Can anyone tell me where there is a link or explain to me how the oil flows thru the check valves,pistons,on compression and rebound.I am still trying to figure out if the oil just rides in the outside tube then back thru ports ect,just to better understand the midvalve etc.Thanks for any help here and if this has been asked before I could not find it.
explanation of fluid flow thru forks and shock
- Thread starter overbore
- Start date
The following was a response to a previous post and I hope will be of some assistance;
OK, let me give this a try. I'm not even close to an expert (but am learning). First the shock and fork are slightly different, but the principals are similar.
Picture a piston on the end of a shaft, now that shaft is in cylinder filled with fluid. The piston has holes in it that pass oil from above the piston to below and below to above as the piston moves through the fluid (if it didn't there would be a hydraulic lock and the shaft could not move).
Now the damping effect is created by the resistance of the fluid moving through these holes in the piston. The smaller the holes, the greater the resistance. Similar to passing a cupped hand through water or an open hand. In a crude system this is what you've got to control damping.
For our purposes though, we need to better control that fluid flow to give greater control of movement. Shims (thin little washers) are stacked on the top and bottom of the piston, covering the holes, or ports.
Now you must imagine this piston having 8 holes. The shims on the top cover 4 of these holes completely, the remaining 4 holes are covered by the shims on the bottom of the piston, but exposing the other 4. So, on each side there are 4 holes covered and 4 holes exposed.
Now move the piston upwards (as if the shock is compressing). The oil is displaced from above the piston and forced to flow to below. The shims are restricting this movement, but because they are thin, they deflect allowing the fluid to pass by.
On rebound, you will see the same effect takes place only in reverse, wit fluid travelling through the other set of 4 holes, past the shims and into the upper side.
This piston and shim arrangement is the "active" valving that you will hear referred to in this forum. Active in that it is moving through the oil, as opposed to being stationary and having oil flow through it.
...starting to make any sense?
Now, I going to throw another piece of the puzzle your way. Go back to when we compressed the shock. As the piston is moving upwards through the oil, the shaft that is pushing it is also filling the lower chamber of the cylinder. If the cylinder is sealed, then you again have a hydraulic lock situation as the internal area that the fluid occupies has now been diminished by the additional area of the shaft. (like putting an ice cube in your drink, that level in the glass rises).
So we have to do something with this displaced fluid. A second valve is provided, in a similar fashion to the one I just described above, only this one is stationary (ie: passive). This displaced fluid is passed through this valve arrangement and controlled in a similar manner (with stacked shims).
I hope I haven't made this any more complex than it has to be, but here's another curve...
In addition to the fluid that is passing through these valve assemblies, builders have given the oil an additional "out". Bypass holes or bleed are usually provided to allow a percentage of the fluid flow to bypass this valving arrangement. For simplification (if that word can even be used at this time ) I will not get into different types of bleed too in depth.
Remember our piston on the shaft? Make the shaft hollow and provide holes in shaft, above and below the piston so that some oil can flow past valve freely. Now we install a restriction between those two holes and into that restriction we install a tapered needle that can be threaded into and out of the restriction hole, thereby allowing us to control how much fluid travels through our "bypass". Voila! this is your clicker! (rebound that is anyway).
Do a similar bypass in your "passive" valve. You now have your compression clicker.
I am sorry for the length of this post, but I hope you understand my ramblings. If anyone has any additional items to add, or corrections to make, please do.
DBD, if you ever get the opportunity to see inside a shock or forks, take a close look and try to trace the flow of the fluid. It makes much more sense when you can see it in your hands.
Hope this helps!
OK, let me give this a try. I'm not even close to an expert (but am learning). First the shock and fork are slightly different, but the principals are similar.
Picture a piston on the end of a shaft, now that shaft is in cylinder filled with fluid. The piston has holes in it that pass oil from above the piston to below and below to above as the piston moves through the fluid (if it didn't there would be a hydraulic lock and the shaft could not move).
Now the damping effect is created by the resistance of the fluid moving through these holes in the piston. The smaller the holes, the greater the resistance. Similar to passing a cupped hand through water or an open hand. In a crude system this is what you've got to control damping.
For our purposes though, we need to better control that fluid flow to give greater control of movement. Shims (thin little washers) are stacked on the top and bottom of the piston, covering the holes, or ports.
Now you must imagine this piston having 8 holes. The shims on the top cover 4 of these holes completely, the remaining 4 holes are covered by the shims on the bottom of the piston, but exposing the other 4. So, on each side there are 4 holes covered and 4 holes exposed.
Now move the piston upwards (as if the shock is compressing). The oil is displaced from above the piston and forced to flow to below. The shims are restricting this movement, but because they are thin, they deflect allowing the fluid to pass by.
On rebound, you will see the same effect takes place only in reverse, wit fluid travelling through the other set of 4 holes, past the shims and into the upper side.
This piston and shim arrangement is the "active" valving that you will hear referred to in this forum. Active in that it is moving through the oil, as opposed to being stationary and having oil flow through it.
...starting to make any sense?
Now, I going to throw another piece of the puzzle your way. Go back to when we compressed the shock. As the piston is moving upwards through the oil, the shaft that is pushing it is also filling the lower chamber of the cylinder. If the cylinder is sealed, then you again have a hydraulic lock situation as the internal area that the fluid occupies has now been diminished by the additional area of the shaft. (like putting an ice cube in your drink, that level in the glass rises).
So we have to do something with this displaced fluid. A second valve is provided, in a similar fashion to the one I just described above, only this one is stationary (ie: passive). This displaced fluid is passed through this valve arrangement and controlled in a similar manner (with stacked shims).
I hope I haven't made this any more complex than it has to be, but here's another curve...
In addition to the fluid that is passing through these valve assemblies, builders have given the oil an additional "out". Bypass holes or bleed are usually provided to allow a percentage of the fluid flow to bypass this valving arrangement. For simplification (if that word can even be used at this time ) I will not get into different types of bleed too in depth.
Remember our piston on the shaft? Make the shaft hollow and provide holes in shaft, above and below the piston so that some oil can flow past valve freely. Now we install a restriction between those two holes and into that restriction we install a tapered needle that can be threaded into and out of the restriction hole, thereby allowing us to control how much fluid travels through our "bypass". Voila! this is your clicker! (rebound that is anyway).
Do a similar bypass in your "passive" valve. You now have your compression clicker.
I am sorry for the length of this post, but I hope you understand my ramblings. If anyone has any additional items to add, or corrections to make, please do.
DBD, if you ever get the opportunity to see inside a shock or forks, take a close look and try to trace the flow of the fluid. It makes much more sense when you can see it in your hands.
Hope this helps!
Jeremy Wilkey
Owner, MX-Tech
- Jan 28, 2000
- 1,453
- 0
JTT,
Nice Work! :thumb:
JW
Nice Work! :thumb:
JW
- Thread starter
- #8
I guess after looking at diagrams it looked like there was only one check valve for the rebound and the compression and that it wouldnt allow flow both directions so I was confused on how the fluid got back in the cartridge that was pushed out the bottom of the cartridge thru the compression valve.Maybe I am just making this harder than what it is here,but appreciate your time so I can make sure the theory of it all.Thanks
Basically the piston is made up of, lets say for ease of discussion, 8 ports. 4 ports flow one way, 4 the other. If you were to look at the piston on one side, you would see 4 raised shouldered ports...kind of like castle tops...boy, this is hard to explain in words :think: Anyways, the shims sit on top of these shoulders covering the ports. The area below the shouldered ports are where the ports from the other side exit. The same goes for the other side of piston.
Now, the fork compresses....oil flows through the piston from under the rebound shims (remember, they are up on the shouldered rebound ports and oil can flow under then through the exposed side of the compression ports. Fluid flows through these ports, and then hits the compression shim stack, flexing it and flowing through.
On rebound, these shims close, creating a check valve preventing fluid from flowing back the way it came, so it find the "other" route, through the ports exposed by the shoulders.
Imaging the compression shims sitting on four small vertical tubes sitting on a plate with 8 holes in it....now add four more tubes, the same as the first on the bottom side of the plate, only on the "other" four holes. The rebound shims cover these over. This is effectively how the piston is configured.
Hope this makes some sense. I know it is confusing, but if you ever get to see a piston, it will all make sense...I hope. :eek:
Now, the fork compresses....oil flows through the piston from under the rebound shims (remember, they are up on the shouldered rebound ports and oil can flow under then through the exposed side of the compression ports. Fluid flows through these ports, and then hits the compression shim stack, flexing it and flowing through.
On rebound, these shims close, creating a check valve preventing fluid from flowing back the way it came, so it find the "other" route, through the ports exposed by the shoulders.
Imaging the compression shims sitting on four small vertical tubes sitting on a plate with 8 holes in it....now add four more tubes, the same as the first on the bottom side of the plate, only on the "other" four holes. The rebound shims cover these over. This is effectively how the piston is configured.
Hope this makes some sense. I know it is confusing, but if you ever get to see a piston, it will all make sense...I hope. :eek:
Hondaxrguy
Member
- May 17, 2001
- 573
- 0
So, in the shock, there is the normal compression and rebound stack on the shock piston riding on the shock's shaft? Then there is also another compression stack controlling fluid flow into the resevoir? For the oil that is displaced by the shock's shaft? Or is there only the bleed hole(s) with the compression clicker controlling that flow?
One more question. How does the HSC adjuster work? Does it control a shim stack?
Jeremy
One more question. How does the HSC adjuster work? Does it control a shim stack?
Jeremy
Last edited:
While we're talking abot fluid flow through the valve stacks, does anyone know of any way that the valve stack could fail such that there is no oil flow through the stack? Imagine riding a whooped-out section really hard and getting the shock really heated up. If you stopped and sat down for a while, could the shims possibly melt together and sieze themselves to the valve body?
Jeremy Wilkey
Owner, MX-Tech
- Jan 28, 2000
- 1,453
- 0
Eburatti,
It won't happen that way but, I've seen shocks that "blew up" from shims getting getting sucked into the oposite port entries.. This happens foristance if the shims begin to vibrate from turbulance, and then get in the way flow path.. Its rather gruesome..
Br,
Jer
It won't happen that way but, I've seen shocks that "blew up" from shims getting getting sucked into the oposite port entries.. This happens foristance if the shims begin to vibrate from turbulance, and then get in the way flow path.. Its rather gruesome..
Br,
Jer
from listening to this string the shims must flex ALOT. with all the flex how are these things designed to where there are no weak points that could cause the shims to break? ie: like when you bend a credit card back and forth in the center eventually it breaks. obviously im just going off of a picture of the shaft motion in my head, so i may not be thinking of this right :silly:
Jeremy Wilkey
Owner, MX-Tech
- Jan 28, 2000
- 1,453
- 0
ATTENTION::::CRITICAL POINT::::
Shims bend very little to create a varibale orfice..
Cool
:)
Jer
Shims bend very little to create a varibale orfice..
Cool
:)
Jer
Ok...lets keep going....
JTT's explaination was more about how a shock works.....forks are a little different.
Lets draw the parallels....
shockshaft=cartridge rod
shockbody=cartridge tube
resivoir(sp?)=area around cartridge tube (lower leg) that is filled with oil.
shockpiston=rebound/mid-valve piston
shock high&low-speed adjuster=base-valve
The big difference is the amount of oil available to dampen and the distance the "shaft" has to travel in a given amount of "time".
Sound right?
Lets keep digging..... :yeehaw:
JTT's explaination was more about how a shock works.....forks are a little different.
Lets draw the parallels....
shockshaft=cartridge rod
shockbody=cartridge tube
resivoir(sp?)=area around cartridge tube (lower leg) that is filled with oil.
shockpiston=rebound/mid-valve piston
shock high&low-speed adjuster=base-valve
The big difference is the amount of oil available to dampen and the distance the "shaft" has to travel in a given amount of "time".
Sound right?
Lets keep digging..... :yeehaw:
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