Jump to content
Sign in to follow this  

Building a shock dyno

Recommended Posts

I am designing a shock dyno for my senior project. Ideally, the dyno is going to be used for the BAJA SAE and Formula SAE teams at school. The Baja team (which I am the VP of) runs Fox Float ATV shocks while the FSAE team (I was drivetrain lead last year) runs mountain bike shocks.

I just have some questions for someone who has experience with shock dynos. I am currently trying to set up a meeting with an engineer at Bilstein and/or Fox since my location in San Diego makes that relatively easy but I am not getting my hopes up. 

I also have to pay for this project all myself so I can't get too crazy with it unless I get some sponsorship money. 


As of right now, I am thinking of a simple design like this http://www.shockdyno.com/ 

If anyone knows what kind of timer that dyno uses I would really appreciate it. I need a way for this thing to automatically start timing when motion starts and stop timing when motion stops. 

 

I would rather do a design based off the fancier machines using an electric motor and a load cell but that increases the complexity 10 fold and also drives the cost way up, but it would be cool and could be done. Plus I would get a chance to learn about electric motor control and a better understanding of computer programming.
Something like this would be really cool and is in the realm of possibility but cost and complexity (time) are limiting factors.  https://www.youtube.com/watch?v=Sz-vaZzTB1g

 

 

Share this post


Link to post
Share on other sites

Why do you want a shock dyno for FSAE/BSAE exactly?

 

A SAFE shock dyno that produces any meaningful data isn't going to be cheap.  What's your definition of cheap?

 

Which mountain bike shocks?

Share this post


Link to post
Share on other sites

I see the dyno being used mainly just to verify that the shocks are balanced and don't require a rebuild, not necessarily shooting for the functionality of a $5K+ machine. That is if it gets used at all. 

I do not see a huge safety issue here. I am not sure on cost yet, it depends on what my team is willing to spend, if we can fund raise any, if we can get any materials donated, etc. 

I am unsure which model shock it is exactly. 

Share this post


Link to post
Share on other sites

I am designing a shock dyno for my senior project. Ideally, the dyno is going to be used for the BAJA SAE and Formula SAE teams at school. The Baja team (which I am the VP of) runs Fox Float ATV shocks while the FSAE team (I was drivetrain lead last year) runs mountain bike shocks.

I just have some questions for someone who has experience with shock dynos. I am currently trying to set up a meeting with an engineer at Bilstein and/or Fox since my location in San Diego makes that relatively easy but I am not getting my hopes up. 

I also have to pay for this project all myself so I can't get too crazy with it unless I get some sponsorship money. 

As of right now, I am thinking of a simple design like this http://www.shockdyno.com/ 

If anyone knows what kind of timer that dyno uses I would really appreciate it. I need a way for this thing to automatically start timing when motion starts and stop timing when motion stops. 

 

I would rather do a design based off the fancier machines using an electric motor and a load cell but that increases the complexity 10 fold and also drives the cost way up, but it would be cool and could be done. Plus I would get a chance to learn about electric motor control and a better understanding of computer programming.

Something like this would be really cool and is in the realm of possibility but cost and complexity (time) are limiting factors.  https://www.youtube.com/watch?v=Sz-vaZzTB1g

check this out http://sportdevices.com/shockabsorber/index.htm it is the best way to go if you want to have a working use full thing at the end of it...

Share this post


Link to post
Share on other sites

As of right now, I am thinking of a simple design like this http://www.shockdyno.com/ 

If anyone knows what kind of timer that dyno uses I would really appreciate it. I need a way for this thing to automatically start timing when motion starts and stop timing when motion stops.

 

That dyno is only going to measure rebound, so it's not really that useful.  You could potentially get it to work in the other direction by using the air to compress instead of to extend, but you still need a bunch more data to come up with anything useful.  Timing the stroke doesn't give you speed information because you only get the average speed, not the maximum or the shape of the curve.  You really need to datalog position and force so you can come up with useful speed vs. force numbers.

 

You want an electric motor spinning an eccentric/crankshaft.  You can probably get an electric motor off of some old pump or something if budget is a concern.  Measuring the rotational speed and position is cheaper than measuring linear speed/position, but then you have to do some trig to figure out the shock velocity at each point.  Then measure the force with a load cell.  If you can't afford a load cell, you can construct one for virtually zero cost as long as you have access to a machine shop and a couple of strain gauges (cheap).

 

You'll need to do some engineering to figure out the offset of your crank, shock travel, the shock velocity vs RPM, and then figure out how much motor torque you need, what reduction ratio (if any) you'll need to run between the motor and the crank, etc.

 

Data acquisition is where the costs can get big, quick.  Your school should have a lab with DAQ cards and something like LabView, find out who is in charge of it and use that.

Share this post


Link to post
Share on other sites

#1 most important item:  Data Aquisition. 

 

http://www.ni.com/

http://www.omega.com/prodinfo/straingages.html

 

Also, you need a way to move the shock shaft at a target velocity, irregardless of the damping forces resisting movement.  Most dynos for automotive racing use a rotating yoke which creates a sinusoidal motion.  Forces are matched up to velocities to create a plot. 

 

 

The high pitched whine is a VFD driving the motor.  You can see the yoke creating sinusoidal motion down in the base of the unit.

 

The problem with a low power actuator, as I see it, is that when faced with differing levels of damping, you are going to get different velocities.  Your velocity is going to be a function of your damping.  You can not control it.  Contrast that with a professional shock dyno with a motor and yoke.  As long as the motor has enough power...you are going to get a known velocity at the shock shaft.  It will be a function of the motor speed, and the yoke offset.  This also give very consistent results, when you compare two tests on any standard shock dyno.

 

Really really high end dynos use high power high speed actuators to force the shock to undergo both a speed AND displacement profile that is more impulsive and realistic than a sine wave.  A good example would be low speed weight transfer a race car might see entering a smooth corner....with a square edged bump overlayed.  The reason to test like this is that the damping characteristics can change depending on the displacement profile.  A step / trapezoid profile will not give the same results as the sinusoidal profile....  These sorts of dynos are what we call shaker post rigs...and cost millions.  To use a linear input, you need a  very high power...very fast actuator...like you see here.  Even for your app...you would probably need something at least 1/4 the size of what they are using here.  Those actuators are driven by high end motion control cards...probably similar to what is used in high end CNC machine tools.  This is essentially a 7 axis machine tool....

 

 

This video also shows how critical tire pressures and tire characteristics are for high downforce formula cars.  A huge percentage of your "suspension" is the tires...

Edited by Blutarsky

Share this post


Link to post
Share on other sites

The trouble with these shock dynos is they do nothing about high speed damping and that is one of the more critical elements of shock performance.  You need something to impact the shock like a swinging pendulum.  The goal is to minimize energy transfer to the chassis during compression and allow for a fast enough rebound to keep the wheel in contact with the ground without causing it to bounce.  I don't know, shock dynos don't really seem to prove anything without a wheel mass attached to them and without hard impacts.  If you look at any testing done by big OEM's for the car world they use full suspension setups with all control arms, masses, wheels, etc... or they use real instrumented cars on proving grounds to get any meaningful data.

 

The test you see for that Redbull car is to stress test the chassis, it's not a shock dyno.  Do you think the wheels move that much on the track?  No, they have to make them move that much to simulate g-forces seen on the track in order to stress the chassis properly.

 

All of these machines make cool graphs but what do they really mean?

 

It seems the most useful aspect of these machines would be to measure degradation of shock performance after a certain number of hours of operation.

Edited by 1987CR250R

Share this post


Link to post
Share on other sites

We do have data aquisition. Which admittedly is the aspect about which I know the least ammount. 

I am aware of how the current shock dynos work. I have looked at several and almost built one but cost was the number one factor this this design.

We will be using a load cell and a LVT to measure force and velocity. As long as these are measured and sampled at the same time, what does it matter how the shock is cycled? When the shaft is moving at 5 inches a second it should behave the same no matter how it accelerated to that speed (baring any cavitation or any funky business like that)
Also, since our shock is linear in nature, once the high speed dampening takes over (at about 2-3 inches a second for our shock) you can extrapolate out to what ever speed you wish.

Look at this graph produced by a $8,500 dyno for our shock
DynoGraphFoxFloat_zpsa3346869.png

If we can produce something close to that, I will be happy. 


For this dyno, I am not after replicating real world forces the shock will see. A much better way to do that is onboard vehicle DAQ. This dyno has a different use. 

 

Share this post


Link to post
Share on other sites

I'm not a shock dyno expert, but I bet you could use a weight/lever/spring type setup to actuate the shock. For example a pulley with a bunch of weight on it, attached to a concrete flywheel, which has a lever which actuates the shock.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×