[928uk] Re: 18" wheels for 928

[Phil Chadwick]

John Kennar wrote:
>
> looking at replacing my 17" 993 style cup wheels with 18" of the same style.
> 
> Has anyone any thoughts on how achievable that is and what tyre sizes are
> best for front and rear, pressures to run etc? Are spacers needed perhaps?

Hi John,

Whole bunch of issues there....

0.  Never underestimate how serious Porsche was about unsprung
    weight.  Note the aluminium wheel nuts were not an accident.
    GT wheels were about 16 lb, S4 about 17 lb.  Many after-
    market wheels weigh *considerably* more.....

1.  18" rims will require 35 series tyres.  Expensive.  Also, pot
    holes are a likely to crack rims.  The car is likely to track
    every road undualtion and ride like a dray.  Some do it and
    appear to be happy.  I'd listen for the experience of others.

2.  The maximum sensible metal wheel width rims are 9.5" front
    and 11" rear.

3.  Maximum tyre width is about 265 mm front and 285 mm rear.
    Required offsets to achieve these are +65 to +68 mm front
    and +52 to +60 mm rear.  Outside these offset ranges, you
    will have to reduce tyre and rim width, and may sufer handling
    problems if you reduce the front offsets much below +55.0 mm.

4.  16" wheels 36/44psi front/rear; 17" wheels 36psi all round;
    and 18" wheels 30/32psi front/rear.

5.  Spacers change offset.  In particular they reduce a positive
    offset.  So a standard rear GTS wheels with a +55.0 mm offset
    and 35 mm spacer has an effective offset of 20.0 mm.  So,  
    you need to know the offset before you can figure out whether
    you need a spacer or not.  Note that anti-matter spacers are
    not available -- you can't increase a positive offset with a
    spacer (like many before you, you'll probably feel the need
    to do this when fitting other Porsche wheels to a 928).

To approach the limits, you will need professional fitting with
wheels (3-piece or custom 928) made specifically for the 928 (wheels
from some other Porsche models may fit the rear).

My (conservative) choice would be standard 17" GTS wheels and tyre
sizes.  Front 7.5"x17" wheels +65.0 mm offset and 225/45-17 tyres.
Rear 9"x17" wheels +55.0 mm offset and 255/40-17 tyres.  Good prices
in these sizes and adequate performance.

Finally, price the tyres *before* you buy the wheels....


-- 
Phil

Recently my family and I spent a week in London, which is a popular
foreign place to visit because they have learned to speak some English
over there.
        -- Dave Barry