Fork oil unwell?
BSA fork problems seem to be reaching epidemic proportions! This month Paul Rogers emailed to say that while his BSA A10 forks work well, his Goldie is harsh and my mention of trying thicker oil interested him because trying thinner didn’t help. He asks: “Can you clarify the logic behind changing viscosity?”
In a perfect world, you should stick to the standard grade as selected by the fork’s designers, although on old bikes wear can change the game. Hydraulic damping systems work by moving oil around, forcing it through small holes as the forks go up and down. The thinner the oil, the quicker it moves, helping the damping stay
efficient when the fork moves quickly (when, for example, a rapid succession of bumps gives inadequate time to recover between impacts). That’s why lighter oil is recommended for harsh action. My problem seems to be inadequate damping; thicker oil should (in theory) slow the oil movement, preventing ‘topping out’, but it hasn’t and I’ve realised that I’m trying to get around wear in parts that cannot easily be replaced.
But there are other possible causes of Paul’s harsh fork action. The most obvious is unduly stiff springs; BSA made sidecar-rate springs and it’s possible a pair of these are fitted. A way round this is to invest in a set of SRM Engineering’s improved ‘Progressive’ fork springs. It’s also worth checking the fork legs aren’t pulling in or out from one another, which would stiffen the action. The BSA fork has a pinch bolt on one side that grips the axle: after fitting the wheel you should pump the forks up and down a few times before tightening it, to ensure the forks settle parallel.