Pinion driveshaft angle

[Roger King]

Any of the engineers ever had any thoughts on pinion driveshaft angle of a Jag-axled car?
I ask because I'm currently addressing this problem on my Mustang, which has a lot of hefty US competition bits on it which I am only now getting around to fine-tuning. This website:

http://www.vibratesoftware.com/html_hel ... ifications

...gives the acceptable spec. Having sorted the Mustang, I thought I'd check the Cobra - and wasn't altogether surprised when I found it out of spec.
The trans angle is great, about +3.0°, with the propshaft at +4.0°. This is OK. The pinion angle, though, was at -1.0°. Coupled to the prop angle, this gives a rear working angle that is well out of spec.
I do have a marked vibration at around 80 which is not wheel-balance related, and am now wondering if it's down to this.
I've had a good look at the mounting frames and brackets for the Jag rear and to alter the angle looks like being a seriously big job. Simply removing the diff without removing the body looks pretty tricky, but shimming the framework to alter the angle by, say, 4.0° upwards is daunting to say the least.
Any thoughts?

[allan horsfall]

Hi Roger,
Correct me if Im wrong but I thought the idea was to get the trans and pinion parallel to each other, so that the prop was forming the same angle at both ends.
As with a Jag everything hangs off the Diff, if you alter its mounting angle the suspension geometry and everything else will be be thrown out.
From the figures youve quoted wouldnt it be easier just to raise your gearbox mounting by a degree?

[Roger King]

Hi Roger,
Correct me if Im wrong but I thought the idea was to get the trans and pinion parallel to each other, so that the prop was forming the same angle at both ends.
As with a Jag everything hangs off the Diff, if you alter its mounting angle the suspension geometry and everything else will be be thrown out.
From the figures youve quoted wouldnt it be easier just to raise your gearbox mounting by a degree?

You are not wrong. But, starting from the back and moving forwards, the diff points DOWN 1°from horizontal. The gearbox points UP 3°. The prop links the two, obviously. I can't tip the entire engine the other way by 4°!!
Gerry's kit mounts it this way, so they must all be the same.

[allan horsfall]

Roger,

Surely you only have to tip the engine down by 2 Degrees to get them parallel ? Could only be few mm on the mounting height.

[Roger King]

Roger,

Surely you only have to tip the engine down by 2 Degrees to get them parallel ? Could only be few mm on the mounting height.

They are both pointing downwards facing each other - one by 3.0°, the other by 1.0°! There is a 'dip' in the middle, so to speak! Draw it on a bit of paper and it all becomes clear...

[Karl]

As I just have the center tunnel off, I could make some pictures. You can see the driveshaft going to the left in the picture from top and you can see it going down on the horizontal picture. Hope this helps to clarify the situation.

[Roger King]

Thanks, Karl - the question is, at what angles? (...assuming you have standard Hawk engine mounts, a small-block Ford and a Jag rear diff unit).

[Karl]

Roger, I only have a digital level. I could only measure the gearbox, which is 2,7 deg. up. I cannot measure the drive shaft, because the safety loop is in the way and for the diff the exhaust system is in the way. I have to wait until the car is on the lift next time and the exhaust system is off. (Should be happening some time in March)

[Roger King]

You only need to access the universal joint caps. If you have a look at the link in my first post you'll see how to measure - it's extremely accurate if you have a decent gauge.

[James Baxter]

How interesting! How about alignment in the other plane? The photos above appear to show that the axis of the gearbox is displaced sideways to the differential (in a vertical plane, as well as horizontal). Does the same theory (are we talking carden shafts here?) apply?

I always thought that the two axes must be parallel (not necessarily in-line) to each other in order to absorb less power.