Important ----rear axle issue

[griffo]

Hi, a while ago I posted a message about a knock from the rear of my 2004 4200 coupe. Having checked out tie rods, gearbox mount and shocker top mounts I decided it was the rear anti roll bar bushes. The clonk seemed to be coming from the nearside (UK) and there appeared to be a mark where the roll bar was just touching the frame of the car from time to time. Took a pair of new bushes to my local (and very experienced) back street garage man since I couldn't get my ancient pinkies up behind the
brackets and he made a very interesting discovery -- nothing wrong with the roll bar bushes but the near side of thge bar was touching the frame because the whole axle/subframe had moved. By measuring wheel centre to wheel centre there was a discrepancy of almost one inch. The axle/subframe is held by hefty allen bolts (three a side) and though they were not loose you could see where the nearside had moved by bright metal around one side of the bolt head. By taking the weight off the wheels and loosening all the allen bolts it was possible to lever the subframe back to its original position and get an equal track each side. They then need to be torqued up again very tight. Result -- no more clonk from the rear. It seems with all the torque these cars have and a tightly set up lsd the subframe can shift slightly under fierce acceleration, particularly on a surface with a differential of grip. Hope this helps someone.

 

[conaero]

Phew, that was close, could have been a nasty one if not addressed.

Anyway, thanks for the heads up, ill check mine next time it up in the air.

 

[CatmanV2]

Indeed. God stuff Griffo. Totally sure mine is coming from the tie-rod, but surely worth bearing in mind.

C

 

[ENZ525]

Good info...wonder if something like the wet handling day where we go from a slippy surface to very grippy / and vice versa, or the kick plate could have an impact?

 

[Emtee]

Hi Griffo, thanks for that. Interesting to read.

Can you describe the knock in any more detail? What were the circumstances when you got it? Braking? Cornering? Gear change?

Would be good to get a bit more detail if you can.

With thanks, Miles.

 

[griffo]

Conaero,
I've not ready anywhere else of this happening so it may be rare but, if you want to check, there's no need to have the car on the lift. Just, with the help of someone else, very carefully measure from the wheel centres of the front to the rear wheels with a tape. Should be the same each side. If there's more than a tiny difference it could be the sub-frame has moved.
cheers

 

[allandwf]

If it moves sideways and the wheels remain parallel would the lengths not remain the same?

 

[CatmanV2]

If it moves sideways and the wheels remain parallel would the lengths not remain the same?

No, as you'd end up with a triangle, and you're measuring the hypotenuse. Whether a tape measure is accurate enough, I'd not like to guess.
A spot of maths indicates that *assuming* the wheelbase remains constant at 2660mm, a 10mm lateral shift in the rear wheel would increase the distances between the wheel centres by about 0.02 mm
So probably not accurate enough.

C

 

[griffo]

Yes but the subframe had not moved materially sideways. It had moved fore and aft. The nearside seemed to have slipped forward a bit (bringing the anti-roll bar into contact with the body of the car) and the offside moved back a bit. IE it sort of swivelled, which could be from the force applied to it under hard acceleration, particularly with a very tightly setup LSD (which mine seems to be) if one wheel was gripping and the other not. With the bolts holding the subframe loosened a small amount of front to rear movement each side is possible. The holes in the subframe through which the allen bolts pass are larger than the diameter of the bolts -- that is why movement is possible. That is why measurement from wheel centre to wheel centre each side is appropriate. Hope I've explained this clearly.

 

[allandwf]

Yes, makes sense if it's twisted.

 

[CatmanV2]

Yep, with you now

C