17LikesRC Shock Dyno Test Results
02-06-2016, 07:13 PM
#91
I had the original split design tested at MIP years ago, since then we have had 2 different designs with the latest being The flocontrol. I would love to send some out for testing. This is cool stuff
02-06-2016, 08:52 PM
#92
They look intriguing. Some testing could probably be arranged. My current priority is to complete the basic piston characterization. After that, I'd love to try out some more specialty pistons. I tested several on the dyno already, but I need to get pack data, which I've now created a test setup and am collecting data for.
02-07-2016, 12:21 AM
#93
Sorry to be confusing... Google is correct, and yes, the hole diameter is in the numerator.... BUT... When you are substituting the velocity based on the flow difference between the piston velocity and the velocity of the oil in the hole, the diameter of the hole itself ends up in the denominator.
The key equation that is missing is the conservation of flow. Qp=n*Qh, where Q1 = Vp*Ap and Q2 = Vh*Ah. (Ah=pi/4*Dh^2) Since the Re formula needs the velocity of the oil through the hole (Vh), you must rearrange the flow formula for Vh. It ends up being:
Vh = (Vp*Ap)/(Ah*n).... and that is how the diameter of the hole ends up in the denominator. The Diameter of the piston is just used because you know the velocity of the shock and you are using that flow ratio to calculate the flow velocity through the hole.
I hope that's a little clearer???
The key equation that is missing is the conservation of flow. Qp=n*Qh, where Q1 = Vp*Ap and Q2 = Vh*Ah. (Ah=pi/4*Dh^2) Since the Re formula needs the velocity of the oil through the hole (Vh), you must rearrange the flow formula for Vh. It ends up being:
Vh = (Vp*Ap)/(Ah*n).... and that is how the diameter of the hole ends up in the denominator. The Diameter of the piston is just used because you know the velocity of the shock and you are using that flow ratio to calculate the flow velocity through the hole.
I hope that's a little clearer???
That is because he is showing velocity as a function of the ratio of the hole diameters. The significant length in the Reynolds number is always on the top of the ratio, meaning as length increases, Re increases as do the influence of inertial vortex forces.
An important fundamental of Reynolds number is that it is a ratio of inertial vortex forces to viscous forces. Low numbers mean viscosity dominates and high numbers mean vorticity dominates....often referring to the tendency of a fluid to roll up into vorticies instead of staying smooth and laminar/viscous.
It takes a professor who really understands the subject to teach this appropriately in college/grad school so that the student gets a good handle on the fundamentals.
** looks like we were posting at the same time!
An important fundamental of Reynolds number is that it is a ratio of inertial vortex forces to viscous forces. Low numbers mean viscosity dominates and high numbers mean vorticity dominates....often referring to the tendency of a fluid to roll up into vorticies instead of staying smooth and laminar/viscous.
It takes a professor who really understands the subject to teach this appropriately in college/grad school so that the student gets a good handle on the fundamentals.
** looks like we were posting at the same time!
One thing that needs to be considered besides just the Re No is the orifice discharge coefficient (Cd). For short tube orifices, which we have, the discharge coefficient varies with Re No. and the L/Dh ratio (piston thickness/orifice diameter). The graph below shows the Force vs Piston Velocity (F-V) characteristics for two pistons with the same total orifice area. One has 6 holes and one has 3. As you can see the piston with more smaller holes produces much higher force than the piston with fewer larger holes. This is a result of the Cd coefficient.
Note that this damper model does not account for fluid compressibility. What compressibility will do is significantly reduce the force the damper produces during the first half of the compression stroke. This effect will be much much more pronounced at higher velocities. Pack velocities as they are referred to.
This model is being validated along with Icecyc1's work so I am very confident the effect of hole size is real. We see this effect in the dyno results.
Note that this damper model does not account for fluid compressibility. What compressibility will do is significantly reduce the force the damper produces during the first half of the compression stroke. This effect will be much much more pronounced at higher velocities. Pack velocities as they are referred to.
This model is being validated along with Icecyc1's work so I am very confident the effect of hole size is real. We see this effect in the dyno results.
All comparisons below assume equal piston speed, equal velocity of oil passing through the holes.
1. With equal pistons but with just a difference of thickness. The longer hole (thicker piston) will pack at a lower piston speed. Tapered/thinner pistons will have a tendency to pack at a higher piston speed.
2. Given 2 pistons with equal hole area but different number of holes. (6x1.5 vs 8x1.3) the 8 hole piston will pack at a lower piston speed.
3. A piston hole shape at the entry/exit (Cd) has little affect at lower piston speeds, but has a greater affect on the RE (Pack) at higher piston speeds
02-07-2016, 09:41 AM
#94
One thing that needs to be considered besides just the Re No is the orifice discharge coefficient (Cd). For short tube orifices, which we have, the discharge coefficient varies with Re No. and the L/Dh ratio (piston thickness/orifice diameter). The graph below shows the Force vs Piston Velocity (F-V) characteristics for two pistons with the same total orifice area. One has 6 holes and one has 3. As you can see the piston with more smaller holes produces much higher force than the piston with fewer larger holes. This is a result of the Cd coefficient.
Note that this damper model does not account for fluid compressibility. What compressibility will do is significantly reduce the force the damper produces during the first half of the compression stroke. This effect will be much much more pronounced at higher velocities. Pack velocities as they are referred to.
This model is being validated along with Icecyc1's work so I am very confident the effect of hole size is real. We see this effect in the dyno results.
Note that this damper model does not account for fluid compressibility. What compressibility will do is significantly reduce the force the damper produces during the first half of the compression stroke. This effect will be much much more pronounced at higher velocities. Pack velocities as they are referred to.
This model is being validated along with Icecyc1's work so I am very confident the effect of hole size is real. We see this effect in the dyno results.
Now, the graph shows speeds of up to 2000mm/s. Do you have any data regarding average piston speed? If not I would gladly include that in my list of projects to-do.
Thank you. All good info and the explanation of the formula modeled clears things up for me. The math now matches what we experience on the track. Based on just the math I'm taking away the following in laymans terms:
All comparisons below assume equal piston speed, equal velocity of oil passing through the holes.
1. With equal pistons but with just a difference of thickness. The longer hole (thicker piston) will pack at a lower piston speed. Tapered/thinner pistons will have a tendency to pack at a higher piston speed.
2. Given 2 pistons with equal hole area but different number of holes. (6x1.5 vs 8x1.3) the 8 hole piston will pack at a lower piston speed.
3. A piston hole shape at the entry/exit (Cd) has little affect at lower piston speeds, but has a greater affect on the RE (Pack) at higher piston speeds
All comparisons below assume equal piston speed, equal velocity of oil passing through the holes.
1. With equal pistons but with just a difference of thickness. The longer hole (thicker piston) will pack at a lower piston speed. Tapered/thinner pistons will have a tendency to pack at a higher piston speed.
2. Given 2 pistons with equal hole area but different number of holes. (6x1.5 vs 8x1.3) the 8 hole piston will pack at a lower piston speed.
3. A piston hole shape at the entry/exit (Cd) has little affect at lower piston speeds, but has a greater affect on the RE (Pack) at higher piston speeds
That's why shock consistency is important, if the track develops holes and ruts after half an hour of continuous racing and the shocks provide, say less 20% of damping, our car would be bottoming and catching more ruts towards the end of the race, something we have to account for and the test with the hair drier is important, at least to me and others who race long races (more than 15 minutes).
Last edited by 30Tooth; 02-07-2016 at 09:52 AM.
08-10-2016, 02:39 PM
#95
Tech Initiate
Who makes the red and blue tapered hole and piston shown in the piston types presentation?
08-10-2016, 04:55 PM
#96
Tech Rookie
This is fantastic. Thanks for sharing!
08-11-2016, 08:11 PM
#97
CSI. I still see you can get them on amain, but I don't see much activity on the thread anymore. Like the results show, these are still the only pistons I have tested that actually have a different rebound damping vs compression damping without any moving parts.
08-14-2016, 02:14 PM
#98
Tech Initiate
Thank you. I have ordered some to try. I let you know how we get on with them.
02-20-2018, 08:59 PM
#99
Ok, after a couple years of being lazy, I've finally got a useful tool for predicting damping available to download. It is an excel sheet that will provide a prediction for damping based on the number of holes and hole diameter, as well as at different viscosity oil.
It is designed to allow you to make comparisons of up to four different setups. Trying to make it as simple as I could to understand, I made a chart that gives you a percentage change for both the low speed damping, as well as the impact damping (pack). This means you don't have to know what the actual damping coefficient is, you only need to understand how much your change will affect your damping in terms of percentage.
The data used to create this calculator was collected with a shock dyno as described in this thread, as well as an impact tester I made (but have yet to publish that data). I tested 12 different pistons using three different oil viscosities on each piston to identify trendlines, which were used to create equations. Due to the method, this calculator is specifically accurate for 1/8 scale 16mm flat pistons, but will not be absolutely accurate for any other type or size. Thickness of the orifice is a critical factor in damping coefficient, which is why tapered pistons will not work for this.
I hope this does provide some insight into how damping is affected by changing the number of holes, hole diameter, and viscosity. I'm quite confident in these results (they matched a concurrent theoretical modeling effort by RC Crew Chief). Playing with this spreadsheet, it will become apparent that the damping is not directly related to the hole area as many believe. I will try to explain this soon in an upcoming report.
Be sure to download the spreadsheet after clicking on the link to it. You cannot edit it in the webpage, you need excel to use it. I also protected the sheet to help prevent messing it up. Only the red text is able to be edited.
https://drive.google.com/file/d/1Q6x...bSxl1JhWA/view
It is designed to allow you to make comparisons of up to four different setups. Trying to make it as simple as I could to understand, I made a chart that gives you a percentage change for both the low speed damping, as well as the impact damping (pack). This means you don't have to know what the actual damping coefficient is, you only need to understand how much your change will affect your damping in terms of percentage.
The data used to create this calculator was collected with a shock dyno as described in this thread, as well as an impact tester I made (but have yet to publish that data). I tested 12 different pistons using three different oil viscosities on each piston to identify trendlines, which were used to create equations. Due to the method, this calculator is specifically accurate for 1/8 scale 16mm flat pistons, but will not be absolutely accurate for any other type or size. Thickness of the orifice is a critical factor in damping coefficient, which is why tapered pistons will not work for this.
I hope this does provide some insight into how damping is affected by changing the number of holes, hole diameter, and viscosity. I'm quite confident in these results (they matched a concurrent theoretical modeling effort by RC Crew Chief). Playing with this spreadsheet, it will become apparent that the damping is not directly related to the hole area as many believe. I will try to explain this soon in an upcoming report.
Be sure to download the spreadsheet after clicking on the link to it. You cannot edit it in the webpage, you need excel to use it. I also protected the sheet to help prevent messing it up. Only the red text is able to be edited.
https://drive.google.com/file/d/1Q6x...bSxl1JhWA/view
Last edited by icecyc1; 02-20-2018 at 09:01 PM. Reason: added download instructions
02-21-2018, 12:27 AM
#100
Tech Addict
iTrader: (1)
I'm glad you updated this thread. I wouldn't have known it existed if you hadn't! Just the other day I was musing that maybe I should build a shock dyno.
But what I really want to do is a little different than what you've done. I want to build a dyno that acts against the complete suspension of the car so I can better understand how much damping is best. And maybe also figuring out a good way to pick spring rates?
I'm debating the utility of testing. I'm not sure how useful it will be? I thought instead of measuring forces, to instead use high speed video to analyze the behavior of the entire suspension system.
I'm sure that tires and foams, and to a certain extent the wheels themselves, play a significant role in the suspension, but I don't know how they contribute to the suspension performance.
My first thought was to attach a chassis to a frame and place a platform under a wheel that moves up and down to simulate bumps. The test can be without tires, to see how fast the suspension can move while keeping in contact with the platform. And the test can be done with tires and foams, to see if the foams significantly change the results. And finally, if the platform has rollers of some variety, the variable of the wheel's rotation at various speeds could be added.
Another variable is how much energy is absorbed by the suspension and how much is transferred to the chassis. That would be possible to test, but again, I'm not sure how much could be learned from doing this.
I get excited thinking about how cool it would be, but then I think about the time, effort, and cost, and then I wonder if it would change anything or really help.
But what I really want to do is a little different than what you've done. I want to build a dyno that acts against the complete suspension of the car so I can better understand how much damping is best. And maybe also figuring out a good way to pick spring rates?
I'm debating the utility of testing. I'm not sure how useful it will be? I thought instead of measuring forces, to instead use high speed video to analyze the behavior of the entire suspension system.
I'm sure that tires and foams, and to a certain extent the wheels themselves, play a significant role in the suspension, but I don't know how they contribute to the suspension performance.
My first thought was to attach a chassis to a frame and place a platform under a wheel that moves up and down to simulate bumps. The test can be without tires, to see how fast the suspension can move while keeping in contact with the platform. And the test can be done with tires and foams, to see if the foams significantly change the results. And finally, if the platform has rollers of some variety, the variable of the wheel's rotation at various speeds could be added.
Another variable is how much energy is absorbed by the suspension and how much is transferred to the chassis. That would be possible to test, but again, I'm not sure how much could be learned from doing this.
I get excited thinking about how cool it would be, but then I think about the time, effort, and cost, and then I wonder if it would change anything or really help.
02-21-2018, 08:43 PM
#102
I'm glad you updated this thread. I wouldn't have known it existed if you hadn't! Just the other day I was musing that maybe I should build a shock dyno.
But what I really want to do is a little different than what you've done. I want to build a dyno that acts against the complete suspension of the car so I can better understand how much damping is best. And maybe also figuring out a good way to pick spring rates?
I'm debating the utility of testing. I'm not sure how useful it will be? I thought instead of measuring forces, to instead use high speed video to analyze the behavior of the entire suspension system.
I'm sure that tires and foams, and to a certain extent the wheels themselves, play a significant role in the suspension, but I don't know how they contribute to the suspension performance.
My first thought was to attach a chassis to a frame and place a platform under a wheel that moves up and down to simulate bumps. The test can be without tires, to see how fast the suspension can move while keeping in contact with the platform. And the test can be done with tires and foams, to see if the foams significantly change the results. And finally, if the platform has rollers of some variety, the variable of the wheel's rotation at various speeds could be added.
Another variable is how much energy is absorbed by the suspension and how much is transferred to the chassis. That would be possible to test, but again, I'm not sure how much could be learned from doing this.
I get excited thinking about how cool it would be, but then I think about the time, effort, and cost, and then I wonder if it would change anything or really help.
But what I really want to do is a little different than what you've done. I want to build a dyno that acts against the complete suspension of the car so I can better understand how much damping is best. And maybe also figuring out a good way to pick spring rates?
I'm debating the utility of testing. I'm not sure how useful it will be? I thought instead of measuring forces, to instead use high speed video to analyze the behavior of the entire suspension system.
I'm sure that tires and foams, and to a certain extent the wheels themselves, play a significant role in the suspension, but I don't know how they contribute to the suspension performance.
My first thought was to attach a chassis to a frame and place a platform under a wheel that moves up and down to simulate bumps. The test can be without tires, to see how fast the suspension can move while keeping in contact with the platform. And the test can be done with tires and foams, to see if the foams significantly change the results. And finally, if the platform has rollers of some variety, the variable of the wheel's rotation at various speeds could be added.
Another variable is how much energy is absorbed by the suspension and how much is transferred to the chassis. That would be possible to test, but again, I'm not sure how much could be learned from doing this.
I get excited thinking about how cool it would be, but then I think about the time, effort, and cost, and then I wonder if it would change anything or really help.
Regarding your idea, I definitely feel it's interesting, but I do believe that it would be significantly more complex than a simple dyno. The dyno part was actually quite easy, it just took finding the right instrumentation and putting it together. The impact though, as simple as it seems, was MUCH more difficult to get repeatable and meaningful data. I'm still not totally certain in how best to define impact, but the results I am finding I'm able to reproduce, so I feel comfortable enough with it. But I still want to find a better, more "normalized" way of defining it. My big challenge after this is to figure out a cheap and simple way every racer can determine what their actual damping coefficient is without using expensive fancy equipment. I believe I can do that.
There is definitely an influence in tire/foam/wheel compliance in a suspension. To keep things simple in suspension calculations, the spring rate of the tire assembly is often assumed to be infinitely stiff, which we know is not true. It might be possible to measure the effective spring rate of the wheel assembly, and it could then be modeled into a suspension system. The difficult part though is it is unlikely that spring rate is linear (like ALL of our shock coils). This means, the rate depends on the amount of deflection. This makes modeling significantly more complex because now the actual position of the suspension must be known as well. Not stopping at just the wheels, you would also want to consider the flexibility of the suspension arms, and the chassis. You can see this is starting to get out of control. Building a full chassis dyno could help you figure out some of that, but you'd have to put in significant research into how they do them for full scale cars, and see what is required or relevant. I don't want to throw water on your fire, but in the end, it may not be worth the effort.
With that said, with basic geometry measurements of your suspension, and mass of your components, you should be able to get 90% there to predict the best spring rate combination for your car. But, due to the unaccounted for flexible components, you'll have to test or drive to tweak the last bit for the perfect suspension. One idea that's used in the autocross world is to outfit your suspension with linear displacement transducers, and monitor your suspension travel for a few laps. You will notice how active your suspension is, and how skewed from ideal it is. Then, you should be able to dial the damping/spring rates to align it to balance the data, and theoretically, that should be your optimum spring/damping for your car's mass.
02-22-2018, 01:52 AM
#103
Tech Addict
iTrader: (1)
One idea that's used in the autocross world is to outfit your suspension with linear displacement transducers, and monitor your suspension travel for a few laps. You will notice how active your suspension is, and how skewed from ideal it is. Then, you should be able to dial the damping/spring rates to align it to balance the data, and theoretically, that should be your optimum spring/damping for your car's mass.
So this measurement is to make sure the shock is not particularly up-jacking or down-jacking? That makes sense. I wonder if there is a sensor that would work on an RC? My first thought is that measuring the angle of an A-arm would be easier than a sensor that measures shock shaft position
This issue of actual piston travel is one of the things that I think has been giving me issues. I've been using the fioroni dual rate pistons (version w/ bb valves), but while the car is fast, it gets unstable seemingly randomly. I think the shock pistons are up-jacking, lifting the ride height on rough sections. It's hard to tell exactly what's going on. My perception is that sometimes it wouldn't "settle".
I'm playing with the idea of trying to correct the problem with springs. I just ordered a bunch of small diameter springs to put inside the shocks, around the shock shaft. The goal is to create the effect of a dual rate spring. The intention is that at normal ride height, the springs won't be compressed at all. Rebound from full compression to ride height will be unrestricted. But these springs will slow rebound past normal ride height, which is roughly middle of the shock travel.
I'm hoping this will encourage the car to stay at it's intended ride height, counter-acting any up-jacking from the pistons.
At this point it's mostly guess work. I made a little chart plotting spring rate vs length, and I ordered a bunch that fall into the range that will likely work. They should arrive Saturday, so I know what I'll be doing this weekend!
I'll post about that project elsewhere, though. I don't want to take this thread and run it completely into the weeds!
02-22-2018, 05:46 AM
#104
Great stuff to both you and Ray - thank you for doing the work. I'd mostly be interested in the "pack" stuff. In my mind, we are trying to achieve what the HSC adjustment does on bicycles and other more adjustable suspension when you have a high shock piston velocity - it makes an incredible difference when you are going down a fast trail at a bike park.
02-22-2018, 06:54 AM
#105
Thank you a lot for all these precious informations !
Do you know where I could find something to hook my computer and measure strain like you did ? Could only find expensive specialized computers to do so, which isn't surprising.
I would have the project of making a motorized table that would oscillate front/rear left/right (roll and pitch) and I would then be able to see what happens dynamically on each tyre, giving me the graphic curve of the roll resistance. And then I could even see the actual relationship between the 3 roll actors (viscous, kinematic and elastic) .
Do you know where I could find something to hook my computer and measure strain like you did ? Could only find expensive specialized computers to do so, which isn't surprising.
I would have the project of making a motorized table that would oscillate front/rear left/right (roll and pitch) and I would then be able to see what happens dynamically on each tyre, giving me the graphic curve of the roll resistance. And then I could even see the actual relationship between the 3 roll actors (viscous, kinematic and elastic) .