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I was looking at some data from a recent test. I noticed that my car seems to have a pretty consistent ~3 hz oscillation in the suspension pretty much throughout. This is based on shock-pot data. The traces smooth out when the car is loaded, but it is very prevalent in the data. I'm wondering if this is normal, or if I'm way off on my damper settings. 3 hz is probably not too far off from the ride natural frequency, but I haven't actually run the numbers. The 2 snapshots below show damper positions down the straight and in a long loaded corner.
Edit: This is 200 hz data that has a 10 hz low pass on the damper positions.
Clay, are your pots zeroed? Seems like the baseline is consistently off with the traces.
The channels are biased to be roughly 0 at ride height. On the straight the rears are compressed a bit (0.1 inch or so +) on average due to accelerating and fronts are in a bit of droop (-). The yellow box is a long right hander (the bowl at chuckwalla) so the lefts are compressed and rights in droop.
I'm no shock guy, but if you're seeing oscillations when there's not any big inputs, that go away when there's significant displacement... I'd wonder if maybe your dampers just aren't damping any until they've moved more... makes me think could they need service, just flopping around on seals, that kinda thing? Though +/- 0.3" is quite a bit.
What brand/type, when last serviced?
Also, maybe compare early vs. late laps, see if it's something that gets worse over the course of a session?
disclaimer: I'm just an engineer who has never looked at shock pot data in this application before.
I only say anything because the data piques my curiosity and I'd like to learn something
I think the 3 Hz oscillations correspond to the 'chatttering' felt when the tire friction circle is exceeded. I say this because the oscillation peaks correspond to changes in lateral G. 3 Hz is also probably close to the natural frequency of your sprung mass.
Your unsprung mass seems underdamped. In the same yellow-boxed portion of the plot there are constant 15-20 Hz oscillations. This is about the frequency I would expect your unsprung mass to excite. However, this could also be an artifact of filtering, intrinsic to how your shock pots are attached, or something else. But I'm inclined to ask...have you changed tire type since last adjusting your dampers? Or significantly reduced/increased the unsprung mass?
Not knowing the vehicle it is tough to fully comprehend, but 3 hz sounds very plausible to be in the range of the sprung mass natural frequency. 10 hz would be close to the wheel hop frequency, so it appears that there may be an issue with your sprung mass control. Can you plot the velocities? It'd be interesting to see at what speed these are occurring. A swag but my guess is you're either way low in low speed damping, or you've got dampers that are in need of a rebuild.
I'm no shock guy, but if you're seeing oscillations when there's not any big inputs, that go away when there's significant displacement... I'd wonder if maybe your dampers just aren't damping any until they've moved more... makes me think could they need service, just flopping around on seals, that kinda thing? Though +/- 0.3" is quite a bit.
What brand/type, when last serviced?
Also, maybe compare early vs. late laps, see if it's something that gets worse over the course of a session?
Might be time to talk to your damper guy...
Originally Posted by dgm8138
disclaimer: I'm just an engineer who has never looked at shock pot data in this application before.
I only say anything because the data piques my curiosity and I'd like to learn something
I think the 3 Hz oscillations correspond to the 'chatttering' felt when the tire friction circle is exceeded. I say this because the oscillation peaks correspond to changes in lateral G. 3 Hz is also probably close to the natural frequency of your sprung mass.
Your unsprung mass seems underdamped. In the same yellow-boxed portion of the plot there are constant 15-20 Hz oscillations. This is about the frequency I would expect your unsprung mass to excite. However, this could also be an artifact of filtering, intrinsic to how your shock pots are attached, or something else. But I'm inclined to ask...have you changed tire type since last adjusting your dampers? Or significantly reduced/increased the unsprung mass?
Originally Posted by 2BWise
Not knowing the vehicle it is tough to fully comprehend, but 3 hz sounds very plausible to be in the range of the sprung mass natural frequency. 10 hz would be close to the wheel hop frequency, so it appears that there may be an issue with your sprung mass control. Can you plot the velocities? It'd be interesting to see at what speed these are occurring. A swag but my guess is you're either way low in low speed damping, or you've got dampers that are in need of a rebuild.
Good thoughts. The dampers are Ohlins TTX 4-way through rods. They have very few hours since last service (~10) but the car has not been driven since late 2015. There's no change with time in the session. I also did pull up some old data from the same track right after the shocks were serviced and similar thing. I'm thinking it is just the way the car is responding to the bumpy track (Chuckwalla). I did just also look at some data from COTA which is pretty smooth and I'm also seeing a similar thing.
I do not think it has anything to do with the friction limits of the tires because Lat G is basically near 0 constant on the straightaway and that's where I'm seeing it.
The car is a 996 gt3r (basically cup car). Weight on track is ~2700, 1350# rear and 1030# front springs. I was thinking under damped as well. I was mostly curious if this is common in shock data or if I have something poor in my damper settings.
Unfortunately I don't have "Pro Logging" so I can't take the derivatives within i2 standard (unelss there is some trick I don't know?). I need to export the data and do it offline in python or matlab in order to get the velocities. I'll do that sometime soon and post up the velocity plots.
On edit: It looks to me like it's the way the car is responding to bumps. There appears to be a phase lag with the front hitting it first, but it's like the car never settles down when there is no lateral load.
I'm too cheap to buy pro analysis for my old original Motec ADL, but I rolled my own shock analysis script and it definitely looks like I need more low speed damping.
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Not knowing the vehicle it is tough to fully comprehend, but 3 hz sounds very plausible to be in the range of the sprung mass natural frequency. 10 hz would be close to the wheel hop frequency, so it appears that there may be an issue with your sprung mass control. Can you plot the velocities? It'd be interesting to see at what speed these are occurring. A swag but my guess is you're either way low in low speed damping, or you've got dampers that are in need of a rebuild.
12-11-2017, 10:16 PM
Suspension Oscillations - Normal?
