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bouncing front forks

Hi,

First thanks for all the help from everyone. Now my next of 3 problems:)

When I apply the front brake the front hops on my 1944 bsa. They are girder forks, and I have tightened the dampner on the side as much as possible but next steps or checks?

email (option): jeff@infinitymedspa.com

Re: bouncing front forks

Jeff, I don't think this is anything to do with the forks. It's back to basics, meaning check the front brake for adjustment, drum ovality, tyre pressures, etc. You might want to check the bolt securing the front brake stay. When that comes loose, you get a very distinct judder. Your fork damper is unlikely to be involved. But it might be dust in the drum. Check the headstock bearings while you're there. Could be a combination of factors.

email (option): dannydefazio@sumpmagazine.com

Re: bouncing front forks

Hi Jeff, the points raised by Danny are worth checking, particularly the ovality of the brake drum.
If you ride along and lightly apply the front brake you will feel the movement at the handlebar lever as the braking pressure varies.
If your problem is a rapid up and down movement of the forks under heavy braking it certainly could be caused by wear in the side damper friction discs, contamination of the discs with oil or weakness in the star shaped spring washer that applies pressure to the damper when you tighten the damper knob.(all of which reduce the amount of friction in the damper assembly)
A weak or shortened (with age) main fork spring could also be a factor...if the spring measures around 8 1/2" in the forks it is probably weak. Original length before fitting is 10". Saying that ineffective damping is generally the main cause....Ian

email (option): ian@wright52.plus.com

Re: bouncing front forks

Hi Ian. How does wear in the damper cause bouncing? I've experimented over miles with the damper in various "settings". Can you explain it, please?

email (option): dannydefazio@sumpmagazine.com

Re: bouncing front forks

Hi Danny..Under heavy braking the front fork spring is compressed. The more it is compressed the more energy is stored in the spring. As the bike slows and the braking loads are slightly reduced the spring then releases the stored energy and rebounds.
The energy in the spring is then less than the braking loads and the cycle begins again, recompressing the spring etc.
This cycle occurs rapidly in practice, causing the rapid 'pattering' of the front wheel and forks as described by Jeff.
The damper slows down (dampens)the movements and reduces their effects by exerting an additional resistance to movement.
Taken to a ridiculous extreme if enough friction was present in the damper the forks wouldn't move at all...while at the other end of the scale they move too easily.

email (option): ian@wright52.plus.com

Re: bouncing front forks

Thanks Ian.

email (option): dannydefazio@sumpmagazine.com

Re: bouncing front forks

It will be your worn fork links and bushes.

Re: bouncing front forks

It shouldn't be...the links and bushes shouldn't play a significant part in the action of the forks. When new they shouldn't have any resistance to movement and when worn they definitely won't have any resistance to movement. That should all be controlled by the forks main spring and the damper assembly. A set of forks assembled without the spring and damper should move quite freely.....Ian

email (option): ian@wright52.plus.com

Re: bouncing front forks

Thanks I Ian and everyone for the help.

email (option): jeff@infinitymedspa.com

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