MG Sports Cars

My GT came with an e-type irs and cage. A shop had partially installed the cage with four mounting bolts. Upon measuring the offset angle of the differential and the angle of the transmission, I find that the differential needs to be tilted down about three degrees. I can do this by shimming the front edge of the cage, but this will cause the wishbone to be 3 degrees off of parallel with the road. I had planned to correct this with the Concours West Inc. adjustable side brackets, but their web site no longer exists and their phone is disconnected. My questions are:

1. Is the 3 degree downward tilt of the wishbone significant enough to keep the irs from working properly?

2. Are there other solutions available? I don't have the skills or equipment to engineer a mounting system like Jim Blackwood did for the Roadmaster Project or the funds to hire someone to do it.

Thanks in advance for your help.
Bill

I'm surprised that you have to adjust the pinion angle at all. I did several MGB / XKE installations with the cage in 1970's and they came out right on the money with the cage bolted into the ledge in the trunk and the front supported by brackets welded on the front of the cage that bolted into angle brackets that bolted into the old shock absorber holes. With the cage located as I described, the extension of the lower control arm inner pivot rod comes out about 1 1/2" below the floor pan at the welded in crossmemberunder the tailshaft of the trans. This is important because you need to have the trailing arms forward pivot at this point to prevent binding as the suspension travels up and down. The intersection of the floor pan and the crossmember gives a strong point to take the loads if you plate the mount above the floor and weld tubes into the crossmember to prevent the bolts from crushing it.

My understanding on this subject is that the faces of the two flanges should be parallel and the drive shaft can run at an angle.

If you have no drive line vibration I wouldn't worry about it.

You hear a lot about pinion angle on US sites but I have yet to see it mentioned anywhere in the UK which does't mean that we are right and you are wrong!.

Kevin.

The driveshaft should run at a slight angle to keep the U joint bearings from always banging on the same spot. You are correct that the flanges should be parallel but the offset can take place in the vertical or horizontal plane or both. The rear pinion is offset from the transmission horizontally so you could have the centerlines perfectly in line when viewed from the side and still have the required U joint action.

Jim,

Thanks that helps me understand the principle which I had missunderstood before, and is completely logical.

Kevin.

any pictures?

Thanks for your comments. I'll try to take some measurements and take some pictures to upload tomorrow.

Bill

I tried to locate some pictures of at least one of the Jag installations I did but no luck. I'll try to describe what I did. You have to use the 6 cyl XKE cage, the others are too wide. I made some 1/4" plates that fit in the areas circled. There were 2 or 3 bolt holes that went through the top of the Jag cage underneath and through another piece of 1/4" steel inside the cage. The Jag cage was back as far as it would go against the vertical wall just below the plates. I used 1/4" rubber sheet between the Jag cage and the MG body metal.



The forward mounts were made from 2 1/2" or 3" angle iron that bolted through the shock mounting holes. The lower part of the angle was just below the bolt holes and extended toward the center of the car. Support the front part of the cage so that the centerline of the lower control arm pivots intersects the crossmember about 1 1/2" below the floor. Sandwich a piece of 1/4" rubber between the frame bracket and another piece of 1/4" steel that extends back to the cage. Drill a couple holes through the two plates and bolt the sandwich together. Now all you have to do is weld the lower plate to the cage and build a couple braces to support it that also weld to the cage. Do this on both sides obviously. All you have left is to build the trailing arms that pivot at the intersection of the control arm pivot axis and the crossmember. Don't use the front leaf spring mount or the system will bind.



Edited 2 time(s). Last edit at 04/13/2011 06:37PM by Jim Stabe.

Evan Amaya had a site that shows how to install the Jag IRS, don't know if it is still on the web or not. Found the link: [rebornco.com]



Edited 1 time(s). Last edit at 04/13/2011 07:00PM by mgb260.

After reading your comments I put my car on jack stands and leveled it. Then I leveled the bottom plate of the irs.


The rear of the cage is attached by four bolts with 1/8 inch plates welded to the underside of the car. The following picture shows the location of the holes in the "trunk" of the GT. This modification had been made before I purchased the car by a garage in California.


I drilled two additional holes is the front edge of the cage and through the floor by the back seat of the GT. I have 1/8 inch plates fabricated and drilled, but I will not weld them in until I am sure of proper fit.


I also have 1/4" rubber between the cage and the bottom of the GT to reduce vibration and noise.

I found that the front edge of the cage is down slightly when the differential is level. When I measured the angles earlier I had the front edge of the cage tight against the bottom of the car.

Measuring the angle of the flange of the differential with my expensive Harbor Freight magnetic protractor, I obtained and angle of about 6 degrees, which agreed with earlier measurements made before installation.



Since the rear of my transmission has no flange, I was not able to take a direct measurement of the transmission shaft angle, but measuring the valve covers of the Rover engine resulted in an angle of about 5 degrees. This angle might change slightly when I install the rear transmission mount, so the final angle will probably be 4 to 5 degrees. I believe I can work out the one degree or so of difference by making adjustments with motor mounts, rear transmission mounts and the cage angle.


My earlier measurements had a difference of three degrees because I assumed the differential lower wishbones would be parallel to the ground if the cage was solid against the bottom of the car.

This leaves the following questions:
Will the two bolts holding the front of the cage be adequate or should I build brackets as Jim suggested?
I noticed that Evan used the spring bracket to mount the forward end of the control arm. Can I get away with this or do I need to mount it on the extended line of the lower wishbone pivot as Jim suggested? I see the geometry involved and mounting it as forward as Jim and others have suggested is probably the proper way to do it, but I know that Jaguar and several fabricators used a control arm that went straight forward. Fabricating the longer angled control arm would be much more difficult. On the other hand I want it done right.

Thanks to all who responded to my post! Your comments were very helpful.

Bill

The reason that Jag got away with running short trailing arms straight forward is that the cage was soft mounted with rubber mounts and it would swing back and forth in these mounts. That flex allowed them to use the straight trailing arms without binding. I mounted my cage rigidly to the car (which is how you are doing it also) so the cage will not rock back and forth following the arc of the trailing arm and the suspension binds up. When I did my first installation I used short arms to the leaf spring mount and guess what - it bound up! If the cage is rigidly mounted you need to have the trailing arm pivoted somewhere along the extension of the lower pivots so the whole thing acts like a very wide spaced A arm.

As far as the 2 bolts holding the front, I can't say if it would be strong enough. All I know is that you have the full torque of the engine multiplied by the low gear ratio times the rear end ratio. That adds up to a lot of twisting force and I elected to make the front mount stronger than weaker. Take a look at a drag race car with slapper bars launch and see how hard the snubber smacks the spring.

Thanks Jim. What you say makes a lot of sense, I will build it as you suggested.

Bill

Bill

Be sure to post pictures as you go. I'm always bugging people to take pictures because I wish I had taken pictures of the stuff I made in the past.

I'll do that. I have been planning to start a journal anyway. Maybe it will help me stay on task.

I always take lots of pictures when taking things apart in the vain hope it will help me get them back together. It's Jim's fault that I have to take off the cage again which means removing the long inner wishbone mounting bolts. They are a royal pain to reinstall. My pictures didn't help much there.

Bill

Bill,

Do you ever go to the show the first Sat of the month in Golden? I went for the first time a couple of weekends ago.

Brad

Personally, I felt the biggest challenge in doing the Jag IRS conversion was to avoid limiting suspension travel. You think about it, the stock axle tube is just a little over 2" in diameter and we already have guys cutting down the bump stops. Unless you can get that big bell of the Jag upright outside of the frame rail you can easily lose an inch or two. Worse still, that inch or two is all at the expense of ride height, meaning the car has to be raised that much. Unless the stance of the car is widened this is intolerable, to my mind, as only a wider stance can really offset a taller ride height. So right away the Jag IRS puts you into some serious compromises. Basically, you have 3 choices. Sacrifice suspension travel, sacrifice ride height, or widen the bodywork. I can see no possible way to avoid one of those three. Even in the best case scenario, trimming the upright bells back to clear the frame rails, you can only get down to the diameter of the U-joints which are still an inch or more larger in diameter than the stock axle tubes so you are still losing half an inch or more of compression. That may be an acceptable compromise but does it allow you to get the tire into the wheelwell? I have my doubts. I think it would take a very special wheel, and one that would likely look odd on the front.

Obviously I chose to widen the bodywork, and do so substantially. On both my roadster and the MG-Roadmaster we have used new sheet metal to increase the wheelwell clearance to allow the use of 17" diameter wheels with wide rubber, but the critical issue was moving the bell outside of the frame. To do this and still make use of the inner part of the wheelwell requires at least a 16" diameter rim (some will not work) and a deep inner offset, giving the wheel a flatter, more contemporary appearance. Because the mounting flange has to be moved so far to the outside this demands a considerably wider rim, so again, back to the widened bodywork. You will undoubtedly notice the similarity of the rear bodywork on my roadster and the MG-Roadmaster. Although we attempted to go for a slimmer look on the latter of the two cars, once the professional bodymen took over the outcome was nearly identical and there is a good reason for this. That style of flare is the only way to truly give the car's bodywork a smoothly flowing appearance that is a perfect aesthetic blend with its existing lines. It took me a dozen years to perfect this on my car, and Terry's men discovered it in a matter of days on the Roadmaster. The result is that the Roadmaster rides on 245 series tires, and my car, which is very demanding in terms of exact wheel dimension, rides on 315 series in the rear and has a front/rear stagger. However, the payoff is that not only is the car capable of being set up at the stock ride height or lower, not only does it have a substantially wider track than stock (+6"), meaning it is much more stable in the turns, but it also has an astounding 8" of available suspension travel. Find that in ANY axle conversion on an MGB short of a 4x4 Franken-monster. The end result is extreme tunability and versatility, smoother ride, and I dare say better roadholding ability, if only because of the widened track.

But of course, we have quite a few good examples of IRS conversions (Jag and Hoyle) which are better than stock but give the stock appearance. I simply was unable to give up the performance that I knew was there.

As for the alignment with the engine, the conversions I've done have uniformly ended up with a 3 degree driveline angle. The Jag IRS comes with 6 degrees offset between LCAs and pinion and matching the driveline angle results in a LCA pivot axis that falls just under the plate across the rear of the driveshaft tunnel. This makes a good location for the pivot of the forward arms. In my design this is done with a cross bar which uses the existing forward spring mounts as part of a bolt-in design, but there is no reason you could not reinforce this area and put your mounts there. It will lack the inherent bracing against torque reactions that my design incorporates but you can add diagonal bracing from the cage to accomplish that task without too much difficulty.

JB

Brad - I haven't made it to the Golden show. As of now I would have to take my bone stock 67 MGB Roadster. The only thing special about it is that I am the original owner.

JB - When I started work on the irs I thought it was from an XJ6. I measured it to see how much it would need to be shortened and discovered it was only an inch or so wider than the stock GT. After some research I and help from friends (thanks, Al) I concluded it was from an e-type. I decided to use the irs as is.

Therefore I can't widen the track as you did, which leaves me with the options of high ride height or reduced suspension travel. If I can find the right shocks I will compromise at an inch higher ride height than stock and some reduction in suspension travel. This will result in ride height about the same as a rubber bumper car. I have a Fast Cars ifs that is currently set at rubber bumper height. With stock e-type springs and shocks the rear ride height is 2+ inches too high. I don't like this but I don't see other options. I will do some experimentation to see how much suspension travel will result. I have some wheels from a Jag XJS (6.5x15x28.5) and will probably run 195/65/15 tires. I think I can do this without fender flares, but we will see.

I'm currently working on the tranny mount and after it is completed I can get a more accurate measurement of driveline angle. I probably won't get it finished this week since I will be out of town most of the week working on the Glenwood Rallye. If driveline angle is around 3 degrees I will have some problems with proper alignment with the differential. I suppose I could modify the side brackets but I don't feel comfortable doing that. Since Councours West seems to be out of business and I can't get their adjustable bracket I may have a problem. I'm hoping that if I mount the irs as Jim Stabe (are all the guys who know about Jag irs's named Jim?) suggested that things will work out. I'm really impressed with your design for the Roadmaster but I can't duplicate it. I will look more carefully at your design using a cross bar for the mount point forward of the forward arms. I have some concerns in fabrication of the pivot arms anyway so having them shorter might help.

Thanks for your help.
Bill

Bill, I recommend you go ahead and get it in there by whatever means necessary without too much body mods and consider a future upgrade. If all goes well, this summer I will be working on the Mark-II version which is expected to have aluminum fabricated LCAs and tubular half shafts, also possibly aluminum. Once these parts are available it would be feasible for you to widen the track by simply swapping parts. It will mean bodywork, undoubtedly, if you can work that into your plans, but gives you the prospect of lowered ride height and more suspension travel, as well as a wider track and more rubber.

I have heard that the use of only one OEM shock/spring per side works out well. If you haven't tried that yet you probably should.

Perhaps wedges can be used to give you the right pinion angle, then project the LCA pivot axis forward and see where it hits. I wouldn't get too concerned about LCA pivot angle just yet. There is a thread on this forum where we were discussing anti-squat/anti-dive but we'll have to search for it. maybe Curtis or someone else remembers where it is. The discussion was about front end geometry but there was also some discussion of rear end geometry along with a very nice drawing showing front and rear projection lines and intersects superimposed on an open wheel race car. That is probably very relevant to your concerns.

Bottom line is, it can and has been done before. I'm not aware of anyone who used anything other than the stock LCA inner pivot brackets up to now. 3 degrees is an extremely common driveline angle and I would be surprised if yours is very much different.

JB

Bill, from the photos of the angle gauge it appears that your alignment is nearly perfect. The pinion doesn't have to be perfectly level, but it should be parallel to the axis of the engine crank and transmission centerline. If the pinion is angled up 6 degrees and the trans output shaft is angled down 6 degrees then they are parallel and the drive shaft will work fine. In fact it's probably better that they are not directly in line as that can lead to non rotation of the roller bearings in the u-joints and early failure of those. This site will give you all the information you might need to get your power train in the best alignment you can. [www.iedls.com]
As far as measuring the transmission angle on my T5 the end of the output shaft was flat and true and I just placed my magnetic gauge directly on it. It's hard to read the gauge up in the tunnel though. As an alternative I bought a small digital angle gauge from HF that has a hold function, that way I'm able to set the gauge, get a reading, then press hold and remove the gauge to record the reading. [www.harborfreight.com]
Definitely a handy tool to have in the tool box.

Bill

I thought I had seen every tool HF had to offer and then you come up with that handy little angle finder. I may have to get one of those.