Adding a check valve to the rebound chamber on a KYB OC fork?

[turbo dan]

I'm wondering if anyone has tried this?  I know some folks have modified them to perform just one function per side to address the cavitation issue but this seems like another way to do the same thing.  It seems like compression damping wouldn't be significantly affected since the mid still has a trapped volume of oil between itself and the base valve.  Whatever volume of oil not able to pass through the mid could be supplied to the cartridge through a check valve to prevent cavitation and any loss of damping control on high speed hits.

 

It seems too simple for it not to have been done this way from the factory.  Is there something I'm not accounting for?

[Kyle Tarry]

Can you explain exactly what you're suggesting?  Where do you want to put the check valve?

[turbo dan]

I suppose it would have to be integrated into the cartridge cap somehow, just to allow backfill into the cartridge behind the mid valve any time negative pressure develops inside the cartridge.  In theory it seems like it could help but I don't know if it could practically be implemented.

[Kyle Tarry]

Cavitation might not just be a function of static pressure in the rebound chamber.  For example, when the fluid goes through the midvalve ports it may accelerate and drop pressure lower than the static state and cavitate then.  I think you would need to have it open below some baseline pressure, but then you'd have an open flow path at low damper velocities and very little damping.

 

Interesting idea though, need to think more about it...

[turbo dan]

I've got a spare set of cartridges to play with so I may tinker.  It seems like a valve similar to the one used on the rebound side of the base would work well.  The whole concept assumes that almost all of the work the mid valve does would not be affected by allowing a second path of oil into the top of the cartridge.  There is still the volume of oil between the base and mid, and just looking at the shim stacks it doesn't look like the mid would be able to force anything past the base. 

[djidji]

...it is allowed to warm up 3 1/2 year old thread?

I wonder if cartridge back-fill has any drawback, like sucking air during refill or other things, making the rebound damping less consistent?

jeanjean

Edited March 1, 2018 by djidji

[mog]

TC forks got rid of the problem

[djidji]

yes, but a lot of enduro (woods) riders are still on OC forks...

jeanjean

[mog]

No one is developing them

[djidji]

...WP maybe...

I would still consider the Xplor to be a OC-fork development.

Anyway, I know that they are not state of the art. CC forks are now a quart century old (next year, to be more precise).

OC forks disappear from motocross world because they have drawback. I just try to find out and to understand what they are.

Cavitation is one of them, and a back-fill valve is one of the solution to try to avoid cavitation, which was not really followed by the manufacturers, and maybe excepted SPI, also not by the tuners. But I haven't yet found an explicit reason.

That's why I ask.

jeanjean

 

[turbo dan]

Cavitation doesn't really seem to be that big of a problem.  It may be happening but I have experienced no discernable loss of damping control.  More of a problem in theory than in practice, and in theory it occurs in twin chamber forks as well. 

I was riding with a buddy a couple weekends ago, he's on a 2018 YZ450.  He had constructed a ~45 foot tabletop with a left hand rut about eight feet from the bottom of the landing.  It took a few tries but once I got it dialed in I could make the corner every time.  If cavitation were an issue here I should be pogo sticking right over that rut.  I have no issues with braking bumps and turning inside of these shit hot brand new fuel injected 450's with the twin chamber forks.  I also have excellent bottoming resistance, equal to or better than the stock Yamaha 450 forks.  I'm sure with revalving these twin chamber forks could exceed my flintstone forks in performance but you would be surprised how well they can work.  The stock valving just sucks.  Leaves a lot of room for improvement.

Seems like the real advantage of the twin chamber is the ability to blow off high speed hits through the base valve and then push the oil back into the cartridge with the ICS.  Makes square edges disappear.  I'd rather have a simple, easy to work on open cartridge fork that still works very well.  I certainly understand why Kawasaki, Yamaha and Honda stuck with them as long as they did.  For the majority of riders they can be made to work very well.  I don't race for a living, but I do enjoy beating up on bikes that are a decade newer with 20 year old technology.

[djidji]

Indeed, cavitation should not be a problem in CC forks due to the pressurization. It works like a "de carbon" damper as the rear damper does.

Cavitation may also not be an issue on a well tuned OC fork. 

I wonder if it would be an issue with a midvalve setting à la SPI phase 4, with blow-off and zero float, but without the cartridge backfill. It may be OK as long as the midvalve and the base valve are well balanced.

In a CC, such a midvalve construction should not be a problem, thanks to the pressurization. Rear shocks also have no float.

On another side, emulsion dampers don't have float as well...

jeanjean

 

[turbo dan]

Open cart forks self pressurize too.  The cartridge builds pressure internally due to rod displacement and the whole fork builds pressure the deeper into the stroke you go.  Unless you have the base valved so soft that the mid is pushing oil out I don't think its really a serious problem.  You would be much more likely to get a spike in damping than any significant cavitation, which is what the blow off mid's are designed to address.

If you're riding woods you have the added option of converting the mid valve to a check plate as well.  This more primitive form of cartridge fork cannot cavitate unless there is enough of a restriction in the mid piston to generate pressure.  Again though, if the oil isn't going through the mid it must be pushed through the base for cavitation to occur.  A fork valved to just utilize the base valve for compression damping will have an even stiffer stack than the standard fork.

The issue I have with backfilling the cart is that there is no way to assure that it won't be sucking more air than oil.  Could result in even worse damping than the standard fork.

[djidji]

On 1.3.2018 at 7:10 PM, turbo dan said:

The issue I have with backfilling the cart is that there is no way to assure that it won't be sucking more air than oil.  Could result in even worse damping than the standard fork.

That's exactly my fear. 

So the by-pass flow of the midvalve needs to be higher than the by-pass flow of the base valve, the same for the flow path trough the shims at all suspension velocities.

And the transition from one flow path to the next should be properly set, so that the shims open when, or before it comes to oil lock through the by-pass.

On top of this, the midvalve and the base valve should be balance so that, as you said, the midvalve doesn't push more oil through the base valve than the volume of the rod entering the cartridge.
This last step can be skipped if you have a back-filling, but yes, if the cartridge sucks air, it's bad for the damping and it's even worst because the air may no leave the cartridge easily.

Sucking air may happen when you comes from a state where the fork is fully extended into very high velocities, like landing flat or landing in the ramp of the following sprung.
 

Conclusion, I'm not gonna trying the back-filling...

jeanjean

[mog]

I have seen times where a TC fork cavitate , a local rider was riding a enduro type Mx race , this was on a 08 RM 125 ,the field had hundreds of small ripples , after around 10 minutes the forks lost damping ,once the race finished the damping returned , each race was like this , I don't think a fork can be designed for every situation ? A higher ICS rate may have fixed it ,or a stronger base and less mid

[djidji]

That's interesting!

So a warm oil would start boiling (or cavitating) earlier. Pretty sure the higher ICS rate would have solved the problem.

jeanjean

[turbo dan]

That may be a situation where an open cart fork actually would work better.  Open cart forks continuously purge the cartridge and have a much larger volume of oil available inside the fork than a twin chamber.

I've never experienced a perceptible loss of damping with an open cart fork.  I have experienced terrible stock valving and piss poor suspension action but this is fixable.