Analysis of shock force vs. velocity graph ?

[TrackRat]

This is a force vs. velocity graph for a performance OE front shock with only 10K miles on it so I do not believe it is worn our, but it does have unusual damping IMO. Testing was conducted by an experienced tech at Koni's facility. Any thoughts on why the relatively high bump at low velocity and the great lack there of damping for rebound?

Thanks for any input as I've not seen this profile before and the rear shock also has an odd profile.

 

[GregLocock]

I'm not entirely convinced it is good data. The friction line, through 0,0 should have the same gradient both in rebound and compression.

Maybe that is an artefact of the number of datapoints.

Yes the rebound curve looks very odd. I don't remember seeing any production shock with a concave characteristic before.

There again the bump/rebound ratio is odd in itself, I'm used to seeing bump of roughly 50% of the rebound damping.

Cheers

Greg Locock


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[TrackRat]

Definitely odd that's for sure! <LOL>

Below is the rear shock graph which kinda makes me think they were trying to compensate or induce something specific but who knows what?

 

[GregLocock]

Random guess- they're trying to put the wheel back on the ground as quickly as they can after it has powerhopped.

Cheers

Greg Locock


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[TrackRat]

Thanks Greg. Yeah I considered the low rebound an attempt to get the tire back on the pavement especially with a low wheel rate but it seemed unusually low for a relatively low powered primarily FWD Haldex based AWD coupe.

 

[GregLocock]

I'm more puzzled by the difference between the front and rear bounce/rebound ratio. It looks as though they want to increase initial roll understeer, bit odd in a sporty car. What's the ride like?

Cheers

Greg Locock


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[TrackRat]

I agree on the front/rear bump/rebound ratios and that's why I was puzzled by the whole deal. Ride was actually decent. The roll was about what you'd expect with modest low damping but not bad.

Everything kind of looks reversed from what I'd expect that is why I posted the graphs for feedback. As best I can tell at the moment the front and rear suspension natural frequencies are proportionate to the 58% front weight bias so that's why I don't know what to think. It may just remain an unsolved mystery? <LOL>

 

[TrackRat]

The attached graph is of a performance, rebound adjustable shock calibrated for a natural spring frequency of 2.0 Hz for performance street and occasional track use. These profiles seem to be pretty typical of many "performance" oriented shocks. Some companies seem to use even more bump and rebound damping below 2"/sec. velocity.

Is this how you pros would calibrate a performance shock for spirited street and occasional track use?

 

[TrackRat]

This is the rear shock graph.

 

[hemipanter]

In order to say anything about the curves we must know about all the weights and motion ratios of the car. If you like to compare your crurves to what I use on my car, take a look at

http://www.hemipanter.se

75% scroll down the site. The car is 3 Hz and weights 1150kg, 62% rear weight. Sert up is for real sportscar driving, not for giving nice girls a ride.
Goran

 

[TrackRat]

Goran, now I understand the Hemipanter handle. You are a master craftsman plus, plus! I don't know how you could find the time to achieve all that you have in several lifetimes! The guitars and Pantera are works of art that I truly appreciate. Well done!

As far as the shock graphs are concerned, the vehicle weight is 3500 lbs. with a 58% front bias with a front MR^2= .92 and rear = .94. Front and rear natural suspension frequencies = 2.0 Hz.

 

[TrackRat]

No more comments from the pros?????

 

[hemipanter]

Thanks for your kind comments. I am just a little busy for a coupple days, but I will be back later.
Regards
Goran

 

[TrackRat]

No problem Goran. I'm just curious how other folks would approach calibration on this type of damper/application as I see a number of variations from companies like Ohlins, Koni, etc. and I'm always interested in the thought process employed.

 

[JahanAssadi]

The curves do look odd, for reasons already mentioned above. The shocks probably have some kind of "velocity dependent" (marketing speak) damping feature where they blow off above a certain speed to help ride quality. With that C/R ratio, you'd probably need it.

I've also seen odd C/R ratios used as a bandaid to make up for springs that are too soft or other deficiencies in the system.

Based on the 3500lb and 58% front bias, this clearly isn't a pure sports car like an elise or even a Corvette. 2.0Hz seems to be on the high end, and I'd expect the rear to be higher than the front from the factory. Is this your modified setup?

If you are looking for recommendations on how to calibrate, we need quite a bit more information, starting with, but not limited to, what behavior you want to change.

 

[TrackRat]

The vehicle in question is a "sporty" stock front engine AWD model developed from a typical FWD model. It turns out that the soft low velocity rebound damping is used primarily to keep the tires on the ground for optimum traction with soft OE springs and the higher bound damping helps to reduce roll and pitching. It's all OE and works reasonably well so I expect the OE engineers spent considerable time in development.

The 2.0 Hz natural suspension frequency is a consideration for performance driver's education type events run on road course tracks. The OE set-up is around 1.32 Hz front and 1. 37 Hz rear. The goal is to reduce roll and pitching with the higher traction tires typically used for the HPDE events and still retain acceptable street ride comfort in a dual purpose vehicle.

It appears that some performance shock suppliers have had good success for both ride and handling improvement by using more low velocity bound damping, say below 2-3"/sec. This seems to allow for lower rebound damping to keep the tire planted over bumps and still support the car to limit roll and pitching. This approach is reported by some folks to improve the ride and handling over the traditional approach of increased rebound with minimal increase in bound as the Koni Sport shock uses for performance street and HPDE type activities.

 

[NCRick]

Do we know why the graphs are one half the cycle? There is no indication of what half (Rebound open / Compression closed or Vice-versa). Greg, I am not very knowledgeable about all the ins and outs of the dynamometer chart, can you explain "friction line" ? Is that the same as the force line? I also have a graph of a Koni twin tube strut that caused me to question the forces. If folks are still reading this thread and interested I can post them.

Great forum folks, Thank you.

 

[GregLocock]

The force at very low velocity is just friction, but having just measured that I think that the usual graph doesn't have enoug resolution, or data at slow enough speeds, to really show it. So I don't think that was helpful.



Cheers

Greg Locock


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[NormPeterson]

NCRick - each of those charts is only one possible shock dyno plot (there are others). Try to avoid thinking of rebound and compression in terms of suspension position.


Norm

 

[NCRick]

Yes, I am more accustom to an absolute force vs velocity plot, those may be average force vs velocity (not as useful).