MG Sports Cars

Since the JAG IRS has the pinon off set from center by about 1 inch and the GM T5 trans tail shaft that I am using is centered in the MGB chassis and trans tunnel I have a problem.... What have other folks done in the past?
1). Cut an inch more off of the driver side 1/2 shaft and LCA and center the pinon in line with the drive shaft (introducing differential left and right side camber changes and other issues under bump and droop conditions, 2). Keep the pinon off set (by ~ 1 inch to the passenger side) with no modifications to 1/2 shaft and LCA lengths or 3) Combination of 1 and 2 (cut off 1/2 inch more from Driver side LCA/1/2 shaft and have pinon mis-alinged by 1/2 inch to the passenger side or 4) center pinon but use rear wheels with different back spacing (left and right) to correct the issue? I'm concearned that neither is a good option considering the car will be a dedicated road race track day and Auto-X car. Any thoughts or experience would be appreciated. Things to consider are u-joints breaking, use of wheel spacers or rear wheels with different back spacing on left and right side, diameter of a typical drive shaft used in V8 conversions, strange handling etc..?

Thanks

Neil

Neil, we addressed this issue early on in the Roadmaster and 340 threads (both cars use the Jag IRS) and came to the conclusion that completely symmetrical handling only exists on dedicated racers with the driver sitting in the center of the car. In every other application, even if the weigh is exactly distributed, the driver's perception will not be. So it is quite clear that the driver adapts to the characteristics of the car at least to some degree. In our case we opted to place the pinion in the center by varying the control arm lengths. We sought to minimize that difference by using the longest control arms possible, cutting, if memory serves, 2" from one side and 3-1/4" from the other (from the XJ6). In hindsight I would reduce both of those by 1/2 to 3/4" and this will allow the use of 17" wheels manufactured to fit the late model Corvettes, guaranteeing a ready supply. These will then fit with no modifications to the inner half of the wheelwell, but of course the outside must be flared. Hope that helps,

Jim

I think having unequal length suspension parts from side to side would cause more problems than it would solve. As fas as having the pinion offset, you need to have some offset of the centerlines to have the u-joints operate properly. If you Google driveshaft setup or universal joint phasing you will find that most people suggest having the centerlines parallel but offset about 4 degreesto allow the bearings in the joint to move slightly and not work against a single spot all the time. The 1" offset with a 24" long driveshaft would give an offset angle of 2.4 degrees.

Jim

There's also a vertical component, I can give you some idea in about a week of how much that would be. Offsetting the pinion is really only limited by the tunnel, by the handbrake, and by the battery boxes. If you use a small diameter driveshaft, remove the battery boxes, grind away the end of the handbrake attachment bolt, and go to the Dana 44 style pinion yoke instead of the bolt together drive-shaft flange you should be able to squeeze a half inch or more of offset out of it, I think it takes 5/8" to center the axle. But Jim, perhaps you would like to elaborate on those problems created by unequal length arms? What exactly is that going to do to create problems? In terms of lateral movement of the tire contact patch it's effect is so small even the best driver would never feel it. It pales by comparison to the lateral movement over the suspension's 8" of vertical travel. Suspension travel on each side is near identical, as is damping and the springs are adjustable so you can easily get even weight. I could see it being a problem if there was, say a 2:1 ratio from one side to the other but there isn't anything remotely close to that. Tell you what, why don't you come to the V8 meet in June and we'll see if you can take the MGB-Roadmaster around the autocross course. That should satisfy your concerns.

Jim

a driveline can have to offsets, one is the center offset where two invsible parallel lines, one from the trans and the other from the pinion, The second invisible parallel lines are the ones on the side such as the offset of the pinion (very common) the lines must be parallel and do not touch at any point. The side offset is a bit more forvigen than the center lines.

This is found in many vehicles such as, Muscle cars (factory engine swaps) such as the Chevelle LS6 where the engine has 1" offset to the passenger side, many big block mopars etc. Then they corrected this by offseting the pinion to align, not really needed.
The point is that is not un common. Other driveline systems like the C5-C6 corvette are symetrical due to the tube houses that driveshaft. The same theory that is used on the dirveshaft alignment is used to the side alignment but as mention it is more forgiven.

I am very surprise and if correct, that the Jag IRS has an offset pinion, most IRS would appear to be symetrical, for other reasons, apperantly not important. Very interesting. I bought an IRS for a project Non-MG I will check it out!

Jim (Blackwood)

With the Jag lower arms you have a choice of where to take out the width. If you take it out between the coilover mounts and the inner pivot point you change the leverage ratio between the wheel and the springs and you also change the working angle of the springs and shocks unless you also change the upper mounting points of the coilovers on that side (not the best choice). If you take the width out between the coilover mounts and the wheel you also change the leverage exerted on the springs but to a lesser degree. Either way you will have a different camber change curve on each side of the car because of the change in the ratio of lengths between the upper arm (halfshaft) and the lower arm on each side of the car. Will the average driver be able to tell the difference??? Probably not but why put yourself through it when you need the offset in the driveline anyway.

I would love to be able to attend the V8 meet but my car doesn't have a prayer of being running by then much less being sorted out enough for a 6,000 mile round trip across the country (I'm in San Diego). The car also has no windows or provision for a top (I'm from San Diego remember) so you have to promise that it won't rain.

Jim (Stabe)

OK, I promise! ;-)
Those points are true, but I discovered that when shortening the LCA's and half shafts it's been common practice for decades to remove the same amount of material from each one (on a given side) which told me that variations in camber change are not a terribly big issue with this axle, or in other words the design is very forgiving. One of the very good things about the design is the way it handles camber change with suspension travel. It's often been said that the design was way ahead of it's time and the more I work with them the more I'm convinced this is true. It has very good geometry. Now this is not changed in the least by the coil-over attachment point, as that only really affects spring rate and damping, both of which are rather easily controlled, and a difference in the mounting point of an inch and a quarter over a lever length more than 10 times that has little effect on either. In fact, after moving the upper mounting point inboard a greater distance than that (both sides) I could tell no difference in the qualities of the suspension when bouncing up and down on the bumper in the shop. I really expected to be able to, but I couldn't. That's not to say a difference wasn't there (it would have to be) but to point out that the results are sometimes much less than you expect.

Jim

Hi Jim(s) and Bill,

As usual, thank you for your thoughts and advice. A good discussion with multiple valid points. I have a few more items to consider before the IRS goes in. I have complete 2 JAG IRS rear ends (53" and 61.5") so I have some options. I need to know if the JAG hub will slip inside a 16" wheel but I'll ask that on a seperate message.

Thanks again

Neil