Effects of increasing wheel size on new Porsches

[SonnyM]

Here's an interesting article on the effect of increasing wheel size on a car:

http://www.caranddriver.com/features/effects-of-up...

As each generation of Porsche iterates the wheels increase by 1 inch. Questions are:

1. What effect does this have on performance if the tyre wall is not decreasing below 35 profile?

2. Is there an optimum wheel size for track racing / fast road driving - e.g. 19" with 35 profile on a 7GT3? Or does it completely depend on the car, e.g. a 6GT3 performs better on 18s than it would on 19s, whereas a 7GT3 performs better vice versa?

3. Would there be a limit to wheel sizes based on physics - e.g. would Porsche stop at 24 or 25 inches on the sports car range in a couple of decades?

[mrdemon]

it's a limit on my pocket forking out for 20" tyres, have you seen the N spec 20" tyre price !!!

[gsewell]

It all depends on where you want to draw the compromise. The best handling car I have ever owned had 13" rims with 70 profile tyres (Yoko A021R's). The car was a Caterham Seven that runs rings around the Cayman in ability despite the cheap wheels/tyres.

Many years ago, Ford did some research based on handling vs NVH and came up with 60 profile tyres as a best compromise for a road car (See the book written by Jackie Stewart for details). So anything below that is to serve one of two purposes:
- bigger brakes
- aesthetics

[thegoose]

gsewell said:

Many years ago, Ford did some research based on handling vs NVH and came up with 60 profile tyres as a best compromise for a road car (See the book written by Jackie Stewart for details). So anything below that is to serve one of two purposes:
- bigger brakes
- aesthetics

Or could it be that cars have advanced so much in the meantime that that info is out of date?

Don't forget, the 60 is just the aspect ratio expressed as a percentage, so you could have the same height tyre wall with a 50 profile on a wider tyre.

[MadMark911]

Best handling car (out of the box) I've ever had was a MK2 Golf GTi 16V on standard 14" rims. When coupled with super sticky Yokahama 008's in a 185/60VR14 it was amazing! Superb ride and (at the time) what seemed like touring car levels of grip on a dry road! However these days my 997 would murder it, albeit probably not on our cart track" B roads (recently described by a Porsche engineer as being more like a rally stage)!

[gbrownzen]

SonnyM said:

Here's an interesting article on the effect of increasing wheel size on a car:

http://www.caranddriver.com/features/effects-of-up...

As each generation of Porsche iterates the wheels increase by 1 inch. Questions are:

1. What effect does this have on performance if the tyre wall is not decreasing below 35 profile?

2. Is there an optimum wheel size for track racing / fast road driving - e.g. 19" with 35 profile on a 7GT3? Or does it completely depend on the car, e.g. a 6GT3 performs better on 18s than it would on 19s, whereas a 7GT3 performs better vice versa?

3. Would there be a limit to wheel sizes based on physics - e.g. would Porsche stop at 24 or 25 inches on the sports car range in a couple of decades?

It is an interesting article, thanks for posting the link.

It seems to me that their comparison has three main variables, the wheel diameter and the tyre profile, which are obviously linked, and also the wheel/tyre width.

As they change the wheel diameter they are also widening the wheel/tyre width, which will exagerate any effect. In fact the width change is the most dramatic (from 6" to 8.5" at each extreme) and could well be responsible for the majority of the effects, especially the weight. They should really change one variable at a time and re-measure.

When Porsche upped the wheel sizes from 19" to 20" they left the tyre profile and width the same so only one factor changed. I would imagine this would much reduce the effects noted.

Even with all three parameters changing, for each step the effect they noted seemed small, and it was only going from one extreme to the other that gave real change. Going up from 18" to 19" and reducing profile from 40% of 225 to 35% or 235 and increasing width from 8.0" to 8.5" (225 to 235) only added 0.1 sec to 0-100 mph while reducing economy 0.8 mpg and grip 0.01 g.

Although the sound level was 1 dBA higher, this doesn't seem to be measured very precisly and for all the other steps shows no change, so I would imagine it is a small increase each time and would probably mainly come from the extra width, which is not a variable that has changed between the 997/987 and 991/981. Wheel weight went up 3 lb, also probably mainly from the extra width.

With the new cars increasing the wheel size by 1" without changing width or profile, this would give smaller effects and add less weight. Would anyone notice?

When choosing between the new 19" and 20" options, the tyre profile will change at the same time as the wheel diameter but the width stays the same. I would say much the biggest effect will be from the reduction in profile as this is around 12.5% where as the diameter increase is only 5% and the width stays the same.

The effect is likely to be mainly on the ride quality and will depend on how much give is in the sidewall of the lower profile tyres and how much the ride quality depends on this (as opposed to springs, dampers, PASM, etc.)

My 987 had 235/35 R19 and 265/35 R19 tyres (without PASM) and the 981 has 235/35 R20 and 265/35 R20 tyres so the profile is the same and I guess the ride would be similar, before the addition of PASM.

Cost will of course be a factor, and the 20" rear tyres do seem to cost a bit more but strangely the front 20" tyres seems to cost less than the higher profile 19" tyres, probably due to supply/demand differences.

I have come to the conclusion that although smaller wheels/higher profile tyres will give better ride quality and save a bit of money, so long as the variation is only an inch or two the difference is small enough to choose the wheels that you think look best (or are easiest to keep clean!).

If the 19" and 20" wheels were the same design, I would probably have the 19" ones.

As Porsche give 1x 19" option and 3x 20" options, most of the time one of the 20" options will appeal most (and of course add margin for Porsche).

I like the less fussy look of the Carrera S 20" wheels, and they look easier to keep clean as a bonus.

kerching!

Cheers,

Geoff

[SonnyM]

Thanks Geoff!

What they have done on the 981 19" wheels is increase the profile from 35 to 40. So on a non PASM car I assume the ride would be better on 19"s than it was on the 987.

I wonder if the suspension / PASM has been optimised for 20" wheels on the 981. And if 20" tyres sizes are the norm across the range it may be easier to get hold of those in N rating rather than 19s on a 40 profile.

Either way the 991 19s look better than the 981 19s - but the car costs a little more!

[gbrownzen]

SonnyM said:

Thanks Geoff!

What they have done on the 981 19" wheels is increase the profile from 35 to 40. So on a non PASM car I assume the ride would be better on 19"s than it was on the 987.

I wonder if the suspension / PASM has been optimised for 20" wheels on the 981. And if 20" tyres sizes are the norm across the range it may be easier to get hold of those in N rating rather than 19s on a 40 profile.

Either way the 991 19s look better than the 981 19s - but the car costs a little more!

I nearly went for the 19" wheels (with PASM) with the belief it would give a better ride, but I agree they are not quite as nice a wheel as the 19" 991 wheels.

Upgrading the car was about £30K but upgrading the wheels was £1K! :-)

Since the 991 S only has 20" wheels, and all the ones I drove or was a passenger in seemed to have a decent ride (with PASM as standard) I guess it's not too radical a choice.

I'm sure there is a Cayman R in my local OPC with indentical looking Carrera S wheels, but 19" as it is a 987. I reckon if I decide the 20" wheels are wrong I should be able to sell them (as I guess more people trade up than down in size) and perhaps fit a set of 19" ones, hence having a higher profile tyre but a wheel that could look exactly the same.

Most likely, the 20" wheels will be great and with PASM as well the ride will almost certainly be better than I am used to. I was very relieved I got the 19" wheels on the 987 as it always looked better to me than similar cars with the 18" or 17" wheels, and just felt better to own. Shallow, I know.

I did look for tyre prices, and actually the 19" tyres seemed harder to find than the 20" ones, perhaps because most 19" wheels have the lower profile tyre fitted? If you are running such large wheels, it seems likely they will have very low profile tyres on them.

Cheers,

Geoff