Shock Tech, 101
 

I want to start a new thread on Shock Tech. It seems there are a lot of questions pertaining to both the technical side and the performance side of shocks.

Shocks aren’t based on black magic, and there’s no reason for anyone to remain in the dark. The better you understand how your shocks work the better you will be at setting up your car for your driving style, and the happier you’ll be with them and your own performance.

In this thread I want to go into detail about the parts that make up a shock, along with their function. What changes in a shock to make a graph look a certain way, and why. We’re going to drill way down into the workings, even down to what I torque the piston nut to.

To get the ball rolling, does anyone have a shock question they can't get an answer to? Maybe something you have been wanting ask, but felt like everyone else already knew. Trust me, you’re not alone and certainly not dumb or inexperienced for not knowing.




MODERATOR NOTE: off topic discussion will be eliminated. questions and shock discussion only please. ie don't post that you subscribed.

Subscribed, with many thanks to Bernie for starting this.

I guess the question I have is how much can increasing low-speed compression damping help the transitional behavior of my car? I'm in a CSP Miata, light-prep (for now), and 550/350 springs. It's too light a spring for the grip of the A6 (it rests on the bump stops in front on course), and my complaint is that it isn't as sharp as it should be in transitions, like slalom. It takes a little long for the body to take a set, sometimes still going one way while I need it to go the other.

Thanks for the feedback.

I'm subscribed as well. I am a complete novice when it comes to shocks and would appreciate all the info you can provide.



I think we should keep the brands out of this thread. The info should pertain to any shock (correct me if I'm wrong). I just don't want to see a "brand war" and diverge away from the useful data.

When going around a track, what determines if a spring is sufficient, too stiff, etc? Is faster rebound always better because the car is usually transitioning very quickly, or can this lead to loosing traction? how does someone start to gauge the optimal setup for there car based on feel alone. i don't have a large budget for shock trial and error? As springs get stiffer, does lateral grip diminish because the weight transfer over the tires around a corner becomes less?

Sorry about all of the questions, but i'm curious.

Good idea-brand edited out. It pretty much doesn't matter, as long as the valving is suited to the spring rate, which mine definitely is.

I am on a never ending quest to save my girlfriend gain understanding.
http://images2.wikia.nocookie.net/__.../91/Xandir.jpg

Starting with the basics of how a shock works and the components that make up the shock would be helpful to the less knowledgeable like me.

Symptoms of too much/little valving for your spring rate: Hops, skips, shutters etc.

I have a question. One of the primary reasons I chose your shocks was that you don't install a Schrader valve unless the customer asks for one. I had heard of valves breaking off, and of valves leaking gas, two things I wanted to avoid.

But given there's a couple hundred pounds of nitrogen pressure in it's own chamber within the shock body, would you please explain how I can be sure the nitrogen hasn't leaked past the floating piston into the oil side of the shock?

Perfect question. This is exactly where I would like this thread to go. The debate on handling can be found many times over on multiple sites. Let's get technical, the stuff nobody else talks about.

A sealed monotube shock, like Bilstein, would have to push out all of the oil before the nitrogen would mix with the oil or leak out of the shock. The reason it would have to push out most of the oil is that the separator piston has equal pressure on both sides, the reason it floats. The o-ring in this application is mainly to prevent cross contamination, not for sealing purposes. Only when the separator pushes out the oil and is pushing on the end of the shaft will there be a pressure differential, and then the nitrogen could leak. For the nitrogen to come out, 2 seals would have to fail first.

By dynoing a shock can you tell what spring range it can handle if you have the motion ratio of the vehicle/weight? How do you read a shock dyno? Can you open up a PSS9 and actually make the settings do something worth while for a track guy?

Here's a great source of info on how a shock dyno graph is generated and what it's measuring:

http://www.roehrigengineering.com/Do...s/dynograf.pdf

I hope Bernie doesn't mind me jumping in, but since he gave me this info I feel I'm on reasonably safe ground.

As for the other two questions, I'll stay out of them.

So, the gas charge is pushing on one side of the floating piston with a certain pressure, and the shock oil is pushing on the other side with essentially exactly the same pressure. Without any reason (pressure differential) for one to go to the other side of the piston they pretty much stay were they are unless you loose the oil, which you would see as it would be all over your shock. Pretty simple.

Follow up questions:

How much Nitrogen gas pressure is in a shock and what are the ranges of pressures and why?

Given that the gas is there to prevent cavitation of the shock oil as it passes through the damping piston, do you use different pressures depending on the viscosity of the shock oil?

my question: where on a shock dyno is "high speed" and "low speed" and what are some examples of driving conditions during which you experience both?

ie. high shaft speed is over freeway reflectors, low shaft speed is over a speedbump or hard left turn

Low speed is that which can come from the springs as they return to center after being comrpessed or extended. High speed is outside that. Large sharp bumps generate hi speed shaft motion.

I've never understood how you can have differing damping rates for high and low-speed with one valve in the shock.

Different slopes through the force-velocity curve. Shocks can have blowoff valves.

I thought I should ad this interesting note, Koni sells there shocks with a nitrogen charge, but when they rebuild them they do not recharge them, when I asked them why they said that they only sell them initially with nitrogen because it is the industry standard, but they do not recharge them when they rebuild them because they say that they see no advantage in there testing.

At low speeds the shock oil passes through the damper piston through a low speed circuit. At higher speeds the disk springs (shim stacks) are deflected open by the increased oil pressure which opens additional passageways, thereby allowing for a different rate of damping.

Have a look at page 15, here:

http://www.penskeshocks.co.uk/downlo...TechManual.pdf

You've been fed an extra large portion of bullshit from the rebuilder.

The nitrogen pressure is to prevent cavitation within the shock oil as the piston moves through its stroke. Without positive internal pressure the oil turns to an air/oil emulsion with very poor damping characteristics in comparison to the pressurized oil. Go to the 3 minute mark of this video and you'll see the problem:

http://www.youtube.com/watch?v=W4ZZYqRVzLc

What the re-builder meant to say is that they don't have the equipment necessary to recharge the shocks, have compensated with a different viscosity oil or valving, and most customers don't notice. They've taken a high pressure shock system and converted it to function as a low pressure system. High pressure is the industry standard because it's so superior.

They use vacuum instead of nitrogen pressure. so that there is no emulsion.

I don't think that's possible, and I think what happened is that you've mistaken vacuum as a part of shock function vs vacuum as an aid to bleeding air from the pressure cylinder.

Essentially all liquids at room temperature turn to gases at low pressure. That's why cavitation occurs. Were you to assemble a shock with any sort of low pressure (below atmospheric), you'd turn the shock oil into an emulsified mix of liquid oil and gas, which would be comprised of lighter and more volatile constituents of that oil.

But here's how you can tell. If a shock were installed under less than atmospheric pressure, you'd find the shaft wants to retract into the body. In the case of my own Koni's, the shafts were fully extended, as would be expected in a pressurized shock. In a non-pressurized shock the shaft would stay wherever you left it.

Here's how a twin tube shock is built:

http://www.youtube.com/watch?v=9fWfFiKBxSc

At 4 minutes, the machine that pressurizes the shock does it's job. That's the machine your re-builder doesn't have.

Yes they where and do return to the bottom, I don't want to argue their point I'm no shock expert. I just thought I would share that experience, the builder is Koni's authorized west coast warranty and service center, they built my front shocks to a custom valving and length, I use the factory set race shock for the rear.

Interesting. So you're using the race version of their double tube shock (yellow), or their mono-shock?

my fronts started as the yellow sports, rebuilt to the race shock standard. the rears are the yellow race, not modified.

The "low speed" on a graph is from 0 - 2.5 (inches per second). The high speed is determined by the speed you dynoed the shock, but is from 2.5 to the highest velocity on the graph.

An example of low speed would be the roll of the car, or the speed of weight transfer. Remember, the shock spends most of it's time in this range considering is has to go thru "0" every time it changes direction.

An example of high speed would be the spike the shock see when hitting a bump.

Typically, you tune the low speed side of rebound, and the high speed side of compression.

There has to be a charge of some sort to allow for compliance when the shaft displaces oil during compression (shaft entering the body). If there was not a charge the shocks would hydraulic, just like an engine. If there was a vacuum the shock would cavitate under compression due to no fluid resistance, the same as low gas pressure.

What you my be referring to is the practice of pulling a vacuum during assembly to make sure the shock is properly bled.

Even though there is only one valve, you can change the low speed and high speed of each side (compression/rebound). One does affect the other, but this is in the middle of the graph around the 2.5 ips area.

The low speed is the bleed, oil viscosity and the preload on the shims. These all change how quick the shock reacts to movement. Tuning with bleed is the most common way to change the low speed. The reason I don't tune low speed with oil viscosity is that it makes the shock more temp. sensitive. The lower the weight of the oil in the beginning, the less it will change with temp.

The high speed is the shim thickness, along with the "top off" shim. The purpose of the top off shim is to limit how much the main shim(s) can open.

To break this down let's think about the movement of the shock on compression. As you start to compress the shock the oil starts flowing through the bleed holes/notches. As you start to max the flow rate of these bypasses the faster you compress the shock, the more the pressure builds against the shim until it has enough to crack open the shim. As you compress the shock even faster you start to push more oil through the shims. The more oil flowing through the shims, the more they open. The more they open, the more resistance they place on the oil.

The amount of gas pressure in a shock varies depending on the compression valving. The more compression a shock has, the more pressure you need to keep the oil from cavitating. This is the only true purpose for changing the nitrogen pressure.

The nitrogen pressure does not affect rebound. The reason being is that the resisting force for the oil on rebound is the "dead" end of shock, or where the shaft enters the body. Since this is not a moving part of the shock, it provides infinite force against the oil. A shock will NOT cavitate on rebound. The only scenario where the rebound will see cavitation is when the compression has cavitated the oil and the rebound has to take up the "slack", or apply pressure to the oil, to stabilize the cavitation before the shims will react.

Any chance you can take pictures of monotube shock parts and tell us what they are? I'm fascinated by this stuff.

AST has an acrylic damper w/ an acrylic external tank that I want to go look at again.

On huge shafts like these, where does all the oil go (through the piston) when compressed? It looks like its so fat that it will displace all the oil.
http://www.rallycarsforsale.net/photos/p45060n3.jpg

Although…Penske guys state that additional gas pressure briefly acts like additional spring rate or preload during transitions…and that in slippery / rainy conditions to slightly lower gas pressure, keeping it above the cavitation threshold.

I will try to get some pictures together in the next couple of days with some detailed descriptions.

What you are looking at in this pic are inverted struts. The "shaft" that you are questioning is actually the shock body with the shaft going into the larger body (note the location of the remote reservoir). Basically an upside down shock using the body for the shaft. The purpose of this is to have a stronger shock for side loads, typically in strut applications, while keeping the weight down.

There are, however, some shocks that use a large diameter shaft. JRZ is a brand that comes to mind. Their intent by doing so is to create a compression sensitive shock. With the larger shaft you have more displacement, therefore more fluid to control allowing you to tune more with the compression at lower shaft speeds. This, in my opinion is the old school way of tuning shocks. I tune with rebound and try to leave the compression alone. Typically, I want the least amount of compression needed to control the unsprung weight of the wheel.

Note: Another application of larger diameter shafts would be air shocks, such as ones on a monster truck. These shocks run extremely high gas pressure with a large shaft. The large shaft displaces more fluid, moving the separator piston more so the shock acts like a self damped air spring. This allows them to remove the coil spring.

Also check out some of Penske's literature, such as:

http://www.penskeshocks.co.uk/downlo...TechManual.pdf

Pics and charts and some concepts are explained, much of which is conceptually the same for similar shocks from others…you can get a sense of what bits in a damper affect which areas of the dyno curves, and some of the terminology.

How about a step by step dissasembly of a popular German shock where you describe each part as it comes apart? Photos, of everything, of course.

Then, how about the hows and whys of swapping those parts for tuning, More photos, naturally.

Than a reassembly and dyno test of the freshly tuned shock. Yeah; we want graphs....

I don't ask for much; do I Bernie?

Those aren't huge shafts. Those are huge, inverted strut inserts.

Not too much, but please be patient. I am loaded up with shocks at the moment.

I pressed Bernie a bit about how much pressure that nitrogen is under in a gas pressurized shock. I was looking for a number...

It turns out the amount of pressure is determined empirically during dyno testing. The shock is put on the dyno and the tuner adds or deletes gas pressure until the lowest pressure that doesn't allow for cavitation is achieved. Too little pressure = cavitation. Too much pressure = additional friction and wear at the seals.

How do you know if the shock oil is cavitating? It shows up as hysteresis on the response plot as the oil changes from a fluid to a fluid/gas emulsion.

Hopefully Bernie will provide some guidelines as to numbers, but that's the process as I understand it.

Speaking of jrz, what issues can one run into with they're old school style of compression tuning. Could one run into a lot of complications trying to adjust the triple adjustable jrz's in between autox runs?

Double and triple adjustable shocks have always dumb-founded me. I guess I need to do more research on them lol.

What is this "old school" theory in the JRZ's?

I'm not really sure either, lol. I was just basing my question off of what Bernie said about them.

I get to drive on a set of jrz next autox season. I'm anxious to try them out.

Any thoughts on why some like to use very little nitrogen, and companies like AST use 175psi+ on teh nitrogen charge?

What I mean by old school is that none of shock guys that I've worked with tune using compression any more. I've worked with two NASCAR championship teams and we always would use a rebound adjuster, never compression. Compression tuning is more to "band-aid" an issue than to fine tune handling.

Back in the early 90's it was real common to tune the suspension with compression, but as car set-ups starting leaning towards softer springs rebound became increasingly more important. As I posted earlier, I want the least amount of compression needed to control the unsprung weight. Why would I need to tune this if it doesn't change?

So what "changes" to warrant adjusting rebound? I was a little let down that my AST's only had rebounding adjustment, but after running with them I appreciate the simplicity and the results from "one click" that made a real difference. My buddy has 3-way adjustables going in on his car and I'm happy to avoid that impeding doom...however he will probably get some help from AST to get them up and running.

A lot of shock companies build their shocks for worst case scenarios. They also dyno their shocks at ridiculous speeds. Some companies using speeds as high as 20 ips. At these speeds you need nitrogen pressure that high to control cavitation. But in the real world, we know these cars will never see 20 ips unless you drive off the road, into a field, just plowed, at 100 mph. So, in my opinion, it is unnecessary.

This question also ties into your previous one concerning compression tuning. A lot of the standards that shock companies go off of today were set in the early days of tuning. This was the era of higher compression valvings where the higher nitrogen benefited the shock. The reason some still do this today is that, for some, change is bad.

If you only have one adjustment, you will have a bigger window of adjustment with rebound.

Until you get into shocks with high-speed adjusters, the adjustment that you are using controls an adjustable orfice. Some use a needle style valve while others use a slide valve to meter flow, but the results are the same. You are metering the amount of oil that can flow past the piston and not the shims.

Since most shocks have more rebound than compression, the metered oil has a greater affect on rebound giving you a bigger range of adjustment per "click" of adjustment.

Bernie, in a serious autocross car, one that is a little "lazy" in transition like mine, would you suggest an increase in compression damping, an increase in spring rate, or both to match?

I would do an increase in spring rate and lower the ride height.

Thanks for answering all these questions, I'm fascinated by suspension.

There is the opinion that too much rebound will cause jacking down, and if I understand correctly, it is preferable to maintain a ratio of compression to rebound that isn't too excessive in rebound. What's your take on this?

Gotta say that this thread is very much appreciated.

You are correct. Too much rebound can cause the suspension to jack down. However, I would not put a relation between the compression and rebound.

Compression and rebound are independent. I've never been in a situation where I had too much rebound and needed to add compression to solve the problem. If you have too much rebound to where the car is jacking down, the only two things can fix that are a stiffer spring, or less rebound.

too much rebound or rebound damping? What does "jacking down" feel like?

Thanks for starting this, I at least am learning a lot.

I have two questions:
1) My current double adjustable are in about the middle of their compression range but seem to be happiest with the rebound maxed out or almost there. Would it be advisable to have them revalved so that the rebound adjustment is "best" near the middle setting to allow more adjustment for varying surfaces and conditions? My car is used exclusively for autocross.

2) Do you have a simple guideline for adjusting rebound? Something like (just and example and I hope it makes sense)
Weight skipping front to rear - add more rear rebound
Fronts skipping in constant radius thanks in advance
--Chris

Thank you for the thoughts, you pretty much nailed the root of my question. Cheers.

That'd be rebound damping.

it feels like all bumpstop all the time

This jives with what I was told by a multi-championship winning tuner in regards to increasing shock compression.

What do you feel are the best springrates for NA Bilstein HD's? Is getting a set of revalves going to dramatically improve on what Bilstein offers out of the box?

I'm curious as to why there is no dual rate rebound adjustment on automotive suspension. Beginning and endind stroke. A slower rate for body roll and a higher rate for preventing jackdown. Bicycles have dual rate rebound and the effective adjustment range is huge.

Its obvious that a track specific car would just need to be properly set up with springrate and rebound adjustment, but for a daily it would be an adjustment I would appreciate. Either way, how could having two adjustments be bad?

The only thing I can see being an issue (and a common one) is that people generally have no idea how to setup their suspension, and thus more adjustments would generally feel worse.

Even if this is the case though, just make both adjustments the same and it will still feel like there is only one rebound adjustment.

There is dual rate rebound adjustment for automotive suspension, but it is mainly on the high end shocks such as Penske. Here is a link to one: http://www.penskeshocks.com/files/60...H%20MANUAL.pdf

As you can see, it is very complicated and expensive. It is much harder to do a high speed rebound adjustment than a high speed compression.

The rebound adjuster that is readily available from most brands does adjust the low speed and high speed, but not independent from each other.

FWIW I am of this opinion.

Big rebound to bump damping ratio is good for top heavy cars on soft springs with lots of bump travel.

I was of the same mind as you, Jason, but I'm beginning to see that upward travel and control of the wheel is it's own independent problem.

You want to optimize this parameter just like you want to optimize downward motion. Given a spring rate, the unsprung weight of the suspension system, and the road properties you'll arrive at a compression damping that controls the bump of the wheel as best suits your goals. Travel in the opposite direction is an independent problem.

There might be a range of compression to rebound damping ratios that work because they do share variables (spring rate, vehicle weight, unsprung weight), but they're not so connected that you'd design one based on the other. Properly designed you might find the ratios are similar between similar cars, but the ratios more valuable as a sanity check than a design tool.

For example, on an extremely smooth road you might want little compression damping and little rebound damping; the ratio might be 1:1. On a very rough road you'll need more compression damping and more rebound damping, but the ratios may begin to vary from 1:1 to 1:1.5 or 1:2. I don't know, I'm just trying to reason this through. Thoughts?