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Discussion Starter · #1 ·
whys spv need pressure in the chamber? is the ball valve and orifice the reason? ive done the "devolve" mod and would like to take it further and remove the spv all together and shim it. id rather not run pressure at all.
 

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"El Whatever"
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I don't know if it cavitates with no pressure, but I seem to recall it loses all damping as the passage between rebound and compression remains open if no pressure is on the valve.
 

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Discussion Starter · #3 ·
yeah, theres not really any compression shims.. just the big crappy valve holding the ports closed. i suppose the wild undamped pogo'ing could cause some cavitation :lol:

i have no proof that it is/isnt cavitating, but it was brought up in another spv thread. i went back and looked at the diagrams and dont see a reason it would need any pressure at all if the compression had shims, and the bypass port was plugged off.
 

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Cavitation occurs more readily if the pressure is reduced below atmospheric pressure (a vacuum). Is there anything about the SPV that would cause a vacuum? During rebound?
 

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The pressure is to keep the spv valve closed, not to prevent cavitation. The valve is held open when air is trapped between two o rings at atmospheric pressure. About 15psi. When you pressurize the fork leg above this pressure, the valve closes over the damping piston providing damping. More pressure, the harder it is to move the valve off the piston.

Forks with shimmed damping typically don't need to be pressurized because the damping shafts speeds are slower than shocks due to their 1:1 damping ratio. Shocks need it because they are leveraged by the suspension design, and the shaft speeds are much higher.
 

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eat your porridge jordie
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The spv valve works having an aluminum cup on the underside of the damper held in place by the air pressure. If you are referring to cavitation being lack of damping when the chamber is not pressurized, this is because the orifice size is so big from the aluminum cup not being pressed against the valve. As you pressurize it the cups are held closed against the valve. If you are referring to normal cavitation during the rod stroke that would either be from badly shaped ports or too heavy weight oil.
 

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Discussion Starter · #8 ·
One Pivot said:
would like to take it further and remove the spv all together and shim it. .
im aware of how spv works, thanks guys :)

i went back and looked at the diagrams and dont see a reason it would need any pressure at all if the compression had shims, and the bypass port was plugged off.
thats more where i was trying to steer it.. so theres no reason for pressure if the compression circuit is shimmed and bypass blocked off, right?
 

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The SPV system works the same way as a monotube rear shock, they need positive pressure in the damper to prevent them pulling a vacuum on the seal-head side of the piston.

Other damper designs use a set of compression pistons/shims to create the positive pressure under compression and negate the need for extra air pressure. On some designs (like manitou TPC) virtually all the compression damping is done by the second piston (or pair of pistons), which means the moving piston cannot pull a big enough pressure difference to cavitate.

Cavitation can be hard to pick when riding, but there are some circumstances where it's very obvious. One would be a friend who landed a jump and was catapalted straight over the bars and bounced off a tree.
His rear shock compressed faster than the valving would let the oil flow, so the oil stretches out to a gas bubble which then implodes. Because there is little oil to move through the piston and damp the motion you get a very fast compression, followed by a very fast rebound.
 
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