Yikes... Movement in moustache bar mounts

[720fast]

I would like to see the front, if my rear uprights move that much the front of the control arms would clunk the back side of the front diff cross member and the heads of the bolts for the control arm pivot point would hit the back of the sway bar. the front should be much stronger than the rear and not move or flex. 

[Chickenman]

I would be more concerned about the flexing of the LCA uprights than the Diff MB. Run the video and put your finger on the center of the LCA pivot bolt. That will provide a reference point. Watch towards the end of the video as he gets on and off the power. The centerline of the LCA bolt is moving a LOT!!. That is a separate issue from the Diff movement, but you're going to have to fix the LCA pivot movement as well.

 

Diff movement seems to be just an issue of two soft of rear Poly mounts and possibly the front mounts are too soft as well. Or the front fabricated X-member is flexing. Either way the rear Poly mounts have too much flex. I'd try some Delrin mounts is there. Or you could try Trimming the ends of the Poly bushing flanges down and putting in metal spacers. That should reduce the Axial compression of the Poly bushing. Should help some, but I'm not sure if it will help with any Torsional compression.

[NewZed]

It's been noted over the years by several, how the bushings allow the rear wheels to move forward in the wheel well.  As the LCA bushings compress.  There's pictures out there.

 

Many people have problems when they install a short nose diff behind a high torque engine.  There are threads out there about the Q45 diff, and the short nose R200.  Maybe the R230 also.  Leverage is the key.  The shorter the lever arm, the harder it is for the mount to hold it down.

[Chickenman]

It's been noted over the years by several, how the bushings allow the rear wheels to move forward in the wheel well.  As the LCA bushings compress.  There's pictures out there.

 

Many people have problems when they install a short nose diff behind a high torque engine.  There are threads out there about the Q45 diff, and the short nose R200.  Maybe the R230 also.  Leverage is the key.  The shorter the lever arm, the harder it is for the mount to hold it down.

True... but he's running aftermarket LCA's which should have come with Poly bushings on the Inner pivots. The outers are Heim joints. . And as the inner LCA Poly bushings are fairly small, they shouldn't flex that much???

[JMortensen]

Many people have problems when they install a short nose diff behind a high torque engine.  There are threads out there about the Q45 diff, and the short nose R200.  Maybe the R230 also.  Leverage is the key.  The shorter the lever arm, the harder it is for the mount to hold it down.

I think you have that backwards. The shorter the lever arm, the less leverage. The problem with the S30 is that it was built for the long nose, so the mount is in the proper place for that length diff. When you put a short nose diff in, then you have to have a longer mount, which provides a longer lever arm. In essence, a Ron Tyler mount with a long nose takes the loads nearly vertically. I don't know exactly the length of most short nose mounts, but if it were theoretically 12" long, then you would DOUBLE the force applied to the chassis, and you're not putting those forces into the chassis vertically, the way they're designed to take it. You're trying to twist the mounts out of the chassis with all that force. I know I've seen at least one thread where a guy had ripped up the chassis. That's why I think the diff cradle is superior for a short nose install. Make it strong enough to handle the loads alone, then bolt that sub-assembly to the chassis.

Edited July 13, 2016 by JMortensen

[NewZed]

I had it right, I believe.  The mount or body is essentially trying to hold the nose of the diff down.  If the nose is long, it's easier to hold down.  Just like trying to stop a spinning wheel by grabbing the spokes near the hub or out by the tire.  Much easier out by the tire.  The body and mount is trying to stop the diff from spinning.

 

The longer lever arm of the new mount also puts more load on those two small bolts in to the body.  Not only more load, but more of a twisting, unequal load.  As the nose of the diff pushes up on the new longer lever arm of the mount it either pivots around the back bolt levering down on the front bolt, or you can think of the front bolt as the fulcrum levering up on the back bolt.  Pretty sure people have had those bolts tear out or twist the metal (I wrote this on impulse from the first few sentences, I see that you've addressed the weakness of the sheet metal).  SUNNY Z maybe.  The original design pulls up on both bolts but with a longer diff nose applying less load.  These short nose designs all kind of introduce two new weaknesses.

 

No offense, the work looks great.  The end result is just weaker than the original design, to be used with a much stronger engine.  More power through a weaker mount.

 

I've though about just building a steel framework that "adds" a long nose to a short nose .  Basically converts it to a long nose.  The new "nose" would need to be stiff and strong.  Then a person could use the original long-nose mount area, both top and bottom (the crossmember).  This GM diff might be amenable to that because it has four mounting holes.  The Q45's don't have the structure to do it easily, they're round and smooth.

 

I feel bad criticizing hard work.  Everyone has been stuck trying to work with those four small holes.  

[JMortensen]

The longer lever arm of the new mount also puts more load on those two small bolts in to the body.  Not only more load, but more of a twisting, unequal load.  As the nose of the diff pushes up on the new longer lever arm of the mount it either pivots around the back bolt levering down on the front bolt, or you can think of the front bolt as the fulcrum levering up on the back bolt. 

You have it right here.

 

You had it wrong here: "The shorter the lever arm, the harder it is for the mount to hold it down." The lever arm is actually short on the long nose because there is not a big offset between the holes in the front of the diff and the holes in the chassis. The lever arm is long with the short nose because the mount is offset however many inches from the diff mount holes to the chassis mount holes, and the mount becomes a lever arm.

 

To put it another way, there are plenty of short nosed diffs out there. The long nose is the oddball. You don't see a lot of cars ripping the chassis apart because the mounts are built correctly to handle the load.

 

I've though about just building a steel framework that "adds" a long nose to a short nose .  Basically converts it to a long nose.  The new "nose" would need to be stiff and strong.  Then a person could use the original long-nose mount area, both top and bottom (the crossmember).  This GM diff might be amenable to that because it has four mounting holes.  The Q45's don't have the structure to do it easily, they're round and smooth.

That's essentially the cradle idea that I suggested too. Some people here have done it, and it would be possible to tie into the mustache bar mounts as well and replace the whole thing, use bushings between the diff and the cracle to isolate NVH if desired, or just mount it solid. I was talking to a guy for a while about doing a bolt in setup for the F8.8, back when I had my diff biz running. Ended up folding the biz before I got to it.

[NewZed]

You had it wrong here: "The shorter the lever arm, the harder it is for the mount to hold it down."

 

That's essentially the cradle idea that I suggested too. Some people here have done it, and it would be possible to tie into the mustache bar mounts as well and replace the whole thing, use bushings between the diff and the cracle to isolate NVH if desired, or just mount it solid. I was talking to a guy for a while about doing a bolt in setup for the F8.8, back when I had my diff biz running. Ended up folding the biz before I got to it.

You misunderstood me in the first example - the arm is the nose.  Or the nose is the arm...the lever arm.  That's the thing about levers, you have to pick the right side, for reference.  I thought I was clear which side I was talking about, but may have left out a few words.  I talked about both sides, separately.  Another way to consider it is that the assembly has what is essentially a hinge in the middle, of what should be a rigid beam. Which is what one the Q45 guys saw in his mount.  Hinging where the nose met the mount, nothing above it to stop the motion.

 

The cradle idea is great for the short-nose R200 and Q45, because they have nowhere to mount anything.  I like the nose extension because it allows you to leave the car alone.  Make the diff look like a long nose and use the long nose mounts.

 

Arizona Z Car used the LCA arms as the cradle.

 

 

Anyway, Oz Boy is probably working up some new thoughts  It'll be interesting to see where he ends up.  The first draft looks well-made, but may just not be right for the application.  Probably work awesomely with my 150 HP (what they say) L6.

[Boy from Oz]

Firstly let me offer my genuine thanks to all who have contributed to this thread, some really useful input. Constructive criticism never hurt anyone.

 

I think I really need to film the front of the diff to try and establish a fuller picture before making changes, if any.

 

A bit more information:

1. The inner bushes on the LCA are OEM rubber - some may have missed that earlier. (See point 4)

2. The outer ends are 'bushed rod ends', replacing the metal spherical rod ends. (See point 4)

3. Despite the LS1 the car is for touring and not for performance use - driver is too old, too slow, and too stupid!

4. My major initial concern was related to metal fatigue. I'm happier to see energy dissipated in bushings, and accept the associated suspension geometry change, rather that through the S30 shell and OEM or fabricated mounts, etc.

 

Sorry if I'm reinventing the wheel here but I haven't followed too closely other non-standard diff installs.

 

Again, my thanks for the input.

Edited July 14, 2016 by Boy from Oz

[Chickenman]

 

4. My major initial concern was related to metal fatigue. I'm happier to see energy dissipated in bushings, and accept the associated suspension geometry change, rather that through the S30 shell and OEM or fabricated mounts, etc.

 

That is my concern as well. If you place a reference mark on your monitor screen against the uprights ( at lower point ) you can see that the metal plates are flexing as you get on and off the power. Eventually this is going to cause metal fatigue and failure of the " L " shaped mounts.  Likely at the top bend of the " L ".

 

As JM mentioned, these thin metal upright plates may be OK for the Torque and Power of a stock L6, but they are no where near adequate for a stout SBC.

 

I would highly consider building better and stronger versions of these plates. Perhaps with Gussets along the spines. Triangulating the Mounting plate uprights would also add immensely to their strength and would distribute the loads to the chassis. 

 

Adding Doubler plates to the Chassis at the mounting points of the original upright plates would be a good idea as well.

 

All that Torque has to go somewhere... and eventually it all ends up at the Chassis mounts. The rubber pivot bushing can only dissipate a small amount of energy, and once they compress, all the energy is transmitted to the Chassis.

 

The system is only as strong as the weakest link.....

 

Edit: I'm not overly concerned about the movement of the diff. That's dissipating energy and really isn't affecting anything but Pinion angle. And I don't think the movement is excessive. If you took pictures of the IRS in a Corvette, you'd probably find a similar amount of movement.

 

What is concerning me more is the flexing and movement of the LCA uprights that connect the rear LCA Crossmember to the Chassis. Those are flexing and that is not good. They will eventually fail from metal fatigue IMHO.

Edited July 14, 2016 by Chickenman

[NewZed]

Here's a not-too-old thread with many of the same actors commenting on the same general topic.  Started on page 4 with SUNNY Z's pictures, but there's 19 pages total, plus a vendor's forum or group buy thread out there somewhere .

 

http://forums.hybridz.org/topic/117668-the-ultimate-irs-swap-for-s30s/page-4

[NewZed]

Post #11 here has an interesting picture of a way to directly address the load transfer problem (I think it's from JMortensen's Diff thread).  More discussion also, can't agree with all of it, about what is strong and what is levered.  Not sure how the guy's two piece mount came out in the end.  Notice that the thread's from 2006.  Ten years of development work on the short nose problem.

 

http://forums.hybridz.org/topic/50794-my-q45-diff-mount/

[Boy from Oz]

...As JM mentioned, these thin metal upright plates may be OK for the Torque and Power of a stock L6, but they are no where near adequate for a stout SBC.

 

I would highly consider building better and stronger versions of these plates. Perhaps with Gussets along the spines. Triangulating the Mounting plate uprights would also add immensely to their strength and would distribute the loads to the chassis. 

 

Adding Doubler plates to the Chassis at the mounting points of the original upright plates would be a good idea as well....

 

Totally agree.

 

NewZed. I can't add 'likes' so I'll say "thanks".

Edited July 15, 2016 by Boy from Oz

[sebaskir]

not really related but can anyone help me with the size bolts for the front differential mount on a 71?  

the ones that go in here^^

Edited August 8, 2016 by sebaskir

[JMortensen]

10mm x 1.25. Most off the shelf 10mm bolts are 10 x 1.5, so they can be a bit hard to find.