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MG MGA - front end shimmy

Hi,

I replaced all the rubber bits on my front suspension (bushings, boots, etc.) and since then my steering shimmies at highway speeds, but only on some roads. I just noticed that a tie rod boot is now ripped, but I don't think that's the cause since it looks recent. (A new one is on order.)

Other than the rubber parts, the whole front end is original and is likely worn somewhere. The other possibility is that my alignment is out, but the car doesn't pull to the side. (Not even when it shimmies. It just shakes and scares me...)

I imagine that this is not a hard problem to solve, but something is adjusted incorrectly. Any ideas would be greatly appreciated.

Thanks,

Darian
Darian Henderson

Darian, have the toe checked. Likely it was set with the worn bushings allowing the effective tread width to be wider and now is closer to 0 than toe in. That would cause the shimmy.
Bill Young

Unbolt the rear shockers from the chassis and rotate the lever arms to confirm a steady resistance throughout their travel. Its funny how front end suspension/steering problems can be caused by rear shock absorbers.
J H Cole

Bill: I'm sorry, but what do you mean by the "toe." I can't find anything on Moss's page in relation to this.

I'll definitely check out the rear shocks.

Thanks,

Darian
Darian Henderson

Never mind. I found it:

http://www.auto-ware.com/setup/toe_hand.htm
Darian Henderson

Darian
Just to be clear, shimmy is felt by a left-right movement of the steering wheel when traveling straight.

Can you say what speed this occurs at? It is possible that in your rebuild a wheel has become unbalanced, in which case the phenomena is most likely to occur at 65-80mph. Frequency of shimmy will match wheel rotation speed.
If at lower speeds then the shimmy can come from the toe being close to zero and some play within the steering system. Shimmy not necessarily related to wheel speed.
The rear axle mounting should also not be ruled out but normally a rear axle issue will only cause vibration and a lower frequency shimmy.

Hope that helps. I have set up a few of these scenarios to train people in problem solving on vibration.

Best regards
Neil
Neil Purves

Neil

Not wanting to steal Darian's thread, but on my car I have started to suffer what feels like front wheel out of balance vibration in the speed band you describe. However, the onset coincided exactly with a rear wheel change following a puncture. Any comment?

Steve
Steve Gyles

I have noticed what seems to be "shimmy" on my 59 roadster. But it only happens on certain roads. I've come to the conclusion that it's the texture of the pavement that is causing it. On very smooth roads there is no problem at any speed. On other roads it can be quite noticeable at times. You get a lot of "road feel" in an A.
My TCW.....

GTF
G T Foster

Hi Darian, I think Bill has hit the nail on the head when he says check the toe-in. Assuming that you have only changed the front suspension and that there was no shimmy effect before then the only thing you have altered is the alignment of the front wheels. It should be zero toe-in on the MGA - that is the wheels are exactly parallel - also one wheel may be more toed in than the other which would make the car shake. cheers Cam
Cam Cunningham

Darian,
A few more suggestions / data points My MGA used to shake it's head occasionally. In my case it would start shaking and continued until you hit the brakes.

MGA wheels are not hub-centric. The disk wheels use the wheel studs for positioning, not the center hole. On wires, the "outside" registration surface is on the outside of the hub. Result- they don't sit straiget and true on modern balancing equipment. I've heard that on-car balancing works better but my experience hasn't been positive- it didn't help my old SAAB 96.

Spec is zero toe. I run about 1/32 inch toe out. This is supposed to help initial turn in although I haven't notice a difference. On the other hand, my steering at speed is "more weighty" than typical.

I rebuilt my front suspension and my rear suspension is "OK". The rebuild included bearings, bronze outer bushings, MGB lower arm bushes, ti rod ends, lower arms, one shock, rubber, and steering rack reshimming. Result- many new rubber parts split within a year and the shimmy didn't improve.

In the end, dropping the front tire pressure to 28psi did the trick. My kind of fix, cheap and completely reversable in case the next owner feels eliminating the "authentic" shimmy is too big of a nod to modern convenience.

Have fun,
Bill
Bill Eastman

Thanks everybody. I'm 99% sure the alignment is out, but I thought that when that was the case it always pulled to one side. I guess I have to cough up the money and have someone align it.

When I go there, I just tell them "0 toe-in?"

Thanks,

Darian
Darian Henderson

Darian, you can check the toe in and set it yourself. All you need is some sort of gauge that will slide under the car and then come up to a point even with the axle or use the string method as described on other boards. I like the gauge, it's simple and quick. All you need is some sort of "U" shaped bracket that will reach from side to side of the car and a measuring tape. What you are comparing is the distance from the edge of the tire tread on one side to the same spot on the other, both front and rear. The front measurement should be about 1/8" to 3/16" narrower than the rear. That's toe in, which will give you more stability. I don't know the recommended toe for the MGA for sure, but most cars run about 1/8" or so toe in. My midget manual says 0 to 1/8" toe in for it, I like the 1/8" setting as it also tends to preload any looseness in the steering components which reduces any "shimmy" from those. Here's a photo of a gauge I made up for my shop from scrap steel I had around.


Bill Young

Thanks, that looks easy enough and the picture made what you were talking about clear. If I remember right Moss sells a gauge as well.

Thanks,

Darian
Darian Henderson

Yes, Moss has a gauge, but from the pictures of it I've seen in their catalogs I doubt if it is very useful. The cross piece is mounted high and would foul the chassis on most LBCs especially to the rear of the tire. I've only seen photos of it being used in the front of a Spridget which has very little front overhang, looks too high to measure at the rear of the tire.


Bill Young

This thread was discussed between 22/07/2008 and 25/07/2008

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