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  1. #1
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    Converted my Reba to Dual Flow piston - need valving advice

    So after searching around for a rebound piston to upgrade on my 05 Reba, I just decided to modify my own. Luckily, I was able to get one of those red shimmed pistons from my brother, who had a perfectly good Revelation that was modified by push.

    The original piston was molded on, so I had to cut it off on the lathe. Then turn a few steps on the shaft so the new piston will fit, and finally, thread it so a screw can hold the piston on. There's a hole in the center of the screw so the orifice adjuster still works.



    OK, so this is the part I still need help on. It seems that with the default shim (17.75mm x 0.15mm) the "Dual Flow" part is not duly flowing. (Pun) Although I haven't put the fork on the dyno to prove it, I can tell that the shim isn't really opening because I can turn up the low speed damping all the way, and the fork still feels "constricted" when I compress and release it (I'm applying about 150 lbs on it).

    My understand for how this setup is supposed to work is that the orifice gives the low speed damping, but its force vs velocity curve is quadratic, so by the time you reach higher speeds there is too much damping. Thus there is always the tradeoff where you want more low speed damping for crawling over rocks, while less high speed damping so the fork doesn't pack down during fast hits. The shim is supposed to help in this regard because its force characteristic is linear + offset, so at some point the shim opens and reduces the pressure to let the fork quickly extend.

    What I did was replace this 0.15 shim with a 0.10 shim I had laying around. (I think that is supposed to reduce the damping by about 3x) That helped, but I feel as though the fork still needs to "loosen up" a bit. Maybe the problem is that the rebound flow holes in the piston are pretty small. At some point, they will limit the flow more than the shim.

    Anyway, after all this blah blah, I am wondering if it would help to enlarge the holes in the piston, or is there something else I can do?

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    Quote Originally Posted by beanbag View Post
    ...I can tell that the shim isn't really opening because I can turn up the low speed damping all the way, and the fork still feels "constricted" when I compress and release it (I'm applying about 150 lbs on it)...
    Top work on the mod.

    Rebound adjuster operates on a bypass port. Rebound should feel constricted when you turn the rebound damper all the way up.

    Put it on a dyno. You can't tell how the high speed rebound circuit works by pushing down and letting the bike go back up. Your high speed rebound events involve the wheel going from weighted to no weight at all (ie. even the weight of the bike). You can't really "feel" this rebound speed by compressing and releasing it.

  3. #3
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    OK, here it is on my dynometer, with me turning the rebound knob from all the way open to all the way closed. (The open setting has more damping than stock because that screw that holds the piston on has extra constriction. Anyway, as u can see, the shim part of the damper doesn't really seem to be opening up, or at least if it does, it's not doing a lot. My fork only makes about 200 lbs at full compression, btw.

    From what I read so far, you want a digressive damping curve, and what is supposed to happen is the shim part opens up and the rebound curve flattens out.

    Edit: I think this means I need to make a higher flowing piston.
    Last edited by beanbag; 06-29-2011 at 11:47 PM.

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    Are you using a small clamp shim between the main shim and the head of the piston bolt? If not, maybe the shim doesn't have a "pivot point" to bend across. It may sound counterintuitive to add a shim to make it softer, but if you don't have the clamp shim, then the face shim will have to bend back around the entire head of the piston bolt (which is larger in diameter than a clamp shim would be) which would make the stack effectively stiffer.

    The shim size that I'm talking about is very small, probably 8-9 mm (as small as a shim as possible that will fit on your piston bolt) and maybe .25mm to .5mm in thickness.
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  5. #5
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    The original clamping nut has a little step on it where it contacts the shim. This step is 9.x mm in diameter. My bolt has that step cut in it too.

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    I'm not sure that I'm reading the graph right. Do the three different colours correspond to three runs at different rebound settings? It looks like the rebound adjuster doesn't do anything other than change the compression damping by affecting the free bleed on compression through the rebound port.

    Is the checkplate on the back of the rebound piston setup properly?

    Also, I'm pretty sure that on the blackbox rebound damper you can pretty much entirely close off the rebound adjuster port if you dial in enough rebound damping with the adjuster. At this point the shim is doing all of the rebound damping. Does your modified setup do this? Is there a plot for this scenario on the dyno graph?

  7. #7
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    Quote Originally Posted by TigWorld View Post
    I'm not sure that I'm reading the graph right. Do the three different colours correspond to three runs at different rebound settings? It looks like the rebound adjuster doesn't do anything other than change the compression damping by affecting the free bleed on compression through the rebound port.

    Is the checkplate on the back of the rebound piston setup properly?

    Also, I'm pretty sure that on the blackbox rebound damper you can pretty much entirely close off the rebound adjuster port if you dial in enough rebound damping with the adjuster. At this point the shim is doing all of the rebound damping. Does your modified setup do this? Is there a plot for this scenario on the dyno graph?
    No, the rebound knob really does adjust the rebound damping and not compression. Maybe I plotted the graph upside down.

    I'm pretty sure the checkplate is setup right, otherwise the damper would be giving compression damping. The checkplate isn't big enough to cover up the rebound inlet holes on the other side.

    My rebound needle adjust might be different than yours because it wasn't designed for blackbox. So even if the knob is screwed in all the way doesn't mean the rebound completely closes off. Also, the piston itself has a little notch on the compression side that lets fluid bleed across even if the checkplate is closed.

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    Can you explain the graph some more? Normally the neg force plot will be the dyno pulling on the shock/fork, so I'm looking at the neg part of the plot to see what the rebound looks like.

  9. #9
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    Quote Originally Posted by TigWorld View Post
    Can you explain the graph some more? Normally the neg force plot will be the dyno pulling on the shock/fork, so I'm looking at the neg part of the plot to see what the rebound looks like.
    just flip the graph upside down then

  10. #10
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    Truth be told I seldom if ever mess with RS stuff. Not since MAG 21 days or if I need to work on my SS with a MAG 21.

    Not even considering the graph, and if it is right or wrong.

    This is stated as a rebound piston.

    Do you have photos of the entire damper layout. There may be concerns on the compression piston causing cavitation which may be something else offered with high flow conversions.

    FWIW, the area of the red piston seems correct, at about 3:1

    The notch in the piston is another bleed, it runs parallel in circuit to the rebound freebleed. These are often utilized for 2 reasons. One you can never fully lock out the rebound, Second, many rebound clickers can not freebleed enough fluid, this supplements it.

    As for the poor rebound control and the idea of a digressive piston setup, why on rebound? On compression that is sometimes good to unload pressure, but rebound is only combating spring force, never more.

    A normal stack would allow the feel of digression on account of diminishing force.

    Opening the holes is probably not correct. You need to change the clamp area to a smaller diameter. What size thread is that bolt? I'm guessing , and based on some experience with this on other application, you will need to run delta shims. Probably in a two stage format.

    What is the .15 shim ID?

    The valve body attachment dia is a concern also. It appears you have access to a shop, seeing mention of a lathe and a dyno. Could you possibly redesign the valve seat to be as is for the check plate, but machine a short stepped OD with smooth sides to seat the valve, threaded and bored to be secured in the rod and give freebleed. The machine a more delicate smaller dia bolt to thread into this new valve adapter, it would need some thin walls for rebound freebleed.

    This could allow using smaller ID shims.

    I would also consider obtaining material and punching some custom shims of .05mm thickness.

    This should not be a big deal to accomplish, at least in regards to shim, making them custom and some as deltas.

    BTW, what dyno plotted that? Suspecting a Roehrig Does it have capability and enough HP to run faster than 45 IPS?
    Guessing it's this unit
    http://www.roehrigengineering.com/Pr...per-tester.htm

    Still marginal at best for off-road forks.

    PK

  11. #11
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    If you'd like to see some ludicrously small rebound ports, this is from a manitou swinger air.

    Too small, even with just a cover shim and 2.5wt oil it's dead on rebound.
    Attached Thumbnails Attached Thumbnails Converted my Reba to Dual Flow piston - need valving advice-msa-damper-piston.jpg  

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  12. #12
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    Quote Originally Posted by PMK View Post
    Do you have photos of the entire damper layout. There may be concerns on the compression piston causing cavitation which may be something else offered with high flow conversions.

    FWIW, the area of the red piston seems correct, at about 3:1

    As for the poor rebound control and the idea of a digressive piston setup, why on rebound? On compression that is sometimes good to unload pressure, but rebound is only combating spring force, never more.

    A normal stack would allow the feel of digression on account of diminishing force.

    Opening the holes is probably not correct. You need to change the clamp area to a smaller diameter. What size thread is that bolt? I'm guessing , and based on some experience with this on other application, you will need to run delta shims. Probably in a two stage format.

    What is the .15 shim ID?

    The valve body attachment dia is a concern also. It appears you have access to a shop, seeing mention of a lathe and a dyno. Could you possibly redesign the valve seat to be as is for the check plate, but machine a short stepped OD with smooth sides to seat the valve, threaded and bored to be secured in the rod and give freebleed. The machine a more delicate smaller dia bolt to thread into this new valve adapter, it would need some thin walls for rebound freebleed.

    This could allow using smaller ID shims.

    I would also consider obtaining material and punching some custom shims of .05mm thickness.

    This should not be a big deal to accomplish, at least in regards to shim, making them custom and some as deltas.

    BTW, what dyno plotted that?
    The damper layout is the same as somebody else posted in the other thread, i.e. just like a regular blackbox damper except I have a bolt holding the piston on instead of a nut.

    I haven't figured out yet how to calculate the flow thru the rebound holes, but I do know that each individual rebound hole is smaller than my free bleed hole opened all the way, so it most likely doesn't give more than 2x the flow.

    What kind of a curve is desired for the rebound? At least in the automotive world it is often digressive, so the wheel can extend quickly after hitting a bump, yet have enough low speeds damping so as not to get bounced around at slower speeds. That's what I think is the problem with this fork right now - doesn't track super well over fast hits, while still a bit bouncy at low speeds.

    I don't think I can reduce the clamping area by much. The piston and the shim both have 8mm ID, and I need at least a slightly larger step on the bolt to hold both down. I don't think I understand the arrangement you propose to use smaller ID shims.

    I'm not sure it is even a shim clamping issue, though, as I am already using a thinner shim than stock and it pulls back pretty easily with my fingernail.

    The dyno is something I ghetto rigged up myself with a bunch of unistrut and data acquisition hardware. I think I can get it up the 100ips, but in this case there's no point. The compression damper works like I expect and I don't plan to make any changes to it anyway. The rebound damper I showed wasn't digressing sufficiently.

  13. #13
    PMK
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    Quote Originally Posted by beanbag View Post
    OK, so this is the part I still need help on. It seems that with the default shim (17.75mm x 0.15mm) the "Dual Flow" part is not duly flowing. (Pun) Although I haven't put the fork on the dyno to prove it, I can tell that the shim isn't really opening because I can turn up the low speed damping all the way, and the fork still feels "constricted" when I compress and release it (I'm applying about 150 lbs on it).

    My understand for how this setup is supposed to work is that the orifice gives the low speed damping, but its force vs velocity curve is quadratic, so by the time you reach higher speeds there is too much damping. Thus there is always the tradeoff where you want more low speed damping for crawling over rocks, while less high speed damping so the fork doesn't pack down during fast hits. The shim is supposed to help in this regard because its force characteristic is linear + offset, so at some point the shim opens and reduces the pressure to let the fork quickly extend.

    What I did was replace this 0.15 shim with a 0.10 shim I had laying around. (I think that is supposed to reduce the damping by about 3x) That helped, but I feel as though the fork still needs to "loosen up" a bit. Maybe the problem is that the rebound flow holes in the piston are pretty small. At some point, they will limit the flow more than the shim.

    The above taken from your first post. Based on this you are correct and need more flow and it does sound as if the holes in the piston are limiting flow.

    The notch in the piston is to assist in balancing the pressure and provide smoother response. It does ultimately add a second freebleed that is not controllable.

    You are running as per your post, a 17.75x.1 shim with a 9mm clamp. This is a fairly large aspect ratio for a single shim stack.

    My suggestion is to not open the ports using a drill. This will remove the wall thickness where there is none to spare, rather use a small end mill and if possible. Enlarge the port to the same diameter except the shape will now be an oval.

    Adding a different radius to the orifice location will make the shim less effective. This will allow more control onto the shim(s).

    Also, I would suggest hand porting the transition from port to orifice edge.

    Since you have the ability to record data, you could make one dyno run non shimmed on rebound, record rebound data only, and see at what point the flow orifices out and kills the flow.

    FWIW, when others are convinced a dyno tune is a must this topic proves that if you use a dyno, it must be capable of fairly high velocities. This takes HP and solid equipment for a fork.

    PK

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    I hate to ask, but are you sure you got the piston in the correct direction? I know taking apart the cartridges in my GSXR forks it took a few minutes of thinking to get it all back together right

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    Update

    I made my own piston to try and get more flow thru the rebound holes. The goal is to be able to achieve a digressive rebound curve, which I know is at least good in the automotive world. This means that the rebound ports have to flow freely at high shaft speeds.




    The rebound holes are significantly bigger, and I went up to a 20mm shim over the stock 18mm. However, based on the dyno plot, it only helped a little bit at the lower rebound settings where I usually run the fork. At least you can now see the curves turning over. I guess I am kind of disappointed that these changes weren't enough.


    It looks like I will have to modify the piston by extending the holes out to 22mm and give the surface a slight dish to preload the shims. Or just remake the piston with even bigger rebound holes.

    Quote Originally Posted by PMK View Post

    Adding a different radius to the orifice location will make the shim less effective. This will allow more control onto the shim(s).

    Also, I would suggest hand porting the transition from port to orifice edge.
    How much difference will these suggestions make?

  16. #16
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    My suggestion is in order to see the true change you need more IPS. At 45 IPs, that is a fraction of the forces seen during a ride. The larger IPS will ultimately peak the ports flow and the valving will not be a factor.

    If you want digressive, a cupped piston is good or a ring shim setup. Your best option seeing that piston you designed is to scale down or find a Race Tech G2 piston that will work. This will allow tuning port area, preload and stack stiffness. The one criteria the G2 will not embrace is port radial location.

    Also, in regards to increasing the shim diameter, be focused on the shim edge becoming the orifice or creating eddy problems to the flow at higher IPS.

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    I question the digressive curve idea. I hear the argument that you want to keep things from packing down on stutter bumps but slow movement on bigger moves. but what speeds is the shock moving on stutter bumps? it would be from extactly NOT moving, to only an inch out or so, which is not really enough time to gather speed from the spring pushing out. remember the spring force is a constant progressive speed at any point in the stroke, unlike compression forces. depending on the shape of the bump and speed at which you hit it, that will change the compression shaft speed. rebound is only the balance of the compression releasing and the spring pushing.

    you dont want very high shaft speeds with rebound, because thats where it throws you ballistically after a hard hit deep into travel. I do see how you would want stutter sensitivity, which removes the compression force balance entirely, without having a too fast rebound on body movements or rolling movements (though I would argue for no rebound damping on those as well, later though) so maybe this is a good place for a 2 stage stack. bleed for slow, initial shim for mid, but if its a deep hit and rebound spikes up higher from more spring force deeper, you dont want it to buck you so you have final shim stack to firm it back up.


    really each one is just a knee point though, it will always be firmer faster, it just takes a lot more to get there with more open ports.

    actually, there is the answer, use port tuning for the final damping stage, and a light shim for in between. which. I guess. is what you are doing.... nevermind. happy tuning

  18. #18
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    Quote Originally Posted by Dichotomous View Post
    I question the digressive curve idea. I hear the argument that you want to keep things from packing down on stutter bumps but slow movement on bigger moves. but what speeds is the shock moving on stutter bumps? it would be from extactly NOT moving, to only an inch out or so, which is not really enough time to gather speed from the spring pushing out. remember the spring force is a constant progressive speed at any point in the stroke, unlike compression forces. depending on the shape of the bump and speed at which you hit it, that will change the compression shaft speed. rebound is only the balance of the compression releasing and the spring pushing.

    you dont want very high shaft speeds with rebound, because thats where it throws you ballistically after a hard hit deep into travel. I do see how you would want stutter sensitivity, which removes the compression force balance entirely, without having a too fast rebound on body movements or rolling movements (though I would argue for no rebound damping on those as well, later though) so maybe this is a good place for a 2 stage stack. bleed for slow, initial shim for mid, but if its a deep hit and rebound spikes up higher from more spring force deeper, you dont want it to buck you so you have final shim stack to firm it back up.


    really each one is just a knee point though, it will always be firmer faster, it just takes a lot more to get there with more open ports.

    actually, there is the answer, use port tuning for the final damping stage, and a light shim for in between. which. I guess. is what you are doing.... nevermind. happy tuning
    I guess it depends on your point of view and how big those repeated bumps you are riding are. Personally the inch high stutter bumps aren't a concern where I ride, but 3-6 inch repeated bumps are. These are using most of the stroke so a fast rebound from deep in the stroke is needed or it packs up fast.

    I haven't dyno'd the forks or shocks I revalve for myself, but I'm generally running the high speed rebound circuit loose enough that full closed on the rebound knob is a smooth return when unloaded. I'd expect this to be digressive, but I haven't been able to check. There is no instability problem with running high speed rebound this fast as the high speed rebound is letting the fork extend fast to keep the tyre planted on the back side of the bumps. When the fork is loaded the balance of forces leaves you with only a small extension force which gives a low speed rebound.
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  19. #19
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    I don't know much about motorcycle suspension, but the few times I looked for info I saw that a digressive curve was recommended. If I am wrong and that is in fact not a goal, then, ummm, nevermind. My complaint about the fork so far is that for slow speed crawling up rocks uphill the fork feels underdamped, but for fast bumps downhill, I can feel the front wheel occasionally lose grip when braking. I assume this is because the fork doesn't extend fast enough after a bump. My understanding for the rational of a digressive curve is that slow shaft speeds have to do with the suspension sprung weight involving the rider. That includes the rider getting bobbed and pitched around. Because the mass is more, you need more damping.

    High shaft speeds have to do with the unsprung weight of the wheel only, to extend quickly following a hit in order to track the terrain. Since the mass is lower, you need less damping.

    I think I still need to go to bigger ports because I am already using the thinnest shim possible. Last night I put a 1 degree dish in the piston, and that seemed to have no effect at all on the preload. So I am a bit confused about that.

    Maybe I should go to a digressive type piston, in which the shim seals a continuous ring round the face of the piston. Then when the shim lifts up, the effective port opening is much bigger.

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    Quote Originally Posted by beanbag View Post
    My understanding for the rational of a digressive curve is that slow shaft speeds have to do with the suspension sprung weight involving the rider. That includes the rider getting bobbed and pitched around. Because the mass is more, you need more damping.
    That is exactly right. Your suspension has two naturual frequencies. The fast one which is it moving the wheel up and down. The slow one is moving the rider up and down.

    A digressive rebound curve lets you seperate these two even further, allowing for control of the slow natural frequency (stability) while keeping fast response of the wheel (better traction).
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    Quote Originally Posted by beanbag View Post
    Maybe I should go to a digressive type piston, in which the shim seals a continuous ring round the face of the piston. Then when the shim lifts up, the effective port opening is much bigger.
    What about going for a spring loaded blow-off instead?

    Once open, it will be definitively digressive. You can play with the spring rate and preload on it to control the knee of the curve.
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    In my experience it is hard to make a spring blow-off type valve that isn't too digressive.

    But maybe it could work.

  23. #23
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    New digressive piston.

    Dyno and ride report coming up.
    Last edited by beanbag; 08-26-2011 at 10:53 PM.

  24. #24
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    Quote Originally Posted by beanbag View Post
    New digressive piston.

    Dyno and ride report coming up.
    The thing looks pretty.

    Difficult to see details, did you make the compression side parabolic?

    You may also find that the port locations may be placed slightly to close to center for allowing small changes via the stacks.

    You will know soon enough. You already know this is sometimes the effort needed to get dialed in. Hope you have great results.

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    The compression side is flat. It is only for a check valve with a weak spring anyway.
    The port location placement was not easy. Because the rebound side is digressive, it needs to have a continuous lip that seals all the way around. (i.e. the sealing surface must still work if you change the preload on the shims.) Between the size of the shim and the groove cut in the side for the plastic band, there isn't a lot of room to place the ports.

  26. #26
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    Quote Originally Posted by beanbag View Post
    The compression side is flat. It is only for a check valve with a weak spring anyway.
    The port location placement was not easy. Because the rebound side is digressive, it needs to have a continuous lip that seals all the way around. (i.e. the sealing surface must still work if you change the preload on the shims.) Between the size of the shim and the groove cut in the side for the plastic band, there isn't a lot of room to place the ports.

    Yes, I agree and understand. Noticed the pocketed port on the one side. Sometimes this is done towards the periphery to move the port without moving ports orifice. Always a compromise. Still a nice piece of work.

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    Looks great beanbag. Really looking forward to the ride report.

    I do not have access to the awesome machining equipment you have but do you know where I could acquire a thinner shim from so I could get that little bit more of performance?

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    croak:
    I got some of my shims from a local shop that does motorcycle suspension tuning. mx-tech.com also sells shims, but you have to pay a lot upfront for shipping.

    I haven't had much time to ride except for the smoother trails close to me, but so far things are looking good. The fork has more low speed control so when I ride over bumps uphill it doesn't bounce around as much. Braking and wheel tracking over fast bumps is improved as well. The only slightly strange thing is that over fast little hits, the fork has a little bit of a rubbery feeling. Usually I take this to mean that it needs more compression damping somewhere, but not sure. Maybe it is that effect PMK mentioned due to how the Motion Control compression damper works. The damper body has a little bit of flex in it so you get about 1/8-1/4" inch of free travel before the actual damping kicks in. This creates a bit more plushness, but maybe a lack of control or "road feel". Maybe I could use some Push style midvalve compression damping LOLOLOLOL.

    I don't have a dyno plot yet because I did a last minute shim reshuffle. However, I am sure that the curve is digressive.

  29. #29
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    Quote Originally Posted by beanbag View Post
    Maybe I could use some Push style midvalve compression damping LOLOLOLOL.
    That or switch to a BlackBox Motion Control Damper. The tube is Titanium and not as flexible. It has a shim built in that I'm sure you can tinker with given your mechanical skills.

    The damper is riveted, so some elbow grease is required to do mods to the stack.
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    Thanks Beanbag, ill call up some local shops then.

    I have the blackbox damper and while it works pretty well I do have an idea to tinker with it(I do not feel like it has a good range of LSC adjustment, feels locked and then completely open within one click). The damper has 3 port orifices, that open up progressively when adjusting the LSC knob, with a shim stack right behind it (shim has a bypass port drilled in for 1 of the 3 port orifices). The first three or so clicks out from lock opens up only 2 of the ports which lead directly to the shim stack with the subsequent clicks opening up the bypass port as well.

    I think if I were to beef up the shim stack, while still maintaining the bypass hole, it would give me the speed sensitive LSC that I want (when only 2 of the orifices are open) while still allowing the HSC to flow freely when the floodgate opens up.

    The port orifice/shim/foodgate combination makes for some interesting dynamics jeez ha.

  31. #31
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    I did some more riding, and I think my initial impressions were correct. Low speed control is much better and the wheel tracks well over bumps at higher speeds as well. However, I now think the compression damping is a bit off. There's this feeling that energy isn't being absorbed, but if I turn up the compression some more it starts to feel like the fork doesn't want to compress easily. It might be because the Motion Control valving is too digressive. From my automotive experience, I like a compression curve that is only slightly digressive.

    Maybe I can fit in a Manitou ABS+ damper because that is user tune-able.

  32. #32
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    Quote Originally Posted by beanbag View Post
    Low speed control is much better and the wheel tracks well over bumps at higher speeds as well.

    However, I now think the compression damping is a bit off.

    There's this feeling that energy isn't being absorbed, but if I turn up the compression some more it starts to feel like the fork doesn't want to compress easily.

    It might be because the Motion Control valving is too digressive.

    Beanbag, not to be nit picky, but can you explain each statement to better clarify?

    #1, if low speed is better, I am assuming this is low speed compression, not low speed rebound based off the other statements posted. so with the new valve you machined, I was under the impression it retained the same checkplate style setup.

    #2, if the compression is a bit off, I'm assuming this is based on the change in port size? Was anything else altered on compression during the change.

    #3, the feeling that energy is not being absorbed, this is LSC or HSC? You added more LSC and it got worse, that seems to backup the previous statement.


    Keeping the checkplate setup places full compression control into fluid volume / displacement changes.

    Can you take one photo of all parts "stacked" so I can fully see where the flow paths lay and how you have altered them.

    The comparison to automotive is fine, provided the automotive uses a passive and active set of valves, assuming these do live inside your fork.

    Sorry for the lack of info, it is a simple setup but some of the description is now getting confusing.

    PK


    Re-reading the entire topic, yes, I agree, the other valve (compression valve) is not shimmed correctly, preloaded correctly, or correct in the aspect ratio of rod dia. to port flow. The rebound is dialed in, now you need to reset the compression portion of the damper.
    Last edited by PMK; 09-03-2011 at 04:08 AM.
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  33. #33
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    Quote Originally Posted by PMK View Post
    Beanbag, not to be nit picky, but can you explain each statement to better clarify?

    #1, if low speed is better, I am assuming this is low speed compression, not low speed rebound based off the other statements posted. so with the new valve you machined, I was under the impression it retained the same checkplate style setup.

    #2, if the compression is a bit off, I'm assuming this is based on the change in port size? Was anything else altered on compression during the change.

    #3, the feeling that energy is not being absorbed, this is LSC or HSC? You added more LSC and it got worse, that seems to backup the previous statement.


    Keeping the checkplate setup places full compression control into fluid volume / displacement changes.

    Can you take one photo of all parts "stacked" so I can fully see where the flow paths lay and how you have altered them.

    The comparison to automotive is fine, provided the automotive uses a passive and active set of valves, assuming these do live inside your fork.

    Sorry for the lack of info, it is a simple setup but some of the description is now getting confusing.

    PK


    Re-reading the entire topic, yes, I agree, the other valve (compression valve) is not shimmed correctly, preloaded correctly, or correct in the aspect ratio of rod dia. to port flow. The rebound is dialed in, now you need to reset the compression portion of the damper.
    1) I was referring to rebound and not bouncing back after hitting a rock or pothole.

    2) The new rebound piston still doesn't make much compression damping, so I was complaining about the Motion Control foot/head(?) valve.

    3) I dunno what the effects of LSC and HSC feel like until I've tweaked the amounts personally.

  34. #34
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    Beanbag, seeing the compression portion as posted in the other topic, and truth be told I hardly ever work on these model forks, it appears the system has no means to isolate effectively the compensator from the fluid.

    Correct me if I'm wrong, but isn't the black or red material MCU. If it is, the designer has utilized the material to act as a compensator for fluid volume changes.

    While not a new idea, most suspension tuners have often noted that MCU is a poor way to respond to fluid changes.

    Some of the problems are the lack of initial head pressure on the fluid. The limited range of non linear compressibility. The MCU deterioration from exposure to the fluids and its natural breakdown, even when not in an oil environment. (BTW, it is not so much the oil per say in the fluid, but the additives to make it a better suspension fluid or the synthetics.)

    In short, I wonder if the traits you feel on the trail, are issues with the dampers inability to be effective on the higher speed compression and more important rebound events.

    The damper design is somewhat similar to a unit referred to as a Twin Chamber design. Basically it is very similar to a rear damper, but runs less pressure. If you can envision running your rear damper, or for example one of the dampers on the car stuff you work, being deficient in gas pressure. Not dead flat, but say half what should be in there, compound this with a slow response from the IFP let's say from an oversize "O"ring. This would somewhat mimic an MCU.

    Ultimately, if you had the ability to push the damper rod in (compress), it should feel smooth and have a linear resistance, starting with some pressure at the initial movement. Extension should be smooth and steady until fully extended.

    If the material is not MCU and used in that application, disregard the above comments.

    PK
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    It's not MCU. There are a couple of thread about how the Motion Control damper unit works. It's basically a bleed plus blowoff assembly that has a spring in series with it due to it's flexibility. But u r right that it does operate similar to a twin tube shock.

  36. #36
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    Quote Originally Posted by beanbag View Post
    It's not MCU. There are a couple of thread about how the Motion Control damper unit works. It's basically a bleed plus blowoff assembly that has a spring in series with it due to it's flexibility. But u r right that it does operate similar to a twin tube shock.
    If that's the case and it is rigid plastic, what means is there to drive the fluid back across the compression piston?

    PK
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  37. #37
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    check valve + suction from rebound rod

  38. #38
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    Quote Originally Posted by beanbag View Post
    check valve + suction from rebound rod

    Give up on the design. With no positive pressure, except trapped air, I doubt you will overcome the problems you have. Sounds like if ridden hard or frequently over square edges it will be a cavitation monster.

    On the flip side, as simple as an open style damper is, at least the positive pressure from the tubes telescoping provide refill for the damper.

    PK
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    There's nothing for me to "give up" on because that's just how the Rock Shox compression damper works.

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    Beanbag do you have the motion control with floodgate adjustment? It sounds like you just need to lighten up on the floodgate. It make the compression feel a lot less digressive.

    Hell ive been running my Blackbox MC locked and "adjust" the LSC with the floodgate to make the port slightly open already. This gives me a medium amount of LSC that filters out pedal bob and brake dive nicely but still opens smoothly on anything larger than a small hit. The shim stack behind helps to not use to much travel during HSC it feels like.

    And I think PK got a little bit confused

  41. #41
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    PK is not confused, just very surprised to see this type of damper run with no pressure system at all. As I have posted, these are not dampers I really work with. The design is similar and better executed on other applications or brands.

    I did some searches about these Rebas, as one person noted, the design is very simple, not pressurized, and when compared to an automotive equivalent, pretty much the least performance of oem car shocks.

    For the performance Beanbag desires, he may be hard pressed and limited by the oem design. The lack of pressure also makes it difficult to effectively add a mid valve that won't cavitate.

    Three choices, figure out a way to pressurize the system, ride it unpressurized and make it as good as possible, switch to a pressurized fork from another brand.

    Also realize, the fork will build pressure when compressed, this makes things better, just not in the realm of where Beanbag is looking to improve performance.

    From what it appears, this design mimics a Showa Twin Chamber fork from the 1998(?) Suzuki RM motorcycles...RockShox forgot one item...the pressure spring and IFP, BTW, these were some very cutting edge and decent performing forks. Now similar designs exists, albeit USD style though.

    PK
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    What XC forks have a pressurized damper section?

  43. #43
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    In regards to the pressurized damper section, many times it is not so much where a huge positive pressure is applied via N2 or the IFP spring.

    Much depends upon if the cartridge is recirculating or non recirculating. For example, a recirculating systems that was just slightly pressurized is an old school Bomber. The damper shaft leakage would constantly bleed fluid at a somewhat controlled rate during rebound. This would minimize emulsified fluid as it was ejected out the cartridge upper end. Pressurization was derived merely from compressing the fork.

    A FOX fit system uses a bladder to isolate the fluid and entrapped air from assembly. The bladder is able to absorb displacement, while the fork compression again offers the pressure to drive fluid back to where it needs to be.

    Open cartridge style FOX units with a basevalve aka footvalve, also count on the forks compression to drive the fluid back into the cartridge under pressure.

    Some of the Cannondale Leftys, utilize a truly pressurized bladder.

    While not quite a true xc fork, the ATC fork uses a cartridge assembly. The cartridge does not use thru shaft design. It may be the most basic cartridge of any bicycle fork I have worked on. Yesterday I was modding another one for consistent fluid volumes and easy bleed design with the addition of a bleed hole. While bleeding the damper and cycling the unit for proper operation, the wife came out into the shop. Since we run one of these forks on our Fandango tandem, she knew what was going on with the mod. That damper uses an IFP and coil spring to provide pressure. She watched as the damper was fully compressed and then extended without any assistance. Certainly she was impressed. On a serious side, she is a finicky stoker and does notice changes to the bike. She may not understand but can tell good vs bad. She does know that some of the lesser forks get confused. Being more simple than these forks, there is no external clickers or lockout. The valving is a beyond simple with a short travel checkplate / orifice piston and no piston band. The checkplate is closed via a curled spring washer. The design is stupid simple, damping changes are done via fluid viscosity. Performance wise it is obviously less than our Fox 40, but is 1/3 the cost. Also, once the fluid viscosity was dialed in, the fork became an Energizer Bunny, and ironically does nothing stupid on the hardtail tandem. The damper is pressurized.

    The photo of the two compression assemblies side by side appears to show that Manitou uses some form of foam to compress and then extend based on fluid volume change. This is most likely their pressure system to drive the fluid out of the compression assembly.

    There should be something in that Rockshox being overlooked, that should help drive fluid back out of the compression assembly. If not and it is as you say, suction makes this happen, then the fork is as we say "it sucks" often in more ways than one. If it does not have a means to drive the fluid out make that a primary goal. With that the fork will come alive, also it will then work even better with a midvalve.

    BTW, I give photo credit to LyNx, as I borrowed it from the other topic.

    PK
    Attached Thumbnails Attached Thumbnails Converted my Reba to Dual Flow piston - need valving advice-p1060435.jpg  

    Last edited by PMK; 09-06-2011 at 05:29 AM.
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    Quote Originally Posted by PMK View Post
    The photo of the two compression assemblies side by side appears to show that Manitou uses some form of foam to compress and then extend based on fluid volume change. This is most likely their pressure system to drive the fluid out of the compression assembly.

    There should be something in that Rockshox being overlooked, that should help drive fluid back out of the compression assembly. If not and it is as you say, suction makes this happen, then the fork is as we say "it sucks" often in more ways than one. If it does not have a means to drive the fluid out make that a primary goal. With that the fork will come alive, also it will then work even better with a midvalve.


    PK
    Just for reference, this is an old Manitou TPC+ Compression damper. I think there's nothing pressurizing the fluid, other than the air trapped in the cartridge.



    However, Manitou had a damper called Intrinsic that had a floating IFP. They had a spring loaded version and an air loaded version. The intention was to drive a kind of compression valve (in gold in this picture) but it's rather position sensitive as opposed to speed sensitive.




    Here's a good explanation by Udi on how the Compression Section works in a Motion Control damper.

    Question for Boxxer Guru's - Udi, Sprocket, etc. - Ridemonkey.com





    I don't think there's anything driving fluid back during rebound. It's actually sucking all the time.


    Looks like Manitou is finally listening to its customers...

    Manitou's Marvel Pro and Real Custom Tuning - Eurobike 2011 - Pinkbike.com

    Last edited by Warp; 09-06-2011 at 08:14 AM.
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    Quote Originally Posted by Warp View Post
    Looks like Manitou is finally listening to its customers...

    Manitou's Marvel Pro and Real Custom Tuning - Eurobike 2011 - Pinkbike.com

    The tuning booklet was posted in the ABS+ shim tuning thread:

    http://forums.mtbr.com/shocks-suspen....html#poststop

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    Quote Originally Posted by mullen119 View Post
    The tuning booklet was posted in the ABS+ shim tuning thread:

    http://forums.mtbr.com/shocks-suspen....html#poststop
    Yeah... and the kit(s) have been available for a while... I was just bringing it up to this thread.

    Sorry for not giving the credit where it was due.
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    Manitou needs a big thumbs up for making those kits available and giving out the supporting documentation. Excellent info.

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    Quote Originally Posted by TigWorld View Post
    Manitou needs a big thumbs up for making those kits available and giving out the supporting documentation. Excellent info.
    Their forks have always been the choice for geeks who like to tinker. It's good to see them fully supporting that. I have been asking for the better part of a decade.
    Owner of www.shockcraft.co.nz and NZ Manitou Agent.
    www.dougal.co.nz Suspension setup & tuning.
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  49. #49
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    I can believe they are counting on the trapped air. Post #38 mentions it.

    The problem lies in that Beanbag is pushing the limits of the dampers design. Once exceeded, he then deals with some type of loss in fluid flow, or if extreme, cavitation.

    Suffice to say, that probably any brand or model with a compression adjuster on top, that utilizes entrapped air, will battle various issues of air across the shims, emulsified fluid, and more than likely, migrating fluid with use.

    Truly these Rockshox and Manitous are very simple forks. They probably work well within the design spectrum. Those that have trails or ability to go beyond this may find the faults.

    PK
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    Quote Originally Posted by Dougal View Post
    Their forks have always been the choice for geeks who like to tinker. It's good to see them fully supporting that. I have been asking for the better part of a decade.
    Agreed, when they would bring back a 160mm fork I would be all over it.(2013 lineup is what I was told)

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