Mid valve float - seeking knowledge/guidance

[Jakobi]

So, straight up before we get too carried away we know that forks need to work together for best performance. Everything from seal stiction, mid valve stacks, base valves, spring rates (and/ICS), and fluid viscosity and volumes. Thats a give in, and may restrict the answers I'm looking for to a certain extent as well.

 

What I would like to know in a round about way is.. when would you look at opening the float up, and when would you tighten it? What negative trait/rider feedback would initiate the thought to change the float.  

 

In theory, less float = firmer across the board, and more = softer. Obviously these cross mx/trails etc.

 

In my instance, I'm working with a set of 48mm CC Marzocchis with some stacks that have been modified from stock, and the float tightened up from .65mm to .4mm. Overall action feels nice, but the fork still spikes/deflects off stray rocks/short square edges. More noticable with the bike laid over as the deflection then changes your direction. Hitting square on standing it can be felt through the bars but is bareable. Reducing LSC doesnt particularly help with said behaviour, and adding LSC causes everything to feel too firm. Rebound is good, bike settles in turns, doesn't bounce back up after landing, and doesn't seem to pack down. I'm thinking less MV Comp/more float, but would like to hear others opinions before hooking into it.

[Kyle Tarry]

Midvalve is a bigger player at higher fork speeds.  Base valves tend to be linear or digressive, midvalves tend to be quite progressive.

 

Keep in mind that, when comparing floats across different forks, you need to take into account factors like piston diameter.

[Jakobi]

Thanks Kyle. Agree with what you have said., but still kind of dances around the particular question. If a fork is performing well most of the time, but occassionally spikes in the faster terrain on the sharp hits (random square edges objects), what would be the suggested approach. I'm pretty confident that the issue is too much damping as there are no signs of it blowing through too fast. It doesn't feel divey, it just doesn't soak up the entire impact and either deflects to the side or pushes up some. I'm personally thinking of opening the float up another .1-.15mm, but I'm not sure what the better approach would be. More float, or less MVC, or both... or if my thoughts are actually even in line with what others would do to address the handling behaviour I'm trying to remove.

[Kyle Tarry]

I think that spikey feel on sharp hits like that is a good time to work on midvalve.comp.  However, I think it also depends on the fork some; for example, the KYB OC forks on my WR have a huge midvalve piston so they need to flow a ton of oil, and they need a lot of float; on the other hand, the CC KYB forks on YZs and TXCs (among many others) have a much smaller mid piston, so the flow is less and the mid is a smaller percentage of the overall damping.

 

You said you're trying to decide between more float or less MVC; I'm not sure that I understand the distinction.  By "less MVC," do you mean a softer midvalve shim stack?  That may or may not help, you'd need to look at the setup; I say this because once the midvave snaps fully open and hits the backer shim, it doesn't really matter how stiff or soft it is any more, just how tall the taper is, and how much float you have.

[Jakobi]

Thats what I meant, softer compression stack. The forks I'm working with are very similar to KYB SSS design wise, however seem to function a little differently in what they want regarding the stack which I believe due to differences in the ICS spring preload.

 

I think I will open up the float .1mm next time I pull them down and then experience the difference. Stock they had .65mm and felt a bit vague/soft for my liking. I went to .4mm looking for a sharper more precise feeling knowing it would be a bit stiffer. I don't mind a firm setup, as long as it doesn't deflect.

[Kyle Tarry]

Yeah, the biggest thing to keep in mind is that once the stack gets to max deflection (when the shims hit the backer), being softer won't make a difference.  When I add float, I always do it by either removing face shims or spacing out the piston (longer sleeve), because I want to let the shims flex.

 

I think adding a 0.1 mm shim to space the piston is a good idea!!

[Jakobi]

The stack I'm running has a reverse taper to back fill and close the float up (2 x .15 shims currently). Will be quite easy to pull a single one out, and I think I have enough spare shims I should be able to remove both and replace with a .2

[numroe]

Jakobi, I think you can have your cake and eat it. Try a tighter float but much softer stack.

 

Less float does not have to be "firmer across the board". At least not with the KYBs on my YZ250 and KX450F. These stacks can bend a long way.

 

My kawi has 0.15mm float and I can smash into anything sharp at decent speed and rarely feel a thing. My YZ has 0.17mm float gap and it's similar, except the much lighter 2 stroke can get knocked around easier.

 

With this kind of setup, the mid valve gives me feedback for the initial wheel movement when initially pushed up by the ground. Giving that sharper more precise feeling.  Not harsh. Just some feedback.

 

My stock 2010 SSS KYB CC fork setup had 0.5mm float. Sure it was soft and supple over the little chatter bumps, but I didn't like it because I cannot feel the tire. I find it more reassuring to feel just a little of what I ride over.  So I know how I'm going with wheel placement.

 

I guess the soft mid stacks will fatigue and wear out (break) sooner, but I see no problem replacing those shims every 150 hrs or whatever.  The stock stacks are probably good for 500 hrs. I speculating of course. Just saying what works for me - at present. ?

Edited June 2, 2014 by numroe

[Kyle Tarry]

What is the piston diameter on the CC Marzocchis?  You may be comparing apples to oranges, trying to compare them to late model KYB CC forks.

 

0.5 mm of float is a lot on 2010 KYB CC forks; it's not a lot on some other forks.  It's all about flow.

 

If the stack stays the same, less float will be firmer across the board.

Edited June 2, 2014 by Kyle Tarry

[Jakobi]

Numroe, whats your definition of a soft mid stack? I do like what you say, some feedback, as long as its not jarring/deflecting dangerously.

 

Kyle, MV requires a 20mm OD shim to close the ports, BV requires 32mm OD. All 8mm ID.

[numroe]

Kyle,  I have no idea about his Marzocchis beyond that he said they are similar to KYBs.

The float gap mm figures is not my point. Probably best if I never mentioned them.

 

My point to Jakobi is to try a softer mid stack. Much softer. And avoid the wide float, because of the feeling he said he is seeking.

 

If the stack is softer, then less float does not have to be firmer across the board.

 

For example.  Stock:

4 x 20

18

16

14

12

11b

 

Mine (does not feel progressive to me. This stack must bend a long way):

2x20

19

18

17

2 x 11b

Edited June 3, 2014 by numroe

[H-B-R]

I agree with numroe, I finally found the set-up I was looking for when I went with low float/soft mid valve. I would have to dig up my notes to find the actual stack, but it was similar to what is used in many of the DDT set ups. My float was .1

I should mention I ride woods only.

[Kyle Tarry]

I think it also depends on where you're having trouble with the bike.  A softer mid stack will definitely decrease damping force at low/mid fork speeds, until the mid blows fully open, then the forces may spike (depending on float) regardless of what the mid stack is.

 

From the shim dimensions he gave it sounds like the Marz CC has similar internal geometry to the KYB stuff, so I take back what I said about it not being apples to apples.

 

I think ideally you want a low (zero?) float mid with a soft stack and lots of deflection, but the problem is that the deflection is limited by the shim size and cupping limitations; that's where sprung mids can be a cool solution...

[MarekB]

I think the basic question to be asked here is whether one wants progressive damping ratio. Because that's basically how midvalve works. From my experience in the woods progressive damping ratio is not good, because hittng hard objects with high speed will cause damping spikes and deflection. So for the woods I'd get rid of midvalve alltoghether or run large float with stiffer base valve...

[H-B-R]

I think the basic question to be asked here is whether one wants progressive damping ratio. Because that's basically how midvalve works. From my experience in the woods progressive damping ratio is not good, because hittng hard objects with high speed will cause damping spikes and deflection. So for the woods I'd get rid of midvalve alltoghether or run large float with stiffer base valve...

 

I never got rid of the MV, but I tried large floats for a long time.  I now realize why I was never happy with that - the high float was nice for absorbing the little stuff, but when you "ran out" of float, things got harsh when the MV and BV kicked in.

 

I personally like the soft MV damping to be in play right away.

[Jakobi]

Stock MV was

Comp (from piston)
12x.10
20x.15(2)
11x.20 (2)
15x.15
16x.15
18x.15
0.65mm float

 

I've already started moving in the direction suggested, but hadn't really softened the stack a whole lot. More so just made a more progressive stack.

 

Currently at

 

12x.10 - Bleed
20x.10 (3)
18x.1
16x.1
14x.1
12x.1
11x.20 (2)
15x.15
16x.15
0.4mm float

 

I could always pull another face shim, or possibly the 12 and 14..

[numroe]

At high speed comp, wide float doesn't make a big difference. What matters is the ability of the mid piston to flow oil, and the ability for the mid stack to bend (many mm) out of the way.

A bonus of softer mid is less cav on hard compression hits, so more predictable rebound.

Edited June 2, 2014 by numroe

[Kyle Tarry]

At high speed comp, wide float doesn't make a big difference. What matters is the ability of the mid piston to flow oil, and the ability for the mid stack to bend (many mm) out of the way.

 

I disagree with this.  The midvalve shims aren't going to bend "many mm."  The stack will only open until it hits the backer and/or cup washer, which is usually on the order of 1.0 mm (Jakobi's stack in post 16 can open about 0.8mm).  If they opened a lot more than that, they'd quickly cup anyway.

 

So, a float of 0.5 mm is a very significant amount of flow!

 

The ability of the piston to flow oil isn't a factor at low to medium floats, because the primary restriction is in the shims, not the piston.  I haven't done as much work on the CC forks, but on traditional OC forks it takes something like 1.5 mm of float to transition to piston flow restriction.

[Jakobi]

Loving the conversation and hearing peoples thoughts. Even though the piston sizes are similar to the KYBs its not entirely apples to apples. These forks require softer stacks. All I ride is bush (woods), but the terrain changes around every corner so I'm after a versatile setup and happy to make the compromises as long as they aren't dangerous/unpredictable.

 

Kyle, you mentioned a sprung MV. Could you elborate on this. I'm only new to the game and learning as much as I can.

 

Would anyone be willing to share their views regarding how the midvalve works? My understanding would be at lower forces/shaft speeds the float would allow free bleed until reaching the backer, at which point the damping characteristics of the stack would begin to work. Total flow is a combination of both shim deflection and float. Is this correct? or does the shim stack start to deflect before the float is totally consumed?

 

Numroe is saying by having a lighter stack and less float (bleed) you can effectively have the mid valve damping kick in sooner (removing the mushy feel), and the lighter stack allows for more deflection (at the expense of some added shim fatigue). Kyle makes the point that depending on the configuration that total flow can be limited by the stack before the pistons haven reached max flow, which should be noted.

 

At .4mm I am happy with the feel and initial travel of the fork. It is at the higher speeds that I'm feeling some spike. Say crusing 70km down a fire trail and there is a pothole or a rock edge sitting out of the groud (maybe 10cm). Hitting that will indeed cause a spike. Likewise, coming around a corner and hard into the gas and hitting a single root feels similar. Its very close to where I want it, just needs a little tweak.

 

I think from what I've read, more float will make things more mushy initially, but will also achieve the extra flow I need. On the other hand, keeping the float the same and softening the stack some more may achieve the same thing too, while retaining what I have. Won't know until I put them into practice.

[Kyle Tarry]

Great post, I think that you are on the right track.

 

With regard to sprung mids:  What you want (well, I think it's what you want) is a midvalve that has a linear deflection from a very small opening (low float, for response) to a very large opening (maximum flow to avoid spiking on big hits).  Problem is, you can't really have this on a conventional mid, because the shim can't physically bend that far back.  So, the traditional solution is to make up any additional lift you need with float, but the downside is it can be mushy (depending on the forks, the setting, the base valve, etc).  As an alternative, you can run a spring behind the mid, so you get a linear rate of deflection with a much longer travel.  I've run some back-of-the-envelope calculations on what rate the spring should be, but I never got any further than that.  I think that the SPI Del Taco setup is something like this but I've never seen one.

 

I think Numroe is on a good track, with a few caveats.  For one, when you talked about a softer stack, you've got to be a bit more specific; the reason I say this is because you care about both the softness AND the max lift.  You could have a stack that was soft but hit the backer really early, and that probably wouldn't be good for highspeed harshness.  I try to build mids with a healthy taper and some good spacing to the backer shim, but on the other hand if you go too far the shims will get damages (cupped).  The other is that some forks (with larger midvalve pistons) simply can't bend a shim far enough to get sufficient flow for a plush woods setup with low float, IMO.  His forks and your forks might not fall into that category,.

 

IMO you're pretty close about how the mid works.  Be careful not to think about it as having distinct phases (bleed, then shim deflection), although I don't think it's that far off, any force on the spring is also force that will be deflecting the shims by some amount.  Spring rate, preload, etc can all have an effect.  Normally the check springs are pretty soft, so you probably don't get a ton of deflection until the float is used up, I would guess...