XJ6 & XJ12 Series I, II & III 1968-1992

Rear camber & tire wear, performance, etc

 
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Old 05-21-2014, 11:49 PM
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Default Rear camber & tire wear, performance, etc

This is a general question though direct experience with a an XJ6 or XJS would be helpful:

I lowered my 1985 XJ6 a little over an inch with lowering springs. (It looks great!) I took it to the shop to get it aligned. (I have 225/55-16 high performance summer tires.) They dialed the front in great, but I'm not sure about the rear. The rear camber is -1.3* and -1.1*. According to the alignment machine, the suggested rear camber range for my 1985 XJS is -1.3* to -0.3*. It looks like I'm at the very edge of the suggested specs.

They did not touch the rear suspension at all. I have camber shims but they did not use them. I know that putting in those rear shims is no treat! Any thoughts on this? The biggest thing I'm concerned with is that I will improperly wear the tires.

I also understand that camber effects how the car handles? I really like to push the car around, but I often drive it nice, too... sometimes on long uneventful stretches of highway.

Any input would be helpful! I still don't have the car back - they had to put a rush job in front of me so it's still not a done deal.
 
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Old 05-22-2014, 08:01 AM
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Originally Posted by FastKat View Post
The rear camber is -1.3* and -1.1*. According to the alignment machine, the suggested rear camber range for my 1985 XJS is -1.3* to -0.3*. It looks like I'm at the very edge of the suggested specs.


Within spec is... within spec . Don't worry, be happy.



They did not touch the rear suspension at all. I have camber shims but they did not use them. I know that putting in those rear shims is no treat! Any thoughts on this? The biggest thing I'm concerned with is that I will improperly wear the tires.

I also understand that camber effects how the car handles? I really like to push the car around, but I often drive it nice, too... sometimes on long uneventful stretches of highway.


A bit of neg camber is generally considered a good thing for cornering and, as the specs reveal, Jaguar specifically calls for it. You shouldn't have a tire wear problem.

All three of my Jags have had *visible* negative camber on the front or rear. By that I mean you can just look at the car and easily see the neg camber. A little disturbing at first glance....it's tempting to say "sumthin' don't look right".... but I've not had unusual tire wear problems on any of them.

Since you're interesting is cornering/steering you might also ask the shop to set up as much positive caster as they can while staying within spec. This gives a crisper steering response and stronger self-centering.

Cheers
DD
 
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Old 05-22-2014, 08:10 AM
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I remember getting my XJ front aligned and the mechanic freaked out when he THOUGHT he had to align the rear cage.

 
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Old 05-22-2014, 08:24 AM
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It's a complex question with no easy answer. For tire wear you want as close to the middle of spec as possible. For the best handling you may need more or less camber depending on how your car is setup. Usually a bit more negative camber helps grip or traction when the car leans but some times you want a car loose in the back, to a point, to help it rotate and some times the opposite is true. Loose and getting the car to rotate would be the domain of the expert driver and since your car is a driver and handler that leaves you with trying to get it back to within spec. Typically more negative camber would make the rear tight and allow the front to push which is considered safer for a non expert driver so you should be OK where you are. Adding the shims to the upper control arm / axle is a pain but considering all the other effort you've put in to the car its relatively easy.

Do you get axle tramp under hard acceleration? I would imagine if you have power-loc the axle trap and cage rotation are pretty bad with the LS1 and that can make the car handle erratically.
 

Last edited by icsamerica; 05-22-2014 at 08:29 AM.
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Old 05-22-2014, 09:38 PM
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Thanks for the replies guys.

I'll keep an eye on the [front] caster. When I set it up last time I had it on the high side of the range to improve the on-center feel. I think they actually still had to move a shim to get the caster even again after adjusting the front camber.

Regarding the rear camber, that's all good information. The nice thing about this alignment is that the price will be the same regardless of what needs done. If I want the shims in there they'll do it - I'm just not sure if that will get me what I want. Right now I like the car's handling characteristics except for a little too much body roll during spirited transitions - which I hope to fix with a small rear sway bar. Otherwise, I'd like to keep it the same as long as I don't get any funny rear tire wear.

Indeed I am very much a non-expert driver. I would rather have the car push than have the rear end come around!

The 5.7L LS1 + trac-lok does put a pretty substantial load on the IRS. The bottom of the IRS did lurch forward significantly with the old rubber bushings. The new bushings have largely taken care of that. However, I am building a 6.7L LS2 stroker + 2500rpm converter. (Crank installed, rods next!) That will be a very torquey setup and I am pretty sure I'll need some sort of torque arm to keep the IRS in place!
 
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Old 05-25-2014, 05:11 PM
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Within OEM Spec.

You'll be fine.

Run slightly lower cold tire temps, with whatever tire heat comes from the camber (which will be marginal), the tire pressure will equal out while driving, due to thermal expansion.

OEM suspension ranges/ specs. can not be treated as absolute, because those specs were for the tire compounds of yesteryear.

Same goes for G's performed under breaking & acceleration.

Example:

In 1993 the Mazda Rx-7 was able to pull a .96 lateral G under acceleration.

In 2006 a privateer who came across a untouched Mazda Rx7 (approx. 15,000 miles), with the original tires & re conducted the test. the result on the old tires was a .89 lateral G.

The next step in his test was to swap the old tires for a set of modern street/ sport tires & he was able to pull 1.06 lateral G's

Then R-compounds & was able to pull 1.13 lateral G's


What I'm trying to say, is that those windows of tolerance, have been extended, so not to worry too much, especially if you're within OEM spec, when the Spec's were originally released.
 

Last edited by AristoCat; 05-25-2014 at 05:22 PM. Reason: got some numbers wrong.
 
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