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MG MGF Technical - Shaking with acceleration - no not a boast

1999 T VVC, 52mm TB and K&N97i - normal tracking, no wheel deformation, normal wheel balancing, no hydragas changes

Problem: When accelerating (foot to floor) through 50 to 70mph the car shakes, most noticeable when passengers seat empty (and shakes rapidly although not loose). This also happens when slowing through a similar speed range.

Question: What could this be? is it my bushes on the rear suspension?
Rob Greig

An old car I had got a bad wobble, and it turned out to be warped brake discs. Must say though that accelarating hard or soft never made any difference to it.

Leigh Reid

Have had same problem for some time now and have been unable to resolve problem yet I have been down the same route as yourself tracking, wheel balancing etc but still no joy.
The vibration is in the same speed range but occurs only when accelerating not decelerating, if you have any joy in resolving this I'd be very interested in the solution

cheers Stu

How about this for a hypothesis, the wobble is caused by something revolving which is not quite in balance, as you've looked at all the normal things, wheels etc. have you check the drive couplings for wear ?, a little bit of play may cause the imbalance and thus the juddering.
Just a thought, discuss.

Mike, M2 VJN, Yellow Trophy

I agree with Mike - this does sound rather like an imbalance on a rotating part... discs, as Leigh suggests, could be the culprit.

Another cause of a vibration is a problem with the tyre - often caused by a flat spot on its circumference. But this is usually more noticeable at lower speeds.

I'd check the discs as my next line of investigation.
Rob Bell

Sounds like a balance weight has come adrift from one of your wheels.
That is the place I would start before spending too much money.

Tony H

Thanks, due in next week for 36K service/MOT will keep you informed (Stu) (Tony H the wheel balancing is normal but cheers anyways)

did notice the dog lead still attached the rear bumper. couldn't find the dog mind..
Rob Greig

If it's a balance thing (eg. wheels) then surely the vibration would be there all the time, not just when changing speed. Drive shafts?

Tim Jenner

Tim, strangely it does not happen that way, it's to do with oscillation frequencies, which means at certain speeds the oscillation is amplified, with front wheels out of balance always happens at around 45-55 mph IIRC not all the time. Mike

Its called resonnance.
One part of the car naturally oscilates at a certain frequency. The extra weight of a passenger will change this natural frequency. When the out-of-balance wheel(or whatever it is) vibrates at the natural frequency it will drive the natural vibration of the car and it will get very big very quick. It will also do it at half the natural frequency, quarter, double and so on.

Look for vibration at around 30mph too.

I expect you've noticed the washing machine vibrations get very bad as it spins down and goes through one of these natural frequencys.

Its obviously related to road speed rather than engine speed so must be Wheels or Drive shafts.

Good Luck.

Russell, do I get one point for a good effort? :-)

and don't forget to check 120Mph as well ;-)
Will Munns

But Rob says the vibrations occur when accelerating or decelerating between 50 and 70 - if, say, a wheel was out of balance then it would still vibrate at, for example, a steady 60mph. My own experience of wheel balance has been that it is most obvious at particular speeds and accelerating or decelerating doesn't make things worse.

Tim Jenner

For what it's worth, I had a similar problem, new brake discs cured it. Strangely it was not that noticeable through the brake pedal.
anita roberts

Mike, Full marks for conciseness!

Will, Good thinking - a very thorough approach. Try 180mph too.


This thread was discussed between 27/05/2003 and 29/05/2003

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