Can the Jag IRS Differential Be "Bottom Mounted"?

[MrWumper]

I know the Jag IRS unit was made to be top mounted (i.e. hung from the top of the differential as opposed to mounting to the bottom of the frame), but is it possible (or advisable) to "bottom" mount it, i.e. do away with any top mount and instead solidly mount the differential to frame rails below the diff (and mount it with bolts through the lower control arm brackets that originally bolt to the bottom plate)...?

Of course the rails would have to pass under the diff north-south as opposed to a top mounted set up of east-west (like the CWI set up, or any other Jag based derivative), which would set the frame rails very narrowly apart under the diff (to avoid interference with the lower control arms), but this set up would make it possible in my set up to not only not have to cut into the body on my car, but also keep the driveshaft angle correct by mounting it higher than would be possible by top mounting it (and thus doing away with a 2" frame rail above the diff.)

Every independent rear suspension I've seen, however, has mounted the diff to a kick up in the rear of the chassis, and top mounted to it. Is there a valid engineering reason for this? Would bottom mounting the differential give a weaker overall structure, and/or allow too much twist? Does a kick up in the frame with a top mounted differential put less stress on the frame, the body, or the differential, (or all three) or is it just done out of convenience or ease of design? Of course this would all make for a very "flat" chassis, but again, I don't know if that would be a problem in terms of strength or rigidity, or not. Any advice is much appreciated.

[daltondavid]

Heavy topic for your first Post!! You obviously have been researching the subject to some degree to reach these questions. What type of "CAR" do you have? are you seriously thinking about making a Change in Design?? This is what the Hot Rodder spirit is all about, that is for certain. I hope this gets a lot of interesting responses. sounds like some real Engineering intellectual types need only Reply!

[Excaliber]

Interesting indeed. Looks like Wumper is the current "leading authority" on the idea so far!

[strictlypersonl]

I doubt whether the casting would take the stress in the long run - it wasn't built for just a single narrow mount. Too much torque reaction to absorb through bolts only 4.2" apart, especially in shear.

[mr0077]

Maybe he could make a small cage where the Diff bolts (at the top) to the cage and the cage would bolt to the frame at points below the diff, with the lower mounting points spread apart a bit?

[chopper]

It may be possible to do, but keep in mind that (assuming you still have the inboard brakes) the brakes will now be upside down and the bleeder ports will be on the bottom. I have taken my Jag IRS out of the car to replace pieces, but I don't remember whether or not it would be possible to remove the calipers and either rotate them 180 or switch from side to side. If I recall correctly, the mounting clearances are different from side to side due to the offset of the input pinion, so take a look at the brake caliper mounting before you cut metal.

[SCOBRAC]

It would break enless you constructed a cradle which distributed the load from the bottom to the sides equally.

Think of a Salisbury like an egg sitting on its side. You can't crush an egg by equally applying pressure through it's axis by hand, conversely push down on that egg with nearly any force and it breaks.

[MrWumper]

Some interesting responses here...As for the differential "cracking" like an egg---that's exactly what I want to try and avoid in my design, although you may be fully correct nonetheless.

Chopper---I guess I don't see how the calipers would interfere with the frame rails (if that's what you were referring to) in this set up. If it makes any difference, the set up I'm using is from a '64 XKE (with a Salisbury differential), so it may be slightly different from the set up you that are looking at. The frame rails would be completely below the entire differential casing, and my calipers don't extend beyond this, but possibly I don't understand what you meant.

daltondavid---"What type of "CAR" do you have?"--- I was hoping it wouldn't be asked, because this IS a Cobra forum, however this seemed the only reasonable place for me to ask this type of question, especially since Jag rear ends are so extensively used in Cobra replicas. But, with hopes that the moderator won't delete this post for being off topic, the car is actually an (in progress) LS1 powered Triumph (TR4A---the original car already has an IRS, but it would never take that kind of power). I'd think my set up is actually much more closely related to a Cobra replica than to that of a Hot Rod (which of course also commonly use Jaguar IRS's), which is why I came here instead of a Hot Rod forum...

Anyway, I'm thinking of possibly having the frame rails cross under the diff. in an "X" type fashion (like an x-brace), intersecting directly under the center of the differential, and widening out from there (then boxing the center section under the diff.). Whether or not the appropriate angles are even possible for this using 3"x3" square tubing without interfering with the lower control arms (and without notching the rails at all) I am unsure of thus far.

Any support from the top of the differential would be extremely minimal, as having frame rails running across the top would necessitate cutting into the body, and should I do that, I might as well just mount it in the traditional fashion. It could bolt to the sheetmetal above, but since this is not a unitized structure (unlike the Jaguar), any excessive amount of forces feeding into this area would be trouble (i.e. should the lower mounts not be sufficiently rigid.) Virtually all of the torque would need to feed into these lower frame rails for this set up to be successful.

As far as the distance between mounting points, as mentioned by strictlypersonl, 4.2" seems to be the distance between the top mounts if I understand correctly. Or, do you mean the distance between the bolts holding the control arm mounts to the differential casing)? As for mounting the whole set up through the lower control arm brackets, the mounting bolts are 8.5 inches apart from front to back, and 9" apart side to side. But again, if you were referring to the distance between the bolts mounting the diff. to the control arm brackets, then you do make a valid point (and even if you were not referring to this, it is a valid point I hadn't yet considered.) Should those be too weak (or should they feed too much torque into too small an area of the differential casting itself), then the concept would certainly not work.

One more thing about this design is that it would need to use either shorter coil overs, or more steeply angled coil overs---of which either situation might potentially lead to an undesirable geometry (I'm unsure on this aspect). The shocks would mount to a frame rail above just as is generally done, however it would need to be lower than the top of the differential, unlike the standard set up, in order to keep the clearance low enough so as to not force cutting into the body.

Any further insight is much appreciated. I wish I could draw a picture of what I was saying and post it but hopefully you get the idea. From the gist of the responses thus far, I assume that no one here has ever seen this done before (and I'm trying to determine if that is for a valid reason.)

[strictlypersonl]

I was talking about the bottom holes, since those were what you wanted to use.

By the way, the Jag differential isn't just mounted to the top holes. The mounting system which absorbes the torque reaction and side load includes both the top and bottom/side mounts. I predict that using only one mount (upper or lower) would result in a broken casting or differential failure.

Note that the original cars' Salibury, while using a different casting, also used widely spaced mounts to absorb the torque reaction.

[MrWumper]

strictlypersonl: Thanks for the response; how about the CWI set up? It does away with the subframe and just mounts a urethane bushed rail across the top, mounted only to the top mounts. Aren't most set ups run this way? Doesn't it only use one mount? I haven't actually seen the CWI set up in person but that's what it appears to me to be, as do most aftermarket Jag based derivatives like Kugel and Heidt's.

[mickmate]

I say if those ribbed bottom mounts for the lower arms can handle the tension loads of the leverage on the lower arms then they can handle the torsional and shear loads of a mount. If you can break those mounts off the rear end then I want to ride in this beast! If I was going to do anything to it that would be maybe drill and tap the holes to the same size as the top mount bolts or just run a good grade hardware (8 might be too brittle) and run a stout piece of angle along each pair of side holes and build your mounts from that. It'll work, just do it! The lower control arms are already boxed and pretty rugged from memory. Make sure the arm mounts are good as there are a couple of places they hold road dirt on top and get a little tender. Put a good urethane bush in them to control the movement there.

[MrWumper]

Thanks for the response mickmate. That was my original reasoning---if the lower control arm mounts could take all the stresses of the control arms, then surely they must be up to the task of taking the torque from the engine. What some of the other posters were saying, however, is that the differential casting itself will just crack like an egg with all that force fed into such a small area, as opposed to be spread out throughout the casing. What I still don't see is how solid mounting from the bottom is any narrower a base than solid mounting from the top (in both cases of course, doing away with the original sub-frame cage). It seems to me that there isn't really any difference...but then again, I don't want to be the first one to try, and find out the hard way with a destroyed rear axle...Surely someone must have tried this before?

[Cobra20646]

I'm trying to visualize the frame rails under the differential....... and not having a ground clearance problem, while trying to maintain the normal geometry of the half shafts/lower arms.

[mickmate]

If you mounted the bottom of the pumpkin with a couple of angles along each side they won't be any lower than the bottom of the diff. This setup doesn't need the Jag lower control arms or brakes they are already on the car. That reminds me, you could also connect to the bolts that normally hold the caliper. That is the bolt holes that can handle the stopping force of a big heavy Jag. Remember braking is more severe than acceleration. Those are also the bolt holes that mount the side plates that hang the diff in a real 427 Cobra rear end with a little bracket to hold the front from twisting. The stock rear may also surprise you. I had a Salisbury 4 cyl rear axle with a hot 302W in front of it that I couldn't break or afford the tires trying to. It's no egg, it's a cast ribbed 105 pound lump of steel. If you can break the case in this application I'll give you another one

[strictlypersonl]

The original Jag defined the location of the LCA bracket with the subframe, not the casting. The differential was shimmed to fit inside the brackets. Longitudinal braking forces (translated into lateral and shear forces at the brackets) were taken by the subframe, not the casting. Reactions from the rotor/caliper were distributed between the top and bottom casting mounts.

In short, the top and bottom bolts were there to locate the differential in the
subframe. Of course, you must always regard the dif casting and subframe as a system, with each complementing each other.

Braking forces on the rear are considerably smaller than the motive forces from a big engine. Remember that on decelleration, weight is translated to the front, leaving only about 30% under maximum braking. Conversely, under accelleration weight goes to the back, possibly ending up with 90+% there, depending on the stickiness of the rear tires.

[mr bruce]

Why are you bent on the jag diff? have you looked at the 8.8 from the Ford Mustang, T-Bird, Mark III Lincoln.

[MrWumper]

mickmate--"If youmounted the bottom of the pumpkin with a couple of angles along eachside they won't be any lower than the bottom of the diff. This setup doesn't need the Jag lower control arms or brakes they are already on the car."Can you elaborate on the setup you are describing? I can't actually picture it in my head, but it sounds interesting

strictlypersonl--I understand that the original setup relied on the subframe, yet there are many cars out there using Jag IRS without the subframe, so then it must be possible to design a setup that doesnt use the original subframe yet is still able to hold up...

mr bruce--I'm not bent on the Jag diff, I'm bent on the Jag IRS set up in general because that's what I have, and, for the most part, it fits, and doesn't require narrowing of the halfshafts. I'm still using the inboard brakes, original halfshafts, control arms, hubs, etc, and only considered ditching the subframe

[mickmate]

If you drill a couple of pieces of angle iron to fit the lower control arm mounting bolts you can bolt the angles to the lower side with angle in or out. I have a pumpkin bolted to a shipping skid at the moment. If you want I can send you a picture. You could use heavy plates and tie that into a mounting subframe.

[Tom Howland]

MrWumper, My diff. is top and bottom mounted. I have a large plate tying the bottom together. I'm not sure if this is normal. But this might give you some ideas.
The plate is attached to the frame about a 1/2 inch higher than the bottom of the frame. The plate is then tilted up at the rear about 1 1/2 inches.

[Michael C Henry]

I have a Jag rearend it is mounted from the top,but also the front bottom has two short ears that come out from the frame and are bolted to a cross strap in front and rear has two longer rods with ears and a heim joint on each to the corners of the rear frame.I have those all padded and covered with shrink tubing . The fuel tank has a forward sump that hanges between the rods .Each strap on the bottom of the diff housing crosses the diff housing and has an ear bent on either end to engage the ones from the frame.It looks and has acted rock solid. The picture above looks like a skid- jack plate.I sure could use one like it .I have fuel strainers and checkvalves and a good jacking point has to be looked for elsewhere.
What gear cover and gasket fits the Jaguar housing ? has anyone used one of those transparent covers that I've seen in the Summit catalog.I have the front seal problem fixed .Now I have a slight leak from the rear cover. Sitting inthe garage for two years it has formed three drops.