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?
...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.
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.
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.
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?
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.
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.
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.
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.
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.
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.
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
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.
Originally Posted by PMK
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.
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.
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 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.
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.
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).
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.
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.
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Originally Posted by beanbag
