Prop shaft alignment and velocity

[ECR]

I found This video to be a very good illustration of potential propshaft installation problems. It's worth a watch.

It strikes me that raising or lowering the engine by means of the engine mounts only for instance, causes the input and output flanges to become non parallel  (asuming the oe installation was parallel in the first place!) and induces one of the conditions shown . Im sure it's not to any great degree and as no problems have been reported, not significant.

Interesting though

[ScottR400D]

This was discussed at length last year in one of the diff threads. That exact video was posted. 

As you say, an important point is whether or not the installation is properly aligned in the first place. In my case, and others, it isn't. 

As an example the duratec points downwards at the front to clear the hood. This probably isn't enough to cause an issue, it's about 1 degree, but in the cases measured it seems that the diff points down at the front even more, about 2.7 degrees or so in those measured. So the inclination of the engine actually aids the alignment which could be worse where engines are not tilted!

As you say though, changing the angle of the engine could affect the alignment and the respective attitudes of the engine and diff need to be considered. 
 

 

[ECR]

I missed it last year...,

a simple to understand video .....

[ScottR400D]

Yes, it's very instructive! As far as I know, the angles we might get 'normally' on a 7 won't do any harm but if you raised the front of an engine that was already 2-3 degrees out of line, perhaps there could be an issue. 

Ideally you'd alter the diff angle to suit the engine but as far as I can see it's not possible. 
 

[Wrightpayne]

What is the difference with an alignment issue of a fixed diff/gearbox and a constantly moving prop of live axle? 
 

I would have thought a little mis-alignment of a fixed diff would have less consequence than an Ital axle bouncing up and down!

[ScottR400D]

Well at least the live axle would be right some of the time!! 

I guess it's one reason the designers try to get the rear axle moving in a straight line which would maintain the alignment but in truth it probably doesn't make much different unless the angle gets significant. 

There's a fair bit of stuff on the www about the issue, with acceptable angles for various rotational speeds. IIRC, up to our sort of speeds, a few degrees is acceptable which confirms our general experience I guess. 

Having said that it really shouldn't be that hard to get it right with a fixed diff. 

[aerobod - near CYYC]

With the current Caterham BMW diff install, it may not be possible to get it exactly aligned without some chassis changes. The rear cover is extremely constrained by the deDion tube clearance, with the carrier and top of diff millimetre positioned to clear the boot floor without being too high, yet ensure driveshaft movement on full suspension articulation doesn't cause suspension contact if the diff is too low. The front of the carrier also has to clear the tubes behind the propshaft tunnel, but the mounting holes and shape of the carrier are defined by the nose of the diff and it's mounting hole positions. The propshaft also has to be in the correct position to clear the tubes in the tunnel.

I'm not sure if 15mm or so diff nose up to get to the perfect alignment is possible by changing the shape of the front of the carrier and moving the front mounting holes up on it, without causing other clearance issues.

[nickh7]

A well known 7 "expert" has had loads of issues with propshafts ended up spending a lot of money on solving it . Perhaps this video might have saved him the trouble

[Tom_Arundel]

As a matter if interest....when I was involved in dyno testing F1 engines using uj propshafts they had to be out of line or the needle rollers in the joints would hammer the same spot on the bearing and self destruct in a very short time.....prop shafts should be run out of line to avoid this......here endeth the lesson 

[Shortshift]

"...prop shafts should be run out of line to avoid this..."

Indeed, but the second important point is that the end faces should have a common alignment (orthogonally aligned??), even  though the shaft itself runs at an angle between the end joints.  Hence the need to get the engine and diff (in this example) mounted as closely as possible to the same x / y / z inclinations, and the joint mounting faces parallel to each other in three planes, even if there is a physical offset between them.

Hard to describe in words, though I know what I mean!

James

[ScottR400D]

I understand that James and you're right. The flanges of the prop should, ideally, be parallel, though offset. 

[Shortshift]

There you go: explained perfectly in one short sentence - 'the flanges of the prop should, ideally, be parallel, though offset'.

Well out, that man.

James