track; Slicks in warm weather on 18" wheels, Hoosiers R-comps when cold, nice big contact patch compared to 15's

Contact patch area has nothing to dow/ 15,16, 17 or 18

It is determined by tire pressure and sidewall stiffness

contact patch shape is determined by the profile

a 225/50 x15, 225/50 x16 225/50 x17 and 225/50 x18 will all have the same contact patch shape and area if the sidewalls and tire pressures are the same

What will be different between all of these tires is the height 23.9", 24.9", 25.9", 26.9"
The taller heights mean the sidewalls are taller and thus have more play in them during cornering, this results in decreased grip as you go from 15 thu 18

To counter act this trend as the wheel increases in diameter the profile needs to be reduced

so a 225/50x15 will be roughly comparable(grip & height) to a 225/45 x16 will be roughly comparable to a 225/40 x17 will be roughly comparable to a 225/35 x18 all else being equal

Quote:

Originally Posted by WP0ZZZ (Post 10734300)

Thanks Bill, it's interesting to see the torque loss due to inertial effects.

Regarding gearing, I'm not overly concerned because I will eventually re-gear the car. Also, as we discussed a while ago in another thread, a lower diameter tire will produce more thrust at a given car speed and gear BUT it will force the driver to upshift earlier which means that, at certain speeds, the car with low diameter tires will be using a higher gear and have less thrust compared to the car with large diameter tires.

That's correct the shorter the gearing (including tire height) the sooner and more often shifting will be needed. When the shift point is at an awkward spatial position is where the issue is worst. Ideal gearing for one track will likely not be ideal for a different track & the shorter the gearing the more likely an awkward place will appear. But even w/ stock gearing there can be a peevish corner or straight. My car w/ an RSR trans is just about perfect at WGI but at LRP the front straight is just long enough to require an up shift but also just short enough to require an almost immediate double downshift in the braking zone. Slightly taller gearing would eliminate that extra up shift and make the following corner shift easier

Quote:

Originally Posted by Bill Verburg (Post 10734264)

Here's a comp of 2 efficient R options one in 15 and one in 16, both on 7 & 8 Fuchs, the 15s have a ~20.1lb -ft edge and both will have roughly the same grip and balance.

http://forums.pelicanparts.com/uploa...1580237536.gif

Bill, do you have an estimate of the rotational inertias of the wheel+tire combo for a 205/50R15 and a 205/45R16?

Maybe they are in the table you posted but I'm unable to find them...

Quote:

Originally Posted by WP0ZZZ (Post 10793603)

Bill, do you have an estimate of the rotational inertias of the wheel+tire combo for a 205/50R15 and a 205/45R16?

Maybe they are in the table you posted but I'm unable to find them...

For the tires alone, ie no wheel and no gearing
205/50 x15 runs from ~15-19 lb-ft, depending on weight
205/45 x16 runs from ~17-20 lb-ft, again depending on weight