Bladder Verses Piston Arguments in PDS Shock Absorbers
From Owner's Document
Bladder Verses Piston Arguments in PDS Shock Absorbers.
As we look at the major tuning topics surrounding WP shock absorbers not many are as quick to bring up debate as the Bladder Verses the Piston in WP shocks. We decided to take this to the Dyno and analyze the results. It is important to point out that both shocks where charged to the same pressure 150 PSI and that both where bleed on a WP pump.
Most of the arguments revolve around these points.
- Area is often used as a quick laymen’s explanation. The bladder with its soft composition is more flexible in ones hand, and given its large surface area it would seem logical that the increase in surface area would translate into a reduction of force required to compress the shaft. Assuming the # of molecules of Nitrogen was the same.
- Friction, the piston set-up uses the sides of the reservoir body as a bearing surface. An o-ring is used a seal. The O-ring rubs on the body and creates friction. The total movement of the piston is very small, however friction would seem like a viable option.
Results:
The piston shock actually proves to be more consistent showing less Hysteresis, and a softer damping force curve. This can be more easily viewed in the force vs. Displacment format.
- Area proves to be a faulty explanation. We are not compressing the bladder but the gas behind the bladder. This is a fixed amount and we compress the exact same number of gas molecules whether the side compresses the tops or all of them are compressed. For this reason the force properties of the shock are not a function of surface area. Importantly it was also noted that the rod charge was also higher on the bladder shock, this would indicate that the total volume of gas was down and that the bladder actually results in a decrease in effective diameter which has a significant impact on total rod charge.
- As for friction, we find in measured values that the Bladder set-up produces around 16 lbs of drag, verses 13.3 with the piston. (I personally was most surprised by these findings.)
Presently I can conclude that the bladder system does not significantly impact shock performance if all variables are equal. In present form the bladder shock is less consistent and actually makes the shock stiffer in application. I believe that most of the difference some riders feel in the way of improvements stems not from technical improvements but from an introduction of emulsified fluid in the working shock. This is not recommend for consistent performance or fluid life, althought it could produce a softer feel.
Further testing would be recommended using a Piston and a Bladder setup and using a hand bleed method for the bladder and the piston and comparing performance to that of the pump bleed. I believe this could provide significant further insight into the results that most riders find.
