MG Sports Cars

I wanted to post photos of both of these IRS units together in their own thread, especially since the info in the 340 and Roadmaster threads will eventually get buried in the mass of other information. This first unit is for the Roadmaster. It has a 3.08 gearset, urethane bushings in the forward crossbar, and will bolt into the MGB without any modification of the body whatsoever, including the battery boxes and the bump stop "humps" but may need custom wheels to clear the stock bodywork.

This next one was built to go in my '71 Roadster. It has rubber mounts, 3.54 gears and a few other more subtle differences.






Jim

They do look great Jim!

thats brilliant!! how did you make this, obviously its jaguar components, im looking to upgrade my MG's rear end as its fairly standard, and this looks too good!

How did I make it? Mostly grit and determination I'm afraid, but the details are in the 340 and the Roadmaster threads.

I'm in the process of installing the second unit in my roadster and I'm afraid they need slight revisions as currently some minor tweaking of the battery boxes is necessary in order for the upper shock mount bolts to be removable, and the gas tank has to be dented in to clear the rear torque straps. Those could perhaps be left off, but I think some sort of torque link there is a good idea. Of course there are other ways of doing that.

Jim

Hey,
Nice job on the fabbing. They really make a difference on the ride quality. I used the stock Bilstiens and cutdown late model springs and the spring rate worked out good. Not sure how but............. I cut away the stock battery boxes and put the battery in the trunk as it made life much easier that way. Off setting the gas tank also worked out well.

Great Job,
Bill

The roadster won't have the tank offset since it has side pipes but the GT probably will, and if there's any more room to move it rearward we may do that too as 1/2 to 3/4" will make a difference.

Jim

Jim, What's the weight of that setup?

Oops, just saw your other post on weight.

Right, 90 lbs heavier than stock but with tubular half shafts and aftermarket lower control arms that can be reduced to 50 or less. It's a great option with an iron block engine such as the Ford or Buick, as it easily will balance the extra weight. Many combinations are possible. Looking at vehicle weights, it it common to see a difference between front and rear tire weights of 50 lbs (for example 575 front, 525 rear) so an extra 90 lbs on the rear balances the car almost perfectly. Of course, it is possible to make the car heavier in the rear. A combination of Rover engine, Ted's front suspension and the Jag IRS could do (especially with a full tank and gear in the trunk! ;-) that but in that case one would want to go with the lighter components as well to minimize the difference.

Jim

have you thought about making the custom cradle for sale? Looking for the cradle only.

Jim,

Do you have a picture from underneath. I guess the lower inboard pivots attach to the diff in some way.

Is it correct to assume this set up only mounts to the body at the droop strap mounting, the damper mounting and the front spring eye mounting?

Thanks,
David

Jim, the overall weight is higher, but what about the unsprung weight? I would think that you would have a major reduction there with the IRS.

Yes the unsprung weight has been very considerably reduced. The brakes and diff are chassis mounted of course, leaving only the control arms, half shafts, uprights and spindles as unsprung weight and half of the weight of the control arms and half shafts is deducted as well since they are mounted to the chassis on the inside. The weight of the coil-over units is pretty minimal as well, nearly half of that also being deducted. So where the stock suspension has about 201 lbs of unsprung weight, the Jag IRS has about 50 to 75 lbs. depending on what components you use. I'm sorry, that's about as close as I can get right now as I'd have to disassemble something to get a more accurate figure.

Jim as far as selling the cradle, I wouldn't be opposed to that but it involves a fair number of pieces and has to be made to match the control arm lengths. It's something we could discuss if you want to send me a PM.

David, you are correct, the inner pivots use the stock Jaguar inner pivot brackets, and the completed unit attaches just like you said.

Jim

Thanks for the reply Jim. Did you ever think of using the later Jag rear end from the XJ40 as a base?

David

Haven't considered it David, in fact I've not even had a real good chance to look one over closely though I do know that they are considerably different and I'm not real sure how difficult they would be to narrow. Aren't the brakes on those outboard as well? I realize some folks don't mind that weight penalty, it makes little difference in a 4000 lb car. But in am MG it would be noticeable.

Jim

I question the unsprung weight, I do not think is less, even if the calipers are inboard.
But street use unspring weight is minimal signifcance and it is overshadow by the IRS.
Tuning an IRS is complex and careful thinking must be applied.

I am not against it, it is a reality of the IRS system.

I have a qusetion mainly due to the picture is not clear, does the driveline (driveahsft) clear the front brace?

Well Bill, I just don't know what to think....

But that's OK, at some point I'll get some actual weights for you. For now about all I can think of to say is take another look at it. I'm really confused as to how you could possibly have drawn those conclusions... you're not yanking my chain are you?

Anyway, yes the driveshaft will clear the front brace quite easily with plenty of room to spare. There's not much in the way of adjustments. You can change the shims at the rotors to vary the camber some and tune the coil-overs and that's about it. Everything else has a fixed relationship. There's a slight amount of toe built in and it isn't adjustable, obviously no caster to be concerned with. The camber change is dictated entirely by the LCA and half shaft along with the vertical distance between the pivot pairs both inboard and outboard and we are a little shorter on the control arms than the XKE, by a little over an inch. I can't see that being much of a big deal, especially given the widespread use of this suspension in modified vehicles in a tremendous variety of narrowed configurations. For an IRS it couldn't get much easier.

Jim

My MISTAKE !!! JIM.
What I meant to say is that is heavier than a straight axle, but for street use is..... and I guess that was already discuss, so I was just putting my 1/2 cent opinion, sorry for that.

It looks great. Are you using a single coil over on each side? What is the total width from hub to hub. The XKE is only 1/4" total wider than the MGB. But hey I could be mistaken.

Sorry !!! if I steped on one of you toes.

Oh that's OK, I just figured you were having a little too much fun, ;-) You've helped me though because I had to re-figure the width as I'd looked at the wrong numbers. The XJ6 is 61-3/4" from hub to hub. We narrowed it 5-1/4 so 56-1/2" which is wider than the unit in the 3.8S by 1/2" and wider than the XKE by 3-1/4". But 57-1/2 to 58" would work much better with Corvette wheels. This is only possible using 16 or 17" wheels having a deep backspacing of up to 6-1/2" edit: (or up to 7-1/4" with the extra width) and flared fenders, but it gives excellent suspension geometry as well as a full 8" of vertical suspension travel, practically unheard of in this type of car. I used single coil overs, QA1's and 225lb springs on my convertible, 250 on the GT. The 225's may be a bit on the soft side, but if too much I'll just stiffen up the damping a bit on compression. I also added bump stops on both just in case.

Jim



Edited 1 time(s). Last edit at 04/15/2009 11:57PM by BlownMGB-V8.

Now for the benefit of those who really would like to have better numbers for the unsprung weight,... I did some testing. Put the flat plate on my floor jack, the shipping scales on that and placed it under the bottom pivot of the upright after disconnecting the bottom of the coil-over. After much raising, lowering, jiggling and tapping I came up with a weight of 35 lbs. Double that (70), add the weight of one coil-over unit (about 5 lbs probably, maybe a little more) and you have the total unsprung weight using the stock shortened LCA's and stock half shafts sleeved with heavy wall tubing. I forget the exact dimensions but if you're that curious you can find it in the 340 thread. Those four pieces are right heavy, so when I said I thought 25 lbs or so could be dropped, that's only 6 lbs off each piece, easily bringing the total unsprung weight down to 50 lbs and maybe less. Add about 25 lbs per corner for a good light wheel/tire combo and you're at 50 lbs per corner. Not bad. Of course the front would be less but it isn't providing power.

So there you have it. Pretty close to the earlier guess, and of course I'm not going to claim that this was the most accurate method possible, but I would say it's within +- 5 lbs at the worst. And that's a whole lot better than stock. Stock rear unsprung weight is about 8-3/8 % of the car's weight. Here we're at 3-1/8 %. Or to put it another way, 1/12th versus 1/32nd and we could drop that to about 1/50th. I'd certainly expect that to be a noticeable difference.

What happens when we add the tires and wheels? Well using the earlier weight of 25 lbs each we're over 1/10th versus 1/19th so we've cut it in half, actually better than that. Those of you that have played with light weight wheels and tires will recognize this as a very significant advantage.

Jim