How do you pick the appropriate shock?

I've seen all kinds of articles on shocks and they usually end with the sentence "you need to pick the appropriate shock for your application".

I understand about picking the proper lengths. I understand (mostly) how rebound and damper affect a particular shock, BUT how do you determine the appropriate shock for your application?

At the end of the day I browse shock application and find that most shocks seem to be pretty much generic as they fit a wide disparity of car make and models with wildly varying suspension and spring combinations.

I read the forums where people build the roundy rounds, Locosts, home brew race cars of various ilks, everything up to the FSAE cars and most of them pick a shock due to price, length and what diameter springs it will take. My god, the same, off the shelf, short GAZ shock is used on everything from TBuckets to Locosts to race cars of all sizes.

Then I read posts about not upgrading your springs on your stock shocks because it will "blow" them.

I've seen Koni rebuild and change valving on shocks at the track and even talking to those guys they say you have to start with the right shock and all they are doing is tuning.

There's the math for optimum header or exhaust system sizes so where's the math or at least the options to determine any type of shock size or performance.

I think Mr. Tanner from Flyin' Miata said it best; Pick the spring you want to use, start with your struts at full soft, then crank 'em up till you are happy. Lots of people say to get shocks dyno'd so you know what you are actually doing/what your adjustment range truly is, so having that piece of paper may be worth it as well.

I've been reading through Miata suspension threads for nearly three months, trying to pick the best budget suspension for my car. On the interwebs, there seems to be several schools of thought out there for picking shocks:

• Pick whatever is shiny and fancy and the latest hotness
• Pick what ever the fast guys at Nationals or the Run-Offs are using
• Go with a setup from a respected aftermarket tuning shop
• Get good shocks which can be revalved to suit your particular setup by a respected shock tuner

Since my car spends 95% of its time on the street, I'm choosing the last option because I think that some things are just best left the experts. Bilstien HD's are quality monotube shocks which run about $400 a set for my car, and about the same for custom valving. Add to that a set of inexpensive coil springs and sleeves, and I'm hoping to install the new parts under my car myself for less than a grand spent.

Yeah, but WHICH shock do you put on in the first place? And WHY?

If you are working with a stock car you must at least use a shock that mounts properly but what if you don't start with a stock type car?

The shock dyno's I've seen don't come out and tell you what spring rate that particular shock will work best at or that it will fail or fail to handle spring rates above X lbs.

I understand spring rate is more or less a personal thing when you're building a car and that a CPM of 80-100 is usually for more of the dual duty cars. But that at least gives you a starting point, but how do you tell what shock will handle and at a certain spring rate?

It was once said that American cars were over sprung and undershocked. How would you over-shock a car?

They literally slap everything on Locost cars and they still work great! I'm beginning to think there's a whole lot more to this light weight thing than we give it credence. (This late at night I guess that ought to be Creedence as in Creedence Clearwater Revival, which is playing in the background right now)

"They literally slap everything on Locost cars and they still work great!"

The guys giving their driving impressions generally aren't suspension engineers or test drivers, right? I'd take these anecdotes with the same grain of salt that you'd use when reading any 'I love my new aftermarket suspension!' threads on the forums - sure they like the results, but compared to what exactly?

Two of the most helpful resources I've read recently are the Shocks section of Autocross to Win and the FAQ's/Suspension Tools section of the Fat Cat Motorsports site.

Your shocks are there to control the springs. That's their basic purpose.

Because the shock is there to control the spring, the damping on the shock is chosen to control that particular spring. If you stick stiffer springs on stock shocks, they won't have the damping needed. The shock won't necessarily "blow", but you will find the car will bounce around as if it had worn shocks - the symptoms are the same.

Now, if you crank up the damping on the shocks, they'll work like springs as long as they're moving by resisting the movement of the suspension. That's how you can over-damp a car. It's a trick that stock-class autocrossers can use to stiffen their suspension on stock springs :)

A good way to find out what the useful spring rate range of a shock is - ask the person selling it to you. Personally, I prefer adjustable shocks over fixed rate shocks. The reason is because I like to play with spring rates, and it's very slow and expensive to send your shocks off to get rebuilt on a regular basis. It's also a great way to get a feel for what shocks do - try adjusting them and see if the car gets better or worse. If you're dealing with an unusual car such as a Locost, you're unlikely to find someone who will nail the perfect setup the first time out, especially since you're liable to be changing other aspects of the suspension as you sort out the car.

Most Locosts are pretty much crap in the suspension department, I suspect. It took me two years to get mine working properly. And even though I had the same GAZ shocks used on other builds, it was a matter of getting the spring rate right and then setting the shocks to match. And it's still not perfect, with what I learned on building the Targa suspension I know I can make it work better.

WIth regards to nderwater's decision - remember that the setup from a good tuning shop is also leaving things to the experts. It'll be an off-the-shelf custom setup for your car, if you will. Unless you're going for an unusual application, this is just as good an option as custom valving. Heck, I've driven some setups with custom-valved shocks that weren't as good as off-the-rack options.

I can't take credit for that "start 'em soft and keep turning them up until the springs are under control" advice. That's from Carroll Smith. But I've used it to good effect.

Well first off, are you going to use Penski shocks at $1200 each? How about Moton shocks at $2000 each. If you're in the National points runoff, the answer better be yes.

If you're building a kit car to drive to lunch in, do you "need" to spend $8000 on shocks? That's up to you.

If you're going to track your car, or do autocross, is it for fun, or for serious competition? Do you need coil-overs in order to accurately corner-weigh your car? Do you feel you need adjustable valving? Single or double-adjustable.

You're asking questions to something that has no answer - only YOU can answer this because YOU are paying for them. YOU are putting them on your car to... what? THAT answers the question. If you're prepared to spend $50,000 on the car, prep, travel, engines, and tires, to win a trophy, well there you go. If you're building a car to drive along the beach in, that's a different answer.

It's as if, because it's too hard, you want us to figure it out for you, but only you possess the information to decide.

On top of all this, what is "best" is due in large part on personal preference. Two drivers may drive the same car and come away with very different feelings about how it handles. What's that say? It means that what's perfect for one driver is crap to another. It also means you'll ask 10 people and get 10 different answers - and that's in the same type of racing. Since you haven't detailed exactly what the car's for, its budget, you're mental drive to win, your driving style, your preferences, etc, etc, etc, it's not likely you'll be happy with a cookie-cutter answer.

I'm writing another book about building yet another car, but don't - can't - have dimensions for where the steering wheel goes, or the seat, or the shifter. Why? Because everyone's built different, AND everyone has different preferences. (And shock brands are not spec'ed either for the same reason.)

shocks=voodoo magic

nderwater wrote: "They literally slap everything on Locost cars and they still work great!" The guys giving their driving impressions generally aren't suspension engineers or test drivers, right?

Actually several are and then there are the kits that are driven by a wide variety of people who also have great levels of experience including test drivers and suspension engineers.

You ought to see the trouble they go thru on LocostUSA to get their suspensions just right.

I've driven a number of the Locost and the kits and my skill set hasn't exceeded the level of performance of any of the cars. Some have been better behaved than others, but all could do things in a corner or a bumpy surface that scared the crap out of me.

As Keith said, shocks are to control the springs, but unlike what Keith said I've never found a person selling shocks who had a clue what spring rates they could control NOR have I found any literature from any shock manufacturer that says it. The people selling shocks all want to ask what kind of car it's going on and then tell me what type of shocks they sell for that type of car.

Shocks = Voodoo magic is apparently what the manufacturers want us to think.

Set up is another beastie too. Yes, set up is a personal preference. I prefer my car slightly loose, but that's handled with alignment, springs, bars, etc. I can get specs on all those pieces.

The question I'm asking is not a matter of how am I going to use it? It's also not as if it's too hard for me so I'm asking someone else to figure it out for me, I'm asking HOW DO YOU FIGURE IT OUT?

As in spring rate calcs there has to be someplace that gives you some sort of idea of how to chose a shock. There don't appear to be any specs for shocks as there are for cams or other parts.

There's so little hard shock information that it becomes the a trial and error situation when it shouldn't be that way. We can calculate spring rates, we can calculate suspension parameters, we can calc metal loads and failure points in a chassis or suspension, but we just bolt on any old shock that will fit? Why bother with any of the rest of the calcs if it doesn't really matter?

Where do we find the info to calculate a similar data point for shocks? This appears to be the last black hole of data when building a car. It is good for the shock manufacturers, but bad for us.

So for the purposes of my question let's presume my question is about a purpose built car and not any type of modification of a stock vehicle.

It doesn't matter what purpose you build the car for, how can you determine anything other than packaging constraints? Where's the science? Where's the math? And more importantly where's the info that allows you to buy that way?

carguy123 wrote: I'm asking HOW DO YOU FIGURE IT OUT?

You need to calculate critical damping force. Start here:

http://www.iwsti.com/forums/gd-suspension-handling-stiffening/102766-critical-damping-analysis-ohlins-tein-ground-control-more.html

Critical damping is a start: some say 70% critical in rebound and 30% critical damping in compression is a very good start.
Damping is misunderstood by many, INCLUDING many in the shock industry. I read an article recently that compared three European compact cars that used completely different damping philosophies, but all rode and handled exceptionally well.
Manufacturers spend days and weeks tuning dampers with the help of the damper manufacturer. I do (developing military vehicle shock absorbers) find that the driver prefers a vehicle that is "overdamped", which arguably produces better driver feedback and rides more roughly than what the "simulations" predict.
Within the industry, opinions vary widely with respect to progressive and digressive damping, and where to the curves should diverge, and, and and.
There's no easy answer: driver preference is a massive factor.

Thanx MCarp2 I'll read the link you gave me.

Njansenv Dampening is one thing, but I'm talking capacities. You could set the same 70/30 the same dampening force on a 1/4" piston as a 1" piston but would it last or give as much control? Is piston size THE biggest determining factor? How do we know a Koni is better than the JIC or Monroe or whatever? Or even that a particular model from the same brand is better or worse than another model?

To my feeble brain it seems like shock science would give us something similar to tire sizes where if we are going +1 or +2 we must maintain a certain amount of air capacity to maintain a certain load limit.

You can use tall skinny tires or short wide tires, but they must contain a certain volume of air to maintain the load limits.

From the shock's load limits that would allow you to see what's the appropriate shock size for the springs you are using and then use the adjustabillity to fine tune it to the way you like it.

But as far as I can find there is zero information on shock sizes and capacities nor a way to buy a different size or capacity.

For most of my life I've read in magazines or heard from techs that you must match your shocks to your springs. Or if you upgrade your springs you need to upgrade your shocks. How can you do that if there's no data?

People talk about buying a "matched set" of shocks and springs but how do you know that you have a matched set. From what I can see the vendors/manufacturers "match" just about anything together and call it a set.

So are you guys saying that shocks basically all come in a capacity range that simple tuning (changing the dampening) will control just about any spring rate changes we can throw at them? Therefore all we really need to worry about is some sort of tuning knob (or the capacity to have the sent off and revalved) and any packaging constraints?

Bilstein has some interesting information buried somewhere in their site about rebuilding dampers and how to valve them. It really isn't an exact science: trust me, I REALLY wish it were.
Generally, 'they' say that larger piston dampers are more responsive, but that said, I've used a 2" bore damper on vehicles ranging in weight between 12,000lbs and 72,000lbs. Obviously, the damping curves varied tremendously. Generally, there isn't nearly that range on pass car, so a family of dampers will be suitable for 'most' applications without resorting to changes outside of 'revalving'. -that is also probably why you don't see too much information on size.
The important stuff involves the length of the dampers and the damping curves. Also important are things like lag and hysteresis, but if the damper is tuned properly, those aren't likely to be obvious to anyone without a very tuned bottom. Most matched sets will be put together using experience: someone like Jay at Ground Control simply knows what valving works very well on what vehicle for various use.

I think you'll find that that OTS Koni's and Bilsteins (for example) can be revalved for any reasonable high performance street application.

I would start with the techies at Koni or similar. If you have the precise numbers and measurements it should be a fun process.

Buy the most expensive one. Duh!