Calling all Porsche wheel size and handling experts

[CarreraLightweightRacing]

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

Edited by CarreraLightweightRacing on Tuesday 18th October 23:02

[CarreraLightweightRacing]

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?

Correct OC. I think Steve may have meant:

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

[CarreraLightweightRacing]

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...

[CarreraLightweightRacing]

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 ?

No H, here I am safe to choose at will 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]

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 225/40/18

Very 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.

Regarding your first point: The reason is, with rim width you either have clearance or you don't. But by playing with tyre sizes you potentially mess with the RR which the ABS, Speedo, TC are calibrated to. They are really picky here.
Also a major factor for the 205 front means an instant 10% less force required for steering input

[CarreraLightweightRacing]

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...

[CarreraLightweightRacing]

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)?

[CarreraLightweightRacing]

Hi Steve, spring rates are 60N/mm front and 120N/mm rear. I expect the car to come in at 1120kg (All fluids, no fuel or driver) or 257kg lighter than where it started if you prefer
20 click rebound adjustment.
In terms of positive rake angle are you talking 1-2mm or more like 5mm plus deviation front to rear?

[CarreraLightweightRacing]

So effectively shift the weight forward to get closer to a neutral balance. 20mm, will look like a top fuel dragster

[CarreraLightweightRacing]

Finally managed to make some headway with this project today as some of my parts arrived:


So here goes… As weight is the single most important factor for this project, you may have to counteract your natural urges and focus on this when I mention things like why I have gone for narrower wheels/tyres. As speaking to some of the tyre manufacturers and F1 Engineer’s for Michelin they couldn’t grasp the fact that I am willing to lose a %age of ultimate grip for instance, in order to reduce gyro effect for better steering feel etc; especially when there is sometimes 3kg difference in a tyre and I have no PAS.
So here are a few critical details about the car:
1998 996 C2 started at 1377kg. When finished will be 1116.2kg (1087.2kg dry), retaining full interior and 4 seats…

From research thus far, the lightest tyres possible in the sizes I want are Conti Sport Contact. Ok, they are no R888R or CUP2 but this car is not a 100% track car; it is primarily designed for the road but of course must be useful when the need arises on track. Just something to bear in mind at this point. But thoughts on whether or not this will work would be appreciated.

FRONT:
Track
First question: I am currently using 15mm spacers with OEM wheels 7.5x18 ET50. I am having bespoke custom Forged T6061 alloys made (205/50 17 on a 6J rim). Would there be much difference between ET16 & ET21 in terms of weight at the steering wheel. (16 would effectively sit the same as the OEM wheels + 15mm spacers, ET21 would be like having a 10mm spacer instead). Reason this is important is purely to reduce the input force required at the steering wheel. No PAS = heavy. I am just wary that perhaps the extra 5mm track per side might be important to retain. Other issues arise here also like arch clearance… Wont know about this until I have the wheels fitted to check clearance.
Rebound & height
I am using Ohlins R&T but I have designed my own Adjustable top mounts to suit. What rebound setting would you recommend (how many clicks, currently set at mid point 10 clicks of possible 20)? Same again for height?
Castor
I am also having some adj Castor arms made (stock are fixed and 369mm long). How much castor is recommended; my thinking is by extending the length of the arms (i.e reduce positive castor angle), this should have an effect of reducing steering weight; where is the optimum (the point where handling is optimised but the steering is also light)?
Camber & Toe
Any recommendation?
 
REAR:
Current 265/35 18 on 10J ET65 with 15mm spacers. These will be swapped with 255/40 17 on 8.5J ET31. This is effectively the same stance as currently fitted but will equate to a 6.3kg reduction per side. The only other modification to the rear will be the Ohlins R&T with custom pillow ball top mounts. So the only question relating to the rears are what geo settings are recommended? And as there is no castor adjustment, will this cause me any problem (do you think I also need to make adj rear castor arms?).

Also what settings for: Toe, Camber, Height & Rebound?

Any other potential issues I may have over-looked?

[CarreraLightweightRacing]

Jim1556 said:

Don't know if you've emailed Michelin, but they're replacement for MPSS comes out in January (MPS4 S).

Lots on here hold the current MPSS in high regard (me included). Maybe worth asking for info?

Although, launch sizes are 19" upwards, but they'll probably release 18" at some point next year...

http://www.tyrereviews.co.uk/Article/Michelin-Pilo...

I'm after 17's Jim and they don't appear very fashionable nowadays with everyone going 19,20 or even 21". The trouble I'm finding is I can get a front but not a rear or vice versa from the same tyre brand.
I weighed a 255/40 17 CUP 2 today and it was relatively light at 10.5kg. By contrast the lightest tyre I have data on is 10kg dead in this size. Some are 12.6kg...
Anyway tyres are only one issue, would still like some ideas regarding geo set-up. Just adjusted the rear Ohlins dampers prior to fitting to -9.5mm less than how they came. As the car will be somewhat lighter than stock, I just thought it might be an idea to go straight in at the lowest setting then dial them up to suit (if necessary).

[CarreraLightweightRacing]

Gentlemen thank you, I was just looking at my friends 964RS yesterday and as he had his CUP2's removed I weighed a rear one. As stock the 964RS came in the exact tyre sizes I intend to fit and the LHD ones were also Non-PAS. So a good call to use their base figures for geo.
Henry reference the settings above, are they both the same for the UK PAS and the Non-PAS cars?

I will speak to Michelin again and plead with them to consider doing a CUP2 in a 205/50 17. Both 996 and 964 have this size as stock. Were there any other Porsche with this size fitted as stock? (would just aid my cause if I can let them know a significant sector of the market is missing out). I already asked them to do them for me but they have no interest; understandably for homologation and tooling reasons. But if there is a wider market, they might just be more willing.

[CarreraLightweightRacing]

OC, I have sent an email to my contact in Michelin requesting on behalf on the older Pork owning fraternity, to make 205/50r17 and 255/40r17 in both CUP2 and MPS4 preferable in 'N' flavour.