Shock + spring dynos in MN?

Anybody know of any shops that have them?

 

Check on mnautox.com, there was someone there who asked the same question a while ago, but I'm not sure of the response he got.

 

Seen it. It was just suggested to call some of the racing build shops around town. There was also a mention of a shock manufacturer too, nothing specific. Someone posted a pic of a spring dyno but never mentioned where.

I'm starting to contemplate building my own rig...

I could basically build a test stand out of 2x4s, bolt the shock in place, and simply use weights. I can adjust the weight added and record the drop with a camera(30 frames per second) and figure the fall velocity for each over a specific start/stop distance. Then I can simply plot the weight load at each velocity point on a graph. Lumber, bolts/nuts, and a bunch of weights is all that is needed.

I've got a set of D-Specs coming Monday, so I'll be able to makes some initial measurements for the rig. I want to plot out each strut so I have have no guess work for settings. I figure the scaling will be off too so 3 turns on strut 1 equals 4 turs on strut 2 and 4.5 turns on strut 3, etc. I don't want to make my life that hard fine tuning across all 4 dampers. I want specific settings. I can also measure my AGX struts on all settings too.

I can incorporate sort of a spring cage too where I can load the same weights onto the spring and measure compression. The downside is I won't have a lot of weight. I may only have about 500 lbs max, so it may be tough getting accurate results on a progressive spring that's designed to rest on 400-500 lbs in its progressive range. I could use a hydraulic jack and a scale, but the scale too would need to be rather heavy duty when you're talking upwards of a ton or more of force. I would need to fabricate something pretty heavy duty out of steel to feel safe testing more then 500 lbs or so. For the dampers, more really isn't needed. Most aren't tested much over 0.3 m/sec, and that's generally under a 400 lb force. Something for that is much easier to produce with simply plywood. I'm not willing to fabricate some large scale device out of steel.

 

If you use wood, won't the flex in the rig negatively effect the results?

 

Not really. The only real change will be the distance between the start and stop points once loaded and moving. The rig will most likely be sort of a 4 post support with the damper bolted in the middle. There will be a small amount of flex but not a ton. 400 lbs. won't do much to a compact structure. I'm familiar with beam structures, so I can design it with low deflection.

The problem I'm starting to figure out now is speed of travel. I have a very specific downside to this design versus the motor/cam setup normally used. This is time. Under load, the strut will move pretty quick. If you figure 0.3 m/sec, that's 118 inches per second. You have a strut with only 5 or 6 inches of usable travel you must stay under. That gives you about 0.04 seconds of time from start to finish at a 300-400 lb load, or about 1 to 2 frames of a video camera. This poses a serious problem to the test rig.

There's also one other issue. The test starts from a stopped position. I may be testing a specific load, but the damper and weight won't start at max velocity. I have no specific way to measure the acceleration rate of the damper. The closest I could do is anticipate the rate of fall of the weights under gravity and work backwards the amount of influence the damper has on the intended fall rate. However, that's quite messy, and I have no tools to measure accurately enough.

It seems the best way to make this work correctly is with the motor/cam design and some form of load scale. It just needs a motor that is speed stable up to 70 ft-lb of torque, i.e. 400 lbs on a 2-in radius cam (4" damper stroke). It's certainly sounding messy now, lol, and expensive. At the very least, I would need to be hacking apart an old treadmill for a motor and speed control.

This is appearing more complicated then I want to get into. :laugh:

 

I don't understand why you are trying to dyno the damper with the spring on it. The point of the dyno is to plot damping force vs. speed.



We have a shock dyno at MNSU. You just need to know the right people to have them test them.

 

Not with the spring on it, spring separate. I own a set of Prodrives and have no clue the spring rate since Prodrive likes to be so hush hush about something so trivially measured.

I would look into MNSU but they don't allow anything what so ever on the personal level. I'd love to use the machine, fab, shops, but it's a big NO to anything that isn't purely academic. My car isn't exactly academic. I guess knowing the right people is key.

 

Knowing people is definitely key. Once you can demonstrate to Al that you are capable of whatever, he is fine with personal stuff.



I'm still working on it, since the people I knew last year graduated. They let me use the autolab on thursday nights if nobody had school labs to do. I was able to finish my wastegate pluming in there. The lift was key for that job not taking forever.

 

Hmm. Should I just talk directly to Al then?

Know if they have something suitable for testing spring rates? I assume there's some press somewhere on campus with a load sensor attached to it. :laugh:

I'd just like to get numbers on the AGX struts and Prodrive STI springs because nobody in the Subaru community has done it yet. BlueSkyWRX had the WRX AGX strut tested but only on one setting and only at 0.3 m/sec so no graph for it. Someone he knows also tested the Prodrive WRX springs some time back too but only provided single rates of 175 lb/in front and rear. There was an older linear version and a newer 2-way progressive version. I don't know which one was tested. That's all I've seen so far.

I'm not too concerned about the D-Specs as they're infinitely adjustable. Even if they're off a little, you just turn the knob a little more and you're where you want to be anyways. I wouldn't mind individual graphs for them though, but I also don't really want to do it when they're brand new.

 

Well, the shock dyno is actually in the auto lab, so Al wouldn't preside over it. More like Mead or someone along that nature.






A good set of dyno plots will tell you what the adjusters are actually doing and how much crosstalk you have between bump and rebound.

 

Lots of crosstalk? :laugh: They do adjust compression and rebound along a relatively specific ratio of 1:2.25 to 1:2.5.

I know who Al is, but I don't know if I've met Mead. Where's the auto lab?

 

What theyare supposed to do and what is reality is usually not the same. Unless they are Penske dampers, they most likely will not be linear in their adjustment, nor will bump or rebound adjustments be exclusive in their effect.




Gary Mead is who I'm talking about. The auto lab is where teh lifts and what not are. Next to the EPL.