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MG Midget and Sprite Technical - Big vibrations

Hi All,
I still have a very big vibration at around 65mhp. One of those that goes through the whole car and up your spine, really harsh, a bit like a hammer drill but harder.
I think I have sorted all the obvious things. I have had the wheels balanced properly by Brown and gammons (They are wires) and he steering wheel does not wobble or shake so I don't think it is the wheels. I have replaced the prop shaft (there was quite a large spot of weld on one of the yokes). What else could cause a big, filling loosening vibration in top gear only at 65 ish mph that goes away at 70mhp?
What effect would a knackered crank damper have on this?
Anyone any thoughts? It is really bugging me and taking most of the pleasure out of driving the car.
Thanks for you help.
Dave Brown

A couple of questions...

1. Which model is your car?

2. Did this vibration just appear, or has it gradually developed?

If the former, was there anything done to the car just prior to it happening?
D O'Neill

If you can't feel the vibration through the steering wheel it's unlikely to be related to front wheels or suspension. So what next? Well a couple of questions if you don't mind.
What's the engine RPM range at 65 to 70 and do you hit this in other gears? This will help eliminate the engine damper.
My initial thoughts went to a diff/axle problem but others may have more questions first.

BTW which engine?

M McAndrew

It is a 1275 mk3.
I think it gradually happened. The trouble is I am not driving th car regularly, my daughter is and getting feed back is often quite difficult. I cannot think of a mod or a specific time when it first started.
I don't know the RPM it is but it would be about 4500 - 5000ish. It doesn't happen in any gear apart from 4th (or it is not so pronounced) which would suggest it is gearbox, prop shaft, diff or wheels.
I think I am going to have to drive it more and investigate more fully.
Dave Brown

Agree with the above....what about engine mounts...a broken or worn out rubber or loose can make some vibrations


What happens when you press the clutch when it is happening??
If it imediatly goes away then it is engine or gearbox related.
If it still happens when you coast it is one of the wheel/axle assemblys
Onno Könemann

Not quite - pressing the clutch only eliminates the engine not the gearbox. The separate test is to slip it into neutral when it is vibrating as this will partly eliminate gears.

65 is classic speed for a wheel wobble. I know they have been balanced but may still be worth checking . If you have a spare you could try swapping around to eliminate each one in turn.

Sorry Onno, Not sure how you became Omni, but being short for omnipotent you shouldn't be too offended!
Typing in the dark!

No problem Guy ;)
And you are correct.
I meant to include that but it is to early to have a clear head and write things down completely
Onno Könemann

Hi Dave B. You said,
"It doesn't happen in any gear apart from 4th (or it is not so pronounced---"

If trus, then it's the box.

Take it for a drive yourself, and test it 4th. :)
Lawrence Slater

Thanks for your replies. I will be driving it for the rest of the week if I can and will be trying out the suggestions you have made. I will report back tomorrow morning.
Dave Brown

I had a similar problem, I found it when under the car and looking at the nose of the diff whilst an assistant rotated the prop shaft. you could clearly see the prop/diff flange was out of true.

are you sure the wheels are round?
Dan Cusworth

That is a good point Dan. I think I need to check. The two fronts are new so I hope they are round and not buckled either but I suppose newness is no guarantee nowadays. They replaced one oval wheel and a badly buckled wheel. One of the backs is not perfect, it is slightly buckled. I will check it for roundness. I do have nylatron axle/spring pads, so any out of balance on the back wheels will transmit through to the car as a harsher vibration.
What sort of tolerance should I accept?

Dave Brown

My original Rostyles didn't seem too bad, but when I changed to new alloys it was like night and day!

Spin the wheels (by hand!) when jacked off the ground and check both rims inner & outer anything more than 2mm, weigh them in ( or put on ebay!)

I would still check the nose on the diff.
Dan Cusworth

I brought the car in to work today and found that the vibration creeps up on you at about 60 through your bum and then through the rest of the car. By 70 it has gone. The steering wheel does not wobble at all. When I dipped the clutch there was no change. When I took the car out of gear there was no change.
So I think I can conclude that it is not the front wheels as the steering wheel is rock steady, it is not the engine as dipping the clutch and letting the engine revs drop does not affect it, it is not the input side of the gearbox as putting it in neutral does not affect it and it is unlikely to be the prop shaft as I have changed that and there has been no difference.
So it must be either the output side of the gearbox, the diff/axle/halfshafts/hubs, rear wheel balance, rear wheel out of round, rear wheel out of true or rear tyre mis shaped.
I will check the roundness of the tyres at the weekend or earlier if I can. I think I will also swap the spare wheel round with the rear wheels to see if it makes any difference.
Any other suggestions?
Thanks for you help.
Dave Brown

Sounds like either, rear g/box bearing, prop balancing, front or rear UJs in propshaft, or rear wheel balance/buckle then.

Easy to check prop UJs. just get under and move it about. Most likely the rear if gone.

Wheels, jack up and rotate. You'll have to have them balanced to check that they haven't lost a weight.

Rear g/box bearing means box out to check that I think. no access unfortunately.

But you can pull the prop out to check the front UJ. Just watch the oil.

Lawrence Slater

I am sure the prop shaft is ok. I replaced the UJs on my original one and noticed a small blob of weld on one yolk. I tried that prop shaft again and the vibration was still there so I got hold of another one off of ebay. The new one has good UJs with no play. I fitted that one and it didn't make any difference either.
Dave Brown

Check the inside sidewalls of the rear tyres for slight bulges. I had a similar problem on another car a while back. Even with the wheel off it was not obvious but if you ran your hand round you could just feel it (even the tyre fitter who changed it asked why I was changing a tyre with plenty of meat still on it, so it wasn't obvious) but a new tyre cured it.

T Mason

Both wheels and drive train seem to resonate around that 60-ish speed. To further narrow it, try to work out whether the vibration is at wheel frequency or engine (and therefore propshaft) frequency. Then see if there is any difference between power on/power off/light cruising.

The frequency gives a clear pointer where to look. Power on/off differences usually indicate drive train.

Drive train - a number of possible causes covered above, here's a couple more:

Check the pinion shaft nut (inside the drive flange). We had an elusive vibration like this, drove me mad, rebuilt the propshaft twice, then Arie suggested this could be a cause. It was. I was put off by the fact that every previous encounter I've had with a loose pinion nut has resulted in diff whine, whereas this one didn't.

Check the pinion flange for run-out. It can happen!

Wheels - get the balance checked. Any tyre or other problems will show up while on the machine.
Paul Walbran

I had a similar problem with my MG1300. Changed all tires and the problem was gone. Didn't know it was the tires, changed them because they where worn / not good anymore.
Alex G Matla

I'm with Alex. This is sounding more and more like a faulty rear tire.

Gryf Ketcherside

Meant to add: A simple way to check the wheels is to swap the front and rear tyres and see if the vibration moves.
Paul Walbran

I swapped all wheels in every way, even rotated wheels on the studs. Didn't help. And as it was a front wheel drive I even checked the shafts for straightness in a lathe.
Alex G Matla

Had a bad vibration on a 'B' - turned out to be a rear tyre - had developed an 'egg' shaped lump in the middle of the tread - a nearly new tyre - was quickly changed under warranty !

Was difficult to spot as again loking for mechanical problems.

richard boobier

how old are your tyres?

regardless of tread depth they can go hard and nasty especially with lack of use and sitting for very long periods
N Atkins

Hi Nigel,
Tyres are about a year to 18 months old so not hard and square yet. I will be looking at the tyres for bulges tomorrow.
Dave Brown

right you are Dave

you did buy the tyres brand new, some don't :)

check for true on the car then yeah wheels off for a good look at the tyres all round, unless they have a bad manufacturing fault - very unlikely

unless you can swap the wheels and tyres with another set or on to another car it can be difficult

good luck
N Atkins

Tyre problems of this sort can usually be found by rolling the wheel down the street. Follow along behind it and watch for wriggles squiggles and bumps in the rolling.

FR Millmore

not so easy with all the potholes over here in the roads (pavements?) and pavements (sidewalks?) :)
N Atkins

Here's an idea; jack up the rear axle on axle stands and check that the problem exists with rear wheels off the ground and running at 65 mph.

Assuming problem persists; Now jack up each wheel in turn and run the car at an indicated 32mph, as with one wheel stationary on the ground and the other spinning, the diff gear will double the speed.

Now try each side, to see if you can identify which wheel is the problem. If neither wheel is a problem, then this suggests that the problem is not reoccuring as the prop shaft is only running at an equivalent 32mph not 65mph.

James Eastwood

And you will blow up your diff....
Onno K

Very true Onno, been there done that ... though not with the car in the air, just a K engine on grass :-). Same sort of speed though, lots of dust then a big BANG!
Paul Walbran

My car is smooth on the road, but if I jack up the rear wheels and run it in gear there is a lot of vibration. So from this experience, I don't think that method would necessarily narrow down where the vibration is coming from anyway.

Simple trouble shooting: a try another set of wheels. Borrow a set from someone to test.
Alex G Matla

Could simply be rear wheel balancing.
Lawrence Slater

I think it is a buckled rear wheel. I jacked the back up and ran the car with it in gear and one wheel is very slightly buckled. I removed it and fitted my spare and took it for a run and it was better but as the spare was a bit flat and I didn't have time to pump it up, I didn't want to go that fast. It was a very short test drive too! So not a conclusive test yet but when I get time I will do it again.

Dave Brown

Yeah a buckled wheel could cause it

no need to run with one wheel off the ground to find that out though (I don't like the sound of doing that) taking the wheel off looking at it and rolling it down a road as FRM put would find it

it also reminded you that if you keep a spare then it needs to be kept pumped up to be of use

all said by me with hindsight sitting in the warm :)

it does reinforce checking for the simple first

uhhhhhhm, buckled rear wheel, daughter driving - pothole perhaps

or kerb, bit of a spin perhaps . . .

no, buckled wheel from way when
Nigel Atkins

This thread was discussed between 13/11/2011 and 28/11/2011

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