Propshaft Balancing

[SteveW]

Having inspected my propshaft, I'm going to replace both UJs. Obviously the shaft was balanced when originally put on the car (I can see two balance weights on the shaft). Do I need to get it balanced again, or is it safe to assume balanced for once, balanced for life?

For UK members only, what sort of price do companies charge to balance one? There is a company local to me Nottingham, but I like to get more than one quote as prices seem to vary enormously for classic car restoration services depending on whether they really want the work or not.

[i.thomson]

I would not expect to need to re-balance a propshaft after replacement of the U/Js as any balance problems are likely to be down to the U/J wear itself. Just replace them and try it.
Ian Thomson

[Rob Reilly]

Balanced once, balanced for life is a safe assumption, unless you see missing weights or damage to the center tube or yokes.
You could also make your own balance checker. It is just two parallel level steel bars. Your end yokes have a machined centering shape on which the center tube is fitted and welded, so it is safe to assume the center tube is centered, and you can lay it on the parallel bars and see if it tries to find a low point, or rolls easy from end to end of the parallels. Not as precise and accurate as what they have in machine shops of course, but it will tell you if yours it way off.
Here is a picture showing the two parallels. The principle would be the same for a driveshaft.

[Duncan M]

Rob-
Such black arts as this is the beauty of forums like this. Back when, people would take a well running and newer XPAG engine and completely take it apart. Balance engine components with a very accurate scale, or methods for certain parts as above, was carried out -- etc. before re-assembly. Such techniques are often pooh pooed on other forums, but the difference that such common sense balancing can do can be dramatic. If not dramatic in the short term, then in the long term.

[Rob Reilly]

Ha, ha. I hadn't thought of it as a black art. One might call it "low tech" in that it doesn't require a computer or even electricity. That picture is from the Jaguar XK120 Service Manual, and yes they really expected their dealers shops to check the balance on replacement clutches and flywheels. I imagine MG did something similar with theirs.
I once was in a hot rod speed shop and a guy was there balancing his giant slicks on mag wheels in exactly this way.
Getting back to the driveshaft; an out-of-balance part would create a force trying to cause the shaft to jump around, i.e. vibration. The force is related to the first power of the radius of the offset weight, and the second power of the rotational speed. Say if it was a gram on the driveshaft tube surface, it is only at a radius of about an inch or so, not a huge concern compared to say a gram on a flywheel at a 5 inch radius. Two grams on the driveshaft would be twice the vibration of one gram. But in 4th gear it is rotating at engine speed so a vibration at 4000 rpm would be 16 times the vibration at 1000 rpm.
So while you have it apart with the u-joints out, it can't hurt to try this if you can find a couple of straight things that won't sag to set up as level parallels on a table. This test will show if it is grossly out of balance.

[ROGER FURNEAUX]

Thanks for the physics lesson - I have forgotten most of what I learnt over 50 years ago! What I doremember is Henry Stone coming to one of the early Rebuild seminars run by the T-register here in Olde England: he and I spent a happy hour sitting in the sun chatting about everything from his days in the Abingdon racing preparation shop, to mocking up the prototype TF from a TD chassis. Anyway, our discussion covered prop-shafts and he was quite sure that if it was out of balance, at least 500rpm would be lost due to vibration. So when I rebuilt TC0978, I took mine along to a trading estate somewhere in W. London, with the UJs neatly bolted on: the first thing they did of course, was take them off!

Roger Furneaux

[Rob Reilly]

Another thing to be aware of with drive shafts is that the two yokes must be parallel. There should be a pair of alignment marks at the spline connection, usually arrows.

The reason for this is that the standard Spicer or Hooke joint is not a constant velocity joint. The center section of the driveshaft actually speeds up and slows down twice with every revolution, the amount depending on the amount of offset misalignment of the gearbox main shaft relative to the rear axle pinion shaft. The math equations with sines cosines and tangents is rather complicated for this forum, but you can search Hooke joint if interested.

Usually with an MG this is a very small amount because the gearbox and rear axle shafts are reasonably close to parallel, but if the yokes were out of parallel you would get some vibration.

Failure to realize this is a major concern with dragsters and jacked-up off-road trucks which often have the trans shaft and axle shaft way out of parallel and as a result go through a lot of u-joints.