How to calculate floating piston depth?

[enduro250]

Hi, first off i would like to say that i have recently found this forum and think its great that a lot of people dont mind sharing their info and tips on here and i hope i can learn some more things from you guys.

Ok I would like to know the correct/proper/accurate way to set floating piston depths in remote reservoir shocks?

I am specifically dealing with old WP shocks which are a bit old by todays standards as most of you guys have the latest and greatest bladder technology.

I do have 2 separate lots of information for correct floating piston depths for WP shocks, but they are not entirely applicable to what i am doing. I am custom building some shocks from a mixture of parts and i plan to fit larger reservoirs (longer and bigger diameter reservoir) as i would like to increase the oil capacity some more. These reservoirs are off of WP's with larger shafts and different amount of travel, so going staight off the WP spec sheets for the listed shocks is not going to be accurate/relaible.

I have calculated out a formula to work out the volume of the shaft from the end eye (including where bump stop fits) to the lip of the oil seal and not just the body cap. And from that i have worked out that when the shaft is compressed metal to metal the rise in fluid level will move the floating piston about 10mm in my application. This would obviously be a little less when the bump stop is fitted, but at the moment to be safe i did it including the area of the shaft where the bump stop sits.

I want to take advantage the larger reservoir to increase oil capacity. I have worked out how much the floating piston would move, but i feel i still need to know more.

How large should the gas volume be? is there a formula for this?

If i am right the gas volume needs to be worked out because if its too small the rise in pressure when the piston moves could be too much for the canister to handle or blow the schrader if the pressure exceeds the rating of the schrader?? yes/no? I have valve cores rated at 400PSI peak.

Is there a standard rule of thumb that says piston should be 50% or 75% of the length of the reservoir canister for example?

Is 'Boyles law' used at all?

what about rise in gas temperature, does that need to be known?

Hopefully someone here can help me as i have always wanted to know how to do this CORRECTLY and get it exactly right when custom building/modifying shocks

Thanks for your assistance :)

 

[Richard S.]

enduro, the WP "rule of thumb" is 90/10. In other words, the oil should take up 10% of the reservoir and the n2, 90%.

So, this is easy to calculate: Do not worry about the diameter of the reservoir body, only the length. If the length is 100mm, your piston height is 10mm. ALWAYS measured DOWN from the top-end of the reservoir (closest to the compression adjuster, in other words).

Just always remember, 10% oil, 90% nitrogen.

:cool:

 

[enduro250]

Thanks for that info. So that is a pretty accurate rule then? Is it from a good source? the otherday i tryed to work out how they determine this and to see if there is any patterns and i set up a spreadsheet with all the WP reservoir data i had and all the floating piston depths and calcualted out the percetange of depth of the reservoir the piston is set up to see if they all came out around the same but they didnt, i had depths from 46%-76% so that didnt help me at all as i couldnt see any common spec or ruling there.

Is there any reason why it should be 90% gas 10% oil? Why couldnt i increase oil and reduce gas if i knew that there was plenty of space for the piston to travel still?

for example shock 'A' has 160mm travel and the spec for the reservoir depth is 80mm
if i reduce travel to 130mm and still use the same reservoir then this becomes shock 'B'

i can calulate due to the less shaft travel the floating piston would move for example lets say 5mm less due to less oil being displaced due to shorter shaft, so cant i just increase the size of the oil section by 5mm more in length ( so depth would be now 75mm for the gas side) and there would be no problems?

i just need to know if there is a scientific ruling saying why the nitrogen volume has to be a certain amount in relation to the reservoir if that is the case.

cheers.

 

[Pete Payne]

enduro250,
Richard S. is a very good and reliable source , he worked for White Brothers and also was the WP impoter for a while .Currently he runs MX-Tech in Arizona, better known as Palo Verde Suspension !

 

[enduro250]

yes thankyou, Richard has contacted me.

 

[Hydrolyze]

1st time posting here

Just wanted to say hello all. This is my first post.I expect to learn some good info here.