Wobbly props?

Anonymous · Post by **Anonymous** » Tue Mar 29, 05 11:30 pm

make sure your rear trans mount is in good order and not letting the box sag down, (affecting angle) or touch anything it shouldnt

Anonymous · Post by **Anonymous** » Sun Apr 03, 05 2:26 pm

At last I managed to do it! Took me long enough...

Anyway the car is on my drive and its sloping slightly with the rear in a lower position away from the house. I measured firstly on the flange on the UJ and got an angle of 2 deg nose up from the ground. I then measured on the face of the bellhousing and got an angle of 4 deg nose up from the ground. So it seems the engine is sloping down toward the trans by about 4 deg but the axle is sloping upward! This isnt right is it?

Pete · Post by **Pete** » Sun Apr 03, 05 3:25 pm

I thought the car had to be level in order to check this.....

Alex · Post by **Alex** » Sun Apr 03, 05 3:33 pm

the ground has nothing to do with pinion angle ! you are looking at the relationship between the axle and the engine!

Anonymous · Post by **Anonymous** » Sun Apr 03, 05 3:39 pm

Well i didnt think it would matter as long as you compare both of them on the same surface its relative isnt it. I was wrong whe i said the diff was pointing upwards but the engine angle is definately pointing lower then the diff

Alex · Post by **Alex** » Sun Apr 03, 05 3:48 pm

OK, to find the true pinion angle you are looking to measure the difference between the angle of the prop and the angle of the pinion.

Now contrary to what I just posted

you need to find the static angle of the diff, so with the car on a flat surface

measure the angle of the pinion reletive to the ground, ie it will be 87.5 degrees, subtract this from 90 (square to the ground) in this case the pinion would be 2.50 degrees down.

Now measure the angle of the prop and note which way it is leaning, i.e front should be higher than rear.....i.e 1 deg.

Add both together if the fron of the prop is higher than the rear and the diff is nose down as is usual and it gives you the true angle, in this case 3.5 deg

Dave-R · Post by **Dave-R** » Sun Apr 03, 05 8:42 pm

Your measured angles are within spec so that is not your problem. At least if I understand your post correctly?

Your engine is sloping down towards the back 4 degrees yes?

Your pinion is sloping up towards the front by 2 degrees?

Fine. If the pinion was 4 degrees it would be at the same angle to the ground as the engine yes?

Under power it will be because the pinion will rise more under power.

See this drawing. The most important thing is to get the angle of each UJ the same. But taking into account that the pinion also rises slightly under power.

Anonymous · Post by **Anonymous** » Sun Apr 03, 05 9:00 pm

Dave -top diagrams, thanks

To be honest I didnt think it would be this. Im sure DaveMcB would have made sure this angle was right. The car had a recon 727 box from Duncan so im sure that there is no problem with that, and the rear is a 8and 3/4 axle im not sure of the gearing but thats not important. The axle was a recon too I think and the rear springs have all been re-tempered. The only thing on the rear springs is a clamp plate on the front side the help prevent spring distortion under hard acceleration, im sure this isnt anything to do with it. The prop is a reconditioned prop also, and it has a patch welded onto it so it is possible it has been balanced

What could it be then? I think for peace of mind getting the prop balanced wouldnt cause any harm.

Anonymous · Post by **Anonymous** » Mon Apr 04, 05 9:17 am

No im sure about that. This problem has been one the car since before I had it. If you look back at the first part of this post you'll see a link to a topic in the old message board. Im sure its something other than wheels, Ive had them balanced and the problem was the same before and after. The vibration isnt constant its more of a "mmmmm.......mmmmmmm......." etc almost like something is resonating

Dave-R · Post by **Dave-R** » Mon Apr 04, 05 9:36 am

All you can do is get the prop balanced again.

The way to check if the problem is rearward of the gearbox is to note what mph it happens at in top gear (which you have done) and then put it into second gear and see if it happens at the same speed.

If the vibration is still at the same speed it can only be a badly balanced propshaft, a worm UJ, a bent axle shaft, or an badly balanced wheel. I can't think of anything else it could be.

Anonymous · Post by **Anonymous** » Tue Apr 05, 05 10:35 pm

If I put the car into second at that speed it'll probably overrev the engine and shake itself to pieces! Its quite low geared at the axle. thats why i was asking about those overdrives on the other thread.

Is prop balancing expensive? How much should I pay?

Kev · Post by **Kev** » Tue Apr 05, 05 10:41 pm

You got them manly 4.10s in there, Boy!

latil · Post by **latil** » Tue Apr 05, 05 11:04 pm

I'm with the bent 1/2 shaft

'tween centres in a big lathe , dial indicator on the shaft bearing area and the stud flange. Are your engine mounts good?

Matt · Post by **Matt** » Wed Apr 06, 05 10:14 am

To add my 2p worth (repeating a lot of Dave's points in the process)

the term 'Pinion angle' is a misnomer when used in this context.

The actual angle that either the tranny or pinion shaft makes with the prop is not particularly relevant (although both will be equal under the below ideal condition) .

To eliminate angle -related vibration, the centreline of the diff pinion shaft must be parallel to (although not necessarily in line with ) the centreline of the crankshaft / transmission output shaft

Practically, however , if you set both the pinion& trans shafts parallel whilst stationary , leaf spring wind-up at cruising speed (or under drag acceleration) will tilt the pinion shaft upwards , and result in misalignment .

Hence for a road car, at rest, the diff nose should point down maybe 2 degrees relative to the crank centreline. MOre for a drag car - maybe 5-7 degrees. Better to determine the exact angles with experimentation , but you get the picture.

The only angles you need to measure are the angle of the trans output shaft relative to horizontal, and the angle of the pinion shaft relative to horizontal. This will give you their relationships to each other. I would say that measuring the angle the prop sits at is unnecessary & may introduce errors.

As far as driveline vibration in general goes, there's a few more points:

1) The slip yoke, or its bush in the tranny are often very worn. This results in up & down movement , which can off-centre the front end of the prop and cause vibration. This thing is spinning round at 3000-6000 rpm remember.

2) the props in these cars are long by modern standards . Sixties cars weren't designed with modern comfort levels in mind when driving at 70 mph or so. You can only practically balance a prop this long so well, and at a certain rpm . That's why most modern cars / vans have 2-piece props, made of two shorter sections with a centre bearing.

I'm thinking of designing a 2-piece prop conversion as a possible answer, as I'm fed up with not being able to eliminate vibration .