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Dyno charts, what do you think?

 
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Scooter



Joined: 12 Nov 2006
Posts: 18
Location: San Diego/Santa Cruz

PostPosted: Thu Jun 14, 2007 4:20 pm    Post subject: Dyno charts, what do you think? Reply with quote

These are S13 coils with 9/7 springrates. I don't know how to read these graphs, but I know they feel way under damped in terms of rebound. Do the graphs indicate this also or am I crazy?








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Wiisass
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Joined: 09 Nov 2005
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PostPosted: Fri Jun 15, 2007 12:11 am    Post subject: Reply with quote

Why couldn't they be force velocity curves instead of force displacement? It would be so much easier. The force displacement plots don't really tell the whole story, I mean you could get a lot out of them if you had forces at all points, but without that, it's kind of hard. So as long as their mostly symmetric, that just means the shock isn't seeing much caviation or hysteresis. So there are some things that you can tell from the shape of the curve, but you can get a much better idea about the valving from a force velocity plot, so if there's any chance of getting them, it would be nice.

And another thing, the units on those plots? I mean I guess I've come to accept that kg/mm isn't really a spring rate. But when did kilograms become a force? And I don't even know what they're trying to say in the kg-mm column.

Anyway, I did run the numbers, I made some assumptions. I used the peak forces for each speed tested, assumed linear behavior between each point, so some of my calcs might be a little off and they might not show the true behavior of the dampers. One of the biggest problems is I had to assume it was linear from 0in/s to about 8in/s which totally consumes the range that really controls the handling of the car. Those would be the number I am most interested in. But if the dampers are linear then it might not have been such a bad assumption.

So the numbers show they're way overdamped in rebound. And really underdamped in compression. I mean we're talking about damping ratios of 1.4-1.9 for rebound in the front and 1.8-2 for rebound in the rear. That's like Nascar rebound damping ratios. The most I would run are maybe up to 1, depending on the car and track conditions, but usually more in the 0.8-0.9 range, at least in the low speed.

Compression is soft having damping ratios around 0.2 front and rear. This is about stock compression damping ratios. It should make them ride nice, but it's way too soft for any performance driving. And compression damping doesn't change with shock adjustments. So the damper is a true single adjustable damper.

And coupling that super stiff rebound with really soft compression can't be good for the performance of the car. With that high of rebound alone, you could be jacking down the car over bumps, you could be unloading the tire on quick transitions, both of which will greatly hurt grip.

Why do you say they feel underdamped in rebound? I mean it could have something to do with how soft compression is because both will influence it during transient manuevers. So you may or may not be crazy.

Tim
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Scooter



Joined: 12 Nov 2006
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Location: San Diego/Santa Cruz

PostPosted: Fri Jun 15, 2007 12:42 am    Post subject: Reply with quote

Tim I asked for the Velocity plots too, but this is all they have.

Thanks for the rough calcs, I find it interesting. I say they feel under damped on rebound because say you go over a bump, they feel like they jack back up very quickly. Not smoothly. Not really bouncy either, just very harsh in general with much faster rebounding than I'm accustomed to. The fast harsh pushback from the coils made me think they are underdamped in rebound. I must be feeling something else. I wonder what that is...

As for the compression being way under damped that is a surprise to me. Maybe they have really shitty springs (progressive or something else shitty-esque) that makes them feel wacky altogether. The springs must be stiffer than advertised if the comp. is so under damped because the ride is stiff as hell.
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Wiisass
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PostPosted: Fri Jun 15, 2007 1:00 am    Post subject: Reply with quote

Well it sounds like the plots don't really match what you're feeling. I mean the plots could be wrong, who knows.

With rebound, with that much force, they could be hitting the bump and then it feels like they're jacking back up, but the response is too slow. Basically, the spring is fighting the damper to put the car back to the static height. But then I can't say for sure, I would have to feel it for myself to get a better idea.

As for compression, the only reason I could think they would ride harsh would be too much overshoot and possibly bottoming out or hitting the bumpstops if there are any. Or just because there is so little compression force, the wheel goes up and then doesn't really want to come down becuase of the large amounts of rebound.

I don't know, from the plots, it just looks like a really weird setup.

Actually, if I have some time this weekend, I'll throw these numbers into my model and see if I can simulate any of what you're feeling with these numbers. It might give a little more insight on why this is happening.
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Scooter



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PostPosted: Fri Jun 15, 2007 1:19 am    Post subject: Reply with quote

Well I've only felt them a few times (as its not my car) but I've got another friend who has owned a couple different sets of coils too, and he says they feel really weird to him as well.

Let me know what you're coming up with in your model Tim. From my experience with damping I can't figure out how these coils feel the way they do with regards to the respective damping ratios. Weird. Based on the damping ratios I would expect them to "pack up" bumps. Oh yeah and they're a long ways from the bump stops, so that's not what I'm feeling.
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Dennis



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PostPosted: Sun Sep 30, 2007 12:29 am    Post subject: Reply with quote

Quote:
And another thing, the units on those plots? I mean I guess I've come to accept that kg/mm isn't really a spring rate. But when did kilograms become a force? And I don't even know what they're trying to say in the kg-mm column.

Tim,
I believe other countries typically use kg as equivalent as some in the US use lbs as mass. The correct terminology for force should be kgf, and lbf for force, and kg and slug for mass. Just an FYI for posterity sake, and not to preach. And for the kg*mm column, maybe it is the area underneath each of the football? This would be in units of work. I dont know who might be able to use that, but it is interesting nevertheless.

Quote:
So the numbers show they're way overdamped in rebound.

Aint that the truth! Shocked
It is calling for nearly 1500lbf at ~25 in/s.
This is a perfect candidate for exhibiting the jacking down effect. Something is definitely not right here. I dont know too much about S13 suspension motion ratio, but if it is a coilover design, the damping rates shown on the graph are out of whack.
Dennis
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Wiisass
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PostPosted: Sat Oct 13, 2007 12:20 am    Post subject: Reply with quote

Dennis, I probably should be used to kilogram force units by now after seeing them so much. It was just drilled into my head during my Vibrations class at school that kilograms have no part in spring rates or damping coefficients. So everytime I see it, I think of that.

You might be right about the kg-mm, but I don't know what you would use that for.

Yeah, these dampers on their stiffer setting were showing damping ratios around 2 for their low speed rebound, while the compression side had damping ratios of about 0.15 for the same speed range. On full soft, they were still overdamped in rebound with damping ratios of about 1.4 and compression was about the same in the low speed.

I do have to say, I am really curious to ride on these and feel for myself if someone didn't something stupid when they were testing these things of if they are really that bad.

Tim
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