Dyno data –And what it tells us about how to tune a shim stack and control the shape of the damping force curve

[Clicked]

Valving Logic kxf250 dyno data

After all of that hacking around on the dyno to figure out the effect of the FC compression adjuster spring Valving Logic ended up a pretty cool set of data with separate measurements for the compression adjuster force, main piston force and overall damping force produced by the kxf250 shock. Those component values measured on the dyno line up pretty well with the theoretical values computed for the shock.

After that initial test series Valving Logic went on to test a stiffer shim stack configuration suggested by rdmx and measured the individual shock component values for that stiffer shim stack configuration as well.

The stiffer stack ended up over running the tuning authority of the FC compression adjuster spring. That drove pressures in the rebound chamber down and caused the oil to foam out in test.  

Comparing the stiffer stack used in the above kxf250 test to the yz250 shock tested by John Curea earlier shows the chamber pressure balance difference between the two shocks. For the yz250 shock the 16mm shaft configuration allowed the rebound chamber pressures to initially drop and then recover as the compression adjuster developed more back pressure at higher speed. That initial pressure drop caused a hysteresis loop in the yz250 dyno data. The 18mm yz250 shaft never allowed the chamber pressures to drop and produced virtually no hysteresis loop in the dyno data (after correcting for pressure changes over the stroke).

 

The kxf250 compression adjuster allows pressures in the rebound chamber to initially drop (like the yz250) but as the shaft is driven to higher speeds the compression adjuster never develops the back pressure needed to recover and re-pressurize the shock chambers. That foams out the oil and keeps it foamed out producing in some whacky behavior from that shock. In test, opening the low speed clickers from 10 to 18 caused the damping force to go up by 15 lbf! The reason for that is opening the low speed compression adjuster clicker generated even less back pressure causing the shock to cavitate harder. The effect was only 15 lbf, but when the adjusters start driving damping force in the wrong direction the shock becomes hard to tune.

[Clicked]

Valve Port Geometry:

Dyno testing up to this point has been hacking around on shim stacks trying to figure out which stack is stiffer. MXScandinavia asked the question the other way around:  

• If you ran the same shim stack on different valves, which valve is stiffer?

Webology of valve port design:

The general thinking on valve port geometry is wide valve ports allow the flow to dump as soon as the shim stack cracks open. That dumps the fluid pressure producing less low speed damping. With that thinking the Race Tech valve should have less low speed damping.  

 

Narrow ports, like on the four port valve, are force to dump more of the flow over the sides or “spokes” of the valve port. As the shim stack peels back from the edge more side area opens so the damping force should get progressively softer as the shim stack lifts. That thinking implies the four port valve should get progressively softer at speed making a digressive damping force curve.

What does the dyno say?

Plotting the dyno data as a damping coefficient (right hand curve) shows the Race Tech valve was softer at low speed and the Ohins four port valve produced a digressive damping force profile. Those results were expected.

 

The unexpected result was the Race Tech valve was stiffer producing about 28% more damping force. Visually the wide ports on that valve “look” bigger and MXScandinavia expected the large ports to produce less damping force. Dyno testing shows the results were the other way around.  

[conton11]

Since we don't have higher speed data to compare these two valve styles, what does the FEA say at speeds up to the 120 in/sec range?

Point of interest:

Fox uses a valve that looks a lot like the race tech valve for some sled applications. They call it "high flow" in the part number.

[jotukka]

With wider ports and same chamber pressure shim stack pressure is lower. Right?

[harrperf]

Valve Port Geometry:

Dyno testing up to this point has been hacking around on shim stacks trying to figure out which stack is stiffer. MXScandinavia asked the question the other way around:  

• If you ran the same shim stack on different valves, which valve is stiffer?

Webology of valve port design:

The general thinking on valve port geometry is wide valve ports allow the flow to dump as soon as the shim stack cracks open. That dumps the fluid pressure producing less low speed damping. With that thinking the Race Tech valve should have less low speed damping.  

 

Narrow ports, like on the four port valve, are force to dump more of the flow over the sides or “spokes” of the valve port. As the shim stack peels back from the edge more side area opens so the damping force should get progressively softer as the shim stack lifts. That thinking implies the four port valve should get progressively softer at speed making a digressive damping force curve.

What does the dyno say?

Plotting the dyno data as a damping coefficient (right hand curve) shows the Race Tech valve was softer at low speed and the Ohins four port valve produced a digressive damping force profile. Those results were expected.

 

The unexpected result was the Race Tech valve was stiffer producing about 28% more damping force. Visually the wide ports on that valve “look” bigger and MXScandinavia expected the large ports to produce less damping force. Dyno testing shows the results were the other way around.  

 

 

Again, nice post.

What's the total combined area of the ohlins vs racetech?  I  imagine the flow out the "sides" of the ohlins is greater - as there are 2 more "sides" to exit at higher stack lifts

[Clicked]

Since we don't have higher speed data to compare these two valve styles, what does the FEA say at speeds up to the 120 in/sec range?

Point of interest:

Fox uses a valve that looks a lot like the race tech valve for some sled applications. They call it "high flow" in the part number.

 

You can get an idea of where damping force is going for those two valves at high speed by looking at the damping coefficient curve.

The wide port Race Tech valve has more-or-less topped out. The Ohlins valve is digressive but looks to be flattening out. My guess is that 28% damping force difference measured at 40 in/sec is going to remain more-or-less constant as that shock is pushed out to higher speeds. At some point the throat flow restriction in the four port valve will come into play. The three port valve has none.

 

How all of that compares to the high flow Fox sled valve is hard to say. The Fox valve may look like the Race Tech valve but when it comes to performance the port geometry details are going to make a difference as well as how that valve port geometry compares to the stock sled valve. Those effects are not always easy to guess, which is what MXScandinavia was trying to demonstrate in the above test.

 

With wider ports and same chamber pressure shim stack pressure is lower. Right?

 

Again, nice post.

What's the total combined area of the ohlins vs racetech?  I  imagine the flow out the "sides" of the ohlins is greater - as there are 2 more "sides" to exit at higher stack lifts

 

The usual approach to compare valve port geometries is to look at the port area (to estimate fluid force on the shim stack) and the valve port perimeter to estimate the flow spill area.

Valve port area

A simple estimate for port area is the port width (w.port) times the length (d.port). For the valve port geometries given above the port area ratio works out to 1.56 for the two valves.

With 56% more port area the Ohlins valve should have 56% more fluid force on the shim stack and that should push the shim stack to a 56% higher lift. The port area difference is a little larger than the 28% damping force difference shown by the data. But then again damping force does not linearly scale with port area.

Port perimeter

The spill area around the valve port perimeter is another parameter used to compare valve port geometries. Assuming some kind of triangular spill area along the d.port length of the valve spoke the ratio of perimeter spill area between the two valves works out to 1.27.

 

he Ohlins valve has a 56% larger port area. The expectation there is the larger port area puts more fluid force on the shim stack face causing a larger shim stack deflection. So the difference in shim stack flow area should be something like (port area)*(perimeter). Multiply that out and the ratio between the two valves works out to 1.98 which is a hole lot larger than the 1.28 damping force ratio measured in test. The magnitude is off, but at least the direction is right.

Wave shaped shim stack deflection

Another effect going on here is the difference in performance of a wide and narrow valve port. Those performance differences are caused by the wave shaped deflection of shim stacks, shown in the MXScandinavia finger press photo below, interacting with the valve port geometry.

• Shim stacks deflect into a wave shape. That wave shaped deflection is centered over the valve port allowing the flow to spill outward over the port edge and tangentially over the valve spokes

• On a wide valve port the wave shaped deflection only opens the middle of the valve port. The left and right edges are closed down by the wave shaped deflection. So, wide ports don't flow over the entire width of the valve port

That puts a new dimension on shim stack tuning. Modifying the shim stack clamp, tapper or crossover gap changes the shape of the shim stack wave deflection. Those differences create situations where the shim stack may, or may not, cleanly lift over the entire width of the valve port. When the shim stack does not lift over the entire width the tuning of wide valve port geometries can get tricky.

 

That also emphasizes shim stack lift, measured via a finger press, does not tell you if the wave shaped stack deflection cleanly opened the left and right sides of the valve port. That creates situations where two shim stacks could have the same center line edge lift but generate different damping forces because the valve port edge was open in one case but not the other.

 

A better performance parameter is comparing shim stack flow area. That accounts for shifts in the wave deflection shape and interactions of that shape with the valve port geometry. That requires FEA analysis to integrate the flow area around the valve port perimeter or finger press measurements to provide a whole lot more detail on the tangential and radial deflection of the shim stack to get that data. Measurement of shim stack edge lift alone can't give you that info.

 

Crossover shim stacks discussed earlier are another example where edge lift does not give the whole story, you have to look at the flow area produced by the shim stack deflection to understand how shim stack modifications are going to effect damping force.

[MidlifeCrisisGuy]

Thanks for leading an extremely interesting thread. Unfortunately for me, it generates a lot of questions.

 

I ride woods and most of what I ride is rocky and rooted.  My bikes have 120 to 140 rear tires on 18 inch rims.  7 to 12 PSI, depending on the tire and conditions.

 

The force curves generated by Restackor assume long, steady state, one dimensional strokes.  The shock dynos, from my understanding, don't test short periodic strokes either.  What is really happening in the shock when we ride over a series of 2 inch roots at 25 MPH ?

 

Why don't shocks have float in the main stack like a fork stack does ?   Does float play a part in small movement damping ?

 

I bought a used shock from eBay for parts.   It had a sticker on it from a prominent tuning company.  When I opened it up it had a very small face shim that didn't cover the piston ports.  The shims behind it were covering the ports.  It would have created a very large bleed.   What was the purpose of this ?  Or did someone mess up assembling the shock ?  Would Restackor model this correctly or would it look differently on the dyno ?

 

I find the force diagrams that Restackor generates to be very linear, even when they are progressive or digressive.   The change in force versus linear are much smaller than what I would have expected.   My butt dyno seems to tell me that shocks sometimes aren't that linear and they encounter episodes of severe progressive behavior.  What might be driving this in the real world compared to in Restackor or on the shock dyno ?   Foaming ? Packing ?

 

How far are we away from using CFD to model shock behaviors and how do you think that would change the results ?

 

Is there a way to target shock tuning to isolate braking bumps and trail trash and still provide bottoming resistance on air and big air ?  Seems to me this is where we need some float in the main stack ?  Does flow actually momentarily reverse in the piston when stack float is used ?  Or should this be handled with position sensitive damping ?

 

Thanks again.

Edited April 20, 2016 by MidlifeCrisisGuy

[MidlifeCrisisGuy]

The posts above indicate that changing the HSC spring and stack shouldn't change the damping curve that much, but then there is this thread that seems to indicate otherwise.  http://www.thumperta...r-the-yz-shock/

 

I implemented this mod.  It worked exactly how 2grimjim said it would, removing the jolt from high speed hits.  I used the FC connection spring because it was easier to obtain and I softened the HSC stack and wala, I had a bike that soaked up high speed hits instead of jarring on them.

 

According to this thread, the HSC adjuster only ever contributes a few 10s of pounds of force to the overall damping.  So how is it that it could be creating the jarring that 2grimjim and I both observed ?  Or more importantly, how does increasing the flow of the compression piston decrease the jarring ?  It seems that 2grimjim, myself and others all agree it works, the question is why.

 

This thread is also applicable to this discussion (http://www.thumperta...-kxf-250-shock/) because they actually measures the lack of pressure on the rebound side due to having the compression adjuster running too wide.  Presumably drilling out the piston ports and reducing the spring pressure would achieve the same thing and make things worse, not better ?

 

So why does lowering the HS compression adjuster spring pressure removing the jarring ?  Does it really modify the compression curve on hits, its transient response, or is it cavitating the oil on the rebound cycle which allows a faster rebound ?  Or is something else going on ?

 

Thanks

[Clicked]

You should read the post here again:

Factory Connection High Speed Compression Spring

 

But what does that mean?

By itself the compression adjuster doesn't make much damping force. That surprises most people and doesn't line up with test ride results where a half turn on the compression adjuster makes a big difference in shock performance, way more than the 3 lbf shown in the above dyno data.

 

The reason for that is the compression adjuster generates damping force in an indirect way. The compression adjuster back pressures the shock chambers and that prevents the main piston from cavitating. By suppressing cavitation the compression adjuster increases the damping force from the main piston– by a lot. Anyone who has ever cranked down on the compression adjuster could tell you that.

 

To figure out compression adjuster performance ...

 

The compression adjuster has a bunch of tuning range and it does that by controlling cavitation. So why does a softer HS compression adjuster spring reduce jarring?

 

You mention cavitation, but it does not sound like you have really thought it through. When the compression adjuster does not supply enough back pressure to push the oil through the main valve the oil cavitates behind the main piston (Roehrig video). When the shock cavitates the oil can not make it though the main piston. So where does the oil go?

When there is not enough back pressure the oil that was supposed to go through the main piston into the rebound chamber is pushed out through the compression adjuster into the oil reservoir. We are not talking about the usual small oil flow displaced by the shock shaft. We are talking about a large oil flows displaced by the main piston – Big flows and big flows produce big pressures.

 

Pushing that large oil volume through the small 6x2mm compression adjuster ports creates a big back pressure and that back pressure is working against the main piston area with low pressure foamed out oil behind the piston in the rebound chamber. Big pressure on the main piston area produces big forces and that creates the jarring hit MidlifeCrisisGuy is talking about on square edge hits.

 

The 2grimjim thread linked above made 4 mods to reduce the jolt:

• Increased the compression adjuster port size to 6x3.8mm. That is a 3.6 times more flow area and reduces the port pressure drop by a factor of 13
• Softer FC compression adjuster spring. 2grimjim mentions the softer compression adjuster spring didn't do anything from full in to full out on the adjuster. He needed a third mod to fix that
• Modify the compression adjuster spring seat to operate on the outside edge of the compression adjuster shim stack instead of the clamp shim. The modified spring seat works the same as the one SDI sells. Moving the compression adjuster spring force to work on the outside edge of the shim stack increases the compression adjuster stiffness by a lot
• 2grimjim also mentions he was using a modified main piston shim stack, but he does not say what that stack was

The situation here, as I understand it, is the shock is severely cavitating on sharp compression hits. The 2grimjim mod opens up the compression adjuster ports and softens the compression adjuster spring to allow the shock to cavitate even harder and blow through the stroke on a hard hit.

 

The other way to go, and it makes more sense to me, is just soften the main piston shim stack. That softens compression damping to accommodate square edge hits, prevents the main piston from cavitating, keeps the oil from foaming out and gives you a chance of some tunable configuration on the rebound stroke to control of the suspension motion. 2grimjim mentions he tried that approach but could not figure out how to change the main piston shim stack to keep enough low speed damping while softening the high speed.

 

That sums up one of those basic “How do you tune a shim stack to control the shape of the damping force curve” questions. The dyno data on TT shows you how to soften high speed damping but we have not gotten to those dyno tests yet...........

------------

Oil Viscosity

What the heck does DaveJ's 215.VM2.K5 – SPI-3 vibration cancellation fluid do in terms of shock performance?

 

MXScandinavia had that question and re-tested the Jake01 stack with DaveJ's 215.VM2.K5 – SPI-3 vibration cancellation fluid to see what the oil does in terms of shock damping performance.

MXScandinavia's comment on the results:

• The SPI3 behave like a ordinary thicker oil in the beguinning but ordinary oils use to fall and meet the thinner oil in the end, but it seams that the viscosity rice with the pressure and hold the same slope angel as jake 01

My take on MXScandinavia's comment: Push a high viscosity oil through the clicker bleed circuits and damping force goes up. But here is an interesting artifact of fluid dynamics: Once the flow goes turbulent, oil viscosity does not make much difference in flow resistance. So at high speed thin oils and thick oils end up producing about the same damping force. Whether that makes any sense or not doesn't really matter, it's just the way it is.

 

The dyno data shows 215.VM2.K5 – SPI-3 vibration cancellation fluid does not fall off at high speed. It is stiffer everywhere, both at low speed and at high speed. Those results indicate the effect of that oil is not a viscosity effect.

 

The other oil parameter that effects damping is oil density. It takes more force to push a heavy fluid through a flow restriction. That is a basic F=ma kind of deal and that effect is not going to drop off at high speed like oil viscosity does. Run ReStackor with a 10% higher oil density and the results line up with the MXScandinavia 215.VM2.K5 – SPI-3 vibration cancellation fluid dyno data.

Increasing the oil density effects the damping force produced from both the base and mid-valve. That alters the ability of the base valve to pressure balance the shock and control cavitation. DaveJ tries to come to grips with that in a thread here trying to figure out the fork mods needed to re-balance the chambers with the higher pressure drops produced by 215.VM2.K5 – SPI-3 vibration cancellation fluid.

 

Brand-to-brand differences in oil density have a direct impact on damping force and those oil density differences are larger than most people realize.

The other important thing to get a grip on is cSt values of kinematic viscosity printed on the bottle are not a direct measure of viscosity. Kinematic viscosity is defined as the true viscosity (aka dynamic viscosity) divided by oil density.

 

That creates problems comparing oils. An oil might have a slightly lower cSt value of kinematic viscosity (suggesting damping force would go down), but a higher density (making damping force go up). That gets even worse once you understand the higher density oil also has a higher viscosity because (true viscosity) = (Kinematic viscosity)*(density).

 

Oil viscosity tables splattered around on the web comparing cSt values of kinematic viscosity really tell you nothing about how the oil is going to perform. You have to look at both viscosity and oil density.

[mog]

I'm having a similar situation wanting the same low speed but less high speed on my 16 sxf 250 shock ,everything I do on the main piston results in a drop in low speed and high speed,I've tried single stage ,2 stage ,as the stack is so stiff in low speed shims (12 ) to get the low speed I want,any changes to the cross overs or hs stack do little to change the balance ,I think a ring shim us the only answer but haven't seen one for years

[MidlifeCrisisGuy]

Clicked: one time I revalved my shock and forgot to fully recharge the badder.   I was running with 40 PSI in the bladder.   By your theory that should have caused severe cavitation and thus jarring on even moderate bumps.  Yet I noticed nothing unusual.  Why ?

 

If cavitation really is the issue with these sharp hits, we should be running higher bladder pressures and increasing the high speed compression setting should make the hits better.  I have not found that to be the case, at all.

 

FWIW, I put a softer compression stack in my shock tonight and left the compression adjuster stock.   At a later point I am going mod just the compression adjuster.

Edited April 22, 2016 by MidlifeCrisisGuy

[racerfmx450]

Clicked,

i have tested a new cadj. Design on a crf 250/2016 shock.

The shock works well with the stock cadj. and stiffer compstack.

With the new cadj. The bike feels like a wild horse...

Side by side kicking on bracking and acelleration..

No traktion.

Feels way to stiff.

Do you think i can see what happend on the restackor?

I am not sure about the stock design cadj stack.

I use 6 ports for the Simulation but i think the shims bend only like a 2 port piston ...

Can you tell me how i can compare the stock cadj design with a 4 port piston on the restackor and see what happend?

[EXC Yankee]

I'm not that technical, but have been trying to improve my suspension. I been scouting threads for ways to improve shock.

If I can quote another thread.. regarding modifications to the exchange stack and /or changing to the softer FC cadj spring:

"

#32 10/12/2014

Midlife crisis it turned out good, better than stock but I went through it again and made it much better.

I originally used gray racers exchange stack with the crossover 9 mm shim. It was better than stock but my current exchange stack is much better.

It's the stock shim stack but with the factory connection lighter high speed adjuster spring. It is the best it felt so far."

In a nutshell, should I leave the stock spring on or change it to the FC spring. I am assembling this shock tonight, revalved it for softer compression and beefed up the rebound(2001 YZ426).

I preferr not to tear it down again cause of the spring.

Thanks for your reply.

Edited April 22, 2016 by YZ Yankee

[EXC Yankee]

Clicked,

i have tested a new cadj. Design on a crf 250/2016 shock.

The shock works well with the stock cadj. and stiffer compstack.

With the new cadj. The bike feels like a wild horse...

Side by side kicking on bracking and acelleration..

No traktion.

Feels way to stiff.

Do you think i can see what happend on the restackor?

I am not sure about the stock design cadj stack.

I use 6 ports for the Simulation but i think the shims bend only like a 2 port piston ...

Can you tell me how i can compare the stock cadj design with a 4 port piston on the restackor and see what happend?

I thought that for aome bikes, FC made softer CADJ springs, other bikes, stiffer.

Was your spring suppose to be softer or stiffer?

[conton11]

While we are in the topic of adjusters, I have some snowmobile shocks that appear to have huge range. Two are fox with "dsc" adjuster and the other two are Walker Evans. The range feels like 2 factors at low speed. Though I have never dyno tested them, so I don't know.

Why do much range?

I have another KYB shock with a typical KYB/Showa "dirt bike" style adjuster. The range of adjustment is hardly noticeable.

If these shocks adjust the way they feel, I imagine the shape of the curve is being dragged all over.

Edited April 22, 2016 by conton11

[Clicked]

Clicked,

i have tested a new cadj. Design on a crf 250/2016 shock.

The shock works well with the stock cadj. and stiffer compstack.

With the new cadj. The bike feels like a wild horse...

Side by side kicking on bracking and acelleration..

No traktion.

Feels way to stiff.

Do you think i can see what happend on the restackor?

I am not sure about the stock design cadj stack.

I use 6 ports for the Simulation but i think the shims bend only like a 2 port piston ...

Can you tell me how i can compare the stock cadj design with a 4 port piston on the restackor and see what happend?

 

There are a lot of different compression adjuster valve designs:

The left hand valve is the more typical compression adjuster configuration using a perimeter port. That valve deflects the shim stack Belleville washer style allowing the flow to spill all the way around the valve perimeter.

 

Valves on the right are also for compression adjusters and use discrete ports. There the flow spills over the valve edge (w.port) and port “spoke” length (d.port). So the spill area there is a little different.

 

In either case the parameters describing the valve (r.port, d.port and w.port) are the same. If you follow the process the calculations can tell when N.thrt*w.port extends all the way around the valve perimeter then the valve has a perimeter seat and when N.thrt*w.port is less than the perimeter the calculations are dealing with a discrete port configuration like the valves on the right.

 

Spec the input parameters and you can compare a 3 port, 4 port, 6 port perimeter seat or an 8 port perimeter seat valve along with the shim stack deflections for each.

[Clicked]

Other things to check out on the stiffer modified compression adjuster is the differences in clicker flow area and the cadj spring seat. If the modified cadj uses a spring seat that clamps the outside edge of the shim stack if is going to be a whole lot stiffer than a seat that acts on the stack clamp.

[MidlifeCrisisGuy]

When there is not enough back pressure the oil that was supposed to go through the main piston into the rebound chamber is pushed out through the compression adjuster into the oil reservoir. We are not talking about the usual small oil flow displaced by the shock shaft. We are talking about a large oil flows displaced by the main piston – Big flows and big flows produce big pressures.

 

Pushing that large oil volume through the small 6x2mm compression adjuster ports creates a big back pressure and that back pressure is working against the main piston area with low pressure foamed out oil behind the piston in the rebound chamber. Big pressure on the main piston area produces big forces and that creates the jarring hit MidlifeCrisisGuy is talking about on square edge hits.

 

This doesn't happen.   If the oil cavitates on the rebound side, it is due to low pressure there.   Oil flows into the rebound side from the high pressure on the compression side via the rebound clicker.   The flow through the rod piston is from the high pressure compression side to the rebound side.  None of the cavitated oil on the rebound side goes anywhere.   Even if the rebound side cavitates, there is no big exodus of oil out of the compression chamber into the reservoir except for the normal shock rod displacement.

 

I think the jarring we feel on square hits is due to trying to force the rod volume through the exchange stack quickly.   The high pressure created by this event acts on the main piston, creating the jarring force.  I suspect that the piston port area on the Yamaha kyb shocks is sufficient (8 x 3mm holes), especially on the 16mm shock shafts.   I suspect the problem is that the exchange piston stack is too stiff and it has too much preload from the compression adjuster spring. 

 

I revalved the rebound stack on my 250FX.  I left the compression adjuster stock.   The jarring remains, though it is somewhat softer.  I will test a softer compression adjuster spring as soon as I get one.  

 

FYI, the compression adjuster springs on the newer Yamaha shocks (ie left hand reservoirs on reverse engine bikes) is much smaller than the previous springs.  They are not interchangeable.

Edited April 25, 2016 by MidlifeCrisisGuy

[KPRacing]

When the rebound chamber cavitates, the volume of oil being forced though base valve is far more than the shock shaft displaces. The shock shaft is not displacing any oil during cavitation. It is the midvalve piston basically raising the floor (with a small amount of oil bleeding through). The volume displaced by the piston face is huge in comparison to the shaft volume.

In MidlifeCrisisGuy's tuning, the bigger holes & softer compression spring are allowing the cavitation to continue by letting the oil bleed through the base valve. This is giving a softer feel over square edge bumps but it is not curing the cavitation problem. It is actually just letting cavitation run its course. So while this may feel better on the hits, it is not helping with the rebound side of things or with big compression hits.

So like Clicked stated, going with a softer midvalve setting would be the better way to go. If you go too soft on the mid valve setting, the suspension will kick on small hits because it is actually bottoming or the wheel is getting tossed up into the bike (bottoming) & kicking.

Sorting the mid valve will give more rebound control in multiple high speed hits & make for a more consistent handling bike through a series of hits vs softening the base valve.

Years ago, I tried softening the base valve stack & opening up the base valve ports. I was getting a plush ride over square edge hits so I decided to soften the mid valve compression stack in an attempt for an even smoother ride. My results were a riding style that had me doing random flying "W's" (feet off the pegs). Initially I thought it was still too stiff so i went softer on the mid valve until the flying "W's" were happening on bumps I could barely see (small rounded bumps). The light went on & i realized I was bottoming &/or having the rear wheel tossed up so violently that it was kicking the rear.

Edited April 26, 2016 by KPRacing

[MidlifeCrisisGuy]

When the rebound chamber cavitates, the volume of oil being forced though base valve is far more than the shock shaft displaces. The shock shaft is not displacing any oil during cavitation. It is the midvalve piston basically raising the floor (with a small amount of oil bleeding through). The volume displaced by the piston face is huge in comparison to the shaft volume.

 

This can't happen.   The only path for the cavitated oil to go anywhere is past the main piston.  The oil on the other side of the main piston is under very high pressure due to the piston being moved by the bump.   As soon as that cavitated oil is put under pressure, it returns to its normal volume.   The only place the cavitated oil is going to exist is on the rebound side of the piston.

 

This theory is proven in the Roehrig video.   The accumulator bladder membrane does not move much further when cavitation occurs than when a normal stroke occurs.  It does move faster because the rod is moving faster, but that is all.   

 

I am going to experiment with bladder pressures.   If your theory is true, higher than normal bladder pressures should cure the cavitation issue.  What is the highest bladder pressure that one can run in a stock shock ?

Edited April 26, 2016 by MidlifeCrisisGuy