Club 80-90 Forums

I know this has been done before but cant find on the search

Diesel prop shafts - Doughnuts to the front or rear? Also, anyone know why they are set like this
Cheers all

Doughnut at the front for diesels and rear for petrol, no idea why though but when I put mine on back to front it vibrated really badly
(2.1 petrol)

Nice 1 Andy

It can only really be due to the combined torsional resonance... the inertia of the prop is added to the crank and flywheel before the (decoupling) effect of the doughnut. Think of it like the crankshaft pulley damper - the mass infront of the doughnut (the VC and diff etc) is decoupled by the rubber doughnut.

The diesel defintely has its own unique crank torsional vibration problems... I get the impression that inj pump timing can be involved in this, amongst many other variables.

Then, AAZs can break the No 2 main cap - amongst other things - an indication of what's going on deep inside.

Also, drive some TDs with the prop missing and although there are obviously no prop vibes, there can be very harsh engine vibrations that aren't there with the prop on... indicating its importance in tuning (/detuning) the whole driveline system.

Then there are the few of us with 1.9 DG petrols, no propshaft donut and no harmonic vibration problems either!
It will be interesting to see what happens when it finally gets the ADY with it's 92.8mm stroke...

Thanks for the insight above HM

HarryMann wrote:
Also, drive some TDs with the prop missing and although there are obviously no prop vibes, there can be very harsh engine vibrations that aren't there with the prop on... indicating its importance in tuning (/detuning) the whole driveline system.

Looking through Bentleys give no answer to tuning the driveline as such and I wonder whether anyone has a guide to finding the "sweet spot" in lining everything up?

I notice the aged paint marks on my front diff so it seems there was a procedure at the factory. But there is not enough marking for reference and I've set everything to about midway.

Yes Chilla, might be able to provide some guidance there:-


These go without saying (but worth repeating):-

Minimal/zero wear/play in the UJs at either end, UJs have minimal friction/stiffness & no notchiness
If common (hooke joint) UJs used (not CVJs) then their fixed forks are mounted at 90 degrees either end
Shaft/tube is straight, wall thickness dimensionally consistent, and whole assembly built co-axially & accurately.
Shaft is subsequently (post production) dynamically balanced (including joints )

Alignment:

The joints should run at a small (<5 degrees normally) but non-zero deflection angle (in one plane)
e.g. this implies the shaft should not be aligned throughout from flange to flange co-axially

There are a few basic layouts up cardan shaft systems to minimise vibration.

The one that appears to have been chosen to suit the Syncro, is the equal angles one:

This is where the a projected line from each end (front diff shaft and gearbox shaft) cross when level with the shaft centreline i.e. the angle between the cardan shaft and the drive shaft axis should be the same at either end.

The Syncro apperas to have been set up this way, with the diff and gearbox axes both pointing downwards, when the former is projected backwards and the latter projected forwards.

andisnewsyncro wrote:Doughnut at the front for diesels and rear for petrol, no idea why though but when I put mine on back to front it vibrated really badly
(2.1 petrol)

I found this out, had a lot of viberations the other way, couldnt work it out either