Oscillation trepidation !

[Tony Griffiths]

I had a somewhat similar problem on a Mk.1 Discovery. It was only induced below 20 mph when braking gently on a slight downslope when the car would lurch to the left. I never found out what caused it, as the car was stolen and replaced by another Discovery that didn't do it.

[andrew34ruby]

I'm surprised there is no mention of tyre pressures. I would expect tyres over inflated to allow the steering to flick side to side easily so making the shimmy more likely to happen. Soft tyres might make the steering 'heavy' so making the shimmy less likely to last more than a second or too. At the moment the new Blockley tyres are at 25 psi.

[AustinWood]

Several 30s cars suffered from axle tramp. This is when the front springs hit a natural freqenxy of vibration and start to oscillate violently.
A friend from University had Rover 14s & 16s that used to do this at high speed. I had a 1938 Light Straight-6 Daimler that tended to do this at about 65-70 mph. It was initated by hitting a slight bump and that would set the front end oscillating violently. Each front wheel would bounce up and down in opposition. One would go up when the other went down. The whole car shook violemtly.
The only way to stop it was to decelerate and let it sort itself out. Braking made it worse.
In the case of these cars it was caused by lack of rigidity in the chassis because there could be no bracing round the engine. I think it likely that was one of the reasons that cross-braced chassis came into use.
The Rovers and the Daimlers had harmonic bumpers. These have a horizontal flat spring from side to side with the vertical bumper bar in front. At each end is a cylindrical section with lead weights inside. This spring-mass had a natural frequency designed to vibrate so as to counteract the tramping.
Examples can de seen here:
https://forums.aaca.org/topic/373028-wil...n-bumpers/
It was also essential to have good shock absorbers.

[john williams]

Check the security of the large steering arm nuts where they locate in the axle beam.

[andrew34ruby]

Over the past 4 years I have driven the seven about four thousand miles or more with no hint of a shimmy. Most of the items mentioned were then replaced or disturbed after the crash in March this year, but since the repair it was fine for over 300 miles, then two episodes of fairly violent shimmy within a couple of miles.

So where to start? I'm inclined to try one thing at a time and test drive to see, before moving onto the next item. Having examined carefully, nothing seems loose, but these are the ideas mentioned above by all the helpful people commenting.

Tracking?
Anything with slight wear?
Steering eye loose?
Loose shock absorber mountings?
Loose king pins, axle eyes, cotters?
Loose U-bolts on the spring?
Twisted axle beam affecting castor?

Then I could experiment with tyre pressures?

If it still persists I could try increasing the castor angle.
Looking in the Austin seven companion ...
Fig 3 on page 181 shows an interesting idea to increase castor angle.
The third paragraph on page 180 condemns this method.
Towards the end of the third paragraph on page 182 is another method of increasing castor angle by lowering the position of the ball that attaches the radius arms to the chassis.

This could take months! Especially as I don't want to test-drive the car on the road now, due to heavy salting by our new North Yorkshire Council.

[stuartu]

Andrew, I feel obliged to point out that of the responses on this thread five people have experienced this when there has been some sort of inadequacy in the drag link internals. If you count me as well that makes six. I would start there!

Regards,

Stuart

[Hugh Barnes]

Agreed, Stuart!

However you approach it Andrew, only change one thing at a time and then test. Only then will you have an understanding of what the problem was and not introduce something else at the same time…

If you have to wait until next spring to do this, then so be it..

Best of luck…

[dickie65]

With ref to tyre pressures I have run my Blockleys at 40 PSI with no shimmy on the road and at Wiscombe Hillclimb.

[Robert Leigh]

Andrew, you finally mention the crash, which some of us were unaware of. I think it is important to check all parts of the suspension which were damaged or dismantled and re-assembled during repair. I suggest that something has changed slightly and finally loosened a bit after the 300 miles. Have you done the suggested check?

[Tony Press]

"With ref to tyre pressures I have run my Blockleys at 40 PSI with no shimmy on the road and at Wiscombe Hillclimb."

Goodness- what is so different about Blockleys ?