Calling all Porsche wheel size and handling experts
Discussion
OEM wheels on a 996 Carrera are:
7.5Jx18 ET50 with 225/40 ZR18
10Jx18 ET65 with 265/40 ZR18
Now there is also an OEM option of:
7Jx17 ET50 with 205/50 ZR17
9Jx17 ET55 with 255/40 ZR17
By my calculation using the 17" tyre sizes quoted above, the lightest wheels I could possibly get away with would be the following:
6Jx17 ET16 Front to fit the 205/50 ZR17 front
8.5Jx17 ET31 Rear to fit the 255/40 ZR17 rear
The above ET's would effectively fill the arches by pushing the wheels out 15mm further than the stock 18's, which is just about perfect.
So my question basically is; would using a 205 section on a 6J rim and a 255 section on a 8.5J rim work? My ultimate aim is to find the lightest possible solution but obviously upsetting the handling is not something I can compromise on. The car will be running on Ohlins dampers with custom adjustable top mounts so camber can be added/optimised to suit. Any geo recommendations for fast road settings would also be gratefully received
7.5Jx18 ET50 with 225/40 ZR18
10Jx18 ET65 with 265/40 ZR18
Now there is also an OEM option of:
7Jx17 ET50 with 205/50 ZR17
9Jx17 ET55 with 255/40 ZR17
By my calculation using the 17" tyre sizes quoted above, the lightest wheels I could possibly get away with would be the following:
6Jx17 ET16 Front to fit the 205/50 ZR17 front
8.5Jx17 ET31 Rear to fit the 255/40 ZR17 rear
The above ET's would effectively fill the arches by pushing the wheels out 15mm further than the stock 18's, which is just about perfect.
So my question basically is; would using a 205 section on a 6J rim and a 255 section on a 8.5J rim work? My ultimate aim is to find the lightest possible solution but obviously upsetting the handling is not something I can compromise on. The car will be running on Ohlins dampers with custom adjustable top mounts so camber can be added/optimised to suit. Any geo recommendations for fast road settings would also be gratefully received
Edited by CarreraLightweightRacing on Tuesday 18th October 23:02
Not an expert, but I've tried most OE wheels out in the real world...
986 was factory fit 255/40 on 8.5J so no issues there. I've also run with those on the back of the 993.
I have the same wheel in 8.5J and 9J 17s (Sports Classic 1), but both have different tyres on them so cannot weigh them for you, but the following website usually accurate....
http://www.944racing.de/wheelweights.php
I would have thought that 'handling' could all be sorted by suspension and geo, so is going down to a 6J at the front necessary? How much weight will that save?
A good friend of mine ran 7/9J 17s on his 996.2 and he was very happy with it on circuit. This was the so-called 10-spoke, weighing just 8.2/9.5
Going forwards, I would have thought that finding a tyre-size with enough choice is the critical factor - 255/40/17 is very poor choice - how about 245/40/17 and work backwards?
986 was factory fit 255/40 on 8.5J so no issues there. I've also run with those on the back of the 993.
I have the same wheel in 8.5J and 9J 17s (Sports Classic 1), but both have different tyres on them so cannot weigh them for you, but the following website usually accurate....
http://www.944racing.de/wheelweights.php
I would have thought that 'handling' could all be sorted by suspension and geo, so is going down to a 6J at the front necessary? How much weight will that save?
A good friend of mine ran 7/9J 17s on his 996.2 and he was very happy with it on circuit. This was the so-called 10-spoke, weighing just 8.2/9.5
Going forwards, I would have thought that finding a tyre-size with enough choice is the critical factor - 255/40/17 is very poor choice - how about 245/40/17 and work backwards?
The chassis on your car will offer more mechanical grip than the standard chassis. A higher sidewall will make the car feel more progressive and generally more feelsome up to the limit and easier to correct at the limit. The lower profile tyre will be easier beyond the limit as it will responde quicker as it will not suffer as much sidewall distortion which will need to be overcome as forces reload to the other side of the sidewall.
My view is that you may create too much disparity between the mechanic grip of the chassis and the physical grip of the tyres if you reduce your tyre widths too much whilst at the same time increasing the capability of the chassis. I'd keep the higher profile for more feel and hoonability but would be inclined to keep closer to the OEM tyre widths.
My view is that you may create too much disparity between the mechanic grip of the chassis and the physical grip of the tyres if you reduce your tyre widths too much whilst at the same time increasing the capability of the chassis. I'd keep the higher profile for more feel and hoonability but would be inclined to keep closer to the OEM tyre widths.
Steve Rance said:
I'd keep the higher profile for more feel and hoonability but would be inclined to keep closer to the OEM tyre widths.
The way I read it, CLR is keeping exactly the OEM tyre widths, but is proposing putting them on narrower rims than OEM.The issue with wider tyres on narrower rims is that the tyrewall is able to distort more - a narrower tyre on a wider rim has the sidewall stretched and therefore cannot flex as much....
If you can adjust understeer with the GEO, I would also stick with close to OEM pairings.
My original question though is relevant - how much weight can you save with a 6J vs a 7J rim?
Orangecurry said:
Steve Rance said:
I'd keep the higher profile for more feel and hoonability but would be inclined to keep closer to the OEM tyre widths.
The way I read it, CLR is keeping exactly the OEM tyre widths, but is proposing putting them on narrower rims than OEM.The issue with wider tyres on narrower rims is that the tyrewall is able to distort more - a narrower tyre on a wider rim has the sidewall stretched and therefore cannot flex as much....
If you can adjust understeer with the GEO, I would also stick with close to OEM pairings.
My original question though is relevant - how much weight can you save with a 6J vs a 7J rim?
Steve Rance said:
I'd keep the higher profile for more feel and hoonability but would be inclined to keep closer to the OEM wheel widths.
Which would then make perfect sense. I have collated all the data from the manufacturer:Wheel Diameter Width ET Weight Position
Oz 18" 7.5J ET35 8.3kg Front
Oz 18" 9J ET37 8.9kg Rear
Oz 17" 6J ET16 7.2kg Front
Oz 17" 8.5J ET31 7.9kg Rear
Fuchs 18" 7.5J ET35 9.1kg Front
Fuchs 18" 9J ET37 9.9kg Rear
Fuchs 17" 6J ET16 7.6kg Front
Fuchs 17" 8.5J ET31 8.5kg Rear
NOTES on the above. They are not Oz or Fuchs, but to all intents look the same made to the same Forged lightweight standard but can be customised to spec. Fuchs/Oz/OEM all require 15mm spacers which come in at 0.8kg each.
The lightest Porsche wheels (the 10 spoke 17's), would effectively come in at around 2kg per wheel more than the lightest option by the time you have added spacers.
My thinking right now, based on your comments is that perhaps the 6J would be better as a 6.5J for the fronts when using the 17's i.e 17x6.5J ET22.
Orangecurry that is very kind of you to offer the use of your wheels. From a purely dynamic perspective I think it might be the way forward to do the testing with spacers/cheap OEM 17's. I couldn't really use them as the final solution though, as a nice set of wheels, custom made to optimum spec, also adds the visual benefit.
One thing that concerns me though, going down the 205 front route, is the potential understeer. If the chassis is set up perfectly, could I get away with a measly 205 on the front?
I wasn't aware of this tyre choice limitation. Just how limited is the choice in 205/50/17 and 255/40/17?
anonymous said:
[redacted]
Care to expand on this J 
Many thanks for you comments guys
... but how much weight do you save by having a 6J front vs a 7J front? If it's 'small' then why have the narrower wheel with the 205 section tyre, as you'll introduce more sidewall flex (everything else being equal).
Anyway - for me, I'd be deciding what tyres I want to offer on the R first, seeing what sizes they are made in, and working backwards - so for tyre availability - look at oponeo or tyreleader or anyone...
255/40/17 = no MPS4 no ContiSC5 few GY and Bridges, but you do have MPSCup2 and MPS2 N-rated
245/40/17 = loads more choice
then it gets worse - matching the above into 205/50 is almost impossible, as this size sits 'one layer down' in the sort of car they apply to.... lots of choice but mostly ECO-tyres. There is the ContiSC5 but no MPS4
Obviously you can go with the MPS2 N-rated, but for how long, and doesn't your car deserve 21st Century tyres?
Best *choice* of tyres in 17s is 225/45 and 245/40, and then you are back to 8J fronts and 9J rears..... but at least you've solved the understeer issue
I have to go down the pub
Anyway - for me, I'd be deciding what tyres I want to offer on the R first, seeing what sizes they are made in, and working backwards - so for tyre availability - look at oponeo or tyreleader or anyone...
255/40/17 = no MPS4 no ContiSC5 few GY and Bridges, but you do have MPSCup2 and MPS2 N-rated
245/40/17 = loads more choice
then it gets worse - matching the above into 205/50 is almost impossible, as this size sits 'one layer down' in the sort of car they apply to.... lots of choice but mostly ECO-tyres. There is the ContiSC5 but no MPS4
Obviously you can go with the MPS2 N-rated, but for how long, and doesn't your car deserve 21st Century tyres?
Best *choice* of tyres in 17s is 225/45 and 245/40, and then you are back to 8J fronts and 9J rears..... but at least you've solved the understeer issue
I have to go down the pub
Slippydiff said:
CarreraLightweightRacing said:
Sorry OC, I should have mentioned, for TUV compliance I am pretty much stuck with OEM sizes. Also really surprised there is such a limitation in tyre options. Hmm looks like a rethink is in order...
Evening Ritchie. Have they got to be N rated too ? my understanding is though, the rubber mixture is optimised when going 'N' rated for the inherent 911 weight distribution and handling characteristics. I will speak to my TUV inspector today though and check if I'm able to go for different sizes if I keep to the same rolling radius
CarreraLightweightRacing said:
Sorry OC, I should have mentioned, for TUV compliance I am pretty much stuck with OEM sizes.
I would have though TUV would be more concerned over a narrower front rim than OEM? Even the 993 was 7J and wider.As the car came on 225/40/18 I'd be very surprised if they didn't allow 225/45/17?
But then it is a Government department.
CarreraLightweightRacing said:
Also really surprised there is such a limitation in tyre options. Hmm looks like a rethink is in order...
It's down to what is a popular-manufacturer-choice tyre-size today. 'Sports' cars are all on 225/45/17 (so there is a massive choice) or 18s and above... hence there is also a massive choice in 225/40/18Very very few 'modern' cars have a wide tyre on a 17 - the fashion is for bigger rims.
...and there isn't a huge choice in 265/35/18, but it's much better than 255/40/17.
Orangecurry said:
CarreraLightweightRacing said:
Sorry OC, I should have mentioned, for TUV compliance I am pretty much stuck with OEM sizes.
I would have though TUV would be more concerned over a narrower front rim than OEM? Even the 993 was 7J and wider.As the car came on 225/40/18 I'd be very surprised if they didn't allow 225/45/17?
But then it is a Government department.
CarreraLightweightRacing said:
Also really surprised there is such a limitation in tyre options. Hmm looks like a rethink is in order...
It's down to what is a popular-manufacturer-choice tyre-size today. 'Sports' cars are all on 225/45/17 (so there is a massive choice) or 18s and above... hence there is also a massive choice in 225/40/18Very very few 'modern' cars have a wide tyre on a 17 - the fashion is for bigger rims.
...and there isn't a huge choice in 265/35/18, but it's much better than 255/40/17.
Also a major factor for the 205 front means an instant 10% less force required for steering input
On the 993 it is quite noticable - 205 on 7J makes you feel as if you have your hands on the tarmac. 225/45/17 is still more communicative and 'lighter' than 225/40/18, but still feels slightly clumsy in comparison to 205s.
(you notice it more when going back from 225 to 205, but it could be that 205 is the sweet-spot of the 993 - I don't know where the sweet-spot of the 996 lies)
I'd forgotten you have no PAS - does that not make it even more important to chuck on an array of OEM rims and tyres first, before ordering bespoke rims?
(you notice it more when going back from 225 to 205, but it could be that 205 is the sweet-spot of the 993 - I don't know where the sweet-spot of the 996 lies)
I'd forgotten you have no PAS - does that not make it even more important to chuck on an array of OEM rims and tyres first, before ordering bespoke rims?
Edited by Orangecurry on Thursday 20th October 09:35
Id go 17inch fuch with a 40 or 45 side wall. 215 front and 255 rear. That should give you lots of feel, it wont be too understeery but will be lovely with a light trail and you will be able to coax oversteer if you want it. It will also fill the arch nicely and suit the stance of the car. I think that a 205 front will require too much of a trail and restrict your geo options.
Steve Rance said:
Id go 17inch fuch with a 40 or 45 side wall. 215 front and 255 rear. That should give you lots of feel, it wont be too understeery but will be lovely with a light trail and you will be able to coax oversteer if you want it. It will also fill the arch nicely and suit the stance of the car. I think that a 205 front will require too much of a trail and restrict your geo options.
Steve this kind of cemented my thoughts but the issue according to a comment above is that a 255 is a poor rear tyre choice due to availability of options.Also in terms of fitment for the front, the 205/50 17 would be more compatible as there would only be a -0.06% variation. With the 215/45 it would be -1.87% variation. But I tend to think the 215 is the better overall compromise (would just like to measure the steering force input with a spring balance to be more precise.
With the rears, the 255 would only induce a -1.07 variation, so no dramas there at all for Mr.TUV
Looks like I'll have to do a bit of digging regarding tyre options...
Agreed. I wouldn't worry about grip levels on the road. I'd even look at the Pirelli classic road tyre range as an option if they made the sizes. I'm looking at fitting those with 17" fuches to my 964 road. I'd also have a spare 18' set of wheels with some sticky track rubber as a track day option for the 996. Both bases covered!
to add to this thread. I've just finished the build on my other halves Clio trophy 182. Sharpened the chassis, upgraded the brakes and rebuilt the dampers (ouch). The finishing touch was to remove the factory fitted 16 inch wheels and 205 45 tyres and replaced with 15 inch Speedlines and 205 50 Toyo R888's. The extra grip of the sticky compound combined with the extra progression, compliance and feel from the higher side wall combine beautifully. This combination transforms an already very good car into a truly great one. The increase in side wall from 45 to 50 makes a huge difference.even though the wheel size is reduced, The rolling radius of the wheel/tyre combination is greater than the 16 inch set up so gearing is very slightly effected - not noticeable.
For any driver looking for more progression, compliance, feel and outright fun from his or her car, this is the way to go. I went for a sticky option as the car will be tracked also but on a road car it's not really necessary
I will post up some pictures soon.
For any driver looking for more progression, compliance, feel and outright fun from his or her car, this is the way to go. I went for a sticky option as the car will be tracked also but on a road car it's not really necessary
I will post up some pictures soon.
I had a look at my C2, due to his thread- I knew the tyres were new Michelin Pilot Sports but not the width, 205/50/17 front and 255/45/17 rear. The wheels are from a later car (mine is a 98 C2 cab), what would mine have been originally- narrower at the back I am thinking, maybe 225?
Some great advice here guys, many thanks
I am kind of warming to the idea of a road set and track set:
Road: 205/50 17 & 255/40 17
Track: 225/40 18 & 265/35 18
Trouble is there is no ideal solution. The best I can see is using R888R's in the road dimensions above with either a 6.5 or 7 front and a 8.5 rear. I think that should just about give the best grip/feel/weight combo. But again not perfect as they only have a 'W' speed rating (168mph). I still think this will be the route I take as at least there is no production issues with the Toyo's in these sizes. I have contacted Toyo just to check on the tyre weights. Does anyone know if the new compound (R888R) is light?
Also if going this route, what would be the perfect geo for road with a 30mm or so drop? (Happy to consider other heights from -35 to -5mm front and -29.5 to -5mm rear)?
I am kind of warming to the idea of a road set and track set:
Road: 205/50 17 & 255/40 17
Track: 225/40 18 & 265/35 18
Trouble is there is no ideal solution. The best I can see is using R888R's in the road dimensions above with either a 6.5 or 7 front and a 8.5 rear. I think that should just about give the best grip/feel/weight combo. But again not perfect as they only have a 'W' speed rating (168mph). I still think this will be the route I take as at least there is no production issues with the Toyo's in these sizes. I have contacted Toyo just to check on the tyre weights. Does anyone know if the new compound (R888R) is light?
Also if going this route, what would be the perfect geo for road with a 30mm or so drop? (Happy to consider other heights from -35 to -5mm front and -29.5 to -5mm rear)?
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