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

Questions on high-torque rear end setup

 
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Old 07-23-2014, 10:08 PM
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Default Questions on high-torque rear end setup

I have a SIII XJ6 that had a 348ci LS1 V8 engine. I recently took that out and I'm about to install a 408ci LS3 V8 engine with a 2400rpm torque converter. I expect around 500 lb/ft of peak torque with much of it available low in the RPM range. The converter will multiply that and hit considerably harder than the stock converter. I know I need to make some mods to the rear suspension, but I have a couple questions:

1) Radius arms. I am going to buy some poly bushings for the small ends of the radius arms. I still plan on using rubber for the large end. Should I install the large end bushings with the voids allowing maximum fore/aft movement, or should I rotate them 90 degrees so that they allow minimum movement?

2) I plan on making a brace for the bottom of the IRS. It will run from the trans tunnel to the IRS tie plate. The IRS tie plate end will be a metal-on-metal mount. The trans tunnel side will use one large-end rubber radius bushing. I plan on making the brace out of round steel tube. What diameter and wall thickness still tube should I use? What grade steel should I use?

Most importantly I want some feedback on these two topics. Any other suggestions are welcome as well. I do want to keep this as simple as possible!

BTW, here's an engine teaser!
 
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Last edited by FastKat; 07-23-2014 at 10:13 PM.
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Old 07-23-2014, 11:21 PM
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Drooling @ engine!
(';')
 
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Old 07-24-2014, 06:43 AM
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Thanks! Aluminum block, 4" steel crank, 4.030" pistons, L92 heads, and a cam that will sound stock-ish but pull hard from idle to 6,000+ RPMs. Can't wait to put it in!
 
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Old 07-24-2014, 08:40 AM
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I've seen this topic pop up a few times in various threads within the last few months with pics of various braces in use. I'm interested in best practices also as a tamer LS3 EROD is slated for my XJ6. Interestingly, while researching, I found this link where the owner has eliminated the radius arms altogether and installed an XJ40 brace:

FiBoy's XJ12C restoration; rebuilding a Jaguar XJ V12 Coupe


 
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Old 07-24-2014, 08:49 AM
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Common 1/2 tubing will be more adequate, it worked for me.
 
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Old 07-24-2014, 08:55 AM
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Originally Posted by LnrB View Post
Drooling @ engine!
(';')
X2!!

Should I install the large end bushings with the voids allowing maximum fore/aft movement, or should I rotate them 90 degrees so that they allow minimum movement?
A little bit of a conundrum here. When forces are acting on any part of a mechanical system, they will always find the weakest link and exploit it. By flipping it around you will minimize flex at that point, thus transferring it to another. This may be desirable; that is this allows the rest of the OEM system to do it's job. However, that may overstress those points and prematurely wear out everything. I think I would align the bushings to allow max fore/aft movement. That way you are only stressing the rest of the linkage bushings under full torque situations. Under normal driving the "softer" point of the bushing at the void will not flex much and the rest of the system will be allowed to work as designed.
What diameter and wall thickness steel tube should I use? What grade steel should I use?
I'd recommend DOM seamless tubing (stock is rated at ASTM A519). Are you going to try cutting to exact length, or will you be tapping the tube for heims? If using heims, you will probably use 3/4" heims...? Then you'll need 11/16" ID. Using 1 1/8" DOM tubing with 7/32" wall gets you to the needed .6875" ID. Or, you could just buy this: aluminum-radius-rod-tubing

Opinions will vary on this subject. I'd do plenty of research on what others have done before proceeding.
 
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Old 07-24-2014, 08:58 AM
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Install the radius big bush arm in it normal stock location. It has to have some give because as the suspension compresses the arm will shorten. If the bush is too tight, the effective shortened arm it will cause a toe in condition and somewhat unpredictable rear steer.
 
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Old 07-24-2014, 12:44 PM
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Any suspension components should be made from 0.120" wall DOM (diameter to suit application or constraints)
 
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Old 07-24-2014, 07:21 PM
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Thanks for all of the great info guys. I'll have to check out the 1/2" DOM tube w/ .120" wall.

I think I am going to do a fixed bolt-on or welded connection at the rear tie plate... no heim joints or anything like that. I figure that big rubber bushing at one end will provide enough deflection from otherwise tearing things up.

That FiBoy support worries me a little. I don't like the idea of having the control arms totally unsupported at my power level!

Regarding the bushings, the question is really how much I want the suspension to move... or how much I can allow it to move... before it becomes a problem. I like the analysis here that allowing the bushing to deflect will better protect the chassis, however, I am worried that too much deflection will send the toe out and/or cause the driveshaft to push into the trans tunnel.

Speaking of rear toe, I think the rear wheels toe out under load because the bottom of the IRS lurches forward. When the IRS rolls forward, the LCA's move forward too, effectively lengthening the radius arms. I don't think it has anything to do with the suspension traveling up and down. Building on that, any other thoughts on the radius arm bushings?
 
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Old 07-25-2014, 01:16 AM
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Originally Posted by FastKat View Post
That FiBoy support worries me a little. I don't like the idea of having the control arms totally unsupported at my power level!
?

Agreed, I feel the same way. We need the trailing arms to transfer the thrust of acceleration to the chassis. That said...What fiboy did is what Jag did on the XJ40. At the time...Jag made a big deal about the cars stability in corners with their marketing materials so it must have been an concern they addressed. The trailing arms were gone but the LCA's were also a new and presumably a stiffer design. If I were going to do this I'd gusset the LCA's like this. The early IRS is a marvel but the XJ40 IRS is even better and remained the same even on the powerful XJR










Originally Posted by FastKat View Post
Speaking of rear toe, I think the rear wheels toe out under load because the bottom of the IRS lurches forward. When the IRS rolls forward, the LCA's move forward too, effectively lengthening the radius arms. I don't think it has anything to do with the suspension traveling up and down. Building on that, any other thoughts on the radius arm bushings?

Ok, this may be the case but its somewhat irrelevant to handling. When the IRS rolls forward, both sides would be equally effected so the net result, from a thrust angle or center line perspective, is zero. For example if the IRS rolls and both wheels toe in 3 degrees, the vector forces cancel each other and the net vector is still Zero. This changes when the car leans, especially under power when one rear wheel will toe in and one toe out. In this case the force are additive and the thrust angle is no longer Zero. For example if the vehicle leans hard the compressed side trailing arm will tend lengthen a bit and the uncompressed side will tend shorten significantly. So one side toes in 3 degrees and the other toes out 1 degrees, the net result is 4 degrees of rear steer. Not good. Before I had a brace or rear anti sway bar...I can tell you empirically, I got wheel hop under straight line hard acceleration. While the drive shaft beat against the tunnel because of cage roll, traction was poor but the vehicle still went straight and didn't exhibit any unpredictable directional stability. When I put the vehicle hard into a corner the rear felt like it was steering the car and handling was unpredictable and imprecise. The imprecise handling issue was resolved with the installation of a rear anti sway bar which limits trailing arm travel and thus limits additive toe changes. The wheel hop and drive shafts drum beat was solved with a lower IRS cage brace which stopped it from rolling. Two different problems, two different solutions. Together they will result in much improved vehicle dynamics.
As for the bush position, if you install a rear sway bar, travel will be limited and it wont matter as much which way it goes for handling. There may be a benefit to installing it more solidly regarding the transfer of acceleration forces at your power level assuming you have the rear wheel traction to make it matter.
 

Last edited by icsamerica; 07-25-2014 at 01:32 AM.
 
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