www.jec.org.uk

I find the ride quality very harsh on my 2004 XK8 fitted with 20" Detroit alloys. Are there any issues with downsizing to an 18" wheel to soften the ride? Obviously the tyre profiles will need to be different to preserve the rolling diameter. What are members views?

Your wheels are interchangeable with standard 17" , 18" and 19" wheels , if you use XK8 or XKR wheels fitted with the correct tyres you will not have any problems with rolling diameters ,
however .......................... if your car is fitted with Brembo brakes 17" wheels won't clear the calipers , most but not all 18" wheels will require spacers to clear Brembo brakes ,

Hope this helps ,

Nik

PS Make sure you buy XK8/R wheels as the rears on 18" + wheels are wider for our vehicles

Ride quality on 20" wheels is a regular issue on the BB and in PMs addressed to me. In general, the XK8's ride does become firmer (more sporting) as wheel sizes increase and tyre profiles decrease. However, that's where CATS (the Jaguar automatic adjustable dampers) come into their own. CATS equipped steel cars have two damper settings, hard (the default setting) and soft (employed for non manic driving conditions). Consequently, CATS plus 20" wheels gives an ideal combination of good handling and reasonable ride. Non CATS cars have only a single firm setting which is fine with the smaller wheel sizes and fatter tyres but can be on the stiff side by the time 20" wheels and lower profile tyres are fitted.
As these cars get older, many are fitted with cheaper, replacement non Jaguar dampers. Most of these dampers are less sophisticated than the originals and have stiffer settings. Hence, non Jaguar dampers have also become a contributor to the problem of XK8s with an unacceptably hard ride quality. These comments assume everything else is in good order. Seized wishbone bushes and similar problems can also produce a hard ride before the bushes fail completely. Mike.

Thank you to Nik and Mike for the useful information. My car does not have CATS or Brembo brakes, so I can proceed with confidence.

Glad to be of assistance

Best of luck with your wheels ,

Nik

Neil

The Jaguar or Autologic Diagnostic Equipment can change the wheel diameter settings on the XK's internal system to allow for the different rolling circumferences of the various wheel & tyre combinations

The Jaguar supplied 18" wheels available to you are: (Model year(s) supplied)
Flute (96-2000)
Double Five (99-04)
Impeller (01&02)
Hydra (03-05)
Centaur (03&04)
Aris (05)
Milan (99-02)
17" wheels were the standard fit from 96 up to the 4.2-S (05) & not all those listed above were options for the XK8. The standard fit for the XK8 4.2-S was 19"

However, as far as I'm aware all 18" wheels are 8"w for Front & 9"w for Rear

18" tyres are 245/45ZR (F) & 255/45ZR (R) - see your handbook for full spec.

Regards

Allan Jones