Today some real progress on the vibration. Rather than wait for the rolling road I decided to jack the car up on four axle stands and run it to observe what is going on. I was a bit nervous about running it this way but I needn’t have been, it worked perfectly and did not feel in any way unsafe.
I could use the hand throttle to run the engine through the range of revs in top gear and see exactly what was going on. The vibration followed exactly the pattern I’d found on the road, coming in at 2600 up to 3000 rpm and continuing as the transmission slowed down when I knocked it out of gear and let the engine slow so, as I thought, is unrelated to the engine, clutch or gearbox. The propshaft runs absolutely true as do the new Hardy discs but the off side rear wheel has a wobble.
I had previously checked that the tyres run concentric to the hub and had the wheels balanced when the new tyres were fitted. but had not found a good way of seeing whether there was any swash-plate like wobble. Running jacked up like this, a wobble was very obvious. I then swapped wheels around which showed that the problem is with the wheel not the hub or half shaft. The offending wheel was demoted to spare and with a reasonably dry road I had a test run which showed a significant improvement, with far less vibration. In fact on country roads at 45-50 mph it is barely noticeable and even at 55-60 not intrusive. Freed of this dominating vibration, I can now feel that the engine itself is free of any major vibration period and hope that the remaining lesser vibration is due to smaller amounts of wobble in the other wheels.
The problem now is to see whether the wobble can be rectified which will mean finding a way of measuring it and putting it right. The wheels are made of fairly thin gauge steel and I suspect easily distorted by kerbing or a blowout, indeed some of the rims show signs of this. The wobble however may be caused by distortion of the supposedly flat inner face which the wheel studs clamp tightly to the hub/brakedrum. The offending wheel can be slightly rocked when laid on a flat surface. I can imagine that careless tightening of the wheel nuts as the wheel is fitted could be responsible. I think I can find a way of truing this mating surface relative to the main pressing of the wheel, either by pressure or a light skim, although there is very little thickness of metal to play with.
The Augusta has those very attractive pressed steel wheels of the style which used to be called “Easyclean”. They also are unfortunately of the Michelin rimmed variety, a style introduced to get around patents on the "Well Based rim". The Michelin wheel has a well to assist fitting of the tyre but the well only extends around 60% of the rim which introduces an imbalance, corrected by the factory by three pieces of steel welded to the inner face of the wheel spokes or as on one of my wheel with weights fixed by three studs. I hope that the minor dints in the rims and the alignment of the rims with the hub turn out to be of less consequence as welding new rims to the spokes would be very difficult as the spokes of the Michelin rim vary in radial length to fit the strange profile of the rim.
There is a small possibility that the wheels could be dynamically balanced to minimise the effects of the wobble. When the new tyres were fitted the balance weights which are plentiful(!) were all fitted to the inner side of the wheel so as to invisible. It is common practice when balancing wheels to put balance weights both inside the centre line of the rim and to the outside to eliminate wobble. I shall speak to Longstone for their expertise.
So I think I know the problem, I just have to find how to sort it out. In the meantime though the car is now perfectly usable, very pleasant and great fun just as I hoped when I chose it.
Mike
