New Turbo owner - tyre query

Big Dave · Jan 18, 2009

ORIGINAL: poprock

Bigger wheels affects the handling though "¦ I'm a great believer in sticking with the manufacturer's spec for wheel size. It was chosen for a reason.

Yes they do affect the handling..
They actually make it better...
On the S2/turbo / 968,s they do anyway..
How do it know. Because i fitted some 17",s...+ low and behold it was better...
It might tramline a little bit, but they defo improve the cornering...Less sidewall = faster steering response ...

Hilux · Jan 18, 2009

The original tyre sizes and types would have been thoroughly tested by Porsche (of course)

The whole package will have been put together with spring and damper rates taken into account alongside tyre wall compliance.

Therefore if you have `old` baggy, saggy worn out suspension with all the suspension and ARB bushes loose and worn then fitting a bigger, lower profile tyre will exagerate any suspension deficiencies and not make a jot of difference except to probably worsen the handling although turn in might appear better. eg: for a start - why is the fitting of of 968 castor bushes highly recommended when running 17 wheels?

To summarise, if you want better handling then start with getting the suspension back to standard or upgrading it and only then consider tyres and wheels that will then be able to enhance it.

TTM · Jan 18, 2009

ORIGINAL: Big Dave UK
Yes they do affect the handling..
They actually make it better...
On the S2/turbo / 968,s they do anyway..

17" make the handling and ride worse than 16" - have tried it myself as well.

Fen · Jan 18, 2009

Actually I agree with you Thom; handling is better on 16s. Grip however is less and having tried 16, 17 and 18 inch quite extensively on 944s i would say that 17 is the best grip/handling compromise. The Turbo even standard with a baggy wastegate can easily overome the grip of 16s, sometimes at inopportune moments and without much warning, though it's quite sweet on the limit when you know you're getting near to it. 17s break away when you don't expect it much less often, but the tradeoff is that they let go with less warning at the limit so you need to tread carefully on the edge. Of course the limit of 17s is beyond where 16s would already have let go. That is the difference between grip and handling. 18s are too big and in my experience you need to find the limit where you have a lot of space because while they hang on well they let go very suddenly.

I also agree about 16s being better in straight line performance, but if that's your primary interest then why not fit 15s?

Last thing to consider is the vastly superior (and often cheaper) choice of tyres in 17 that simply arent available in 16 inch.

TTM · Jan 18, 2009

Fen, thanks for finding the right words. I agree with you.
I haven't looked into 15" wheels as grip would be even less good than with 16" and 16" is the original size. I also believe it won't be easy to find a 15" 245 or wider tyre.
Although grip is obviously a factor to look for, too much grip can actually alter the dynamics of a chassis, and having tested many suspension set ups on 944 (Bilstein Cup, KW, Moton, Konis, Bilstein B6), in any case the best compromise, or maybe should I say the one that I consider best, was reached with 16" wheels.
I firmly believe a minimum amount of tyre "slippy-ness" is needed to make the dynamics of a chassis work correctly, this slippy-ness not necessarily being noticable to the driver in extreme conditions such as heavy cornering and weight transfers.
In order to improve grip we could obviously go mad and fit ultra low profile tyres with 18" or even bigger wheels, however it won't make the chassis work correctly and although this will make the car virtually impossible to lose in a corner it may not make it lap a track faster.

ORIGINAL: Hilux
why is the fitting of of 968 castor bushes highly recommended when running 17 wheels?

Funny you should mention that ... Look at the following picture [

]

Diver944 · Jan 18, 2009

Oooooh that is a good picture that I haven't seen before Thom. I have shamelessly saved it into my useful pics folder as it clearly shows where the caster block is at the back of the A Arm

Oddy enough even though it is a 968 cutaway, it does look like the earlier 'rubberier' mount from a 944 has been fitted [&:] Maybe not all 968s had the later mount?

TTM · Jan 18, 2009

Maybe the people at Porsche found out that just fitting bigger wheels was not such a good idea after all and introduced the hardened caster block later to make up for their error [

]

Anyway I certainly saw more than a couple early 968 (1992) with 944 caster mounts.

Diver944 · Jan 18, 2009

ORIGINAL: Hilux

why is the fitting of of 968 castor bushes highly recommended when running 17 wheels?

The theory is that the 968 bush has lots less rubber so the A Arm is more securely fixed in place. The narrower profile tyre on a 17" wheel is more prone to follow ruts and poor road surfaces when the A Arm can flex in the rubbery 944 mount. The 968 mount reduces this flex so the car drives straighter.

This picture shows the 968 mount above an old 944 version: