Cause Of Cracked Diff Bushes

[g6eturbogoodness]

The diff bushes were cracked on my g6, is there a common cause of this? Eg burnouts,driving it hard or is it just wear and tear.? It's a 2008 g6e turbo with 100,000k

Thanks

[.Stripes.]

Have a read of the tech document I've quoted below from one of the members on the Australian Ford Forum (CAT600) that explains what's happening with the factory diff bush setup, axle tramp and what you ideally want your diff bushes to be doing. Gives you a good idea why they break.

There has been a lot of talk lately about the diff bushes in these control blade IRS cars. As many know the earlier BA series of cars use a different diameter rear bush (which in itself I believe is better than the latter “upgrade”) but due to the front ones being the same, you can break rear covers and/or rear diff main bolts as a result of the additional stiffness that this BA rear bush enjoys. The aftermarket does offer bushes to suit the BA’s also.

Now the main issue is that the IRS design is compromised by the requirement to keep the diff centre as well isolated and insulated from the subframe (and thereby keeping it isolated from the solid connection to the cars monocoque frame) The way Ford have successfully done this is by designing a set of bushes that are very, very pliable. Not only did Ford use a very soft durometer rated rubber but the bush is made in such a way that the rubber is porous which in turn allows the central fixed part of the bushes to flex upward and distort the bush to the point where it ends up hard against the outer shell of the bush.

At this point you have distorted the bush considerably and this repetitive distortion over time causes them to crack and tear….. once they are torn the bush offers little to no support and they start banging even on light gearshifts. Another common criticism of this IRS is that it is prone to tramping and banging. This is even more prevalent in wet conditions or with low traction tyres. The reason for this I posted some time ago: Guys, the tramp issue is NEVER going to be fixed by ANY amount of suspension alteration.

The issue is caused by the torque reaction that originates in the diff mounting. It goes like this:

1) The factory diff bushes allow miles too much movement and the applied torque through the half-shafts reacts back through any means it can, which in this instance means that the diff rotates opposite the half-shaft rotation.

2) Once the differential has rotated to the point where the mounts are fully compressed in that direction, the whole assembly is effectively "maxxed out" and the wheels then break traction as there is nothing left to give.

3) When this happens the diff bushes begin to rebound (decompress) and the diff is again able to flop and move, because you have unloaded the tyres somewhat, the tyres will begin to regain traction at some point. This is because the diff is "reverse rotating" and that actually assists with regaining traction as this act absorbs torque and reduces it to the wheels. As traction is re-established, go back to step 1 and repeat the whole process again.

This explanation is all happening in split seconds and sounds like a hammering effect... the sound you are hearing is the rear diff mount bolt banging into the subframe via the rear bush combined with the tyres hopping as the traction is effectively going into "make and break traction" mode.

Now the popular after-market bushes have had many complaints of intrusive road noise (which I would define as diff whine travelling through the bushes). Now to be honest in a factory exhaust car with stock levels of NVH required, you will never 100% escape this at cruise with these off the shelf parts. I have been in many cars with Nolathane’s all round and one car with the Pedders “Harder” option and they do whine noticeably. I have got around this in my own car but the point of this thread is not to talk about that, its to explain what the problem is, why it happens, how you can improve it and what compromises this may entail.

Just fitting a stiffer rear bush will not stop the issue, it will improve it but the drawback is that you have a greater likelihood of snapping the rear bolt or cover...... the trick is to actually make the front two bushes a lot stiffer (as much as you can before you compromise too much in the way of NVH) then choose a rear bush that actually has some give but not enough to allow the "maxxing out" effect... this will not allow the bolt/cover to break anywhere near as easily as the bolt is not trying to bend or shear in the cover, rather it operates like the factory bush but limits the travel.

I found the Nolathane rear to be the best in the combination I run.

Also, see these videos that show how much movement you get with super pro comforts in the first video and much less movement in the second video with the super pro competition bushes:

XR6T RHS Front Diff Bush (Superpro): http://youtu.be/_q0fIFtE3jc

XR6T Superpro Comp diff bushes: http://youtu.be/iWzuG9PV8yY

Edited 29/07/14 02:50 PM by .Stripes.

[demon1300]

I've got CAT600 bushes in my car and filmed them the other day. Moves even less than the last video. I'll post it up if people are interested?

[.Stripes.]

Yeah mate I'd be real interested to see that, if you don't mind posting demon? How's the noise going with them?

[demon1300]

Yeah mate I'd be real interested to see that, if you don't mind posting demon? How's the noise going with them?

Still pretty sh*tty to be honest... Really want to get it sorted as its pretty extreme.

I'll be calling into pedders hopefully tomorrow to get their opinions.

[Ralph Wiggum]

Solid bushes be acting like solid bushes.

[aussie1926]

I want to know how you got 100 000 out of standard bushes? I found standard one lucky to get 10 000ks out of them.

[g6eturbogoodness]

I bought the car at 93,000 so maybe it had already the bushes replaced previously

[gaz097]

I've got CAT600 bushes in my car and filmed them the other day. Moves even less than the last video. I'll post it up if people are interested?

Where do you buy these cat Bush's from?

[.Stripes.]

Contact forum member CAT600 (his name is Daniel) over on AFF. He does have an account on here too, but not sure how often he checks it anymore.