The black art of shock damping ? 1

[TrackRat]

I've enjoyed reading 52 pages of Tips many of which included suspension natural frequency and shock damping calibration, which obviously several people here do for a living. I also "get it" in regards to shocks having the same damping profile/rate but having completely different "feel" because of how the different areas of valving transition.

So the obvious question without giving away any trade secrets is how can one determine what "feel" is possible for a given damping profile? In other words from what little I know about shim stack calibration, it looks like thinner shims in a higher quantity might be preferred over a few thicker shims as the thinner shims might provide a smoother transistion and thus a better perceived ride. I used "might" several times because in all black art there are few absolutes, <LOL>.

Where I'm going with this is when you have a custom shock built for a sports car with performance handling in mind and it's calibrated based on the application and suspension frequency using typical 65% critical damping for low speed damping and a digressive rebound profile, with fixed bump damping, how can you determine:

1. What changes if any will improve the ride or handling from the baseline calibration?

2. Should bump be altered or just rebound to improve ride quality?

3. Is all the above subjective as some folks have very sensitive arses and others do not?

Any and all help appreciated including "speed secrets".

 

[hemipanter]

You raise a difficult subject to explain in a forum. As you can see in a parallel thread to discuss what the reason for a couple of dampening curves look like they're doing. I have not answered the questioner since I do not know the most appropriate way to give a good answer.

Your question number 1 is difficult answered for the same reason. If I start with the question "improwed handling" that is the first thing you sort out what action is better. A large part are therefore of the person driving the car must be able to specify what he wants for the properties, which is often not the case. In the case of ordinary cars, there are standardized solutions for shock absorbers, but if you want to run more sporty driving so will the driver's wishes enter the picture. That view is no shops that sell auto parts are particularly interested to disclose, because it will require personal service and is also becoming too expensive. Moreover, it is difficult even for a shock absorber manufacturer to determine the valve setting that gives the customer the right shock absorber.

If we take sub-question 2 so the answer is a little easier. The car has the weight, the wheel weight and unsprung parts have, and the suspension travel is determined to require a certain amount of damping, which can be divided between the compression and return. On a typical passenger car has been chosen to be less in compression and more rebound damping, since hard bump gives uncomfortable ride. For a sports car we use more of bump and decrease slightly in return for retaining the grip after an uneven ridge in the road as you drive faster.

The damper may have separate adjustability for high and low speed dampening, where high speed is for bumps and wheel movements while low speed setting is for chassis roll and the weight of the sprung mass. One might think that there is a lot "secrets" in the buissnines, and sure there is, but this is not so much about shock technology, but more concerning "team secrets of adjusting". When I is sorting out shocks for a race car, I set up the car in a "shaker rigg" that shakes the car while I observe things. This is far from secrets, just a lot of work.

Regards
Goran

 

[TrackRat]

Thanks Goran.

Yeah I understand quantifying what a driver desires is difficult for the driver and the shock supplier. Your comments mirror my limited experience with shock adjustments. I just didn't know if once you quantify the required damping, how much comfort can be dialed in to achieve that rate via "black art".

 

[gt6racer2]

It certainly is an art - but really not so black. In most simple form, to control the sprung and unsprung masses, we need to dissipate a given amount of energy. We can dissipate in both directions and all speeds as we see fit. For comfort, traditional thinking is to use more rebound dissipation than compression - used to be as much as 6:1, now usually 2:1 - whereas for performance 1:1 is more typical. We can also shape the curve in both directions. This is where the art comes in - where to put the damping such that the masses are controlled and handling response and comfort meet the desired objectives. Trying to explain how to do this ( low speed/hi speed, digressive/linear, rebound/compression etc.. ) would take a book. For your specific questions 1) has no correct answer - all cases are different 2)Both. It is quite possible to improve comfort by increasing bump damping. 3)Beyond mass control is really is subjective. You are always making tradeoffs.

 

[TrackRat]

Thanks for the comments gt6racer2.

I recently read the book Advanced Vehicle Dynamics and Damping by Jan Zuijdijk. It's an interesting read and I understand he's working on a book that's geared more toward engineers and academia.

One of his approaches that appears less common at least in the racing community, is to use more compression damping in the < 2"/sec. velocity range to improve traction. His experience using more low speed compression damping and less rebound seems to work for a variety of applications from SVO Mustangs to F1 cars. While it is perhaps 180 degrees from what many engineers have been taught and use, there appears to be some logic and results to support his approach.

I will be testing both approaches on track soon to see what works for me. Interestingly drivers reported improved ride and traction with the increased compression damping < 2"/sec. Jan concludes that with much better control of the unsprung mass, rebound damping can be lowered thus improving tire contact with the road vs. using higher rebound to control traction.

As long as you're not stunning the tire on turn-in it seems like a reasonable approach worth pursuing. Ohlins, Moton and others seem to use a similar approach as far as the increased compresssion damping < 2"/sec. from the force vs. velocity graphs I've seen though they may use more rebound damping also?

 

[evelrod]

Personally, I do my own thing...pretty hard to describe what I have learned in over 50 years of racing..

However, for starters:

http://www.amazon.com/gp/product/08..._m=ATVPDKIKX0DER&pf_rd_r=1P3WQ4KQMMFW4AAKSJ55

AND:

http://www.amazon.com/Tune-Win-Carroll-Smith/dp/0879380713#reader_0879380713

It really is not "black art". It is very time consuming and entails a lot of "chasing your tail" to get close. Close, cause I never seem to get it "perfect" !

Rod

 

[TrackRat]

Thanks evelrod.

Yeah I've had all of Carroll Smiths books for years. He is an inspiration and a very wise engineer.

For the record Jan who needs a simpler last name... Zuijdijk who started with Koni in the 60's, worked with Carroll and many other notables, on race car development. Jan and a fellow engineering college at Koni developed the first externally adjustable Koni hydraulic shocks. It seems like a lot of Koni engineers eventually started their own racing damper companies. We're fortunate to learn from their experience. With 50 years in the game you qualify to be writing books too if you haven't already. I read many of your comments in the 52 pages of TIPS here and I learned from them all.

 

[TrackRat]

I had a good laugh recently...

I contacted a mfg. of performance shocks and inquired about the usable strut/shock strokes for one of their production kits and they refused to answer. It turns out that the strokes are too short for the application, which was independently confirmed. The performance aftermarket has some real characters in it.

 

[hemipanter]

It does not surprise me, I have similar experiences. If I may allow myself to be a little bit critical, I think that these "characters" are aware that there are many who can not decide a shock absorbers-quality properties, a situation that can be exploited. I meet many people in amateur racing branch, and many are afraid for 3 way adjustable dampers for fear that it can "adjust themselves away".

 

[TrackRat]

You mean there are some unscrupulous purveyors of questionable products in the automotive performance aftermarket? I'm shocked... or NOT ! <LOL>

 

[hemipanter]

I do not think it is appropriate to use to strong words, I believe rather that many vendors are a little (appropriate) unaware of the technology behind what they sell. Many products work to sell that way, but the shock absorbers are probably the rule that confirms the exceptions.
Goran

 

[evelrod]

I'm old enough to deal only with people I know personally or, with companies that I have a good history with...even if it costs me a bit more than some of the so called "names".

As to brand names, I do have several "modern" brands that seem to work well...but...I prefer Koni since I have nearly 50 years experience with them.

Rod

 

[TrackRat]

Hemipanter - I know many retailers lack the product technical expertise we might desire, but you'd expect a manufacturer to know the shock strokes of the products they sell... <LOL>

Rod - I hear ya on working with people you know and trust. I too am of that age and I have no problem what so ever paying for quality products and customer service. I just walk when the seller is evasive as this mfg. was.

 

[hemipanter]

I noted that this was a known brand and a manufacturer, I expressed myself, consciously, on a more general level. It is expected that the shock stroke shall be entitled to the car, it's a fairly simple technical situation. It is expected also that a manufacturer should take note of such a complaint.
We may be in the same age, all of us here, I am myself 66, so our experience is probably a common denominator.
Goran

 

[TrackRat]

OK I have another question for those who do suspension calibration for a living. Have any of you tested the weight of coil springs such as on a coilover shock and been able to tell the difference between two springs of the same rate but with one being slightly lighter as in say 1 lb. diff in weight?

I realize that ligher is nice but I sure can't tell the diff in ride quality between a coil spring that weights 4 lbs. vs. one that weighs 5 lbs.

 

[GregLocock]

A difference of 0.2 kg in unsprung mass would be very hard to detect in my opinion. I have actually persuaded someone to try the forbidden experiment - bolting a 5 kg mass to each spindle. the results were puzzling.



Cheers

Greg Locock


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[hemipanter]

I have driven my Pantera with a number of different tire-wheel combinations with weight from 16 to 26 kilo. As long as the road is flat there will be hard to detect a difference in grip, but if I drive over a bump during cornering it is not very hard to notice a sideway movement with the heavier wheels. The heavier wheel also require another shock setting which in turn affect other areas of driving.

I should say that EVERYTHING should be as light as possible since the sum of all savings will be great. And especially unsprung, and rotating, parts are important.

To see if things are important, we must look at the specific car as a whole.

Goran

 

[TrackRat]

I agree lighter is better bit I doubted that many people could feel a weight difference of .5 kilo on a coilover spring. Most people have a difficult time noticing a 5 kilo difference in wheel/springle weight in my experience.

 

[patprimmer]

Just because the average driver can't feel it, that does not mean you might not get faster times on certain (bumpy) tracks.

Heavier wheel/tyre/brake rotors certainly measurably and even noticeably impacts on acceleration if it's a few kg per wheel, well at least in my experience.

Regards
Pat
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[hemipanter]

Rotating mass is not good for acceleration, or should we say altering the speed of the car. For an 10 sec 1/4 mile, 17" wheels compared to 15" wheels could make 0,5 sec difference. Becouse of mass location on the wheel. So, if weight is to be reduced on the car, unsprung and especially rotating unsprung mass, is where it pays the best.
The argument that we do not feel difference is not so successful, there are no large weights to be removed from an already light car, but about many small things together.
Every single detail is not possible to detect by driving that it was removed, and the action would in that case have been unnecessary. Anyway, you can see it with the timing clock or other measurement tools.
Goran