Are poly diff bushings legal?

[Prof. Chaos]

I have the diff out on my ITA Miata, and I was trying to find if the two diff bushings that hold the assembly to the car are legal to replace with poly units. It seems that all suspension bushings are open, but I wouldn't count the diff as part of the suspension.

9.1.3.5.d.6
Bushing material, including that used to mount a suspension subframe to the chassis, is unrestricted. This includes the use of spherical bearings, so long as no suspension component is modified to facilitate their installation. Retention of spherical bearings by use of tack welds is allowed, as long as the welds serve no other purpose.

Maybe the question is: is the diff part of the suspension subframe?

[Greg Amy]

No.




Da-doo-doo-doo, da-daa-daa-daa, is all I want to say to you...

[titanium]

Are these the bushings in question:
http://iscracing.net/Miata_Brake_Diff.htm

[Prof. Chaos]

Those are the bushings in question. I found it odd that all the suspension bushings are open, yet the diff must be mounted using the factory rubber. I'm sure there's some advantage to replacing the diff bushings in a Miata, but I imagine the gain is far greater when you replace all the suspension bushings (which is allowed). Plus, it's not like the diff bushings are hard to find or very expensive. $50-$60 for two seems to be the going rate.

But I suppose if the GCR won't let me buy them then that's even cheaper.

[JohnRW]

... I'm sure there's some advantage to replacing the diff bushings in a Miata...

Advantage: Weight. Your wallet will be lighter.

What's wrong with the ones that are in there now ?

[JoshS]

I found it odd that all the suspension bushings are open, yet the diff must be mounted using the factory rubber.

I think of the diff as part of the drivetrain; it's in the same category as the engine and transmission -- which also must be mounted with the factory rubber. Seems totally consistent to me.

[Greg Amy]

I think of the diff as part of the drivetrain; it's in the same category as the engine and transmission...

Ergo:

- Since motor mounts are part of the drivetrain, and suspend the engine, they're free;

- Since rods are part of the drivetrain, and suspend the piston, they're free;

- And since the head is part of the drivetrain, and suspends the cams, it's free (and the cams suspend the rocker arms, so they're free too...)

[Z3_GoCar]

I think of the diff as part of the drivetrain; it's in the same category as the engine and transmission -- which also must be mounted with the factory rubber. Seems totally consistent to me.

Probably the best answer would be it depends. I havn't removed a differential from a Miata, so I can't say no, but on the other hand... Josh's car and mine the differential bushing in the single "Mickey" mouse ear locates the sub-frame so In our case a polly/delrin/or aluminum bushing would be perfectly legal, same as polly/delrin/aluminum would be legal in the sub-frame points just inside of the wheel. So, if it mounts only in the sub-frame or only on the unibody they're not legal, but if it mounts in both the sub-frame and unibody then you might have a leg to stand on.

James

[JoshS]

Probably the best answer would be it depends. I havn't removed a differential from a Miata, so I can't say no, but on the other hand... Josh's car and mine the differential bushing in the single "Mickey" mouse ear locates the sub-frame so In our case a polly/delrin/or aluminum bushing would be perfectly legal, same as polly/delrin/aluminum would be legal in the sub-frame points just inside of the wheel. So, if it mounts only in the sub-frame or only on the unibody they're not legal, but if it mounts in both the sub-frame and unibody then you might have a leg to stand on.

Interesting argument. I can see the twist that on this car, the differential bushing (where it mounts to the unibody) is essentially a subframe bushing too. But I use stock.

[Z3_GoCar]

Ergo:

- Since motor mounts are part of the drivetrain, and suspend the engine, they're free;

- Since rods are part of the drivetrain, and suspend the piston, they're free;

- And since the head is part of the drivetrain, and suspends the cams, it's free (and the cams suspend the rocker arms, so they're free too...)

G** D*** it Greg!! Now you tell me and I should have gone with the aluminum mounts from Bimmer World and they would have lowered the motor by 1/2 an inch too

J/K

James

[Andy Bettencourt]

Ergo:

- Since motor mounts are part of the drivetrain, and suspend the engine, they're free;

- Since rods are part of the drivetrain, and suspend the piston, they're free;

- And since the head is part of the drivetrain, and suspends the cams, it's free (and the cams suspend the rocker arms, so they're free too...)

I am not sure what you are driving at here Greg. All Josh was driving at IMHO was that since engine and tranny mounts must remain stock, it seemed consistant that the diff mounts would too.

[Greg Amy]

All Josh was driving at IMHO was that since engine and tranny mounts must remain stock, it seemed consistant that the diff mounts would too.

Ah, roit. Oops.




Da-doo-doo-doo, da-daa-daa-daa, is all I want to say to you.

[JoshS]

Ah, roit. Oops.

Da-doo-doo-doo, da-daa-daa-daa, is all I want to say to you.

Thanks Andy. I couldn't figure it out either. I figured he just failed to be funny, so I ignored him :-)

[Greg Amy]

...so I ignored him.

Ouch. That hurt...

[Prof. Chaos]

What's wrong with the ones that are in there now ?

Nothing's wrong with the ones in there now. In fact, they're in perfect condition. But it's far easier to drop in the better bushings now than do it later since the diff is sitting on the floor in the garage right now.