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Of course they could just run a taller sidewall on a 20. Cup cars do the same. Square wheel diameter, rolling diameter staggered by sidewall height.
R spec are very different to road or cross-over tires e.g. sidewalls are heavily reinforced. Compare something like an Advan 005 (or 006 wet) with a MPSC2 N1. Very different beasts.
Of course they could just run a taller sidewall on a 20. Cup cars do the same. Square wheel diameter, rolling diameter staggered by sidewall height.
And the RSRs go even further, very large OD on an 18” wheel. However they only do that because the rules mandate the use of 18s (Porsche used 20s on their GT1 at Le Mans for one year before they were banned). From a performance perspective it’s better to use a larger OD wheel with a shorter sidewall if you’re going for more grip. And yes, the larger OD does increase both lateral and longitudinal grip- the contact patch is more evenly loaded and rubber is in contact with the road for a longer durration, particularly important as speed climbs.
And the RSRs go even further, very large OD on an 18” wheel. However they only do that because the rules mandate the use of 18s (Porsche used 20s on their GT1 at Le Mans for one year before they were banned). From a performance perspective it’s better to use a larger OD wheel with a shorter sidewall if you’re going for more grip. And yes, the larger OD does increase both lateral and longitudinal grip- the contact patch is more evenly loaded and rubber is in contact with the road for a longer duration, particularly important as speed climbs.
True and true. My point was more that a 19F-20R stagger could easily be configured for customers who want more sidewall, while maintaining the same rolling diameter.
Useful for us few radicals who would want to daily a GT car.
True and true. My point was more that a 19F-20R stagger could easily be configured for customers who want more sidewall, while maintaining the same rolling diameter.
Useful for us few radicals who would want to daily a GT car.
It’s a fair trade-off as long as you’re willing to give up grip, transient resonse and/ or ride quality.
If you’re going for ride quality while maintaining performance I’d go with the larger rim every time. You can pair a large OD rim with a short, paper thin sidewall (ie the Dunlop Sport Maxx Race) and get great response with excellent ride. The only major downside I see beyond cost is the risk of running out of travel (bent rims, pinch flats, etc). Increase sidewall height instead and you need a much thicker sidewall to maintain response, which in turn transmits more NVH.
On many cars I actually prefer the greater slip angles that come with taller sidewalls, especially on the road. I’ve gone minus 1” three times now- two experiences were positive, one less so. In every case, however, ride and/ or performance were sacrificed. These cars are engineered to the point that there’s no free lunch.
... with a shorter sidewall if you’re going for more grip.
Please explain. Certainly - all other things equal - more sidewall tends to increase 'squirminess' due to the contact patch moving with latency with respect to the wheel barrel. So, response is poorer. But, in terms of ultimate grip - lateral or longitudinal - doesn't less sidewall essentially equate to 'stiffer' which is not always the best for grip? For huge acceleration longitudinal grip is increased with more sidewall, specifically more flex. Although, that may be less about contact patch and more about keeping the wheel barrel from rotating on the tire.
Apologies, I believe you can get some19s if you have non PCCB cars (e.g. OZ and BBS and its very tight on the front due to brakes) but you lose width on the rear wheels for anything thats decent e.g. 19x11 - perhaps there is more choice in the US. I'm sure there are some solutions out there with spacers and extended lug nuts etc. Porsche don't offer an OEM 19 inch wheel for RAS and or PDCC/RAS cars. Cup cars don't have PDCC or RAS so brake clearance is the only issue they have to deal with.
Apologies, I believe you can get some 19s if you have non PCCB cars
You can fit 19s over PCCB brakes too. That's the standard move for guys running hoosiers, etc. Clearance is tight on the brakes and you do need the right wheels, but the factory could easily fit them if that was a desire.
Please explain. Certainly - all other things equal - more sidewall tends to increase 'squirminess' due to the contact patch moving with latency with respect to the wheel barrel. So, response is poorer. But, in terms of ultimate grip - lateral or longitudinal - doesn't less sidewall essentially equate to 'stiffer' which is not always the best for grip? For huge acceleration longitudinal grip is increased with more sidewall, specifically more flex. Although, that may be less about contact patch and more about keeping the wheel barrel from rotating on the tire.
The passage is needed for full understanding:
Originally Posted by Petevb
From a performance perspective it’s better to use a larger OD wheel with a shorter sidewall if you’re going for more grip.
The increased grip bit comes from the larger OD of the tire, the response from the shorter sidewall.
From a pure longitudinal grip point of view I agree with you- wrinkle walled slicks running extremely low pressures with extremely long contact patches are the way to go. The tall sidewall deflection is key- it puts a huge amount of rubber on the pavement and keeps it there for a very long time (relatively). The time is key because rubber is visco-elastically flowing into the pavement, so the longer duration it's in contact the more effectively it grips.
The problem from an overall performance point of view is that the same trick when loaded laterally deforms the contact patch into funny shapes, hence the whole switch from bias-ply to radials. However it's likely response that trumps.
The reason I called out "if you're going for more grip" is because grip isn't always the goal. If you're optimizing for fun the answer can be quite different IMHO.
The increased grip bit comes from the larger OD of the tire, the response from the shorter sidewall.
From a pure longitudinal grip point of view I agree with you- wrinkle walled slicks running extremely low pressures with extremely long contact patches are the way to go. The tall sidewall deflection is key- it puts a huge amount of rubber on the pavement and keeps it there for a very long time (relatively). The time is key because rubber is visco-elastically flowing into the pavement, so the longer duration it's in contact the more effectively it grips.
The problem from an overall performance point of view is that the same trick when loaded laterally deforms the contact patch into funny shapes, hence the whole switch from bias-ply to radials. However it's likely response that trumps.
The reason I called out "if you're going for more grip" is because grip isn't always the goal. If you're optimizing for fun the answer can be quite different IMHO.
Please explain. Certainly - all other things equal - more sidewall tends to increase 'squirminess' due to the contact patch moving with latency with respect to the wheel barrel. So, response is poorer. But, in terms of ultimate grip - lateral or longitudinal - doesn't less sidewall essentially equate to 'stiffer' which is not always the best for grip? For huge acceleration longitudinal grip is increased with more sidewall, specifically more flex. Although, that may be less about contact patch and more about keeping the wheel barrel from rotating on the tire.
You're right, of course. Stiffer isn't always better. But we're swimming against the tide.
Anyway, there's more to it than absolute grip. That drag photo reminds me of a study Porsche put out many years ago. Many years before the internet too, so good luck finding it. Anyway, the finding was that tires are very efficient at transmitting power up to moderate hp, but then become increasingly inefficient as power increases. What they found is power transmission is fairly linear until it increases to a point where less and less of that power goes into the road and more and more goes into flexing and heating the tire. That drag tire is an exaggerated example of what they were talking about. Same thing happens in every tire, even very low profile tires, just not so much its easily seen. High power makes tires inefficient. Inefficiency at Porsche is verboten! The inflection point or transition is around 350-400 hp. This is the real reason Porsche started developing awd. That's not my opinion either, its what Porsche published. Looking back on it, this was right around the time of the 959, their awd test bed with a then monster 444 hp.
Being Porsche of course it was not enough merely to show that awd is more efficient. AWD requires extra equipment. The efficiency gain must be more than enough to make up for this extra weight. Which at around 400 hp it does. On paper. The 959 proved it in the real world.
Thanks Chuck, for reminding me that I wanted to respond on this...
Pete, originally you wrote (emphasis added by me)
Originally Posted by Petevb
From a performance perspective it’s better to use a larger OD wheel with a shorter sidewall if you’re going for more grip.
then
Originally Posted by Petevb
The increased grip bit comes from the larger OD of the tire, the response from the shorter sidewall.
These are two very different statements. It was the first statement that caused me pause. The second I agree with. In the first case, it does not necessarily follow that the contact patch is larger simply due to increasing wheel diameter while it the second case it does.
Bottom line question for me is: if all other characteristics are held constant, does shortening the sidewall increase grip? Or, in other words, if I take a 245/XX x 18 tire and wheel combo and replace it with a 245/YY x 19 combo with aspect ratios such that the revolutions per mile are equal for both combos why should I expect more grip from the YY tire than the XX tire?
Originally Posted by chuck911
Anyway, the finding was that tires are very efficient at transmitting power up to moderate hp, but then become increasingly inefficient as power increases. What they found is power transmission is fairly linear until it increases to a point where less and less of that power goes into the road and more and more goes into flexing and heating the tire.
This makes sense. For any specific tire, with specific tread pattern, sidewall and tread stiffness, with a specific loading there must come a point where power transmitted through the tire hits 'a knee in the curve' and goes non-linear. Increasing the stiffness of the tire or size, etc. would likely move that point on the power efficiency curve.
I highly doubt though that it is just one power level for all tires regardless of their other characteristics. I imagine a set of very hairy differential equations.
04-01-2018, 01:13 AM
