bizzare damper curves

[Raphaël Rochette]

Until a couple of days I was believing that at a constant speed a damper was generating a constant force. But then I got this from our shock dyno. Is it normal? It's not something funky that happened only once. The curves I get from the dyno are deadly repetitive and this variation is always present and incresing with speed. I'd really like if someone could post similar data to compare.

http://i15.photobucket.com/albums/a377/r_rochette/shockdynocurve-3inpers.jpg

[Raphaël Rochette]

Until a couple of days I was believing that at a constant speed a damper was generating a constant force. But then I got this from our shock dyno. Is it normal? It's not something funky that happened only once. The curves I get from the dyno are deadly repetitive and this variation is always present and incresing with speed. I'd really like if someone could post similar data to compare.

http://i15.photobucket.com/albums/a377/r_rochette/shockdynocurve-3inpers.jpg

[Storbeck]

What is the static load at those two positions?

It could be just the effect of the gas pressure pushing on the shock rod, and it's the change in position that's causing it, not the change in time.

Measure the force on the shock rod at the two positions while the shock is static, and see the difference is the same as the difference in force you're seeing while the shock is moving.

Might be stating the obvious but I thought I'd throw this out there.

Andy

[Buckingham]

I agree with storbeck.

Additionally,

For this direction of shock travel, the magnitude of the force increases with time. In the other direction of shock travel, does the magnitude of the force increase or decrease with time?

[Raphaël Rochette]

Andy and Donovan,

As you can see on the first pic the static load is not constant either. I also thought it could be the change in static position but then I should see the same delta force in compression and in extension. The second pic clearly shows that it's not the case (compared to the pic in my first post). Furthermore, the delta force I observe in extension increses with speed (pic.3) while it stays almost the same for compression (pic.4)

http://i15.photobucket.com/albums/a377/r_rochette/3in-completerun.jpg

http://i15.photobucket.com/albums/a377/r_rochette/shockdynocurve-3inperscompression.jpg

http://i15.photobucket.com/albums/a377/r_rochette/shockdynocurve-6inpersextension.jpg

http://i15.photobucket.com/albums/a377/r_rochette/shockdynocurve-6inperscompression.jpg

Raphaël Rochette

[Buckingham]

If this is caused by your gas charge, try changing the gas pressure and see if this changes anything. Also, I wouldn't assume that an air spring (your gas chamber) behaves the same in compression as it does in extension. (It may, but I wouldn't assume it, gases are funny little things http://fsae.com/groupee_common/emoticons/icon_smile.gif)

Try changing the weight of your shock oil and see if this changes anything maybe?

Do all four of you shocks do the same thing (meaning this is not a rebuild issue or a bad seal or fatigued spring washer?)

[Steve Yao]

What dampers are these?
Initial load seems low to me. Try running these at 50 or 100 lb preload?

[Raphaël Rochette]

I don't know for the other dampers yet. I'll test them when I have time (at the end of the term). I'm quite sure that it's not a rebuild issue since they've never been serviced. I would not be suprise though if it was for a bad seal since they've seen some serious mileage.

Why the gas chamber wouldn't behave the same in compression as it does in compression?

-------------------

SEY

These are Fox Vanilla RC. Why does the initial load seems low to you? In wich manner would a higher preload (higher gas pressure) affect the force variation I observe at constant velocity?

Raphael Rochette
Université Laval

[Steve Yao]

Sorry, it was late when I posted that. I don't believe preload would have an effect. But i would be interested in seeing if the load variation goes to a steady-state value.

A few of the Fox Vanilla RC's features:
-Nitrogen charged
-Speed Sensitive Rebound Valve Stack
-Force-Sensitive Compression Damping
-12 Click Externally Adjustable Rebound Damping
-12 Click Externally Adjustable Compression Damping

I would seem to me that all of these would play a part in your damping behavior. The damping medium being compressible; if the piston is moving faster than the medium can bleed through the diaphragm then your medium is compressing; providing additional damping load. Once piston motion stops, the compressed volume can take time to equalize with the opposing charge. At least, thats how I would figure it could work. I am no shock expert.

As for difference in behavior between compression and rebound, the Vanilla's appear to advertise dynamic behavior in compression and rebound. Additionally, of course, what compression and rebound settings you have set on the shock make a difference...and whether those 12 settings really affect damping linearly or not.

[Raphaël Rochette]

I guess there's something I don't understand because I don't see any difference between speed sensitive damping and force sensitive damping. Sounds like marketing to me.

I hope testing the other dampers will give me answers. I know the Fox aren't the best solution but that's what I got to work with. I'd like to compare them to a better damper though. Is anyone willing to show me similar data as what I posted? I don't think I'm asking for any secrets here ans I know that many other teams have a shock dyno.

Raphaël