working on a cub to replace the seals ( i think the forks are actually bsa perhaps from a c15) i put 150ml of 15wt oil in and they 'sag' for fun (maybe an inch and a half under the weight of the bike and another inch when lardy boy climbs on board..)

to stiffen them up do i

put more oil in

put thicker oil in

put a longer preload spacer in

put 'fresher' longer springs in ..

Sounds like the wrong springs are in there anyway. Fork oil controls the damping not the spring rate. As a temporary measure try longer pre tensioner spacers but make sure the springs dont get coil bound. If you have any try putting some old valve springs in instead of the preloaders.

Ideally the bike with you onboard should sag about an inch and settle there coming back up as soon as you climb off.

Good luck

stuck a couple of alloy spacers (2/3rd of an inch) in and that seems to have done the trick so getting out the hacksaw and big hammer to get at some valve springs that are on an engine thats been behind the shed for sooo long.

working on a cub to replace the seals ( i think the forks are actually bsa perhaps from a c15) i put 150ml of 15wt oil in and they 'sag' for fun (maybe an inch and a half under the weight of the bike and another inch when lardy boy climbs on board..)

to stiffen them up do i

put more oil in

put thicker oil in

put a longer preload spacer in

put 'fresher' longer springs in ..

<{POST_SNAPBACK}>

If the existing springs are too soft, the easiest way forward is to SHORTEN the spring and make up the difference with a spacer. 1-2 inches off an internal spring make a noticeable difference, but the more you reduce it by, the more chance there is of it getting coil bound. Easy enough to measure the gaps between the coils and work out if there will be enough movement, though.Hands up those who thought shortening a spring softens it?

For those wanting to stiffen up modern forks on a heavier bike, s miller sells some heavier springs. Save your money, the ones i got were no different to the ones which came out....