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MG MGF Technical - Does your MGF shimmy?

This is not going to be easy to describe, but is part of the on-going "sort-out-the-suspension-without-doing-a-Techspeed" saga. Polybushes on dampers and rear-tie bar bush have been done; ride height is consistent and within spec; car is 7 years and 67,000 miles old and she "shimmies" now (even more clearly felt after the mods than before).

What do I mean? Well, going over a rough bit of road, particularly a crack that runs across the road, the car feels as if it is dancing or shrugging its shoulders (one wheel at a time) and particularly at the rear. The steering accuracy especially on bends has markedly increased after the rear tie-bar bushes were done, but this squirmy, feeling has also increased. You might describe this as the car feeling alive and responsive to road-imperfections, but this feels like too much aliveness at the cost of stability and a feeling of security.

One of the things I sort of expected was that in tightening up the bushing with Polyflex, any other suspension issues would become clearer. Until the standard suspension is right, I don't want to go down the lowering and Bilstein route (and cannot afford that anyhow, just now).

There is one possible reason that occurs to me having read the archives and talked to a garage, but I wonder what the experts on the board think?

TIA

Chaz
C Golvala

It's not something as simple as wheel balancing is it?

(start with the easy one) ;-)

SF
Scarlet Fever

Worn sub-frame mounts?
Gaz R

What tyres are you running on ?

Could this be a factor

STU
S.C. Dickens

Scarlet, I assume that once balanced, the wheels stay balanced if the weights stay on? The weights are on and wouldn't wheel imbalance show up on smooth road as well?

Gaz, worn sub-frame mounts sound worryingly expensive. Stu, the tyres are in good nick (though extensive reading of Rob's site means I understand a little about slip angles and what the effect of not having the same tyres front and back could be - the "shimmy" does not seem to be a symptom).

Could this be a worn ball joint problem? If so, how much to fix? Should one do all 4 at the same time (assuming one per wheel)?

Thanks

Chaz
C Golvala

Worm ball joint on the top of the rear knuckle is apparently quite a common failure. I a MOT failure (7 years young 61,000) due to this a few months back. If you got the rear of the car on axle stands and got a friend to move the wheel up and down you can see the play in the joint. About 12 quid from MGR and then about 13 quid for 50mm or so socket to get the old one off...It required lots of swearing and bleeding! Once I changed it thou the car did seem better! Might be worth a check.
R Baker

Chaz,

As Stu mentioned tyres are a no.1 thing to check.
Are all your tyres of the same make and tread pattern.

What about tracking has this been done if so what settings?

Tom
Tom Randell @ work

Chaz, this suspension 'shimmy' you describe sounds very odd - and no, mine doesn't suffer from this (nor do other cars with polyflex bushes).

How much of the polyurethane grease did you apply? Not sure whether these bushes can bind can they?

Regarding the upper ball joints. My car passed its last MOT without a hitch in Feburary, but the lads at Techspeed suceeded in identifying excess play in three of the four upper ball joints. My car has done 68,000 miles in 7 years. Based on this, I'd guess that yours were probably also due for replacement.

The ball joints are cheap (they're also used on Minis and Metros), and appear easy to fit (although I let Greg and co get on and replace them on mine - how lazy of me!!! LOL).

Wear is detected by raising the wheel off the ground, and tugging on the top and bottom to see if you can detect excess play in the vertical plane.

It could explain your car's shimmy, although, as I said, no evidence of a shimmy on my car before these items were replaced.
Rob Bell

Could it be tramlining? When on the el'cheapo tyres my F used to think it was a trolly bus. Now with the new F1's (GSD3's) and a four wheel track its lovely :)

Mark
Mark Catterall

Thanks all for comments to date. The tyres are in good condition (though different tread pattern on front v rear axle, but careful reading of RB's slips angle section makes me think that this is not causing the shimmy). Re-tracking still to be done post fitting of polybushes (Maidstone Sports Car recommended letting the bushes bed in for a few hundred miles first) but no other tracking-related symptoms can be felt. The car runs straight and true and corners like a dream, with no abnormal tyre wear etc etc. Similarly, no tramlining from the standard-width MPi tyres and, if I understand it correctly, this would occur when meeting ruts in the road going in the same or similar direct to travel, whereas the shimmy occurs when crossing ruts/cracks etc.

MSC identified wear in the NS rear ball joint when fitting the polybushes and I'm off to see them and confirm diagnosis (Nick seemed to say that it was not the ball joint, but another bit of the susp at one point) in a few weeks. One way of describing the feeling is that the wheels (rear esp) jink sideways under you, often one after the other is the crack is not straight, as though the contact patch just moved left or right. If a worn ball joint allows the wheel to move when being pushed and pulled in the vertical plane, then may be this is the cause of the shimmy when encountering a rutted but of road? Could the knackered rear tie-bar bushes have previously soaked up some of this motion and changing to polybushes makes it more obvious?

Rob - maybe mine is just worse than yours, may be my bottom is too sensitive! Thanks for all the help.

Chaz

C Golvala

This thread was discussed between 07/05/2003 and 09/05/2003

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