9 replies to this topic

[B40RT]

Hi All
I recently changed the fork oil on my WD B40 and replaced it with 200ml TQF per leg , as before. Now they're topping out, I added 20ml extra per leg but all that did was reduce the movement, and still topped out. The forks are standard issue BSA with external springs. Any thoughts appreaciated.
Ross

[wayne_weedon]

Ross

What type of oil were you using before?

Wayne...

[B40RT]

Hi Wayne
Same stuff.
Ross

[totalshell]

topping out suggests insuffcinet damping effect . ie what the oil is supposed to be doing. if the oil is exactly the same as before it could be that the oil has not yet settled in the leg and air is trapped in a pocket. ride it for an hour or so to see if it improves. adding more oil will only take up space in the fork making it harder to compress.

[alan]

assuming that the forks are air tight, and you put the fork top nuts on with the forks fully extended,

The trapped air works as a very progressive spring, reducing the air gap (i.e increasing the volume of oil) makes the air spring much stiffer,

so, with less oil, the forks will sag further under the weight of the bike and rider..

more oil, the forks will sag less and therefore more likely to top out.

[B40RT]

Thanks for the reply's.
Totalshell, I ended up riding a trial with them and they are still the same.
Alan, if I understand you correctly, are you suggesting compressing the forks a bit before tightening the caps, thus reducing the spring rate effectivly ?
Thanks
Ross

[alan]

errrrrmm no...


Put the top nuts on with the legs fully extended..

What i am saying is that you can use the oil level to fine tune the spring rate of the fork assembly.

Not knowing the forks you are using, i imagine that you will need say 150 ml of oil to get the damping mechanism working, adding more oil will make the spring rate stiffer and therefore reduce sag.

Ignoring the fork springs, If for example you have say 6 inches of air gap with the forks fully extended, compressing the forks by 3 inch would compress the air gap by 50% and double the pressure and therefore force inside the forks.

If you then added more oil to reduce the air gap to four inches, compressing the forks by the same 3 inch would compress the air gap by 75% and triple the pressure and force inside the forks.

This is why some forks have schrader valves built in, to enable the spring rate to be fine tuned by adding air.
For most trials applications a long slow spring rate is preferred, for instance MP forks are vented to prevent the air acting as a spring.

I would reduce the level by say 5 ml at a time until it stops topping out.

[B40RT]

alan, on Sep 22 2009, 06:28 PM, said:

errrrrmm no...


Put the top nuts on with the legs fully extended..

What i am saying is that you can use the oil level to fine tune the spring rate of the fork assembly.

Not knowing the forks you are using, i imagine that you will need say 150 ml of oil to get the damping mechanism working, adding more oil will make the spring rate stiffer and therefore reduce sag.

Ignoring the fork springs, If for example you have say 6 inches of air gap with the forks fully extended, compressing the forks by 3 inch would compress the air gap by 50% and double the pressure and therefore force inside the forks.

If you then added more oil to reduce the air gap to four inches, compressing the forks by the same 3 inch would compress the air gap by 75% and triple the pressure and force inside the forks.

This is why some forks have schrader valves built in, to enable the spring rate to be fine tuned by adding air.
For most trials applications a long slow spring rate is preferred, for instance MP forks are vented to prevent the air acting as a spring.

I would reduce the level by say 5 ml at a time until it stops topping out.

Now I'm really confused. What I though you were saying was compress the forks with the caps off, tighten, and create a vacuum effectivly.
I will try reducing the level and see how it feels.
Thanks for your in-put.
Ross

[alan]

the fork seals are only designed to work in one direction... ie keep the oil in.

if you put the tops one with the forks compressed, when the forks extended they would suck air in through the seals.

i understand that bultaco's riders use the scrader valves to vent the forks as they tend to pump up in use.

[B40RT]

Hi Alan
The forks on my Aprilia are bad for pumping up, to the extent that the oil will continue to seap out in the garage.
Will try releasing the pressure.
Thanks
Ross