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MG MGB Technical - MGB Handling Quirk

I have a 1971 mgb roadster that has a completely new front suspension (bushings, shocks, pins, tie rod ends), four new tires (sumitomo H rated running at 45 PSI), and good wire wheels (spokes tight, wheels balanced and true). The car has a strange handling problem. When I let off the gas at about 60 - 70 mph, the car wants to jog to the left, even though the steering wheel does not move. No, it's not the steering rack either - there is ZERO play in it. I have not touched anything in the rear suspension, so I suspect that it may be worn leaf shackle bushings or something else back there that allows the rear end to shift ever so slightly when the torque is reversed when I let off the gas. Any ideas what I should look for or is this a normal quirk among MGs?
S. J. Daniel

Good Morning; Tighten your u-bolts at your rear axle. Bob Thompson/International Auto
Bob Thompson

Check your wheel centres with a tape rule or trammels if you have access then get a 4 wheel alignment check
Iain MacKintosh

Both suggestions from Bob and Iain are good. Another thing you might want to do is lower the pressure in your tires. 45 psi sounds like w-a-a-a-y too much. I think the original specification (for the 155SR14 or whatever) tires on my 1969 was something like 15 psi front/17 rear. Of course modern tires are different and will require different pressures. But once you get your suspension problem sorted out, you can experiment with tire pressures to find what you need. Here's how:

http://www.mgaguru.com/mgtech/tires/tire1.htm

HTH!
Rob Edwards

I didn't want to comment on the tyre pressures knowing nothing about these tyres but they should probably be standard at 21 front 24 rear. Incorrect tyre pressures won't however cause the car to veer to the left. This is an alignment problem and I suspect that if you could accelerate hard enough it may veer to the right. You mightn't be able to feel this but if you then lift the throttle quickly I'll bet it takes a rapid change of direction. Hold on tight.

Is there any evidence of accident damage in the chassis ?
Iain MacKintosh

You say it drifts to the left when you let off the gas. Does it drift right when you give it full throttle? If so, it's most likely a loose rear end. Tighten the rear end hardware as Bob suggests. It is common for it to be a bit loose.
Steve Simmons

SJ
In the same direction as Bob's comment, you should check to see if the main leaf on one of your rear springs is broken. I went through a rash of that some years back. I had three different springs to break cleanly right at the perch. When accelerating the car would dart one way and upon braking it would return to normal. If I let go of the steering wheel when accelerating it would rotate about 1/8 a turn and the car would crab slightly. When raised on a jack I could, with some effort, move one rear wheel fore and aft.
David
David

Yes, a number of years ago when I first got my car it did the same thing, quite disconcerting. “Classic rear-end steer,” is the comment Roger Parker gave me at the time. He was 100% correct.

I replaced the rear springs (mostly because they were shot,) front eye bushings, and spring pads. This cured the problem. I suspect the problem was mostly the front eye bushings. From the outside they looked ok, but inside they were jelly.

Also, I would recommend that you use the Superflex bushings available out of England. I wouldn’t use anything else.

See ya in the garage…
Bryan

Bryan Prindle

Check to assure the springe center bolt hasn't sheard. which permits the axle to shift even if the U-bolts are tight
greg fast

Thanks for all the replies. You are certainly a helpful and knowledgeable group. I will check the U-bolts and probably go ahead and replace the bushings as suggested. Heck, I've already done everything else, so what's a few more dollars. Concerning the tire pressure, the Sumitomos are designed to be run as high as 55 PSI, and they are a little too soft at normal pressures, so that's why I run them so high. This is the first set of tires I've ever owned that are pure performance units (they are not all-season - we don't get too much snow in these parts of Texas... y'all). They are quite different than the family sedan rubber I'm accustomed to.
S. J. Daniel

That must be max pressure.

If you use this presuure you will have about half the contact patch area of any other B on the road.

The book pressures are as Iain's post, however at sustained high speed these should be increased by 6psi
so 27F 30R. Lower profile tyres usually require slightly higher pressure.

The front rear pressure balance also requires adjusting for suspension set up arb's etc and driver preference under/oversteer.

Paul
Paul

Yes , I once pumped the tyres up when the gauge at the garage was out of cal. The car was darting all over the road .I went home and checked them and they were well over , about 40 psi. Since then I always use the foot pump and gauge here.
Cross plys at low speed are OK down to 18 all round ! It's only for sustained 90 mph that they are recomended to be raised, by modern standrds the B is not a heavy car.
S Best

SJ the tyres on your car are your primary suspension and at that pressure you hav'nt got any.I bet you can tell the date on a coin if you run over one. Race cars use high pressure and soft rubber but there on a smooth track not bumpy roads. Its the air that holds the car up not the side walls and at that pressure will be placing great stress on suspension, body and driver. I would not go over 30lb for these reasons.
Denis
DENIS

You say "the the Sumitomos are designed to be run as high as 55 PSI, and they are a little too soft at normal pressures, so that's why I run them so high".

Like others posting above I feel your problem is most likely to be excessive tyre pressure.

Why not contact Sumitomo's technical suport people and discuss recommended tyre pressures with them.

They may want to know the overall weight of the car, and what modifications (if any) you have made.

Good luck.

NJSS
Nigel Steward

A broken main spring, as Iain and Bryan mention will will give you rear end steering more pronounced than badly worn bushings - so it is a matter of degree.

Do check the top leaf closely, you should see a crack if it is broken, and if not the crack, at least that the top leaf has two contours not just one from front to rear when the car is at rest on its wheels. The two halves, front and back, will bend independently rather than giving one smooth curve to the main leaf from shackle to shackle.
Bob Muenchausen

There is one other possibility. There is a piece of formed sheet metal under the U-bolts. I don't know what it is called. If it is cracked you will see the same power steer that you describe.

To check it, you have to remove the U-bolt.

Werner
Werner

This thread was discussed between 11/06/2004 and 14/06/2004

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