For forks’ sake
At least I’m not the only one suffering BSA fork troubles at the moment. Neil in Australia emailed to ask the cause of his tele fork 1949 BSA M20 ‘topping out’.
Since I’m struggling with the same problem with my Gold Star (see page 94), I’m not the person to ask – but it got me thinking further. My forks are late ’60s with internal dampers, but Neil’s are the earlier type; here’s how I think they work.
The fork stanchion has two small holes, one above the other, that would usually sit between the top and bottom bushes. On compression, the fork bottom bush drives oil down, back up into the stanchion via a hollow bottom nut and out of the two
holes. A taper cone in the slider bottom slows and blocks this exit, preventing bottoming with a hydraulic lock. Coming back up, oil now outside the stanchion is compressed between the bushes and squirts back through (initially) both holes, then – as the top bush covers the upper – just one, giving a rising rate of damping. When the second hole is covered, another hydraulic lock is formed, preventing the two bushes from bashing together.
However, if oil can bypass a worn bottom bush or slider bore, there will be no oil cushion between the bushes to prevent the clash – and while replacing bushes is no problem, wear in the slider would require tricky honing and manufacture of custom bushes.
Interesting to see varying internet opinion on fork oil grades, indicating that bikes differ; probably as a result of wear – low-mileage bikes may be happy with the standard SAE 20, but I’m on 40 and Neil even tried SAE 140, which he says made the noise quieter but didn’t get rid of it!