V1.7 Dec 31 2007
This is a work in progress, and it has been a very long time coming. I intend on working on it for the next little while, dumping as much of my brain and hard-earned knowledge into the public record. It is, however, NOT public domain; I reserve the copyright for myself. You may NOT duplicate this elsewhere without my express permission - and this is the age of Google, folks, I WILL find you if you post it elsewhere. Otherwise, read and enjoy, and put this all to good use. Go fast!. DG
Now its time for damping. I set mine according to the suspension velocity histograms, (which I'll describe later) but roughly 65% critical damping is almost always perfect. Why? I don't know; it just seems to work that way. Set 'em to 65% critical, and be done with it.
One of the more rewarding things about writing this book was seeing the reaction to this particular nugget. This, plus the natural frequency discussion on the suspension page kicked off the usual online shitstorm, but more importantly, got a whole bunch of people measuring cars, dynoing shocks, hacking Excel spreadsheets, and - amazingly enough - independantly verifying this stuff. Not only is that personally very satisfying, it's a great sanity check on my work. Peer review is a Good Thing.
Some of these folks - and I'm looking at you, Scooby and Honda guys - have taken this stuff and really run with it. It's really cool to see, and makes the effort just that much more worthwhile.
Note that I consider knobs on shocks as ways to get the forces to match on the dyno, or to rough in forces while testing; I never, ever, ever muck with the shocks during competition. I've run hundreds of shocks on my shock dyno and the repeatability of shock adjusters is downright horrible on anything except the highest end shocks (and even these have their quirks). When you have the damping right, it's right - unless you do something to change the natural frequency. Changing the shocks to try and change balance is an act of despiration - or of a Stock class car, which has little other options.
Let me make this as clear as I possibly can: THE ADJUSTERS ON YOUR SHOCKS ALMOST CERTAINLY DO NOT DO WHAT YOU THINK THEY DO. Unless you have something high-end, like a Penske, and you've taken the time to clock the adjuster window on the shock dyno, the knobs on your shocks cannot be trusted to work. Most shocks of the same model DO NOT match each other on the same adjuster setting, and each click DOES NOT make the same change in force. Most shocks make very large changes per click near the "full hard" setting and make very little to no change near the "full soft" setting.
Despite this easily verifiable fact, the Cult of the Adjustable Shock certainly has its adherents.
Consider the following:
Happy Fun Fact: Formula One cars use non-adjustable shocks. They get away with this by running the car on a seven-post shaker rig that plays back suspension movements recorded previously on that track, using the data the collect on the rig to tune the shocks, and once the shocks are tuned, those are the shock forces they use at the race. Once they're right, they're right.
OK, so what if they aren't right? Wouldn't it be good to have an adjuster for tuning purposes?
Well, yeah, it would be - if the adjuster worked as advertised. In my experience on my shock dyno, most adjusters DO NOT work the way you think they do - in some cases, horrendously. It's like having a blind man adjust the focus on your camera. What's the point?
And that's if the left and right shocks on the same end of the car act the same - when in reality, they almost certainly do not (unless you've taken the time to match them)
BTW, here are three suspension velocity plots from my car. The top one is from Topeka Nationals North Course, the middle is from Peru, and the bottom is from Toledo. The dashed lines on all three plots show 3 in/sec (so they aren't on the same scale)
Notice the peak speeds on all three plots, and notice how much time the shocks stay below 3 in/sec. Now think about how useful that high speed adjuster really is.
Remember this list:
Not on this list? Almost certainly crap.
Perhaps I should elaborate a little.
For a while, I was the shock engineer for a race team, and was designing, building, and rebuilding shock packages for customers. A big part of this service was running customer shocks on the dyno to set a baseline for where they were currently at. I dynoed a couple of hundred shocks, representing the spread of almost every shock brand extant.
(All the shock dyno plots on this page came off my dyno.)
Amazingly, save those brands mentioned in that earlier list, this was a non-stop parade of horror, including, but not limited to:
The only shock brands I worked with that actually did what they said they would do were the shocks on that list - and even then, they had their quirks:
Bilstein makes amazing shocks - they are mechanically simple, parts are dirt cheap and readily availible, and the innards of their street shocks and their full-race shocks are identical. Crack open a Bilstien street shock, and inside are the same parts as inside their NASCAR shocks. That means that there is a simple conversion process (involving welding a boss onto the shock body to fit a Shraeder valve into the gas chamber) to turn an off-the-shelf street Bilstein into a full-race, user-servicable, user-revalveable NASCAR Bilstein.
The hardest part of getting race shocks onto production based cars is fitment. Real Race Shocks mount with a standardized 1/2" heim joint, so that aside from body and shaft length, race shocks are all interchangeable. The OEMs, however, seem to chamge shock mounting methods and dimensions willy-nilly, so the trick for a production-based-car racer is finding somebody who makes a shock that fits. Bilstein, being a huge company (they are a division of Theissen-Krupp), makes a ton of fitments, so if you can get your hands on one - PRESTO! Convert it to a take-apart and you've got real racing shocks for a bargin price.
Bilstein's tech support was also easily the best I ever worked with. Bill Hindorff at Bilstein would design valvings for me to fit design curves, and he went way out of his way to teach me how to design my own shimstacks (partially, I suspect, so I'd stop bothering him to design me shimstacks). I got tons and tons and tons of help from Bill, way out of proportion to my importance, and the end result was me learning how to effectively tune shocks.
There is only one small problem with Bilsteins: no knobs.
Bilstein does not really make adjustable shocks. They have a rebound adjuster (and they use it on the PSS9 series shocks) but it is really crappy and hard to tune - to the point of worthlessness. It's an on-off switch for the most part. That turns out to not really be a problem for those inclined to tune shocks by revalving, but the average customer wants a knob to fiddle with and so Bilsteins wound up being a hard sell. Those that bought them were universally ecstatic with their performance, but it was tough work getting anyone to buy them when they could buy a shiny quadruple-adjustable from some no-name brand.
And those people were invariably unhappy....
You can add a compression adjuster to a Bilstein, by buying Penske or Ohlins remote canisters, throwing away the Shraeder valve and the separator piston, and attaching the remote to the valve hole - that what my personal shocks were - but that's a lot of money to get more weight and little extra fuctionality. It just isn't needed.
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Penske shocks are works of art, and they have, hands down, the best adjusters you can get. Their rebound adjuster has tons of control authority and is linear over the meat of its adjustment range. The 8100 series canisters are junk, but the expensive double-adjustable canisters have simply massive control authority (although there are some odd interactions between the two adjusters that you have to be aware of.
Fitment, as with all race shocks, is always an issue, but Penske has a munber of off-the-shelf fitments for some common cars (Corvettes and Vipers come to mind) and the design is modular enough that they can probably custom-fit shocks for you if you are prepared to pay for it. They don't make struts, but there is a company (owned by a guy that works out of the Pennsylvania Penske shop) who will build you custom struts.
Penske's customer service is also pretty good; not quite as good as Bilstein, but still good. I never had any problems with Penske, and they were always good to me.
But at the end of the day, a properly-tuned Penske and a properly-tuned Bilstein are fuctionally identical; the only difference is the knobs on the Penske. And you will pay, pay, pay for the Penskes; they are very, very expensive.
The other issue with Penskes is that you have to buy out of the main shops if you want to be sure that they are made correctly. I worked with the PA shop and the MI shop, and they were great. Be very, very, VERY careful about buying "custom" Penskes from other "rebuilders". Some of the worst work I ever saw came out of a particular self-titled Penske "expert" who built a lot of Penskes for National-level autocrossers. He was responsible for the upside-down shimstacks, the one-shimstack-swapped-front-to-rear, and the all time force record bullet points from the Parade of Horror list.
Here's a tip: anybody who builds you a set of shocks had better provide you with a dyno plot for your shocks, as in, "here's the dyno plots for the very shocks I just sold you". If he can't do it (because he doesn't own a dyno) or if he won't do it (because of some secret-squirrel valving he is trying to keep secret) - run away. All he is doing is hiding his own mistakes from you, and there is a reason why he is doing that. A good rebuilder has nothing to hide.
That bears repeating: YOU MUST GET DYNO PLOTS FOR EVERY INDIVIDUAL SHOCK YOU BUY, OR YOU ARE WASTING YOUR MONEY. ANYBODY WHO CANNOT OR WILL NOT PROVIDE DYNO PLOTS FOR YOUR SHOCKS IS A FRAUD OR AN IDIOT
Bottom line here is that if you absolutely must have knobs on your shocks and price is no object, buy Penskes from a direct Penske dealer - and call up the Penske shop on PA and ask them about the guy you are thinking of buying from, if you aren't buying direct.
Also: Penske's Adjustable Shock Technical Manual is required reading if you want to learn about shocks.
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| Penske Rebound Sweep | Penske 8100 Compression Sweep | Penske 8760 Compression Sweep |
I have a love-hate relationship with Koni. Love, because Koni makes more production-car fitments than anybody, the performance of their shocks is reasonable, and they have a knob. Hate, because Koni won't let anybody service their shocks except Koni and a couple of priviliged few rebuilders, and they wouldn't let me become one of those special few. It used to make me mad as hell to have to turn customers away, because they wanted me to rebuild their Konis for them, but Koni wouldn't let me.
Grrr.
That being said, the ubiquitous Koni Yellow is actually a decent shock for the price. The off-the-shelf valving is usually pretty good, the knob is rebound-only with very little crosstalk onto compression, and while the knob is SERIOUSLY nonlinear, it can be worked with: a typical Yellow had 2 1/2 turns of adjustment. The last 1/2 turn to full hard is useless (tiny changes make huge force changes) and the last half to full turn does nothing, but that turn to turn and a half in the middle of the range usually isn't bad.
Koni's quality control on the Yellows is such that the odds on any two shocks with the same part number matching forces are very small - there's quite a bit of shock-to-shock variation. But bought as a group buy and then dyno matched, it is possible to put together matched sets. Be aware that I've seen Koni Yellows with the same part number that matched perfectly when one was 1/2 turn off full hard, and the other was at full soft - I consider the adjuster knob a way to match shocks on the dyno, NOT a tuning tool.
Koni will also rebuild and revalve for you, although the labour and parts pricing is a little high. Koni is "save now, pay later" where Bilstein is "pay now, save later".
Konis that are custom-valved to make a lot of force also have a tendancy to explode... but that's a "Doctor, it hurts when I do this" problem, not Koni's fault.
Where there's no Bilstein fitment and Penskes are too expensive, Konis are usually perfectly adaquate. They are, by far, the best budget shock and better than any of the crap coming out of Japan. JIC, GAB, Tokiko, Tein - synonyms for "crap".
You must, must, must have dyno plots for your shocks - and not just "typical" plots, but the actual performance of each of your shocks. Anybody who cannot or will not provide you with dyno plots for your shocks is a charlatan. Running the dyno was a huge eye-opener for not only what was out there, but also on what shocks actually did.
Not only that, but you need as a minimum plots of the forces at slower speeds, around the 3 in/sec peak speed range. Plots of faster speeds are OK, but once you're over about 10 in/sec you're into "big bumps" mode and you're probably just digressing the hell out of the forces anyway. You want to see the shock operating in low speeds, and that is where I found the majority of shock-to-shock variation lived. Shocks that produce near-identical forces at high speeds can be very different at low speeds - get the low speed plots!
I'll say it again: YOU MUST GET DYNO PLOTS FOR EVERY INDIVIDUAL SHOCK YOU BUY, OR YOU ARE WASTING YOUR MONEY. ANYBODY WHO CANNOT OR WILL NOT PROVIDE DYNO PLOTS FOR YOUR SHOCKS IS A FRAUD OR AN IDIOT
Amongst some people, the hot ticket is to take gas-pressurized shocks, and degass them. The idea being that, with less gas pressure, the car sits a little lower and that lowers the CG height.
Well it does lower the CG height - but it also takes away the little bit of extra spring rate that the gas pressure adds, and worse, it exposes the shock to cavitation - like this:
That's a slow-speed "football" plot of a degassed shock. Yes, you lowered the CG of the car - but the shock stopped working too.
NOT a good idea - especially on a Stock-class car. In a Stock car, you really should go the other way, and pressurize the shock as much as possible to get more spring in the car. (Interestingly, the NASCAR teams use the same trick on tracks that have spec springs)
Interestingly, I've started to see/hear pushback from the Cult of the Degassed Shock about this section... that dyno trace is a slow-speed trace from 0-3 in/sec. Those are speeds you will see with chassis movements, not bumps. No matter how smooth your site might be, you're going to see shock speeds in this area, just as the chassis pitches and rolls due to driver inputs.
A degassed shock cavitates at a very slow speed, and once it starts cavitating, all kinds of crazy crap starts to happen. Cavitation is Bad. Cavitation is Not Your Friend. Cavitation must be avoided at all costs, damn the ride height and full speed ahead. A slightly taller car with functional shocks will be faster than a low car on cavitating shocks, all else being equal.
Of course, all else is rarely equal... which is where we go back to the "people who are fast in spite of their setups, not because of their setups.
For a Bilstein, minimum gas pressure before it started cavitating on the low speed trace was about 80 PSI +/- 10 PSI. I ran 125 PSI in mine, and that never cavitated.
Some feedback on what people consider "crap" - seems I may have gored a few sacred cows. That's not at all suprising, given the amount of disinformation and myth accepted as common sense when it comes to shocks.
The reason why JRZ and Moton aren't on the "good" list is simple - I never dynoed any. In the absence of any dyno information one way or the other, I choose to remain silent.
So then, why "not on this list, almost certainly crap"? Because the amount of crap I found on the dyno is simply staggering. It was INSANE just how much blatent, evil-wrong stuff I found running shocks on the dyno. Stuff that I had always considered decent based on reputation and referral turned out to be crap once tested.
With that track record, any given shock is more likely to be crap than not - especially if the shock supplier can not or will not provide an individual dyno trace for each shock he sells you. If he can not (because he doesn't have a dyno) then HE has no idea what the shock is doing either, and if he will not (he dynoed it but won't give you the data) then he is hiding something and should not be delt with. My experience with shock builders who have Secret Squirrel valvings is that they DO have something to hide - the fact that they do crappy work. Insist on dyno plots!
As far as my criteria for what constitutes "crap", it is actually pretty basic:
Note that NONE of these criteria talk about the shape or the amountof the force curve - that I consider as a "configuration" issue and is up to the engineer. Any given off the shelf force curve may or may not match a given instance of a car, but matching or not matching off the shelf is NOT an indication of quality or crap. That is, of course, contingent on rebuilding/revalving services being available - if you cannot revalve the shock for love or money, then an crappy off the shelf force curve (Japansese stuff again) makes for a crappy shock.
I like Konis Yellows quite a bit - well enough to sell them, when that's what I was doing. They have their foibles, but they are really very good for the price, and there are workarounds for the foibles. If you absolutely MUST have a knob, and you cannot afford Penskes, then they are really the only choice.
The biggest downside to a Koni Yellow - not user serviceable, meaning that you are not Master of your Own Domain when it comes to revalves. This is a big enough PITA that I think you're better off forgoing the knob and going with a Bilstein, where you can field-revalve in minutes.
My car had Bilsteins on it...
The manufacturers expend quite a bit of marketing heat and light over the supposed superiority of a twin-tube shock over a monotube and vice versa. I have a slight perference for monotube over twin-tube, based soley on the simplicity of the monotube, but for the most part, I'm construction-agnostic. It just really doesn't matter all that much.
I've found that people tend to run way, way, WAY too much rebound force. The usual claim is that the extra rebound increases driver feel and makes the car feel more "planted". It does - but it makes the car slower too. The suspension needs to work to work; if you lock it down with a lot of rebound you're depending on the tire alone, like a go-kart.
Again, if you are in the Stock-class game, you may not have a choice but to run tons of shock force; you are prohibited by the rules from changing springs, and stiff shocks will act like "fake springs" in transitions. But once you graduate to classes that let you change springs, shock forces should plummet.
I fell into this particular trap myself. The shocks that I won the 2002 ProSolo SM Championship on were grossly overdamped, in both compression and (especially) rebound - but the car felt great! (aside from occasionally suprise snap-spinning me into the weeds, which was the bad habit that finally forced me into rethinking my setup). When I moved to a 65% critically damped setup, the car felt way less planted; way less tied down. It was actually a little bit scary, because it moved. At turn in, the car would make a definate movement in roll to the peak roll angle, and even though that roll angle was actually pretty small, all that moving around was a little bit disconcerting. The percieved loss of grip was also completely illusionary, as the data proved over and over again that the car not only had better peak grip, but it also didn't get upset over bumps anymore. The day I crashed through one of the big Peru dips at full throttle and nearly full roll, and the car didn't even notice... wow.
Once I got used to it, the car was faster, it was easier to drive, and it eliminated a ton of bad habits from both the car and myself. As a system, we got way faster, more often, more consistently. That's a good thing!
Set the shocks at 65% critical damping between 0-3in/sec. Somewhere above that, digress the shock off to ~30% critical to handle the occasional high speed whack - the exact location of the knee in the force curve doesn't seem critical, as long as it doesn't happen during the chassis roll speed zone. And watch youself go faster.
Here's the super black magic:
Firstly, equip your car with suspension position sensors. Make sure the movement ratio of the sensor is the same as the shock, or if it isn't, calibrate the sensor until you can read the suspension position.
Make sure you are sampling at at least 500 Hz.
Use the math channel to take the first derivitive of the position data, and now you have shock velocity data. This is worthwhile in its own right, but there's another trick coming.
Consider this: for every roll, there is an un-roll. For every bump, there is an un-bump. For every pitch, dive, heave - every chassis movement, there is an equivelent un-movement.
Which means that a histogram of shock velocities for a given shock should be symmetrical.
Which in turn means that the histograms for a given pair of shocks should be identical.
And means that the histograms front/rear should be of a similar shape, but "stretched" on the velocity axis.
Now you have a use for the adjuster knobs! Do a run, tweak the adjuster, do another run, and keep going until you have symmetrical, identical histograms on both wheels of the same axle, and similar shapes front to rear. What you wind up with are shocks that provide slightly different force curves, but identical reaction speeds - and you have optimised your shocks.
MOTeC's software has a wizard set up to do just this - and every major race team I know of uses this technique to tune shocks. Done on the track, done on a shaker rig... this is how it is done. And it is how I did it too.
Amazingly, 65% critical with a sharply digressive knee above 3in/sec typical Bilstein shimstack produced almost perfect histograms on my car. A little compression nudge was all they needed to become perfect. This might be blind luck, so I recommend a shock with a useable adjuster if you want to play this game. Penskes would be perfect.
