two way damping

[tck]

I read somewhere that some post war radiadrulic forks were modified with a disc on top of the suspension piston as well as the bottom thus regulating the upward movement as well as the downward does any one have any details of this? and what models if any had it included

[eddie]

I have also heard that this had been tried, but have no details of the modification or whether it was effective.
     The biggest problem we had with the damping on our Comps was the free movement before the damping valve closed. Having hit a bump, the forks would then recover (quite rapidly) until the damping took effect - by this time, the front of the bike was already lifting, and the sudden application of damping would momentarily cause the wheel to leave the ground. On the standard set up, the damper valve has just over 1/8" of movement before it closes the ports. The leading links are about 6 times the length of the internal links, so the suspension moves about 3/4" before the damping gets it under control. My solution to the problem was to make new, slightly longer guide pins for the damper valves and fit light springs to keep the valves in contact with the seats - the improvement was amazing - to the extent that a lighter grade of fork oil was needed.

  Hope this helps,
                             Regards,
                                            Eddie.
 

[tck]

Spot on Eddie that at least may solve some of my worries  It could well be that someone has some forks with this top mod and never noticed

[eddie]

The problem with fitting a second valve on top of the piston is that it will be either open or shut - so the effectiveness of the damping will be solely controlled by the fit of the suspension piston in the fork tube, and there would be no provision for pressure release on hitting a severe bump - thus making the ride a lot harsher.

  Eddie.

[roger h]

We had a 90+ for rebuild last year. This model was supposed to have 2 way damping in the forks, however when we stripped the forks we found only the single damping arrangement. Yes, they were original 90+ forks.
Roger

[eddie]

I very much doubt that effective 2 way hydraulic damping could be successfully applied to radiadraulic forks. Rebound damping is easy to obtain as the oil is contained in the bottom of the forks, and the damping is controlled by regulating it's escape past the piston - using the hydraulic principal that liquids cannot be compressed. Damping in the opposite direction could only be obtained by generating a partial vacuum below the piston, so the amount of damping generated would be limited by atmospheric pressure bearing down on the top of the piston - at that point a void would be generated below the piston, resulting in a sudden shock load as the suspension recovered. Also, when damping is increased, spring rates often need adjusting, otherwise, on a series of bumps, the suspension may not fully recover before the next bump is hit - resulting in the suspension pumping itself down onto the bump stops!
  When I was researching the 1950 ISDT machines, I was loaned some ex-factory documents that included some details of the experiments that Eddie Withers had carried out. There was mention of 2 way damping but no actual details - so I guess the experiments were not successful enough to warrant the modification being done to production machines.

  Regards,
                 Eddie.

[eddie]

Whilst we are on the subject of front forks and damping - how many have suffered spring breakage on radiadraulics? I am of the opinion that Mr Douglas got it wrong when he fitted the tapered springs thick end uppermost. By taper grinding the O.D. of the spring, it became multi-rate, and the generally accepted rule for these springs is that they should be fitted so that the lighter rated end is static (to prevent the light end becoming coil bound in operation - the cause of most spring failures!). So, the springs in our forks should be fitted light end uppermost, so that it is the heavier end that is being subjected to the shock loads.

  Regards,
               Eddie.

[tck]

My first Mk5 (from this distance not certain)  about 50 years ago when I stripped it the square taper wound springs were in hundreds of pieces literally each coil had shattered

[graeme]

Not Douglas forks, but when I fitted multi rate springs to the forks of my T160 Trident, the instructions said to fit them with the coils closer together at the top, so exactly what you are suggesting Eddie

[Neville Heath]

A pair of Plus forks I have has holes (about 1/8th dia) drilled through the oil filler screws in the top of the forks.
Any views Eddie?
Neville

[tck]

I think there was a recommendation about this in one of the documentation I read recently  about the forks 'pumping up' the screws can be added to ease the problem otherwise the whole nut has to be taken off to release the air

[eddie]

Hi Neville,
                 I cant see the point of drilling these holes - unlike telescopic forks, there is no change in internal volume as the forks operate. The only thing it may help with is the release of pressure due to temperature change - thus reducing the leakage around the seals on the leading links - I got over that problem by applying a smear of waterpump grease to the faces before assembly - my Comp forks are still dry after 25 years!!

  Stay in and stay safe,
                                      Regards,
                                                     Eddie.