I’ve been wrenching on cars for over two decades, including engineering them professionally for a dozen years, and I still manage to learn new things all the time. Some of those things I feel like I should have known by now. Like how a shock absorber works. I know the basics, the main components, and I know why they exist, but I need to rebuild shocks for one project, tune them for another, and buy them for a third, so it’s time to find out what’s actually going on inside. What’s with all these shims? What is an IFP? And why do some of them cost way more than others? To find the answers to these questions I took a wrench to my Icon shocks, I Took a Sawzall to my Honda Grom shock, and I chatted with engineers who rebuild off-road shocks professionally. So join me as I talk about shafts, stroke length, and orifices, all while trying not to giggle.
[Editor’s Note: Matt Brown, engineer, renowned YouTuber, and just overall cool dude, has created a video on how shocks work, and even written The Autopian this exclusive deep-dive. Please subscribe to him and watch his video, and definitely keep reading! Matt, who admits that he’s not a shock expert (but who is clearly great at understanding mechanical systems) is currently engineering an off-road Dodge Viper, and you’ll notice how I didn’t just say “building.” Matt brings true auto engineering know-how to a mass audience in a way that few on YouTube do, and car culture is all the better for it. -DT]
A shock absorber is a part of the spring-damper system that is your suspension. Your car is suspended on springs, which do a nice job of preventing the impact of a pothole from going directly into your spine. The shocks also help keep your spine and the wheels two separate systems by converting some of that impact into heat. Shocks absorb… shocks, but they also damp the oscillations of the system; they prevent the car from bouncing up and down a dozen times after every bump.
I blew out a shock on an off-road trip in Moab and drove 800 miles home with my right rear undamped. It was noticeable. I took apart that shock to see what was inside, because why not. It is a 2.5″ diameter off-road shock made by Icon.
I also took apart the rear coilover I had in my Honda Grom, mostly because I was just curious. I disassembled a mountain bike shock, and I got a peek inside the Fox shocks that will be going into my off-road Dodge Viper project. Here’s a look at that, by the way:
Mountain bikes, motorcycles, 4Runner and Viper shocks. Guess what: they’re basically all the same. Pretty much every automotive shock damps oscillations by controlling the flow of fluid. The simplest type of fluid damper would just be a hole in a plate that restricts the flow of fluid. But most cars and motorcycles these days use a piston with a stack of shims on the top and bottom. We’ll get into the details of that part in a minute.
The main shock body has caps on both ends to seal it off, and it has a shaft that moves in and out. Two caps, a body, a shaft, and a piston assembly (one that allows some, well-controlled fluid to flow from one side to another) would be enough for a shock except that the shaft needs to move in and out. And as the shaft moves in, it takes up space in the body that was previously being taken up by the fluid inside. (i.e. some of the shaft that was outside of the shock body moves to the inside, wanting to displace some of the incompressible fluid).
You need somewhere to displace that fluid as more of the shaft moves into the shock body, so you throw in an internal floating piston (IFP) that separates the fluid inside from some compressible gas on the other side. That gas is usually nitrogen, and usually has some pre-loaded pressure to it so the shaft is always sticking all the way out when it’s not in use. The nitrogen helps keep the fluid pressurized, which will prevent it from cavitating. Cavitation is when small vapor bubbles form behind the piston when it moves really fast and creates a low pressure zone. Cavitation is bad; it makes your damping inconsistent and can damage your piston and shims.
And there you have your basic shock. This is called a monotube. I have this type on a couple of vehicles and they do the job pretty well for a reasonable price.
The Icon shock has a bit different layout. The nitrogen and IFP are in a reservoir stuck on the side (see diagrams above; the red “slug” represents the fluid displaced by the shaft as it enters the. body). It’s a piggyback shock. On some shocks, the reservoir is sort of wrapped around the shock body so you have a tube within a tube, this is a twin tube design and is pretty common on cars. They all kind of work the same, squishing air somewhere so the shaft can move in and out and the fluid can displace somewhere.
There are also emulsion shocks which are monotube shocks without the IFP. They just mix the nitrogen with the fluid all willy nilly. It’s basically like pouring Guinness into your shocks. They’re cheaper, but they’re inconsistent and much harder to tune because the damping depends on how shaken up everything is inside.
There are a few other components inside the shocks that I took apart. There was a spacer to limit the extension of the shock, a couple bevel washers to absorb the impact of a sudden full-rebound, a Shrader valve to fill the reservoir with nitrogen, some seals and bushings, and a nut holding the shim stack and piston together.
But let’s get to the important part.
The Piston sits in between two shim stacks and exists to funnel fluid from the outside of one side to the inside of the other side. This is so one shim stack is reacting to the fluid when the shock is being compressed, and the other reacts when the shock is being extended (rebound).
There are a few different ways pistons are designed.
The one on the top left is from the Icon; it’s two separate pieces with milled pockets and drilled holes sandwiched together. The top right is a Fox off-road shock piston from their Performance series. You can see it is pretty much two funnels going opposite directions alternating four times around a circle. The bottom left is another Fox off-road piston, this one from their factory racing shocks. It has more complex machining for a consistent and more tunable flow. The bottom right is from the rear shock of a Honda Grom. This is about as cheap as it gets.
The Fox shocks that are going on the Viper came from a company called AccuTune that specializes in tuning and rebuilding off-road shocks. AccuTune was nice enough to invite me out and show me exactly what they’re doing when they’re tuning my Viper shocks. Part of what they did was swap out the Performance Series piston for the Race piston. It controls flow a bit better and gives more consistent tuning.
If you look at the pistons on the left, you can see they both have three holes drilled into the outer perimeter.
Pointed out above are just holes; when your shock is moving slowly in and out, all the fluid goes through these. The two pistons on the right of the 2×2 grid shown above — in which you’ll see no such small holes — accomplish the same thing with shims that have slots cut in them. The Grom also has a small groove in the piston on the compression side that does the same thing.
Before we talk about the shim stack, we have to talk about damper curves.
The Horizontal axis is how fast your damper is moving. When people say high speed or low speed in the context of dampers, they’re not talking about the car, but the speed of the piston and shaft moving in and out of the shock. Sometimes when people say “low speed,” they’re talking about the part of the graph where all the flow is going through those small holes I showed before. High speed represents piston velocities above which the shims start to open. But some suspension systems, especially off-road, are a lot more complex than a single knee in the curve, so low and high speed might need some more clarification.
The vertical axis is how much the damper is pushing back against that movement. The higher up we go, the harder it is to compress the shock. If you see a curve for a particular damper, you’ll usually get the compression and rebound side, with rebound negative because the force is in the opposite direction.
The shim stack is just a spring. That’s all it is. If you push on it with fluid, it moves back. If you push harder, it moves back farther. The shims are usually stacked largest to smallest. You can have all the shims be the same diameter; in fact it’s done that way in the Honda Grom shock. But the pyramid stack distributes the force so you can push the shims farther and longer without them permanently deforming. Think of it as a leaf spring rotated around in a circle.
The separate shims slide along each other, so several thinner shims are a lot less stiff than one thicker shim.
A shim that is 0.2mm thick will be much more stiff than the two 0.1mm shims. About eight times as stiff since the stiffness is proportional to thickness cubed. If you imagine taking this to the extreme and just replacing the stack with a cone of solid steel, it would hardly move at all.
Changing the different shims will do different things to the curve. For instance, the last, smallest shim is called the pivot. Its thickness doesn’t really affect anything since it has a solid ledge behind it, but if you increase the diameter, it will shorten the lever arm of all the other shims, making the whole curve stiffer by increasing the rate.
The shim on the opposite side of the stack won’t change the rate as much, but will shift the whole curve up or down. None of this affects what’s going on down near the start of the curve because that is all dictated by those small holes drilled in the piston.
Why do shocks need shims? Why don’t they just use the drilled holes for all the damping? Excellent question. You can make a damper with only holes, but the curve will be wildly progressive, having tons of damping at high speed and almost none at low speed:
But, shims aren’t always stacked largest to smallest. Sometimes, you’ll have a small diameter shim thrown in the middle. This gives you a dual stage shim stack. This is done because sometimes you want more damping at very high speed and you can get that by splitting your stack with a small shim in the middle.
The smaller middle shim is called the crossover shim. It gives you two separate parts of your damping curve that you can tune somewhat independently, although the larger stack will always affect the sorter stack. You can change the location of the transition point by changing the stiffness of the crossover shim. A thicker shim will move the knee farther up the graph.
You can bend this graph the other way by preloading the first shim. Icon and Bilstein do this on a lot of their off-road shocks. This will give you a firmer ride at low speeds. Washboard roads will be harsher, but the vehicle won’t flop around like an old Cadillac in low speed corners. This is a digressive curve, and it will give better handling by itself. But a common approach is to have a more linear curve, that way the shocks are doing the ride quality and the handling is taken care of with the anti roll bars.
My Fox shocks, and most double adjustable shocks, will have another shim stack as well. Remember how we talked about how that shaft moving into the shock body displaces the fluid and pushes it into the reservoir? Well you can control the flow of that fluid into and out of the reservoir to further tune and adjust your curve. This is one of the benefits of having a piggyback shock instead of monotube. The flow is controlled there with some combination of shims, needles, and springs.
My Fox shocks have high speed and low speed compression adjusters, and this (see above and below) is what they adjust. The low speed adjuster is just a needle that blocks off a hole, allowing more or less fluid through. At high speed, the fluid is moving too fast to all go through this small hole, so it pushes the shim stack away and flows through that passage:
That shim stack has a coil spring that pushes down on it. The high speed adjuster just preloads that spring more or less, causing more or less resistance to the fluid flow. The rebound side is often just a one-way valve, but you can also have shocks that have the same thing on the rebound side so you can adjust high speed and low speed compression and rebound. Moving these adjusters in or out just kind of shifts your whole damping curve up or down. It’s a little more complicated than this since the transition between high and low speed moves each time you change the setting, and also it probably doesn’t line up exactly with the main shim stack curve, but you get the idea.
Shocks are one of those things where changing one thing affects everything else. So much so that experience is almost necessary tell you what change needs to be made inside the shock to reach a desired result. If you make a shim change, you probably want to check it on the shock dyno to make sure its doing what you expect it to be doing. Or check it right on the car. Most of the off-road people prefer checking it on the vehicle over the shock dyno. Speaking of off-road…
After disassembling four different shocks, it became clear that the difference between off-road shocks and their on-road siblings is pretty much just size. They all kind of work the same. Even the shock in my mountain bike has a very similar piston with shim stacks on the top and bottom. Off-road shocks are longer because they see a lot more travel, but they’re also larger in diameter. This helps with thermal management. When the shock absorber absorbs a shock, it turns that shock from the ground below into heat, and when you do this in a desert 400 times a minute, a lot of heat is generated. You can add cooling fins and pass-through radiators, but the simple way to keep your fluid cool is to just have a lot more fluid, either by getting a larger diameter shock, or by supplementing your shock with another shock. This is one of the reasons you see off-road racing trucks with bypass shocks next to their coilovers.
The other reason is that those bypass shocks can give you a more complex damper curve, and also change that curve at different parts of the stroke. As the piston moves up the body, some of the fluid bypasses the piston by going around it. But once the piston passes the bypass opening, the shock gets stiffer and your damper curve changes.
Race cars dampers have to deal with body roll, braking and acceleration, and the occasional curb. Off-Road shocks have to deal with a virtually infinite assortment of obstacles. The environments are wildly different, but what’s going on inside the shocks is surprisingly similar. On track or off-road, mountain bike or motorcycle, it’s all motion being controlled by pushing fluids through holes and around shims.
So there you have it. Shocks. And now it’s time to finish rebuilding the Icons. And also to buy a new Grom rear coilover. Partly because the stock one is not very good, but mostly because I opened it with a Sawzall.
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