It could be that you went with a different tire..but to me the car still feels skittish. I did the same thing and went from 18 to 20.

https://www.jaguarforums.com/forum/f...odynes-145635/

 

As far as I know, you are the only base owner who's made the switch and reported that observation. Every other one I've seen perceives a significant improvement.

 

Couple observations:
1. I went from 18" Velas to 20" Storm wheels. These are the wheels that are only offered with the carbon brakes. They are forged wheels, and are supposed to be the lightest of the 20" wheels (don't have the data to back this up though). They are still heavier than the 18" Velas.
2. The tires are definitely different than what you are running. The set came with Michelin Pilot Cup 2 tires, 305/30/20 back and 255/35/20 front. These tires are very sticky when warm and dry, but can get squirly when cold or wet.
3. The difference I am describing is not about wheel slip under high applied torque. While this wheel/tire combo is certainly better in that regard than the stock Vela wheels and Conti tires on warm, dry pavement, it's still possible to break the back end loose with large throttle inputs.
4. The difference I'm describing is more about the feel going around corners with uneven pavement under moderate throttle input. With the stock setup, the back end sometimes felt as if it was bouncing sideways in those situations. The new setup feels significantly more planted in the same situations. The best way to describe it is that before it was semi-random, unpredictable, road surface induced (mild) oversteer vs. now it's predictable right foot induced (mild to wild depending upon right foot position) oversteer.

Also, one other point you mentioned in the referenced post was the diminished acceleration / braking due to the added inertia of the larger wheels. I can't comment on that specifically with the new setup since I (almost) never accelerate hard from a dead stop (ie., drag race). I tend to mash the go pedal only after starting rolling first, or when passing, and I can't feel any difference in those scenarios. I also have not yet really pushed the brakes.

 

A base v6 fitted with gyrodynes/Pirellis will spin its tyres in 1st gear on dry Tarmac even with the traction control fully on.

The increased rotational inertia will have an effect on this, but I switched before I completed running it in, so cannot comment on how much longer the 11 would have been.

In previous cars, fitting significantly bigger brakes had a more noticeable effect than switching between 17" and 18" wheels fitted with run flats.

Seperately, the ride is better on the somewhat rippled roads of the Welsh Marches, but I would still say it's over sprung, and underdamped at low frequencies.

 

F-typical: I totally agree with this statement about spring rate / dampers. The difference between the 18's and 20's, though, due to the additional unsprung weight on the often very rippled roads of New England is, to me, remarkable. I'd still like a tad bit more damping, but I'm no longer afraid of the go pedal on bumpy corners.

 

+1

 

I agree with your statement on line 4. The struts are underdamped for the springs that are on the car. Coming from a track background with supensions, I'd be curious to see what the data is on a shock dyno. I'm hoping a company like Bilstein will eventually offer their adjustable shocks.