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MG Midget and Sprite Technical - drive shaft vibration

My driveshaft vibrates from 2800 to 3700 rpm: I can see it in my rearview mirror and feel it on my gear lever. It's not very much, but it's annoying, especially at night. The axle is balanced. I've tried the differential connection in all positions. Could it be that the engine and differential need to be aligned? This can be achieved by adjusting the upper control arm.

Flip
Flip Brühl

Hi

Assuming all the UJ's are in good condition, Have you had the propshaft balanced?
Bob Beaumont

Flip,

I would be checking the UJs first and if they're in good condition taking it to someone that can dynamically balance it as would have been done at the factory. They're balanced as a separate unit and orientation on the final drive pinion flange shouldn't have any effect on the balance.

"This can be achieved by adjusting the upper control arm". As it's a frogeye do you have adjustable length radius arms? Ideally on a drive shaft with Hooke's joints the input and output are parallel or close to it for homo kinetic drive.
David Billington

Thanks for the suggestions. The driveshaft is balanced. I measured my frogeye's drivetrain. The measured angles aren't ideal, as they're over 3 degrees. Front driveshaft 4.45 degrees, rear driveshaft - 3,7 degrees. My conclusion is: The engine needs to be a bit higher at the front and/or a bit lower at the rear. Does anyone have experience with this? How can I achieve this?


Flip Brühl

If you have a standard rib case gearbox it is difficult to lower the back. If it's a modified box like a T9, there may be some scope for adjusting the mounting.
You could raise the front with packers under the mountings, but with a frogeye you need to be careful about clearance to the bonnet where it slopes down at the front.

Why do you think these angles matter? Shouldn't the prop shaft joints be taking care of that?
GuyW

The difference between the two angles at either end of the propshaft is only 0.75°. I wouldn't have thought that was significant.
Dave O'Neill 2

Resonances are indeed annoying and difficult to track down. I'd be tempted to try a different prop shaft.

As an alternative, could you rotate the rear axle slightly to restore parallelism, perhaps some tapered shims between it and the spring mounts?

[edit...frogeye, quarter elliptic, not so easy - do adjustable radius arms exist?]
AdrianR

Yes a different propshaft would be a relatively easy swap. The standard rear radius arms on a frogeye are not adjustable
Bob Beaumont

Does the sliding joint at the rear of the gearbox move freely in and out? The axle pushes the prop shaft back and forwards as the rear suspension moves. If its binding there it could cause a vibration relating to particular engine revs.
GuyW

Excuse my daft background questions - have you checked the condition and torque of the bolts on both ends of the driveshaft as well as the wheel nuts? Are the rear hub nuts secure? Also are the engine and gearbox mounts secure and in good condition?

Cheers
Mike
M Wood

Yes, all the bolts are tight. I can borrow a balanced axle from a friend. I can try that first. It'll take a while before I can swap the axle. I could also have the current axle rebalanced. I'm also considering a T9 gearbox. The ribcase has >100,000 KM. the synchromesh is non existent. I have a 1275 engine with over 80 hp.
Flip Brühl

If I were doing another t9 installation I would seriously consider using the support bracket that Alan was, I think, trialling a few months ago. It reportedly avoided the need to cut out the centre of the cross member. It would be good to get an update on how that worked out.
GuyW

Hi Flip.

You were asked about the UJs. You haven't replied. How old are they?

Also, you say, "The ribcase has >100,000 KM "

There's a good chance the vibration could be coming from there too.


Yep it would be interesting to know how that T9 modification and support bracket has worked out. It looked really good.

I can't remember which thread that was in. Alan?

anamnesis

Hi Flip
Not sure what axle ratio you're using but at those revs, if it's only in 4th gear then it's getting into the area where rear wheel balance comes into play. Is it a drumming type vibration(driveline) or a thumping/slower vibration(wheels) If you can see it in your mirror the tailshaft vib. just makes the pic go blurry where wheel balance is more of a shakey mirror.
What speed on the road is this happening.

willy
William Revit

I had a similar vibration on our Frogeye a good few years ago. I tried everything except changing the diff, because I didn't have any decent diffs to substitute. In the end I just changed the diff with one of the others I had, even though I knew they were a bit dodgy.

The outcome was the vibration disappeared although a different problem became evident.

A recon diff was eventually purchased and then all was good.

Rob
MG Moneypit

Just a thought, could your pinion nut have come loose and lost preload, is there any up/down play at the flange at all---
William Revit

Yep, that would do it. We had the collapsible spacer fail in two of our cars (K midget and 1440cc A series) from the loads in motorkhanas/autotests. Finding the first one was a mission - overhauled and balanced the prop twice, no improvement. Which was hardly surprising because there didn't appear to be anything wrong. Eventually found a little play just as Wille describes.

With that clue under our belts we found the second one straight away.

Curious thing was no telltale whine from the diff. Even more curious was that despite odd wear marks on the crown wheel due to the trauma, when set up again "temporarily" it remained quiet and so lasted for years.

We now convert to solid spacer for any car which will be used vigorously.
Paul Walbran

I'm replying a bit late because I was sick with a serious skin infection. I checked to see if it also occurred in gears other than 4th: Yes, it's rpm-dependent and occurs most severely with the 3.9 differential around 2900 rpm. Pressing the clutch makes it disappear immediately. My conclusion now is that it's the engine... The engine and the lightened flywheel are balanced, but the clutch isn't.
Flip
Flip Brühl

"Pressing the clutch makes it disappear immediately"
Not the clutch thrust release bearing?
Jeremy MkIII

possible, there is less noise when I press the clutch
Flip
Flip Brühl

Nah, the thrust bearing/pad is doing nothing driving along. If that's dead it will be noisy when depressing the pedal
I'd be taking the fan belt off and start it up see if the vibration is still there or not, it could be a dead or dying water pump or generator
Does your motor run a vibration damper front pulley--if so does it spin true-no rubber dislodged
William Revit

Thank you for the suggestions. The vibration damper is very very old.
Flip Brühl

I've driven a 1275 with a knackered TV damper and the problem RPM area is around 5500RPM for a standard engine, IIRC it's mentioned in Vizard, the issue was quite a noticeable rumble as it went through the range but as I was aware of the issue I didn't stay there and kept it above or below the problem area. Maybe marking the TV damper ring and centre with paint will give an indication of whether the rubber bonding has failed and the outer ring is moving relative to the centre.
David Billington

This thread was discussed between 02/12/2025 and 23/12/2025

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