Morris Minor Owners Club

A complete description of the vibration will help us.
When does it happen and what affects it?

this will be changing the effective difference in height between the output of the gearbox and the input to the diff

It's not the difference in heights that causes vibration. The key requirement is that the output from the gearbox and the input to the rear axle are in the same planes, both vertically and horizontally. Good driveline design of the engine/gearbox mounts and the rear axle mounts creates the correct alignment, and so determines whether this works or not.

The small angle that the engine/gearbox drops towards the back of the car is matched by the same angle that the rear axle is 'kicked up'. The vertical rear spring movement does not change the rear axle angle under normal loads, other than by possibly a minute amount (excluding 'power on', which can twist the springs).

The design means that for a constant output speed from the gearbox, there will be a constant input speed into the rear axle - the prop shaft tube in between the 2 UJ's will speed up and slow down through each 180° rotation. At small height differences this does not create noticeable vibration.

Unless there is an extreme difference in heights, which would cause a significant angle change through the UJ's and thus very high levels of acceleration/deceleration of the main part of the prop shaft, the result will be a smooth driveline (assuming the prop shaft is balanced, which they are as standard).

To revive an old post...

After the rebuild of my 1960 saloon, I am in the process of diagnosing one item that is still a problem: vibration above 40MPH...

brief history of what I think is relevant:

-When I purchased the car there was a similar rumble; since then:
-new engine & gearbox mounts;
-new UJ's and propshaft has been balanced;
-new tyres fitted & balanced;
-rear axle is seated properly on the rear springs;
-vibration is road speed not engine speed dependent;
-vibration frequency seems harsher and higher than one would expect from wheels & tyre imbalance making me suspect the propshaft;
-Vibration is worse on overrun;
-no vibration with propshaft off, gear in top & taking engine up to 60 MPH;

-At present I have taken the propshaft back to have the balance rechecked. Will know tomorrow what the result is. Next step will then to repeat the 60MPH test with wheels & drums off, with propshaft on;
-I have made rudimentary measurements of the diff flange vertical angle compared to the gearbox output shaft (using my iPhone as an inclinometer... ) and end up with an approximately 6 degree difference. This seems too much?
-the diff nose is probably pointing down by about 3 deg.and the output shaft down by the same amount.

I will try and get more accurate measurements but my questions are:

1. By how much can the input and output shaft differ (in degrees) so vibration is not noticeable?
2. It was suggested to me that 1 degree should be the target, in which case I either have to lift the diff nose up by 6deg or lift the gearbox output shaft or both;
3. If this is the need, do I achieve this by inserting wedges at the rear between the axle and springs and/or spacers on the gearbox mounts?

Many thanks.

Erwin

If the transmission output shaft is 3º below horizontal, then the differential input shaft should be 3º above horizontal so that the 2 shafts are parallel. They used to make caster shims (tapered) for adjusting axle angles but I don't know if they're available these days.

Thanks for the reply Katy, that's useful to know. I will get more accurate measurements then get some tapers machined to the correct angle.

OK, by way of update... I got the propshaft back supposedly with the balance only "checked"... when I got home I thought I noticed different weights welded as balances... so thought hmmm... let's just replace the shaft & see how she drives?

Well, I'm delighted to report that without any other changes she now runs as smooth as silk, it's chalk & cheese compared to what I had. Happily cruising at 60MPH when previously I could feel vibrations at 40.

So in spite of the input/output angle difference of 6 degrees, with the shaft properly balanced she runs fine...

Erwin

Well done !