This is related to my '95 6 liter swap into my '74 XJ12.

In my car, I reused the stock 2 piece driveshaft and fabbed up a new support for the carrier bearing. Using a digital angle finder, the front driveshaft is 175.5 degrees and the rear shaft is 175.6/7 degrees.

On hard acceleration it thumps briefly.

So, before I start chasing tenths of a degree all over the earth, what is the advantage of a 1 piece drive shaft versus 2 piece? I feel like nixing the center bearing and support (which I fabbed ) is a good step but there must be a reason Jaguar used two piece driveshafts right?

If this was a stock car then I'd simply recreate what Jaguar did, but it's not soooo...

 

I'll start with a quick lesson on driveshaft angles. You need the joints on a shaft to ideally be within 0.5° of each other, have at least 0.5° of angle so the rollers in the U joints can move, and the true angle no greater than 3 degrees total. Key word here is true, as you need to know the compound angle, or True Operating angle. That is the net angle in two planes, looking down front the top of the car and looking from the side. It sounds like you have only measured in one plane.

The flange of the differential is not on the centreline of the car when viewed from the top, so if the transmission output flange and the diff flange were the same height from the ground, the U joints would still have an angle across them, as the transmission should be on the centreline of the car, and the diff isn't. You need to calculate the true angle taking into account the angles in the two planes that are at 90° to each other. There is a diagram and the formula in some Spicer guidebooks that gives the formula and a diagram. I can't seem to attach it, but the document filename is J3311-1-DSSP. This link seems to work: https://www.waterousco.com/media/wys...311-1-DSSP.pdf

I don't believe it is possible to bring the drivshaft U joints within the tolerances needed with a single piece shaft. With a 2 piece, you can split the angle across the 3 joints and make them all equal, with a single piece shaft you're stuck with whatever it ends up being, and you have no adjustment, other than moving the engine and transmission.

Second advantage of the 2 piece, is each piece is shorter, so that also means that the critical speed is higher with a shorter shaft. Once result of that is you can make the shafts smaller diameter; a single piece needs to be bigger to be stiffer the longer it gets. Short shafts increase the top speed the car can be driven before you encounter driveshaft whipping.

So if you're having clearance issues, then a single piece should make things worse, not better. You can't move it around, and it needs to be a larger diameter than a 2 piece. Maybe take the car to a driveline specialist and have them take a look at it? Maybe see if you can get away with a smaller tube diameter than you currently have to give clearance?

Second thought: Is the centre bearing carrier in perfect condition? There is a rubber part to absorb vibration, and if that is torn or gone soft it can allow the shaft to move excessively.

 

Great explanation Jagboi, many thanks.
Andy
Quite a few guys on this and the XJS forum have had thump problems when uppping the power of the drivetrain. The solution has been to brace the rear axle somewhat, but this too is a specialist area and requires careful design and construction if the rear axle cage itelf, rather than its rubber bushes, is not to become the bending item!

 

Still reading and processing above info, very informative and makes me appreciate this forum!

The drive shaft was rebuilt with new u joints by a well regarded local shop. the center support bearing is new.

And i should use a different word other than "thump" to describe the sound, its not the sound of a worn out center bearing or the drive shaft hitting, just a heavy vibration that is present from a stop in 1st gear.

The other clue that made me think it was driveshaft angle related was that the sound changed (got better) when I shimmed the center support bearing mounting plate to make the front and rear shaft more parallel with each other (previously the rear shaft was about 1.5 degrees off from front shaft, when looking from the side). With this change, the noise/vibe became less pronounced and required much more throttle to reproduce. So it seemed like I was heading in the right direction. But now they are almost perfectly parallel so doesnt seem like there much left to adjust. But for sale of argument, i'll make them dead nuts perfect parallel and report back.

 

Sounds like an adjustment issue. I'd start by calculating your compound angle across all 3 joints and see how close they are. Then you might have a better idea of which way (up-down or left-right) to adjust your centre bearing carrier.

 

Hello Bullit, How did you check the side to side angles of the two tubes of the drive shaft? May I suggest four plumb bobs, Two on each shaft. Long sheet of paper on floor. Look to see if the two lines are running exactly even with each other. Even with each other even and in line, Loosen and push drive shaft to the right to counteract twist.

Larry Louton