Modified differential mount

[konish]

I had this stock mount modified to use both the stock rubber isolation as well as an energy suspension sway bar bushing (donut) or old valve spring. The stock mounting stud was cut off and a hole was drilled in it's place. Essentially a bolt passes through both plates with the head welded to the top plate. It'll bolt up exactly like the stock mount with the suspension bushing/spring mounted between the nut and the differential cross member.

The benefit here is that the top plate can still extend upwards, but is limited by the dampening from the spring or bushing. My hope is that the stock rubber isolator combined with the less compliant urethane bushing/spring will limit the upward moment while providing all the benefit of noise isolation and required compliance from the stock mount...without all the bracketry of the RT mount. If the stock rubber isolator was capable of arresting most of the torquing moment by itself without ripping in half in short order, the welded bolt head shouldn't experience enough force to tear it from the weld. I'm also hoping that the limited compliance in the bushing/spring will keep the whole mess from ripping through the cross member like a solid mount. However, if it does fail, then it'll be an RT mount next.

EDIT:

I guess you could just use a rounded head bolt and make it without welding the bolt to the top plate. With this method you could also jam 1/2 a polyurethane bushing (sway bar link bushing) in between the top and bottom plate then put the bolt in place. The benefit here is that you have a polyurethane bushing both in tension and compression giving the stock rubber pieces a break in both directions.

[Oiluj]

NIce work, but why not just go to an RT mount? They are proven to work great are are not difficult to install.

Just my 2 cents, and proably not worth that much...

[doradox]

That's an interesting idea. That modified mount may want to walk side to side on the cross member as it is no longer solidly bolted to it. Possibly increasing the size of the hole in the cross member and using a poly bushing with a shoulder that would fit in the hole could eliminate that. Something like the shock absorber bushing I circled. At the very least it will keep the bolt from contacting steel.

Steve

[konish]

That's an interesting idea. That modified mount may want to walk side to side on the cross member as it is no longer solidly bolted to it. Possibly increasing the size of the hole in the cross member and using a poly bushing with a shoulder that would fit in the hole could eliminate that. Something like the shock absorber bushing I circled. At the very least it will keep the bolt from contacting steel.

Steve

I thought about that but then wondered how much side-to-side movement the whole drivetrain experiences since the motor mounts and mustache bar mount should prevent that...(?) I like the shouldered isolator idea or I could just have a spacer machined up that does the same thing if I can't find a properly sized shouldered bushing and put the urethane under that, then nut.

[konish]

NIce work, but why not just go to an RT mount? They are proven to work great are are not difficult to install.

Just my 2 cents, and proably not worth that much...

An RT mount just may go in there eventually but I wanted to see if this simpler idea worked any better.

The one thing I don't like about the RT mount is that the end of the diff is always suspended by that big rubber GM snubber unless you install both the stock mount and the RT. If the GM mount *did* fail (tore away from the bracket) I'm guessing the diff could twist freely not unlike the pics you see of a failed cross member using a solid mount.

I'm pretty sure the poly block would last a long time but my design allows for an easy replacement/inspection of any bushings and/or stock mount. Also, it should retain any fail-safe advantages of the stock mount enhanced by the bolt tying the top plate of the mount and the cross member together.

[Arne]

I use my RT mount with the stock mount below and a poly bump stop above - not the GM mount. In this configuration, the stock mount is doing the locating same as designed - the RT w/bump stop is replacing the limiting strap, but doing it much better than the original strap ever did.

[Oiluj]

I did the same as Arne. It works great!

[jmortensen]

An RT mount just may go in there eventually but I wanted to see if this simpler idea worked any better.

The one thing I don't like about the RT mount is that the end of the diff is always suspended by that big rubber GM snubber unless you install both the stock mount and the RT. If the GM mount *did* fail (tore away from the bracket) I'm guessing the diff could twist freely not unlike the pics you see of a failed cross member using a solid mount.

I'm pretty sure the poly block would last a long time but my design allows for an easy replacement/inspection of any bushings and/or stock mount. Also, it should retain any fail-safe advantages of the stock mount enhanced by the bolt tying the top plate of the mount and the cross member together.

If you've got poly in the mustache bar, I don't think the mount is taking that much weight in the front (take the front crossmember completely off and the front of the diff is still pretty horizontal), but regardless, due to the interlock design it really can't come apart. If it gets loose, replace the cheapo poly mount.

Your welded stud should work OK, but if the diff mount fails you're putting all of the vertical load into a pretty small area on the diff crossmember, and I think you're likely to find it failing there. There is a thread on hybridz.org showing solid diff mount failures and one in particular shows the hole in the diff crossmember tore open, and I think that's likely where you'd end up with yours if the diff mount failed. Once the rubber fails I think they'd act the same way on the diff mount for all intents and purposes.

[doradox]

I thought about that but then wondered how much side-to-side movement the whole drivetrain experiences since the motor mounts and mustache bar mount should prevent that...(?) I like the shouldered isolator idea or I could just have a spacer machined up that does the same thing if I can't find a properly sized shouldered bushing and put the urethane under that, then nut.

Just make sure you aren't going to have any metal to metal contact on parts that will move independently of each other and you should be OK.

Steve

[konish]

If you've got poly in the mustache bar, I don't think the mount is taking that much weight in the front (take the front crossmember completely off and the front of the diff is still pretty horizontal), but regardless, due to the interlock design it really can't come apart. If it gets loose, replace the cheapo poly mount.

Your welded stud should work OK, but if the diff mount fails you're putting all of the vertical load into a pretty small area on the diff crossmember, and I think you're likely to find it failing there. There is a thread on hybridz.org showing solid diff mount failures and one in particular shows the hole in the diff crossmember tore open, and I think that's likely where you'd end up with yours if the diff mount failed. Once the rubber fails I think they'd act the same way on the diff mount for all intents and purposes.

Jon,

By failure of the mount do you mean the complete failure of the stock rubber isolators allowing the top plate and bottom plate to completely separate?

If that were indeed to happen, the stud (still dampened by the urethane bushing) would still keep the top plate from separating completely which would at least prevent a catastrophic failure of the mount.

But seriously, how likely is that anyway? I mean my stock mount is crusty and loose but it hasn't completely failed. Besides, I think failure of the stock isolator is more a function of having to take the entire torquing moment it's whole life. This design allows the load to be split between the urethane bushing in compression underneath the cross member and the stock rubber.

I disagree that my design would act like a solid mount in the event of a total failure of the stock isolator. The urethane bushing between the mounting nut and the cross member will still allow for enough compliance to keep the mount from fatiguing the cross member until it could be fixed.

Those pics of solid mount failures (ripping through the cross member) happened over time as the cross member became the "compliant" material in the drivetrain...it didn't simply pull the nut through the cross member on the first (or second etc) application of torque.

If by failure you mean the stud, well yes, that would be a problem as it would simply fall out of the car and the mount would be completely loose from the cross member. Hopefully just the design of the stock mount (essentially a wedge) will keep it in place long enough to discover and repair the problem.

Again, I find it highly unlikely as the mount can handle the torque all by itself for a long period of time...I'm not even sure my street-driven car could generate enough torque to pull that stud out of the weld without something else failing *long* before that event. I would think the torque load would have to overcome the stock isolator first, then the urethane bushing before the stud would have to bear the entire load.

My intent was to make an easy mount (drill hole, weld bolt) that would control the torquing moment of the diff while offering all the benefits of the stocker. The RT mount looks awesome but short of having a welder and the knowledge to use it and/or having someone make it for you, this may offer another alternative to the same problem without the same level of fab work.

[Zed Head]

It looks good in concept. But I think you'll find that when you tighten the nut down the top plate will pull down until it's edge (front or back, can't remember which) contacts the "diff mount front member" (cross member) lip. Effectively giving you a solid mount (metal on metal). The "clunk" will be gone but you might get more vibration.

If you don't crank it down, you'll need a self-locking nut or similar to keep it tight.

Not trying to be joy-killer, just something to watch for when you tighten it up. I have stared at that edge for quite a while trying to figure out how to work around it. the designers made it so tight, it only sits about an 1/8" above the lip normally.

[Nigel1943]

I'm confused, isn't this a modified TRANNY mount not a diff mount ?