Questions on suspension parts

[RETed]

Did you replace the "bushing" that are in the "ears" of the rear diff case?
The stock ones are "hollow" with just a few "spokes" (I think 4 per side?) that allows the rear diff to move a lot.
Did you replace this with one of the delrin units?


-Ted

[infernosg]

Originally Posted by RETed

Did you replace the "bushing" that are in the "ears" of the rear diff case?
The stock ones are "hollow" with just a few "spokes" (I think 4 per side?) that allows the rear diff to move a lot.
Did you replace this with one of the delrin units?


-Ted

Are you referring the "mounts" colored purple in the picture below (picture credited to Racing Beat)? If so, then yes, I replaced those with the "Competition" units along with what's colored red and yellow. I made sure the "voids" or gaps in the rubber material were facing forward/aft per the FSM. Only the subframe mounts (not colored) were replaced with delrin units.

[RETed]

Originally Posted by infernosg

Are you referring the "mounts" colored purple in the picture below (picture credited to Racing Beat)? If so, then yes, I replaced those with the "Competition" units along with what's colored red and yellow. I made sure the "voids" or gaps in the rubber material were facing forward/aft per the FSM. Only the subframe mounts (not colored) were replaced with delrin units.

Yes, those are the "weak points".
You can literally grab the rear diff and sometimes shake it with your hands and see the entire rear diff swaying under there.
Imagine the amount of torque backlashing through the drivetrain?
It's violent.

I've never seen "solid" aftermarket mounts to replace those "Differential Casing Mount" (PURPLE).
I've seen aluminum ones - TOO stiff.
I've seen stiffer replacement for the "Differential Stop Washer" (RED) underneath - i.e. URAS - that don't really do squat either.
If I could, I would try and play around with a SOLID polyurethane mount.
If I ever get off my lazy ass, I might even try and play around with a brand new set of mounts and injecting some of that "gray weatherstripping RTV" into the voids - it's a trick some racer use for those bushings with voids in them.

Those mounts are the cause of why the rear diff moves around so much.
The other extreme problem (mentioned with the aluminum mounts) is that if those mounts are too stiff, you risk damaging the rear diff casing itself.
Those mounts absorb a lot of the rear diff movement.
Once you start to restict that movement, something else gets stressed.
What happens is that the "ears" of the rear diff housing that those mounts are inserted in tend to crack off.
Remember, the rear diff housing is made out of cast aluminum...
To combat this problem, I've seen custom rear diff frame braces made to minimize the ears cracking.
And now we're back to square 1...

Also, if all of that isn't bad enough, the front diff mount is a stupid design.
That mount resticts movement via a shear force of the mount - WTF???
This is where a pinion snubber comes in and does help...but it can only do so much.
There is still a lot of movement up there.
So what can you do?
Most will tend to crank down (reinforce the front diff mount or solidly mount it) the front diff (pinion) to the rear subframe...
Again, now you risk breaking the rear subframe itself - I've seen this happen also.
And now we're, again, back to square 1...


-Ted

[infernosg]

^^^ All good points. I've considered the RTV trick, but hesitated due to the concerns you've expressed. I've also heard of people inserting objects between the stop washers and mounts (like ground down hockey pucks) but that would lead to the same issue.

Everything that's "colored" in that picture has been replaced by the Mazda Competition mounts, which are supposedly 40% stiffer than stock. The only "solid" mounts I have are the subframe-to-body mounts, which are delrin.

I checked for free-play in the drivetrain on Friday. Aside from discovering the left-rear wheel bearing is going, there was a little moving in the driveshaft at the u-joints, but it really is not that bad considering the car, and driveshaft, is 22 years-old. I'm now almost certain the ring and pinion backlash is off. There is a good amount of free-play in the rear wheels before I feel resistance. Time to go have a word with the shop that set the backlash...

[Trots*88TII-AE*]

Originally Posted by RETed

I've never seen "solid" aftermarket mounts to replace those "Differential Casing Mount" (PURPLE).
I've seen aluminum ones - TOO stiff.
I've seen stiffer replacement for the "Differential Stop Washer" (RED) underneath - i.e. URAS - that don't really do squat either.
If I could, I would try and play around with a SOLID polyurethane mount.

...

-Ted

Aren't these mounts from MMR what you're referring to?

That's what I have for my rear diff. I am also considering the MMR spherical bearings, as you cannot use individual camber adjusters with the delrin control arm bushings. My question is how can a spherical bearing give enough play to force camber adjustment while Delrin cannot? I would've thought that the bearing would be under more stress and would fail if adjusted too far under constant driving.

[RETed]

Originally Posted by Trots*88TII-AE*

Aren't these mounts from MMR what you're referring to?

(Pic deleted)

That's what I have for my rear diff. I am also considering the MMR spherical bearings, as you cannot use individual camber adjusters with the delrin control arm bushings. My question is how can a spherical bearing give enough play to force camber adjustment while Delrin cannot? I would've thought that the bearing would be under more stress and would fail if adjusted too far under constant driving.

I assume this is where you got it from?
http://www.mmr-direct.com/catalog1/RX7FC/index.htm

I hate it when vendors use "generic" pics for their pages.

The pic you posted was for an "rx7diff" part.

Yes, it does sound like their "Rear Differential Mounts" part is the part I'm talking about.

Their plastic is still too hard for this application.
It will tend to break the ears off the diff as I have mentioned earlier.

As a side note, the engine and transmission mounts they offer are crap.
I hate when vendors "get cheap" and offer these types of mounts that do not properly mimic (or do a better job than) stock mounts and bushings.
Just because of this inferior design, I would never use their products.


-Ted

[infernosg]

Originally Posted by Trots*88TII-AE*

My question is how can a spherical bearing give enough play to force camber adjustment while Delrin cannot? I would've thought that the bearing would be under more stress and would fail if adjusted too far under constant driving.

Simply the fact the spherical bearings allow rotation/movement along more than one axis. As Josh said, the delrin mounts only allow movement in one direction. I have the spherical bearings installed and I am still able to get the desired toe settings.

Regarding "solid" mounts I'm a firm believer there needs to be play SOMEWHERE in the system. If everything is solid the vibrations get transferred right into the mounts and THEY WILL ultimately fail (low-cycle fatigue). As Ted said, you want some movement at the differential cover or you will break the mounting points. I only went with delrin subframe bushings was because the stock ones needed to be replaced and I could get both delrin bushings for less than the cost of one OEM bushing.