Another review for those interested in hearing about and discussing the XKR-S GT.

Enjoy

 

Nice post, and its amazing that there is nothing changed with the engine!

 

I agree 100% with the reviewer. The news Jaguars don't need more power. They need a way to put down the power! The XKR-S GT looks like such an easy car to drive fast. Randy Pobst isn't having to constantly saw at the wheel to keep it on the track.

"Hold my beer, watch what I can do!" :rofl:

I'm a little sad that Jaguar can't get the same qualities of this XKR-S GT in their lower end XKR models. I mean, the regular XK with the 385hp 5.0L with the suspension/steering improvements of the XKR-S GT could be such an awesome car, while still maintaining the special nature of the higher level models and maintaining pretty decent ride quality.

Maybe I'm just wishing the X150 was more sports car than GT, though...

 

Yes, I am thinking the same. I owned an '05XK: terrible handling. An '08XK; much better than the '05 but, still sloppy handling. Now both gone replaced by a '12 XJL and a '12 XF. Of the four cars, the XF has the best combination of steering and handling. The XJL would be second best, still ahead of the two XKs.

I am certain that the GT's handling could be reproduced by reasonably priced aftermarket parts. If only someone would produce them or, if only Jaguar decided to sell the parts. But, if they did, who would pay $175K for the GT?

Albert

 

If you still leave the boy-racer bits for only the $175k XKR-S GT, and maybe threw in a small engine improvement, I'm sure people would still want it!

It just seems unfortunate because the XKR could've been a 911 fighter if they had improved the steering/handling from the getgo. Instead, they only really improved these things for the very late-cycle XKR-S GT and they're only making 25 of them for North America. I want to smack my head on a wall!

 

It is maddening, isn't it? Like I said, it does not take a scientist to make a car handle good. Why Jaguar had not done it with the XKs is simply beyond me.

Back when I owned the '05XK I resorted to installing a second stock rear sway bar in parallel with the original one, simply because the aftermarket offered zero options. While it improved that terrible handling but, still was not enough. It probably needed a third stock bar of that puny diameter to get the handling balance right. I simply ran out of room to go there.

Solid sway bars cost under $200 for most cars. Just that one single part, like a 3/4" or 7/8" rear bar with poly bushings could make a very significant difference in the handling of the XKs, if only someone would produce it. Combine it with a set of correct (not too stiff) lowering springs while leaving the stock electronic dampers in the car and you would feel a night and day difference in the handling for very little investment. A number of people here on the forum has done the lowering and reported great results. Still, it is not complete without a much larger rear sway bar to correct the factory handing imbalance that is heavily geared towards undesirable understeer.

Albert

 

I agree and why I went down the path of H&R soon to be coilovers and possible sway bars. Although there is nothing I can do about the steering ratio, so far my XKR feels much sharper than it did stock. The springs alone offer the stiffer ride and lower center of gravity but the rear dampers just can't handle the rebound very well over 100MPH. The rear of the car is just too "bouncy" at any speed, I want something that feel more planted to the ground.

You can buy the GT parts but they are insanely priced. Last I checked the rear sway bar was 1800.00

 

Yes, $1800 is insane for a bent steel bar. You could possibly custom make one for less. I used to use tubular, light-weight bars with aluminum arms when sway bars used to be mounted in a straight forward manner under the suspension. With the present generation of XKRs as well as with my XF and XJL, Jaguar managed to bury the bars deep into the suspension where it would take quite a bit of work to change it.

To address you points: The coilover is the absolute right way to go, as far as springs. They tend to use standard racing spring sizes which means that you can choose from an extreme wide selection of rates at very low prices.

The steering inaccuracy/slowness would improve a great deal with a larger rear sway bar. I remember changing rear bars on nearly every car I owned; when changing it on my wife's car, remember her coming home the next day after her usual canyon-drive commute, wide eyed, telling me how much better her car handled, how much "easier" it was to steer and how much she actually enjoyed driving it now. To sense the significance of her awareness to the changes I, at times, described her mechanical sense as: "she would not feel if a wheel fell off her car".

Dampers: this is the biggest dilemma. Since you described the lowered car's rebound as insufficient, this IS A BIG dilemma. The electronic dampers are a great idea. I would think twice before I'd go back to non-electronic dampers. Just disconnect the wires to the dampers and go for a ride to see the huge difference in ride harshness. Those dampers do a good job compensating for the lack of tire sidewalls in modern, low aspect tires.

Rebound control is BY FAR more important than compression. With insufficient rebound the car will wallow up and down, causing unpredictable weight transfer and thus, unpredictable tire adhesion to the road surfaces. I had never driven my '08XK hard enough in corners to probe the damper action simply because the car handled kind of sloppy and I did not feel like pushing it. The best solution would be if an aftermarket tuner hacked the factory damper software and actually gave it some adjustable parameters.

A second possible, and much less expensive alternative, is to avoid placing springs on the car that will overcome the dampers ability to dampen at any frequencies. My guide in setting up my fastest road and even race cars was always to use the softest possible springs that would still keep my lowered suspension off the bump stops. In general, I would NEVER exceed 300 lbs/inch on the rear of any of my cars. At these rates, you can place your body weight on the rear bumper and feel the springs compress, as opposed to today's factory setups (my XF and XJL) which feel rock solid.

Softer springs provide better tire traction, much better road surface compliance and much more confidence inspiring under real world driving. Their secondary benefit is that they will work with stock dampers, assuming that the stock dampers were doing the job with the stock springs. In general, when I lowered a street car I would not go more than 15-20 percent stiffer than stock with the springs. Of course, that is no longer a good measure because my XF, XJL springs, IMHO, are far too stiff to begin with. I have now concluded that my freeway ride vibration in the XJL is largely due to the dampers inability to follow/handle the high speed (high frequency) spring actions and they begin to oscillate over perfectly good road surfaces that come with the usual traction-inducing surface irregularities.

Once those softer springs selected the next thing is the sway bars. Most cars have sufficient front bar sizes and only the rubber bushings may need to be changed. But, most cars have entirely insufficient rear bar sizes and that is where the final tuning of the suspension can be done. With the proper size the body roll will be almost completely eliminated, steering sharpness greatly improved while still maintaining suspension compliance with the softer springs. Be aware of aftermarket spring manufacturers who would not tell you their spring rates. I have seen some aftermarket springs as high as 7-800 lbs/inch, great for drifting, horrible for street performance driving.

I stayed away from lowering my XK when I found out that the rear suspension had no camber adjustment. I measured my cambers and found them to be around 1.7 degree negative. I figured any more would be bad for tire wear and handling. As it appears, the XK(R) may have a very good rear geometry where there is no actual camber gain due to lowering. That is the good news in the otherwise bad news that Jaguar did not think it was needed to provide such adjustment? Again, in a GT (or any performance) car, WHY NOT, Jaguar??? Are you penny pinching on a $100K car???

I love the XK(R) concept and would actually consider going back to a 5.0 XK or XKR, if a complete solution existed for tuning the suspension. The two big questions for me at this point are; damper and rear sway bars selection. Despite not having an XK, I still follow the XK(R) forum exactly to see if such solution becomes available.

BTW - just out of curiosity; do you know the size of the $1800 GT rear bar? If large enough I could even consider swallowing that ridiculous price for the expected huge handling returns.

Albert

 

Albert all great points and thank you for the detailed response! I've been working with Spires and their solution still uses modified(due to higher spring rates) F-Type dampers that are fully compatible with the Adaptable suspension. This was I can still get a decent ride and the Adaptive control system is still working as it should, with a softer ride when Dynamic Mode is turned off. They said to wait on the rear sway bar until we get the spring rates dialed in then possibly move on to a larger rear sway bar.

 

The modified F-type dampers that work with the XK(R) software are good news, indeed. Just looked at their website and the adaptive coilover is listed for the XKR. I did not see any sway bar listings. Have you heard if they do have the larger rear bar already available? Or, are they planning to make one?

I was never able to dial-in a suspension with springs only. If I'd use ultra high rate springs to control body roll, the car setup will be far too stiff. In all of my 4 late Jaguars the rear bars were/are hopelessly too small and definitely need to be changed for more neutral handling. On my present XF and XJL the bars are only 0.55", which I would guess to be no more than 125-150 lbs/inch. I think the car needs around 350 lbs/inch rear bar.

Albert

 

Luckily they can have sway bars made for the car as they have in the past.

Here are two of our many email exchanges

Me:That is fine and expected for custom parts, I just waited 2 months for wheels... One thing I wanted to mention is that I want to go beyond XKRS performance, I want a handling monster of an XKR. I want to wring out as much as possible out of this car .

Matt from Spires:OK. This is going to be a brilliant package for your car. So I propose we use our F-Type derived dampers together with the 30% stiffer springs as per my previous email. All contained in to our coil over system. You won't be disappointed.

Considering these guys are serious racers I feel confident in the package they are supplying.

Those are tiny sway bars for 4k lbs cars. I remember running .75 inch sway bars on the rear of a Corvette with coil overs and that was a 3200lbs car.

 

I'm also going with RBC Yellow Stuff pads and possibly the Spires rotors as autocross season is month away. I can't wait to see how this package ends up handling.

 

Yes, they are. I am also using a .75" bar on the back of my 2600 lbs, 500 HP twin turbo third-gen RX-7. I went as high as 7/8 on that car but, was a bit too much, the rear-end was getting loose in high speed corners. With the 3/4" rear bar, 275 lbs springs (rear) and adjustable rebound dampers the car is on rails with truly race-car quality handling (I raced cars for decades). So, 7/8" for a 4K car does not sound unreasonable, at all. I think a 3/4 bar (depending on the side arm lengths) is around 350 lbs/inch.

Albert

 

Nice to know you have the race experience, I raced Vettes and Porsches about a decade ago and am about to get back into racing. Still trying to decide what to race Nice to have someone with race experience to bounce ideas off of. You definitely know where I'm coming from when it comes to handling. I am going to get as close to GT handling as I can!

Soon as they get back in on Monday I'll ask about the rollbars and get back to you.

Your RX-7 sounds like a beast! I just can't decide what class I want to race.

 

Yes, I know exactly where you are coming from. Once you taste a perfect handling car setup, most stock setup will leave you wanting...

I did SCCA autocross for some years, winning one championship. Then, for many years I raced SCCA GT-2 and GT-3 winning 3 Regional championships. Finally competed in SCCA Formula Mazda and Formula Unlimited (FS), where my best was a 2 place in a championship year. Very difficult to adjust from GT class to Formula class where you have to overcome your mental bloc/panic to cutting your braking distances nearly 50 percent. If you start braking anywhere near you used to in a GT car, half the Formula field will pass you under braking.

I would suggest to you SCCA GT class racing for all around best. These are pure race cars. I raced Mazdas as I was very up on the technology. Also, as a self financed team, it was far less expensive than, say, a piston engined car. Engine rebuilts were $1500 (doing myself) or $5000 (turnkey) vs. a Porsche rebuilt $40K+ (turnkey). Mazda race engines ran on regular pump fuel vs. much more expensive mandatory race fuel for piston engines. Also, GT cars provide far more driver protection (steel tube cage) than formula racing cars and, IMO, are more fun. Formula cars are so very sensitive to setup and to changes in handling due to tire wear, track temperatures, etc. that I found them to be much more work to drive and much less fun. I found it very difficult to come up with a setup that would work throughout a full race. In GT cars that tended not to be an issue.

I'll be more than happy to answer any questions you may have.

Albert