shim changes and damping curves, based on dyno testing

I've never heard of using a pvp test other than in a production damper environment where the dyno is being used as "go or no-go" instrument because of the lack of data points. I'm curious as to why you you're not using a CVP test especially when trying to analyze data? As for the graph type, this is still showing a linear force curve. I've attached a digressive curve for comparison. 

 

 

Is it possible to post up a screenshot of the actual dyno force curve? I'm curious as to what it looks like.

 

Darren

 

 

For anyone interested, the two dyno tests we are talking about are:

 

1. PVP = Peak Velocity Pickoff.  In all my discussions, I show test results based on the data collected from 11 individual test speeds;  1, 2, 3, 4, 5, 10, 20, 30, 40, 50, 60 ips.  The dyno simply runs each test at the selected speed, and then pulls out the peak force for that velocity.  For example, from the above graphs, 115 lbs peak force at 2ips.

 

2. CVP = Constant Velocity Pickoff.  This type of test collects many more data points.  For example, I have my machine set to get 2000 samples per second.  This is why you could question looking at one data point, when you can look at thousands of data points.

 

3. I might as well answer that.  Why PVP tests. 

 - - We use the PVP because it gives a quick snapshot of the forces for the speeds tested.  We have found this works well when trying to understand how the valving changes effect the forces at the different velocities.  We always have the CVP to fall back on as needed.

- - In essence, we are frequency testing, with the different velocities being the frequency.  (frequency = the rate of occurrence of anything, in this case the velocities).

 

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

 

Just the other day I had a long conversation with Scott at Roehrig about this. The consensus was that there is so much data to look at, you have to find some way to make it manageable.  This is the method we've adapted to make sense out of everything.  And when you start pressure testing, the amount of data goes up 4x.

 

When you run the PVP test, you still get the CVP data.  So I can still look at the CVP graphs.  I just get one extra graph, the PVP graph.

- - This first graph is the PVP graph.

 

1579_11.png

 

 

 

- - This second graph is the CVP

- - I have found the PVP shows a little more detail at the lower velocities.

 

1579_12.png

 

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

Edited by kevinstillwell

Yep, that's the idea. The key piece of info is what LSC clicker setting change is it approximately equal to?

 

That depends on the clicker setting.  And that specific type of info might fit into the 'top secret' category.

Starting from 10 or 12 out say? Just interested to know approximately what sort of shift there is. Would be useful to make more educated guesses on drilling out the bleed.

 

 

 I've attached a digressive curve for comparison. 

 

post-60373-0-88160400-1399059173.jpg

 

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

 

Darren,

 

This run is at 14 ips (inch per second). What are the peak velocities you test shocks at?

post-60373-0-88160400-1399059173.jpg

 

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

 

Darren,

 

This run is at 14 ips (inch per second). What are the peak velocities you test shocks at?

Depends on what we're doing. Offroad damper tests are tested to a minimum of 60 in/sec up to 180 in/sec.

For anyone interested, the two dyno tests we are talking about are:

 

1. PVP = Peak Velocity Pickoff.  In all my discussions, I show test results based on the data collected from 11 individual test speeds;  1, 2, 3, 4, 5, 10, 20, 30, 40, 50, 60 ips.  The dyno simply runs each test at the selected speed, and then pulls out the peak force for that velocity.  For example, from the above graphs, 115 lbs peak force at 2ips.

 

2. CVP = Constant Velocity Pickoff.  This type of test collects many more data points.  For example, I have my machine set to get 2000 samples per second.  This is why you could question looking at one data point, when you can look at thousands of data points.

 

3. I might as well answer that.  Why PVP tests. 

 - - We use the PVP because it gives a quick snapshot of the forces for the speeds tested.  We have found this works well when trying to understand how the valving changes effect the forces at the different velocities.  We always have the CVP to fall back on as needed.

- - In essence, we are frequency testing, with the different velocities being the frequency.  (frequency = the rate of occurrence of anything, in this case the velocities).

 

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

 

Just the other day I had a long conversation with Scott at Roehrig about this. The consensus was that there is so much data to look at, you have to find some way to make it manageable.  This is the method we've adapted to make sense out of everything.  And when you start pressure testing, the amount of data goes up 4x.

 

When you run the PVP test, you still get the CVP data.  So I can still look at the CVP graphs.  I just get one extra graph, the PVP graph.

- - This first graph is the PVP graph.

 

1579_11.png

 

 

 

- - This second graph is the CVP

- - I have found the PVP shows a little more detail at the lower velocities.

 

1579_12.png

 

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

I'm confused. Are you saying that when you run a PVP test that you're also getting CVP data? Just because you grab an individual velocity from a multi PVP file doesn't mean that you're getting CVP data. 

 

Also, I'm very surprised that Scott would recommend using PVP files for any kind of data evaluation because it doesn't tell the story of what is actually going on in the damper, but rather just a quick snap shot of the limit of the velocity being tested. Especially in a situation where you have so many dynamics such as bleed, preload, displaced volume, speed sensitive volume, shims, valve springs etc. 

 

You mention that "I have found the PVP shows a little more detail at the lower velocities." but how could that be the case when you're getting such fewer data points and really just an average of the few that are being collected? 

 

Darren

Darren,

 

First, I don't want anyone to get the impression that Scott recommended using PVP.  Obviously, nobody knows who Scott is, but we know he is two notches above the average bloke when it comes to this sort of information.  So I don't want to give the wrong impression.

 

Basically, running a PVP test gives you exactly the same data as running a CVP.  You just get the option of one additional bit of information (the PVP info).  You get this info in the report data as well as in the graphs. 

 

It would be extremely difficult to try to explain this in a Thumper Talk post, and we would be the only two guys even slightly interested. 

 

kevin

I don't know how you could troubleshoot with a PVP, you would need to see the compression opening, compression closing and rebound opening and closing data. The F/D is also good for finding problems.

Keven make that 3 of us lol. I've found the thread interesting I'm just smart enough to know to watch listen learn

Yz295

There are a couple of other points from the dyno data in this thread that are interesting. Nobody brought them up so I thought I would.

 

You see threads around where riders try to reduce seal drag by dropping the reservoir pressure. The dyno data in this thread shows that does not work.

 

The “normal” compression adjuster produces a chamber pressure of 575 psi and 22 lbs of seal drag. The stiff adjuster doubles the chamber pressure to 1181 psi but still makes the same seal drag. Kevin's dyno data shows pressure in the shock has almost no effect on seal drag, dyno verified.

 

seal-drag.png

 

No doubt reducing the gas pressure makes the shock ride softer. But I don't think that is because of seal drag. It is because the lower gas pressure lets the oil foam out in the rebound chamber as shown in this Roehrig video. Once the oil foams out the shock rides softer, no doubt. 

 

chmb-foam.png

Edited by Clicked

Clicked!

You arw rigjt about the seal drag, but i think the N2 pressure has only a small influence aout the cavitation.

The Cadj. Setting is the part the prevent the shock from cavitation.

I have removed the adjusterstack just for fun and put 12 bar N2 IN the Shock.

I feel cavitation just on the floor without the stack..

Definitely true!

 

I've done a few shocks with really soft comp adjusters (DR 650, for example) and had them cavitate with what I would consider to be a really conservative compression stack.

 

If you do some restackor calcs, you'll see that the chamber pressure goes up a lot (like 500+ psi, I think?) fro the cadj, way more than the nitro pressure.

 

Still, nitro helps keep the initial cavitation at bay, and adds a bit of safety margin.

Keep in mind we need the n2 pressure for the backfill and cavitation prevention of the comp chamber on the reb stroke also..

There are a couple of other points from the dyno data in this thread that are interesting. Nobody brought them up so I thought I would.

 

You see threads around where riders try to reduce seal drag by dropping the reservoir pressure. The dyno data in this thread shows that does not work.

 

The “normal” compression adjuster produces a chamber pressure of 575 psi and 22 lbs of seal drag. The stiff adjuster doubles the chamber pressure to 1181 psi but still makes the same seal drag. Kevin's dyno data shows pressure in the shock has almost no effect on seal drag, dyno verified.

 

seal-drag.png

 

No doubt reducing the gas pressure makes the shock ride softer. But I don't think that is because of seal drag. It is because the lower gas pressure lets the oil foam out in the rebound chamber as shown in this Roehrig video. Once the oil foams out the shock rides softer, no doubt. 

 

chmb-foam.png

The seal drag test is not a live measurement, it's a test performed at the same time as the gas test. To get a really accurate measurement it's best performed with the shims removed.

Keep in mind we need the n2 pressure for the backfill and cavitation prevention of the comp chamber on the reb stroke also..

 

Getting compression adjuster backfill flow resistance out of the dyno data is another useful task for a pressure tapped shock like Kevin tested here.

The seal drag test is not a live measurement, it's a test performed at the same time as the gas test. To get a really accurate measurement it's best performed with the shims removed.

 

Right. The standard seal drag test is not a live measurement. It’s a static measurement made during dyno gas pressure tests.

For measuring seal drag I don’t think tests with the shims removed is a good idea. With the shims removed the shock doesn’t build any pressures so the seals operate unloaded and that results in low drag.

 

The other problem with taking the shims out is winging the valve back and forth through the shock fluid produces flow resistance from the valve ports. That creates a force on the shaft and there is no way to sort out the shaft force from valve port flow resistance and shaft force from seal drag.

Valving Logic data here shows a different kind of seal drag test that measures the seal drag during an actual dyno run with the shock producing it’s normal damping force. This is new data for the seal drag of a yz250 with the shock fully loaded through the stroke. 

 

1-norm-drag.png

 

The data shows the seal friction starts off at 13 lbf of drag at 1 in/sec. That’s pretty close to the seal drag measured in the standard static seal drag test.

 

At 10 in/sec the seal drag almost doubles to 22 lbf and then remains pretty much constant through the remainder of the speeds tested. The interesting thing in this new data is the seal drag increases with shaft speed and is almost double the value determined from the static seal drag tests. 

The other thing interesting in this new data is chamber pressure has no effect on seal drag.

 

2-stiff-drag.png

 

For the stiff compression adjuster the chamber pressures almost double at high shaft speeds, but the seal drag is the same. So chamber pressure doesn’t seem to have much influence on shaft seal drag and chamber compliance doesn’t have any effect on piston band drag either.

Edited by Clicked

For low drag piston bands it would be useful if manufactures put out some of their data showing how piston band drag behaves over the velocity range.

 

The data Valving Logic posted for seal drag is much different from the constant seal drag values previously thought.

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