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

[harrperf]

to add - the real solution is a much lighter bike.  Then all things start to come around.

It's funny - have a guy go ride a stone stock yz125 coming off any 450 and they all say the same - wow the suspension is good and the bike handles well.  Bike weight makes everythign worse.

[GHILL28]

But it's rear wheel rate is of course high.

 

Which means higher shaft speeds for the same event

[Vietze]

Crank down your HS compression adjuster and ride up a rocky stream bead to find out for yourself.

Sometimes stiffer is stiffer.

But your work shows quite clear that softening the cadj is not the same as softening the main stack...

 

small/square bump compliance often is related to LS Damping even though it would be a HS movement.

We have to take DISPLACEMENT into account too in some cases.

 

Small Bumps are HS movement without any doubt. However, the shock just moves short travel distances (low displacement).

It has to react FAST to give a compliant feeling.

Edited May 3, 2016 by Vietze

[slotracer577]

I know there was a guy offering a spring loaded base valve a few years back that was supposed to cure the high speed hits. On that setup the high speed damping was pretty low so you could use travel to absorb square edge hits at high speed. The primary users were desert racers.

[harrperf]

Which means higher shaft speeds for the same event

 

 

Yep.

 

For supercross this was a big plus for the teams.  It's hard to create repeatable bleed circuits and tuning.  It's easy to adjust a shim stack.

I personally feel the rear of bike isn't that spectacular in stock form for motocross.  

A little less low speed damping on that bike goes a very long way in our testing.

 

Big picture, people will always chase the "holy" grail of suspension - but no one has yet to define what that "holy" damping curve is.

I personally feel people don't understand that it's the front and the back working in unison to really makes a bike work well - and each chassis has it's own "harshness" character. 

 

Design engineers are always up against a compromise - better suspension performance is as easy as more travel - at the sacrifice of handling.

[JoeT]

Clicked,

 

I played with putting a ring stack in the mid, I find the whole thread very interesting and like to play with the restackor and it probably would lead to an unridable bike but......

 

Anywho,  I can't wrap my head around getting the program to do what I want it to do which is.

 

I want to have a ring stack in where rather than preloading the face shims, sets the float to zero. Essentially it would act as the spring in the mid vavle.  In the fork I would do this by making the stack height the same as the available collar height. 

[conton11]

Clicked,

You talked about a bleed messing with rebound too much or vice versa.

What about asymmetric bleeds, if you have the space. Like a small bleed through one port on the compression side and nothing on rebound? Or the other way around.

I've seen kyb snowmobile shocks use a bleed slot molded into one of the rebound ports but no bleed on the compression side. Or bleed shims for the same effect.

Clicked,

You talked about a bleed messing with rebound too much or vice versa.

What about asymmetric bleeds, if you have the space. Like a small bleed through one port on the compression side and nothing on rebound? Or the other way around.

I've seen kyb snowmobile shocks use a bleed slot molded into one of the rebound ports but no bleed on the compression side. Or bleed shims for the same effect.

[conton11]

The real question on this matter is:

Does less HS damping really enhance square edge bump compliance?

Yes. This is talked about all the time. But how true is it, when is it true and by how much?

Plus when you talk to people, one man's high speed is another's low or mid speed. And I'm always surprised by people who neglect motion ratio. Saying that "oh, a shock will never see those speeds" when comparing a 3:1 dirt bike shock @ 45mph to a snowmobile front end at 1.8:1 at 80 mph, or a road bike at 80mph.

People also may neglect area under the curve leading up to high speed, as there is always acceleration of the shock from low to high.

[GHILL28]

People also may neglect area under the curve leading up to high speed, as there is always acceleration of the shock from low to high.

This is exactly what I think does not get properly represented, or at least isn't commonly focused on, in a F-v curve.

[JoeT]

Clicked,

You talked about a bleed messing with rebound too much or vice versa.

What about asymmetric bleeds, if you have the space. Like a small bleed through one port on the compression side and nothing on rebound? Or the other way around.

I've seen kyb snowmobile shocks use a bleed slot molded into one of the rebound ports but no bleed on the compression side. Or bleed shims for the same effect.

Clicked,

 

 

The bleeds you have listed are all symmetrical, that is they work in both directions.  To get an asymmetrical bleed you have to put check valve in to control the flow in one direction or the other.

[conton11]

The bleeds you have listed are all symmetrical, that is they work in both directions. To get an asymmetrical bleed you have to put check valve in to control the flow in one direction or the other.

Thanks. I get that now that I look at the piston rather than just think about it.

[Piney Woods]

My ever-reliable butt dyno has shown me over the years that slow rocky stuff like that tends to be MUCH more affected by low speed damping increases than high speed damping.  Occasional fully-weighted tall square edges probably generate high shaft speeds, but aren't most of those inputs that aren't massive being taken up by the tires or the bike being moved around to some degree?

 

Not criticizing, I'm genuinely curious.  What I'd love to see is some footage of a rear suspension going through a chopped up section with those two setups just to see what it takes to overcome that LS knee.

I think this issue is part of what I am fighting in my offroad use. I am starting to wonder about putting more emphasis on adjusting my LSC for better compliance on roots, etc. Maybe I've been doing it backwards using the traditional method of backing off on the HSC. I think that may be contributing to the severe bottoming. Thanks, great thread.

Edited May 5, 2016 by YHGEORGE

[GHILL28]

I think this issue is part of what I am fighting in my offroad use. I am starting to wonder about putting more emphasis on adjusting my LSC for better compliance on roots, etc. Maybe I've been doing it backwards using the traditional method of backing off on the HSC. I think that may be contributing to the severe bottoming.

 

I would take that a step further, and first optimize the REBOUND adjuster for your compression feel because that's your main piston bleed.  Might be a pogo stick or it might sit still like a rock through repeated hits, but pay attention to initial feel.  Then LSC for more/less bleed if you still need it.  Then HSC for additional high end relief.  After all that you'll have to valve the rebound around whatever setting you like the most from the clicker.  If you're picky about suspension (which....you are, and so am I), that's just how it has to be done if you're going to use the adjusters as an indicator for direction.

 

 

Remember, for a high speed event, the shaft still has to go through the low speed range.  It's not irrelevant, especially for very quick inputs where there's less impulse/energy being put into the system, which it sounds like you're trying to tune for.

[Piney Woods]

Thanks. I think the typically slower bike speeds in the woods make a big difference if that is what you are referring to with "impulse/energy" . ?

[GHILL28]

Bike speed, and amplitude. You're hitting rocks and roots, not missing the landing of a triple from 20ft up.

Edited May 5, 2016 by GHILL28

[Piney Woods]

Yep, thanks again.

[GDI70]

To jump back on the preload ring stack...we tested a 2010 crf450 last week. The rider really likes the way the bike is working. There are differences in hoop shim placement hc or lc. I've been working on these stacks a bunch as of late. Brings much more questions; leverage ratios, chain torque, application mx or off road. But, we have been having good results so far.

[GHILL28]

Clicked - Was there anything further you were going to dive into with this topic, or was that your conclusion on page 20?  In either case, many many thanks for the testing and analysis and being kind enough to make it public.

[Clicked]

Clicked - Was there anything further you were going to dive into with this topic, or was that your conclusion on page 20?  In either case, many many thanks for the testing and analysis and being kind enough to make it public.

 

In short: Ran out of dyno data.

 

The dyno data on TT ranges by a factor of 6 from the softest to stiffest damping force. That pretty much covers the practical range of tuning and shows the shape of the damping force curve does not change a whole lot between a soft and stiff curve. To change the curve shape you have to do something else like preload the stack as MXScandinavia showed in his dyno testing. The dyno tests also checked out at a couple of different valve port geometries showing how that effects the damping force curve as well.

 

So the data here covers the range, you just have to interpolate 

 

The basic conclusion of the dyno testing imo was it’s not always easy to guess how changes to the shim stack are going to effect the damping force curve. But if you work through how the shim stack structure deflects and how changes to the stack clamp and taper effect deflection of the stack structure you can eventually get an understanding of the why of the data. The dyno tests here show those results are often a little different then you might expect from basic approaches like shim factors.

 

Back from pg. 9.............

 

Shim Stack Tuning Webology:

So far, this thread has looked at six points of shim stack tuning webology. Some theories have worked out, others have not. Here is a summary of webology theories verified with dyno data so far:  

• Shims become stiffer at edge lifts greater than one shim thickness

  • False: The MXScandinavia finger press data shows shim stiffness is nearly linear out to edge lifts of 7 shim thicknesses. For a shock that is equivalent to a wheel speed of 38 m/sec and well beyond the 7 to 10 m/sec speeds generally considered to be the extremes of MX suspension velocities. If you can not see any effect at 38 m/sec it just is not going to show up in a suspension system

• Shim stiffness becomes nonlinear at high lift

  • False: The MXScandinavia finger press data shows shim stiffness is nearly linear out to edge lifts of 7 shim thicknesses. Data from that test is equivalent to hitting a 7 inch bump at 140 mph 

  • A second point here: the data shows shim stiffness is “nearly” linear. It is not “exactly” linear, but far close to linear than the strong non-linear effects commonly expected based on Belleville springs

• A tall stack of thin shims is stiffer than a short stack of thick shims

  • False: The stacked bowl theory and fulcrum theory both assert tall stacks are stiffer. Those theories made sense against the first two MXScandinavia dyno tests modifying the low and high speed stack. Further tests changing the height of the high speed stack did not support the theory. So “sometimes” tall stacks are stiffer, sometimes they are not. Rules that only work sometimes just aren't very useful for suspension tuning

• A tall stack of thin shims is more progressive than a short stack of thick shims

  • True: The stacked bowl theory asserts shims added to the top of the stack are force to bend around a sharper bend radius. That theory supports the dyno data showing tall stacks of thin shims are stiffer, demonstrated by the SF4/SF14 finger press data comparison

• Stiffer high speed stacks increase high speed damping
  • False: Increasing the stiffness of the high speed stack makes the damping stiffer everywhere, not just at high speed. The Ohlins dyno data, the MXScandinavia dyno data and the FEA analysis all show that effect

• A straight stack is stiffer than a tapered stack

  • False: Shim stacks can be straight, tapered or an upside down Christmas tree. All that matters is the stiffness the stack structure produces at the face shim. The Valving Logic data and MXScandinavia data both demonstrate there is no difference in the shape of the damping force curve between a straight or tapered stack

There may be other dyno data out there showing some of these webology theories work. The dyno data here shows they do not work. Theories that only work sometimes just aren't very useful for suspension tuning.

 

This thread is not dead, just resting.....

Pound Like a couple of times to keep the thread moving.

Edited September 15, 2016 by Clicked

[vince46]

Why dont we just back it all up to basics.

The idea of suspension is to take the energy produced from an event such as a bump or jump and neutralize that energy through pressure that is generated by the restriction of oil to flow  mainly.

. Ive been making my own pistons and doing some data logging and I feel that it gets made to be more complicated than it really is. &%$#@! the shims and just think about a shock piston with no shims for a minute. This is still speed sensitive isn't it? the faster you move the piston (with no shims) through the oil the the more Dampening. Why do you ask? Because no matter how fast the shock shaft moves the the amount of oil that needs to be displaced is relative to the amount of stroke used. If we use 100mm of travel at slow speed we have more time to displace the oil. This means that the oil flows through the holes in the piston with less velocity and in return generates less pressure in the shock. If we doubled the speed of the shaft we need to displace the same amount of oil in half the time. This means the oil will have to move through the port at twice the velocity and in return generate even more pressure in the shock, it is the pressure that dampens the shock movement. Low speed, high speed we have become obsessed with these terms. Shims don't know the difference they open as far as they can relative to their stiffness and the pressure of the shock. If you have less shims on a piston the shock will accelerate faster (this is most people idea of plush, Bad Bad Bad) in turn the shaft will reach a higher velocity and it turn become harsher quicker. I am trialing my very own shock piston tonight will see how it goes.