Diff Bush

[thecook]

Just wanted to know what would be the cause of your diff bush to give way after 500 ks of normal driving

[.Stripes.]

Poor design.

[thecook]

mate its suppose to be the best out there supapro

[.Stripes.]

Comfort or solid? I broke a comfort in a stock powered FG XR6T manual within a few hundred kms. From your sig you have a manual, they send a lot more shock through the driveline than the autos. DId you just replace the rear diff bush? Or the front two as well? 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 axle tramp and what you ideally want your diff bushes to be doing.

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.

Daniel

Edited 24/07/14 06:33 AM by .Stripes.

[Peppy_t92]

Poor installation?

[ms700]

^^^^^^ this "may" have something to do with it methinks

[thecook]

Comfort or solid? I broke a comfort in a stock powered FG XR6T manual within a few hundred kms. From your sig you have a manual, they send a lot more shock through the driveline than the autos. DId you just replace the rear diff bush? Or the front two as well? 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 axle tramp and what you ideally want your diff bushes to be doing.

confort bush is what I put in and only the diff bush thank you I have put supapro bushs I think nearly everwhere and yes have alot more axle tramp than I had before.

Edited 24/07/14 12:40 PM by thecook

[.Stripes.]

I might suggest editing your post and removing your phone number bud, given this is a public forum. Flick me a PM if you'd like and I'm more than happy to give any advice I can

[gaz097]

Nice write up stripes. Exactly the conclusion I made about diff Bush's. Hopefully in the coming weeks I will be going to superpro race series front Bush's with a comfort rear

[.Stripes.]

That was all CAT600 mate, I just copy and pasted his tech article from AFF. Just bear in mind Gaz that the super pro race series front bushes will highlight any imperfections and slop in your driveline. It'll make the rear end feel a whole lot better...but just be aware a lot of the time they do make the driveline noisier. Although if you've got a louder zorst and maybe some sound deadening it may be less of an issue.