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I've just been adjusting the spring pressure on my Rock Shox SID XX fork and then adjusted my Manitou Minute Expert because I had the pump out. The SID has the solo air spring and needed 115psi and the Minute has the ACT spring and only needed 20psi. Whats the difference between high and low pressure systems and what is the benefits of each?

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Force = pressure times piston area. (Piston area is proportional to piston diameter squared.) Thus, higher pressure == smaller diameter pistons, for the same rider weight. At the expense of needing better seals. –  Daniel R Hicks Aug 11 '13 at 3:17
    
Just to clarify, these are two forks on different bikes. –  DWGKNZ Aug 12 '13 at 20:01
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1 Answer 1

To add to Daniel's comment, there is no real benefit/detriment to one vs the other, but you are going to find that rear shocks are typically higher pressure for a few reasons:

  • rear shock pistons are typically smaller in diameter and therefore require more pounds per square inch (PSI) to push back against load.
  • front shocks (ie your fork) are acting directly against the weight you're loading them with. For example, if when loaded the front wheel is supporting 50lbs, then the fork just needs to push back against 50lbs (minus the weight of the stanchions, front wheel, and front brake if you want to get technical about it). With many rear suspension designs, the linkage that hooks the shock ip to the rear triangle acts like a fulcrum, with the shock being on the short end of pivot, and thus it's not a 1:1 ratio of weight that the rear wheel is supporting vs the pressure with which the rear shock has to push back.
  • finally, weight distribution across a bike of the type in question is not 50/50 front and rear. The rear tire must support more weight when not under heavy braking. Therefore the rear shock is supporting more weight and must push back harder.
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