Calling all Porsche wheel size and handling experts

[Steve Rance]

If you are going to fit the new Toyo 888 there will be enough grip from a 205. You won't need to go wider. I'd run a fair bit of rake. It is a big help in loading the nose. I run a lot on all of my set ups. Typically very low at the rear and even lower in the front. If your spring rates are not that high then it's even easier to load the front up

[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?

[Steve Rance]

20mm. Try it and wind back if you cant get on with it. Dont lift the back,you will provoke oversteer. Drop the front. You shouldnt get bumpsteer issues

[CarreraLightweightRacing]

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

[Steve Rance]

The car is already in the right attitude as the brakes are applied. I always like to run as much take as I can on a 911.

[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?

[Jim1556]

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...

[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).

[braddo]

On the front castor, perhaps it is worth checking to see what impact bumper cars with no PAS typically run? Another comparator might be a recent Elise/Exige, given they have unassisted steering.

[Slippydiff]

Evening Rich. These are the standard 964 RS geo settings :

From Porsche Service Technik '92 911 Carrera RS, WKD 497 620, page 0-4, Technical data

Wheel Alignment

Front Axle:
Toe (total): +25' + 5'
Camber: -1deg +/-10', max difference left to right: 10'
Caster: 4deg 25' +/- 15', max difference left to right: 15'
Toe difference angle at 20deg steering lock: 1deg 50' +/-30'

Rear Axle:
Toe (each wheel): +15' +5'
Camber: -1deg15' +/-10'

Be aware the 964's rear end is crude when compared to that of the 996. But the front settings should give you some pointers. Front caster could be increased considerably to 7 deg but this will come at the expense of increased weight. I think this is going to be a suck it and see exercise.....

[braddo]

Lotus 211 also has 4 degrees front castor according to this.

http://wiki.seloc.org/a/Geo_Setups

[Geneve]

You could try this experiment......

https://www.youtube.com/watch?v=HPh90yNX-mY

[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.

[Orangecurry]

993


968/944?

Ask them to do a 17" MPS4 in 205/50 255/40 sizes as well - which will address your customer's needs if they want a road/UK-weather-bias.

ETA Michelin already do a 225/45/17 MPS4 which I have on the front, which is good if you have PAS.

Then Michelin will also retain all of the older Porsche-owners market when they phase-out the MPS2.

Edited by Orangecurry on Saturday 10th December 10:35

[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.