How do I analyze shock dynos and motio ratio?

[Nihal]

I was wondering how I could properly analyze a shock dyno graph. I know to look for consistancy between tests and if the adjustments make sense, but what kind of information can I infer from the profile/shape of the curve? Where/ how steep do I want the knee etc? Now would all this determine the desired motion ratio? I am thinking about making a pushrod system to improve our motion ratio, but am not sure which motion ratio to shoot for. I was thinking it would be determined by the shock's resistive force at certain velocities. Is my thinking correct, motion ratio is a function of the shock dyno (to an extent)?
Thanks

[Nihal]

I was wondering how I could properly analyze a shock dyno graph. I know to look for consistancy between tests and if the adjustments make sense, but what kind of information can I infer from the profile/shape of the curve? Where/ how steep do I want the knee etc? Now would all this determine the desired motion ratio? I am thinking about making a pushrod system to improve our motion ratio, but am not sure which motion ratio to shoot for. I was thinking it would be determined by the shock's resistive force at certain velocities. Is my thinking correct, motion ratio is a function of the shock dyno (to an extent)?
Thanks

[Matthew Robinson]

Shock Stuff

You want the low speed and high speed slope to correspond to a certain percentage of critical damping for roll,ride,bump,unsprung. (of course these forces/speeds need to be adjusted for damper movement and not wheel/car movement)
Testing and vehicle analysis is needed to choose these percent of critical damping for each type of movement, and obviously one needs to choose what motion control is of the highest importance.

Generally you design shocks for a motion ratio not vise versa, but a motion ratio of around 1 is generally helpful for design aspects and makes many equations more simplified.

now if you cant rebuild a shock to a certain force/velocity profile, you can choose a motion ratio that will make the forces at the suspension/car be equal to what you want for percent critical so in that sense you are correct.

Does that help?

[95M3Racer]

This might help you http://www.normas.com/SAE/pages/R-176.html

[Matthew Robinson]

Milliken and Milliken is what I use, and its pretty darn good

The shock absorber handbook I found to spend too much time discussing things I did not find important to FSAE, although its advanced theory is both interesting and complicated.

You can derrive all the equations for damper motion with a dynamics book, a cup of coffee and a good long night.

[Nihal]

I am trying to digest the Dixon book right now, but it doesn't seem to go over F(V) curves in that much detail.

It has been a while since I read Milliken, I'll go back to the library and get it.

I was always told that in general you want critical damping for the high speed portions of the curve ( >3in/sec). I have no idea how true that is.

[Matthew Robinson]

high speed transition is of course dependent on motion ratio and your transistion from low speed to high speed motion on your particular car

The F(v) curves are a direct function of your desired critical damping rates. You make a ideal curve based on your rates (ie at roll you want 90% critical which corresponds to 1.5in/s and 55lbs of force) so you make your ideal F(v) chart go through that point. The challenge is then creating a damper that will hit all the points that you desire

Feel free to email me with any questions, Im more than happy to help mjr62@cornell.edu

[Twonius]

One major thing to look at is the classical plot (f vs D) it should be roughly oval, this is very useful for detecting bad rebuilds etc.

[Nihal]

Thanks for the offer. I have to go get to work on it now.

[FPV_GTp]

Hi , been reading and reading information on Shock dynamometer graphs.

I'm no hydraulics expert but just interested in shim stacking and hydraulic oils the different approaches people implement to get their front/rear shockers tuned rite for a specific application.

At the present building a shock_dyno hopefully be operating in the next coming weeks. This will be a big learning curve for me.

this thread has been a good read.

Cheers

[Gino]

Its good to know that I'm not the only one struggling to digest the Dixon book. There are a couple SAE papers out there that are a bit more helpful though

Here's one, thanks Ann Arbor
2006-01-3641