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I have a 1985 Jaguar XJ6. The IRS is stock, and has a forward-facing trailing arm on each side that goes from the bottom of the LCA to the chassis. I was thinking about removing those trailing arms, and installing a brace from a 1988+ Jaguar XJ sedan. This brace goes from the chassis to the bottom of the IRS "tie plate" area, and keeps the bottom of the IRS assembly from creeping forward under acceleration. Here is a picture:
So in this situation, the bottom of the rear end is supported, but the LCAs are not supported. What kind of issues is this going to cause?
I noticed that the 1988 and newer XJs do not have any sort of trailing arms for the LCAs. However, the LCAs on the 1988+ XJ sedans are a little beefier, too.
Am I going to do any permanent damage by running the car with no trailing arms? I want to add some like the guys in the IRS picture threads, but I'm just not there yet - too much other stuff to do on the car. BTW, the car has a 350+hp fuel injected LS1 V8 with an automatic trans, if that makes any difference.
Any info/opinions/suggestions would be appreciated.
-Dave
Last edited by Daze (7/09/2012 7:48 pm)
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I can see the advantage of adding the extra mount like you are talking about, but I would not do it at the expense of the trailing arms. There are a lot of Hot Rod guys that that do a Jag rear with out running the trailing arms, but I am of the mind that it is not a good idea. It added a lot of extra stress to the arms and allow some flexing that will effect handling. Is there room under your car to switch the arms, left and right, then run the arms out the back??
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I never thought to look behind the IRS for the radius arms. Unfortunately, the trunk floor comes down lower than the inner mounting points on the LCAs, so I don't think I can use the rear side.
I don't think I'll be able to use the brace shown in the picture either. The location of the new radius arms would interfere with the body of the brace.
I guess I'll have to sit down and design something when I figure out how I'm going to route my exhaust. I'd like to utilize the two mounting points located in front of the rear wheels, just like the brace shown in the picture. They must be strong because Jaguar used them for the radius arms, and the wide stance will help me keep the bottom of the IRS from moving around - both forward and laterally. I replaced the old rotten bushings in my radius arms last week, so I guess they'll be sticking around for now.
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On the early XJ rear the trailing arms are there to take acceleration/brake torque loads off the cage mounts (they can't handle it) and to 'steer' the cage and induce roll understeer.
The later design gets rid of them entirely, the new mount design is much more solid and spreads the loads further and roll understeer is a function of the revised pivot angles of the lower arms.
The later design also removes the vehicle weight from the cage and its mounts, it has a single long coilover assembly that mounts directly in the upper fender structure - something typical of modern IRS designs e.g. BMW.
Last edited by JEM (7/30/2012 1:51 pm)
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I was intedning to do this mod - I have a later XJ40 rear end and looked at this exact setup and decided to forego this as it required the XJ40 brace to be cut and welded as the radius arm mounts on my XJS do not line up with the XJ40 mounts. I think much easier to fad something from scratch.
The other is the LCA's will flex and this is mostly at the inner fulcrum point where the arms are quite small. The XJS race cars (depending on class) out of the UK solid mount the top of the cage and keep the radius arms. I think forn a road car the rubber mounts with a bottom brace to the chassis high up would be the go.
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