Shock Dyno Stroke?

[Mark Workman]

I have recently acquired a very nice shock dyno. It was missing the whole data acquisition system, although the sensors were still in place. That's what made the price too good to pass up. That part of the deal didn't scare me off, cause the old stuff was outdated anyway, and writing a new dataq program will make for an interesting wintertime project, and in the end, we will have one sweet setup.

It was originally owned by something like an Indycar team, or maybe GTP, or something else that doesn't use a whole lot of suspension travel. The stroke on it is right at 1" total, and can go no higher.. No problem, methinks, I'll just drill and tap a new hole with 1/2" more offset to put the stroke into the 2" range, like most other shock dynos. So as I start disassembling it, I think; Why? 20"/sec is 20"/second, at 1" stroke, or at 4" stroke. Just the rpm needed to obtain it is different. The motor power needed to do it will be the same, cause the same amount of work is being performed. So I stopped to rethink things.

So my question is: Am I out to lunch here? I can see some minor benefits to stroking the shock more, but is it really necessary? I'd be glad to hear your thoughts.

Also, I'd like to hear from those who use or own shock dynos. What software features of your dyno do you like/use the most? What features do you not like/ could live without? What would your ideal software do?

mark

[rooster]

It all depends on the application. High speed/ short stroke is relative to bump control whereas low speed/ longer stroke is more about weight transfer. If you don't already have alot of personal experience/preferences, you may want to consider using whatever method your preferred shock manufacturer uses. Unfortunately, there isn't really an industry standard. Shock dyno interpretation and usage is basically a subjective deal.

[RyonPro1]

Mark,

Depending on what you want to use it for. Do you have an DA on the car you car wanting this for? Do you know how much travel your cars shocks are getting now? I have seen cars go more than 20"/sec, but they were some pretty violent combo's. The ones I got to work on, the car went quicker when we could slow down the "/sec to something that the shock could manage better.

I am assuming this dyno is a electic motor based dyno? Is the motor a VFD?

I like having temp sensors on the dyno as well.

Shawn

[Mark Workman]

Here's a little more info on the setup: Motor is electric, 7.5hp, powered by a Toshiba VFD with the closed-loop control option. Dyno has a Giles & Penny linear transducer for displacement, a Trans-Tek LVT for velocity, a strap-on thermocouple for temperature, and a 5000lb Interface Load Cell. This thing was represented to me as a Roehrig, and it does resemble one of their older versions, but there is nothing on it anywhere that identifies the maker. No serial number, data plate, or even evidence of one being removed. Maybe someone more familar with the brands could positively I.D. it, but not me(yet). It also has the appearance of a prototype or custom-built unit, as the contsruction quality is very high, but there are no items on it produced by dedicated tooling. I'm going to take some pics of it, and send 'em to Roehrig,,,see if they claim it.

Yes, we have DA for the car, but it is new, and on our budget, test days are few and far between, so it will be next spring before I see any useful data from it.

However, I think I did come up with a pretty good reason to use a larger stroke to attain the velocity needed (whatever that ends up being): Since the data taking is based on a samples/per second rate, for a given velocity if you halve the stroke, and thus double the rpm needed to obtain the same velocity as before, the shock is completing 2 cycles in the same time it was previously finishing just one. So you are effectively halving the data resolution rate also. The data card I am going to use is way overkill for this application, and can sample at a pretty high rate, but I think I will still go ahead with my original plans to lengthen the stroke. I don't think I can go wrong, and if I decide to offer a shock testing service at some future time, I don't want to have to explain to every single customer why my dyno only has half the stroke of Joe Schmoe's, but is just as good. I already fight too many of those battles.

[Greenlight]

I had a set of rear Koni adjustable shocks from my NHRA E/SM Comp. Elim. car tested on a shock dyno at American Showa (a Japanese auto manf. supplier) several years ago. I worked for an OEM at the time and Showa supplied shocks to us, so they were more than willing to test them for me.

They used ~ 90% of the total shock travel during the test.

The range of velocity was from 0 to 1.13 m/s (~3.7 ft/sec) and the range of force was 0 kg to +/- 200 kg (~450 lb).

I had them test the RH and LH shock at 2, 6, and 10 "clicks" on both compression and rebound, and they supplied velocity vs. force data points at every 0.01 m/s.

Plotting the data points clearly show the "knee" in the graph where the high speed valve opens and clearly showed a difference between the LH and RH shock (there should have been no difference) especially in the low speed valve performance and the velocity that the high speed valve opened.

The data is interesting to see.

I can scan the documents and email them to you if you would like.

I would post them here, but my skill level at posting files here is very limited.

There is also a chance that I can get my hands on the OEM shock testing/performance specifications.


Let me know.

[STK 758]

Good topic, We are about to do our comp car shocks and the stocker shocks on a Koni dyno. Your info sounds interesting and I have suspected the comp shocks are not the same in the rear. Now we will get to see.

Thanks,
Barry

[Mark Workman]

I can scan the documents and email them to you if you would like.

I would post them here, but my skill level at posting files here is very limited.

There is also a chance that I can get my hands on the OEM shock testing/performance specifications.


Let me know.

We use Penske 7500's on our car, and usually I call Penske to get a recommendation for a specific track. They have been extremely helpful, even giving me the exact setup of some of the track 'shoes' to duplicate, and what to do if we're loose out, etc. Some of the things on their 'to do' list goes contrary to what I expect to see, so I end up in a fog, which is not conducive to improving, or even making the show. I love how some of the better analytical minds in the sport (even some shock makers), pass off shock theory as a 'Black Art', or advise against becoming involved in it. Hell, them shocks cost a pretty penny, and came with a whole box of doo-dads to tinker with, so I'd like to be able to make some sense out of it someday.

We've had a lot of our shocks dyno'ed also, but the dyno is on the other side of the state, and I have never seen it done in person. So I'm fairly familiar with force/velocity plots, but at this point in time, I am a sponge for any and all information, so send 'em along when you have a chance. I will be most appreciative. The OEM testing specs sounds very interesting, also.

email is : mw@workmonster.com

I downloaded a demo of Roehrig's software from their site, and am planing to use that as a guide for the different features to incorporate into my own software. On first examination, they start out testing at a fairly low velocity, and then step up in increments of .5 in/sec, testing for a few seconds at a time at each speed. I'm not sure why this is, because to get to say, 10"/sec, you have to pass through all velocities under that one at some point, twice per revolution. Hopefully this will become more clear as time goes on. Maybe the slope of the acceleration curve? It would be flatter on either side of the max velocity point, and thus more stable? I dunno.
Mark

[WheelSpeed]

Just a few tech notes for you.

On our car(6.70 bigblock stick car) we see about 3.4 in/s in ext on the intial hit and 17.78 in compression which starts at .024s into the run.

Peak ext displacement is .2" and comp is .73" (both from static).

The dampening is also dependent on the accelleration. So the force at 4 in/s when testing at 5in/s will be different than if you were testing @ 10 in/s.