Again, I beg your pardon if I am wrong since I have not done much in car setups for the past few years but, higher rate rear springs WILL reduce rear traction and thus, cause oversteer. That has been the complaints of pro testers running XKRs on the track as they all find a lack of traction with the XKR variants.

Not surprise about the swaybar being puny. Do you have an idea what size it was for the XKR? I just have my '12 stock-XF on jack stands right now and the rear bar is 0.54" which is WAY-WAY too small for a 4100 lbs car. Same size on my XJL. Again, seems like they are using heavy spring rates for roll resistance, instead of proper roll bars. To change the swaybar on my XF (even if one was available) is a non-enviable task. You'd have to remove the exhaust, the driveshaft, a bunch of plumbing and lower the whole rear subframe as they managed to hide the swaybar between the chassis and the whole rear subframe. Maybe that is why no one makes larger swaybars because just to change it would involve a massively major job. What happened to times when you could swap a rear sway bar in 10 minutes??? BTW - I just looked up my RX-7 records from 2005, I paid $53 for an adjustable rear 3/4" bar. Wonder what Jaguar is making their GT sway bar out of to cost that much? Titanium???

As to BMWs using 800 lbs springs on the rear? That absolutely astonishes me. The last use I heard for 800 lbs springs were for drifting and not for proper street/track driving. No wonder that one of the last reviews I read for the M6 said that the ride was "unbearable". I maybe falling behind times but, this seem to be so far away from what I would consider good setup that it is not even close. Like I said, I am more than willing to change my mind if someone can explain to me what changed from the old school, front-stiffer, rear-softer, larger swaybar setup.

BTW - are the springs progressive? That would explain the decent ride.

Albert

 

No the springs are not progressive the are linear.

The BMWs I mentioned are not drift cars but SCCA road race cars, and winning cars for that matter. There is a formula for front to rear spring rate ratio that I'm not fully educated on.

You're right that stiff rear springs cause oversteer but I was doing dry skid pad loops and it felt perfectly neutral. No push and no oversteer at 50MPH. I will say while gathering the information he said the spring rate ratio was different but something he sees on all the BMWs. This guy sets up race cars almost exclusively.

 

I guess, I will not argue with success. If it feels good and handles good, it IS good. Still, can not get over those rates.

I basically used the same philosophy to set up my GT2/3 and Formula Star race cars with the springs. I won 3 championships to show that the suspensions worked. I am simply flabbergasted... of course now we are talking about race cars that weight (GT 2100lbs, Formula 1300lbs)

Albert

 

I'm right there with you and Jag also sets the car up with stiffer springs in the back form the factory.

Let me email Spires and see what they say about the F/R Spring ratios.

 

I just emailed Matt from Spires and he should be able to provide us with an answer as he races BMWs as well. BMWs have plenty of sway bar option to pick from so we are missing something as far as chassis dynamics.

 

Yeah, but you have tenders on the rear, which gives you a sort-of 2-stage progression.

 

Here is the answer from Spires

It's actually about 'wheel rate' rather than spring rate. So it depends on where the spring is packaged and the ratio of the lever from the wheel to the turning moment of the spring.

Even allowing for this relationship they have a tenancy to run higher rear springs to enable the car to have an agile 'turn in' for most customer in most applications. XKRS-GT runs a significantly higher (60%) front spring than your car is now and a 20% stiffer rear. This actually means the car is very good on track but maybe not ideal but still very competent on road.

Having big ARB (sway bars) is OK and normally means you can have much softer springs so the car is more comfortable on road.

 

Yes, I understand the difference between the wheel rate and the spring rate. Of course, not knowing the "lever" it does not mean much to me at this point.

So, indeed, the high spring rate is for "agile turn-in" just as I suspected. I always considered the use of high spring rates instead of larger sway bars a mistake. Like I said the other day, given how Jaguar hid the sway bar above everything, aftermarket people do not even seem to consider changing that and, instead, go for ever stiffer springs. Just as I speculated, I now strongly suspect that those high spring rates are the reason for the lack of traction for the XKR variants.

From what you posted, I derived the rear spring rate for the stock XKR to be 542lbs/inch (or, is this a wheel-rate also?) If that is the spring rate, it is too much already for my taste. I do not have experiences with 4000 lbs cars but, I would definitely like to explore actually reducing the rear spring rates and increasing the sway bar. But, there I go again, trying to replace the swaybar which is hardly available and requires taking the car half apart.

As far as I know there are no disadvantages to the light springs in the back and increased rear bar and not sure why Jaguar is playing with those laughably puny rear bar sizes. I understand the safety issue (understeer is safe) but, you could have a car very safe with a 3/4 bar and 350 lbs springs (just guessing for that much weight) and it would result in better roll resistance, better turn-in, more traction than the present setup.

I told the Jaguar NA rep the other day that, given my experiences with 4 late Jaguars, I thought that the Jaguar suspension team was incompetent. He answered that Jaguar was listening to what customers wanted and this was the result. I replied that when the XKR-S is a full 10 seconds/lap slower than the new base Z51 Stingray at VIR, hardly any customer would be proud of that. Why does the XKR always has to trail the field in track comparisons? Certainly not for the lack of chassis or the lack of power. It is all suspension, particularly the lack of traction. I am certain that if I was given free hand with springs, dampers and swaybars I could easily tune the suspension where it would eat many seconds off the track/lap times and, at the same time, would be actually providing a better, softer ride for the streets.

Ok, rant over. I very much appreciate this discussion and your efforts to lead with suspension mods and share it here.

Albert

 

Albert,

I couldn't agree more about the sway bars I ran a 3/4 rear on my corvette and it was around 3400lbs with #450 rear springs. The sway bars on these area are like rubber bands and useless, on top of that the bushing are super loose, just changing to urethane would make a difference.

The only thing that still gets me is even with the high spring rates the car grips incredibly well. Much better than stock or the H&Rs grip. Before the corner balance and alignment the rear end was way too happy. Now I have to be stupid with the throttle to get it to step out at speed, and that annoying luxury car push is gone.

I'm looking for custom sway bars if I find some I'll let you know, but of course you have to take half the car apart

Let me know if you have any more questions for Matt he's always very informative and knows his stuff.

 

Undoubtedly you are describing a much better feel for the car. That is exactly what the high rear spring rates were supposed to give you, and they do. Not sure how much and how hard have you been able to drive the car but, the real "tell" will be if you start pushing it pretty hard (like, race track hard near the limits) and see if you still got good rear traction. If you do than it is a WIN.

On the other hand, if you do not push your car to near the limits, it is still a WIN, since the car feels so much better.

Albert

 

It will be out on the track soon so we will see. I've pushed it pretty hard but obviously not to the limit, except maybe a few on ramps So far it feels good but you're right lets see how tail happy it becomes on the track.