Z3 Rear Crossmember Bush Inserts

[Mike Fishwick]

Handling is like power – we can always use more of it! My 1998 2.8 already had a strut brace and a ‘Butt Strut,’ both of which made small but definite contributions to reducing understeer and improving steering response.

The next modification was to stiffen the rear suspension cross-member bushes, which incorporate spaces designed to reduce road noise. As the bushes soften with age, these spaces permit the entire rear suspension to ‘float’ backwards and forwards, so misaligning the wheels. This promotes instability and tyre wear, particularly under hard cornering, or in straight lines on rough roads. Fitting the solid rubber type used on the E30 M3, or better still polyurethane bushes, is the real answer, but this is not a simple task.

The alternative was to fill the spaces with PVC stiffeners, purchased from America via Ebay, from 'Skunkworks27'. These little pieces of PVC (drainpipe?) are a work of genius, and even though my bushes appeared to be in excellent condition, they provided a small but definite improvement in steering response, without any penalty such as increased road noise. In the longer term they will doubtless help to reduce wear on the edges of the rear tyres. For $24 plus postage this must be the most cost-effective handling modification available.

Fitting was easy, but there are a some points which the unwary should consider seriously.

As you will be removing the nuts which hold the rear cross-member to the body, there is a risk that it may drop to such an extent that it strains the flexible brake pipes between the body and the cross-member. Before you undo anything, therefore, support the cross-member by jacking the car down until it is supported at its centre on some wooden blocks.
Initial fitting of the inserts – to about the 50% mark – was easy enough, but to fully inset them requires more force than can easily be applied. Hammering them into the bushes is virtually impossible – just like trying to hit a chisel with a rubber hammer! The answer is to press them into the slots, a process which takes a considerable pressure.

I found that the best technique was to first screw a suitable nut and washers along the stud to force the inserts into place, after lightly lubricating them with WD-40, which does not harm rubber when used in small quantities.

I then finished the job by jacking the car up on the insert, by means of a piece of suitable steel tube (I used a box spanner) supported on the platform of a trolley jack. (I would not even attempt the job without a trolley jack)

Make sure that the insert is pressed into the bush sufficiently deep to clear the bracket which braces the stud to the door sill, and also ensure that the insert does not protrude above the bush far enough to make contact with the upper plate. Failure to do this will provide a transmission path for road noise.

On the road the difference is not vast, but for the amount of money involved it is considerable! Steering response is notably sharper, and stability on poor road surfaces is certainly improved. As a rough guide, these bits of sewer pipe probably provide an improvement of 20% when compared with the Butt Strut at 100%.

Road noise does not appear to be any different – at least in a roadster – although CW states that in his ZM Coupe it is slightly higher.

[c_w]

Lol, I wonder if they really are bits of drain pipe!? They do fit perfectly though.

For me this was one of the best changes I've done to the car as it seems so much more stable and precise now that the rear subframe and suspension isn't constantly floating around. And was easy too.

The only thing I'll add is that I changed the allen key bolts that hold the rear plates to the sill for some 13mm hex head bolts as I find these a bit less liklely to round off if I was to remove them again.