i've a pair of conventional forks on my montesa twin shock that are fantastic smooth and smooth as can be , they hold the bike straight over the roughest of rock piles the only downside is..

when they extend fully ( eg front goes light or heaven forbid the wheel leaves the ground) they give a clear 'clunk' as they reach full extension they seem to be ok when i took them apart and both legs do the same thing they have 10wt oil in at the minute

whats wrong and how do i cure it..

Edited September 12, 200619 yr by totalshell

I beleive there should be some form of plastic bushing in there too stop them topping out, maybe they are worn? (all this is dependant on the make of fork etc etc but thats all I can think of)

Twinshock era trials forks usually have "topping" springs that compress just as the forks top out. It is possible to overwhelm these springs by using a lot of preload on the main springs in combination with light oil.

Another possible cause may be insufficient oil ie the oil level goes below the damping holes on full extension.

The plastic bushes you should find in your monty forks are "bottoming" cones which generate a greatly increased compression damping rate as the forks reach full compression.

Easiest starting point is the oil quantity as you probably haven't enough in each leg and the effect is more obvious when on rebound than compression. Quantity required obviously depends on the model and/or forks you have fitted. 10 weight is plenty thick enough.

Try around 180cc per leg which is a sort of baseline for Ossa MAR/Bults and subtract or add 10cc increments until you get the result you want, although over 200cc on old style telescos/betors is probably too much and you'll lose travel through hydraulic locking. Later t/s Monts had Marzzochi style forks, not sure what the quantity in those is.

had the things apart today , really easy job considering.

the allen screw in the base of the slider holds the damper rod. with the main spring out a piece of plastic tube down the slider held the damper rod while i loosened the allen screw

the circlip at the base of the stantion came off easy and let the damper rod slide out easily

the damper rod has a plastic 'piston' at the top and this is restricted on 'topping out ' by a spring similaer in apperance to and as strong as a 'valve spring' and seemed way too hard almost solid, as you'd expect from an engine valve spring, so i have changed it for a spring from a mountain bike fork and added an extra 20mil of oil

i'll test them on thursday hopefully..

i now know more about forks and springs than a springy thing..

the fork 'clunked' when 'topping out' out because the rebound spring was too stiff ( almost a valve spring) so that it provided virtually zero movement/ resistance to the main spring after measuring it all up i've ordered so softer and longer springs to experiment with in the hope that they will compress and slow the fork when it tops out , luckly i kept one fork leg as standard because changing the rebound spring to a softer one albe it slightly longer actually shortened the fork leg,

once resolved fully i'll release all details to the waiting world..

ttlshell, very cool how you resolved yr problem with the fork.

Keep it in mind.