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TOPIC: ZB Drive line alignment

ZB Drive line alignment 5 years 9 months ago #155

  • amartyn
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Have had for about 18 months a ZB with MGB 5 bearing engine and Ford 5 speed box installed.

When purchased there was (in my opinion) too much mechanical vibration in the drive train above about 50 mph, noticeable when either driving or coasting.
Further investigation revealed play in one direction of the rear UJ. This was corrected by measuring the end play and splitting the difference between the 2 circlips and fitting suitable thicker clips.
Balancing the rear wheels also helped to reduce the vibration, ideally this should have be done on the car but there doesn't seem to anywhere locally capable of doing this.
The prop shaft was not re-balanced because I couldn't remove it, when the UJ flange was undone the shaft couldn't be withdrawn because the crossbrace under the propshaft tunnel didn't allow the shaft to be lowered enough to clear the flange on the diff to allow removal.

Driving the car was not a problem but the issue continued to play on my mind.
Further investigation revealed that the engine/gearbox assembly was angled down at the back by about 1.2 degrees and the rear axle was angled down at the front by about 2.7 degrees (I have used 'about' because it's not very easy to measure accurately when laid out on the garage floor under the car,( Oh for a lift!).
As engineers know a cardigan shaft must have it's input and output axes parallel for the speed variations and harmonic vibration to be cancelled out, clearly on my car they are not parallel, there is a total misalignment of 3.9 degrees.

The rear springs seem to be distorted although I would expect any distortion to be in the opposite direction and tend to raise the front of the differential rather than lower it if it's due to torque reaction to the driving force.
The springs have not sagged, the car ride height seems to be fine.
The rear springs are not quite like the workshop manual or parts suppliers shows them, the leaf clips are not bolted over the top of the top leaf, the ones that haven't rusted away are just folded over and there are no corner rubber pieces fitted, are they the correct springs?

I think if the differential was level or, angled slightly upward (ie parallel to the engine gearbox axis) I would have been able to remove the propshaft for re-balancing.

I have to rebuild the rear end this winter, the spring bushes are showing their age, and am looking for guidance from other members who may have come across similar issues.

How much is this extent of miss-alignment likely to cause driveline vibration?
Does any body have the full spec for the rear springs so I can send them away to be re-tempered and re-shaped?
Can anyone recommend a good spring refurbishment company?
The NTG ones at £240 each will make it an expensive job.
I can correct it with wedges between the axle and the spring (maybe with longer U bolts), has anybody any experience of this?
Any other bright ideas?

Am I just being too pedantic??
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ZB Drive line alignment 5 years 9 months ago #158

  • JONES21
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There is a very good article on the Read and Collect section of this website under Mag articles which answers most of your questions.

Lou Shorten does new springs I think, or find a pair of secondhand items on ebay they do come up quite often, the alignment issue you describe may well be the cause or at least part of the cause as the propshaft will in effect try to wind itself up to a small degree.

You may find that if the springs are knackered, the leaf springs are ridged where the leaves have come in contact with its neighbour this is a common problem and will have a detrimental effect on the ride quality.

Regards
Trevor
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ZB Drive line alignment 5 years 9 months ago #160

  • JONES21
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amartyn

Having just returned from the 60th at Wroxall ( Just a stunning event) I spent sometime talking to Andy Dear who has what appears to be a similar problem to you, looking at Andy's engine it seems to be too high at the front, so that the whole drive line is set at a more acute angle, This showed itself on Andy's car clearly because the clearance between the top of the gearbox and the transmission tunnel is 2 - 3mm . The problem seems to be that the front engine mount plate location holes on the 1800 engine are a similar but is in fact 10mm or so lower so when the Magnette mounting brackets are fitted this pushes the engine skywards affecting the alignment. As we had 40+ cars to compare it became fairly evident that this may be the cause of the vibration due the screwed drive alignment. I have just now checked the situation on my 1500 engine with the 1800 I have and sure enough the distance from the sump gasket to the front plate mounting brackets on both engines are 10 to 12 mm different, the 1800 being lower.

Worth a look I think, easy mistake to make I guess unless you compare both plates side by side, and very easy to rectify, just make a new angle bracket and drill the holes 10/12 mm higher to allow the alignment to be corrected
Trevor
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ZB Drive line alignment 5 years 9 months ago #161

  • amartyn
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That's very interesting Trevor, it could also be compounded by the spacers under the back gearbox cross member, mine has about 5/8" in there, to give clearance under the tunnel.
Have spoken to a spring manufacturer/refurbisher who judging by the pics and dimensions I gave him seems to think that there is not a lot wrong with my springs.
He has suggested that I slacken off the U bolts and make certain that the springs are correctly located to the axle by the center bolt and that the rubber pads are sound. He seems to think I should be able to lift the front of the diff by re-seating the axle on the springs.
This issue has some way to run yet!
Thanks for your interest.
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ZB Drive line alignment 5 years 9 months ago #163

  • clacksman
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As amartyn says, this will go on and on. I have made a crude comparison between the engine mount positions on my old 1489cc Magnette engine and the new MGB 1800 3-main and there seems to be no difference! Pic No.1 shows the old engine

SX220-HS-13-Aug-13-IMG_5434.jpg


with a piece of plastic strip sitting on the upper engine mount nut and leaning against the front manifold stud the position of which I have marked with a black marker. It is crude but shows what I mean.

Pic No.2 shows the same strip on the MGB 3-main.

SX220-HS-13-Aug-13-IMG_5435.jpg


I did not switch the front engine plates when I adapted the 3-main for the Magnette as they appeared identical. Of course I don't know the origin of either engine plate but they do look identical. Not sure what the next step should be. Perhaps as Trevor suggests I should get a pair of shorter engine mounts made up. Need to check clearance between the sump and cross-member first. Can anyone tell me what this should be?

Incidentally, we had an uneventful trip home with less vibration than on the southward leg but a bit of a rattle from the transmission on the overrun. Have not had time to check on this yet.

Andy
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ZB Drive line alignment 5 years 9 months ago #164

  • gruntie
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A few years ago I also had propshaft issues, when I originally got the car. It gave all the symptoms of a knackered u/j, such that the previous owner gave me a spare, new, item to fit when the problem was pointed out to him.
The u/js were fine however: like Martyn I then went down the road of drive train alignment etc (I did get someone tell me that diffs can be straightened by wedge-shaped shims?), but then the light dawned.
Having an overdrive gearbox the propshaft had a u/j at each end, with a splined joint in between them. Someone had obviously had this apart and then reassembled it out of phase so that the u/js amplified the oscillation instead of cancelling each other out.
Attempted balancing also revealed a (new) propshaft shaped like a banana, so a complete replacement was called for. With these 2 items fixed the transformation was complete. It didn't do much for my opinion of previous owners, though.
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