Steering imbalance.

[AAC567X]

Driving along normally at about 40 - 45 mph. Suddenly the front end starts shimmying like an epileptic belly-dancer. After 15 or 20 seconds, everything settles down again and it drives perfectly for the next half an hour. Repeat at irregular intervals.

1) Tyres are worn to slightly different diameters and sometimes come into destructive resonance.
2) One or both front tyres have lost balance weights.
3) Loose nuts.
4) Worn bearings.
5) Worn steering mechanism.
6) Other cause?

[Beans]

1 - could be a possibility especially when it occurs in long bends (resonance starts, gets worse and slowly fades away). Balancing the wheels should stop it;
2 - possible, see above;
3 - hope not   but in that case it would be there more or less constantly. And you'll feel it in the steering and hear the wheel knocking (when you are lucky);
4 - could be part of the problem, and easy to check;
5 - see 4 and check for excessive free play in the steering
6 - yes to start with check all parts of the front suspension for wear, especially steering ball joints;
7 - a tricky one to check; weak springs (or broken) in combination with worn shock absorbers;
8 - check the tracking of the car is within the factory limits, as the TR7 is rather sensitive here;

• Camber angle should be -0,25° ±1°. If outside factory spec rather difficult to adjust. Only way to adjust is by moving the top mounts, and not much (if any) adjustment there;
  
• Castor angle should be +3,5° ±1°. Not easy to adjust as you have to loosen the front ARB and move it left or right slightly to get the angles even;
  
• Toe in should be between 0 - 1,5 mm, aim for 0,5 mm. No toe out!
  

[AAC567X]

Quoted from Beans

<lots of very helpful suggestions>

Thanks, as always, Theo.

[Beans]

Good luck with finding the culprit 😉

[AAC567X]

Quoted from Beans

Good luck with finding the culprit 😉

Heard the terrible news about Martin when I took the 7 in for new track control arms, ball joints and bushes today.

[kelvin45]

I would start with wheel balancing first and see if that resolves it.

[AAC567X]

Quoted from kelvin45

I would start with wheel balancing first and see if that resolves it.

It didn't, that's why the job ended up being new track control arms, ball joints and bushes - which did resolve it.

[kelvin45]

Good to hear the parts fixed it.