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MG MGB GT V8 Factory Originals Technical - Prop shafts - UJs or CV

From about 80mph the car starts to vibrate quite profoundly. By 90mph it peaks and you tend not to be able to drive out of it - I've taken the car to 115 and didn't want to go any faster. You can feel it through your feet and through the seat. Tends to stop when you're back down to about 70mph. It's definitely not wheel balancing and is definitely linked to road speed not revs. I suspect the prop shaft. I've had it rebalanced but if there is an improvement it is very slight and is probably my imagination.
The prop shaft man was surprised to see that my prop shaft has CV joints on both ends and no slide/telescoping mechanism. It was taken off a Rover and shortened and balanced. It's been suggested that I have a UJ prop shaft made up using the flanges from my Rover prop shaft. The angle from the gearbox through the shaft into the diff is not a straight line but is very shallow.
Any ideas please? Do UJ shafts have it over CV versions?
Marc

Just to show you how Stone Age I am, I didn't even know they made driveshafts with CV joints. You learn something every day. Regular U-joints are what I've always seen. And it seems axiomatic that you need to accommodate the up and down movement of the rear axle.

As someone noted in the other thread, there is a place over there, I think called the V8 Conversion Company, that sells these for about 130 pounds. As I stated in the other thread, heaven knows why my driveshaft appears to have "aged" about 20x as fast as normal. The V8 is not THAT powerful!
Harry

Marc,

Interesting set up: do you have SD1 rear axle and do you know who modified prop and axle.

I have just replaced UJ's on mine but when they were out the way out I noticed the vibration at low speed as well.

As it happens, I also have a vibration felt through the steering wheel at 2700rpm in 5th but disappears either side. This was not helped by the tyre pressures being unequal. Also tyres need replacing so wheels could need balancing so I will see what happens after replacing tyres.

Paul

Paul Wiley

It could still be the wheels. As your prop shaft spins faster, so does your wheels.

On Porsches, most racers have their wheels, on car high speed balanced. It is their consensus that off car balancing is no good. You'll have to find a company who preps race cars.

My geography of GB is horrible, so I have no idea if London is anywhere near Silverstone or Donington Park, but you should find plenty of people that can do it there.

I'm with Harry as well, I've never seen a RWD front engine car with CV's. Maybe it's a Brit thing :)

Mike
Michael Hartwig

Paul,

The shrotening and balancing of the propshaft was done by A1 fabrications in Salisbury. They shortened the exle for me too. The prop shaft balance was done by Dynoprop in Harolds Wood. They do nothing but balance things and make bespoke propshafts. A very professional bunch. Anyway there was nothing much wrong with the propshaft and the rebalance made pretty well no difference. The vibration is at very high speed but very much through the car and feels very much like a propshaft problem as I had a UJ wear out and snap on me years ago. Very similar thorough shaking of the vehicle.

Mike,
When I refer to prop shaft, I am referring to the shaft which spins in the gear tunnel and links the gearbox to the diff. It's a car thing :)
Marc

Marc, the SD1 prop shaft had a sliding joint ,so what happened to yours ? I would think it's essential! An old cure for this type of fault was to undo the propshaft at one end, rotate it 180 degrees & reconnect it.If you want to go for a UJ setup, I suggest you try a Land Rover shaft. They're strong & skinny & have the correct flanges for your diff. & gearbox. it will probably just need lengthening a bit. CV joints were used on a few Jap RWD cars as well as Rover. HTHs Barrie E
Barrie Egerton

Marc,

A1 Fabrications and Norman Burton Enginering appear to be the only places where this conversion was carried out and both are no longer trading, so why CV joints were used is a mystery. Used to live near Harold Wood Essex. I gather the Land Rover Prop is a good option. Tim at Frontline can also do a made to measure Prop for about 100. I gather that vibrations from the drivetrain (UJ's Halfshafts) occur at regular intervals 45mph 90mph but this does not fit your situation and my UJ,s were well and truly stripped by the time I replaced them so may not be a good comparison.

Paul


Paul Wiley

I'm not sure if the same applies to CV joints, but on a U-joint driveshaft, you want to make sure the transmission output shaft is parallel to the pinion gear shaft on the rear. They may be off in height, but they must be parallel. It may be out due to the slight angle of your motor. It can be corrected by shimming the pinion angle of the rear at the leaf spring mount. This all needs to be done at ride height with suspension properly compressed.

Got to this site for more driveline info:
http://www.iedls.com/guide.html
Brian Kraus

This thread was discussed between 13/02/2001 and 15/02/2001

MG MGB GT V8 Factory Originals Technical index

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