Fork oil un­well?

Classic Bike (UK) - - WORKSHOP -

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.

Don’t tighten spin­dle pinch bolt till legs are par­al­lel

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