What size are your wheels ? I've just swapped my 19" Polaris for a set of 17" Elegants. Ride is now much more compliant and the car definitely feels more settled on the road. Tyres aspect ratio went from 40 to 55.

 

Fraser, funny you should ask about wheel size. My car has 19", with an aspect ratio of 40, same as yours was. I *HATE* the low profile tires that Jag (and, it seems, most other luxury lines) are using now. In fact, I own this car now because I hated the very stiff ride of the 2009 XF Supercharged that I owned. I believe most of that nasty ride was due to the 20" rims (and associated lack of sidewall), and I figured I would swap those out for 17" or 18" rims, until someone (probably on this forum) kindly pointed out that the smallest rim the supercharger's brakes would accommodate was 19". So I dumped the car for this one.

There's no doubt that I'd get a better ride if I came down from 19" to 18" or 17", and in hindsight, I probably should've done that. But I'm glad to know this particular issue is fixed.

I'm now dealing with problems caused by my nice Pirelli P7's being malformed or out-of-round. I have a brand new, very irritating steering wheel shudder that came with the brand new tires. The tire dealer completely replaced one of them and the problem is *mostly* gone, but it still has some steering wheel shudder, and it still doesn't drive as smoothly as my in-laws' 2004 XJ with 17" rims. Perhaps it never will.

 

Yes, I didn't mention that the more powerful XJs had larger diameter brake discs. My previous X350 had 320mm on the front, but the XJRs, and, I think, all the supercharged cars had 360mm. From about 2006 the normal front discs went to 326, and at the same time the caliper became a cast iron item with a single piston.Previous calipers were aluminium twin piston jobs.

I bought the 17" wheels quite quickly on eBay, and then spent a week worrying about the change of caliper and disc size and the possibility they would not fit or the balance weights would hit the caliper. However, in the end they just went on with plenty of space between caliper and wheel, about 1/2" My MG TF has 304 mm discs and 16" wheels with sexy-looking twin piston calipers in red. The gap with these is very close, and I can't get the wheel brush in between the gap, although the stick-on balance weights pass through OK

 

When I first received my Super 8, it came with tires that were rock hard, but had good tread. It basically did the same thing your car did in the rear. Now after putting new tires, strengthening all wheels, balancing and alignment, and doing a hard reboot to the car, it rides really good on 20” wheels, except if I hit a pothole or certain types of manhole covers.

I don’t know about your car, but Super 8s have the comfort suspension, but mine is a Portfolio and I was told by the dealer mine has the sport suspension. But either way, I’m liking this car more and more as it envolves!

 

Unfortunately, I’m learning about bent wheels and maybe your new problem is caused by one of your wheels being bent and/or not balanced correctly. It might be a good idea to have all your wheels straightened.

 

Shame on me for not posting an update sooner, but this problem is now resolved. I went back to the tire shop and complained about the shudder, and they offered to pay to have a different tire shop do a 'road balance' -- supposedly they balance the tire while it's mounted to the car, or at least, that's how the initial tire shop described it.

I took the car to the second shop, asked for a 'road balance', and they looked at me kind of funny. They pulled tire 1 off the car and said that it was 'way off'. So they re-balanced it. Same with tires 2-4. They charged me $60 and sent me on my way with my car driving like a freaking DREAM! Shop #1 was exceedingly puzzled, but reimbursed the $60, and I haven't looked back. I've put 4,000 miles on the car since then and it's still driving like it should. Can I get an AMEN??

 

AMEN to that !!