The 5 car got sent to the back for the start of the race last Sunday at Dover after qualifying third when their shocks didn’t clear post-qualifying inspection. Shocks and springs work together to control the rate at which the body of the car moves. The ideal attitude is the hound dog position: nose down, tail up, as demonstrated in the photo at right by my capable assistant Darwin. That position prevents air from getting underneath the car and it sticks the spoiler up in the air as much as possible, which means that more air hits it and creates more downforce.
A spring – one weapon in the setup arsenal – exerts a force proportional to the distance it is compressed. A 200-lb-per-inch spring will compress by one inch when a force of 200 pounds pushes down on it, 2 inches when a force of 400 pounds pushes down on it, etc. You can add spring rubbers to change the rate, but springs are pretty much straightforward tools.
Shocks (the exterior of on being shown at left) are a little more subtle: They exert force proportional to speed, not the distance. Shocks are great fun to play with: they have everything one likes in a mechanical device – lots of tiny little parts that can be put together in myriad ways, adjustments, oil and nitrogen gas. If you push them slowly, they present some resistance. If you try to push them quickly, they provide more resistance. Shocks make a good complement to springs in terms of damping out some of the energy from the bouncing, and for a crew chief, they also provide the ability to tune how the weight of the car shifts during acceleration, braking and turning.
Each team has a shock specialist who works with springs and shocks. A spring is a relatively simple device: it’s a metal rod that’s been twisted into a coil. A shock is much more complex, which means you can tailor the shock much more subtly than you can a spring. The crew chief will specify what type of behavior he or she wants and the shock specialist’s job is to figure out how to build the shock so that it has that specific behavior. Shock specialists are an interesting breed. Most shops have a display where they show which of their shocks were selected by NASCAR for disassembly. It’s something of an honor. The dissected shocks are displayed for everyone in the garage to see.
Although you are only allowed to use a specific set of discs and shims, the number of ways you can combine them is huge, so shocks represent a real degree of freedom for the teams to exploit to gain an advantage. Although the box that NASCAR allows the team to work within is small, this is an area where a knowledgeable person can make a huge difference for his driver.
From the outside, a shock has a body (shown mostly in brass on the drawing at left) and a shaft (the long silvery part extending downward. Many racing shocks are made to be rebuilt – the body unscrews. I’ve diagrammed the inside of the shock at right. The resistance of the shock to your trying to pull or push on it is because there’s a piston and series of shims that resist moving through the oil the shock is filled with. The piston is a disk with holes in it. The shims (the thinner disks that sit on either side of the piston) flex and bend depending of how fast you try to move the shock. The flexing allows the oil to move through the holes in the piston. You tune the shock by selecting a piston and shim stacks (one on either side) that determine how easily the shaft moves. (That’s the macroscopic view – there are a lot of subtleties that I’m glossing over here).
A shock has two motions: compression, which is pressing down on (or shortening) the shock and rebound, which is elongating it. You can tune the shock so that it behaves differently on rebound and compression by selecting the appropriate piston and shim stack – you’d like the car to resist compression and return to its fully extended position as fast as possible.
In addition to the piston attached to the shaft, there is a floating piston. That’s a solid disc with o-rings that seal it to the cylindrical sides of the shock. If you move the shaft through oil at atmospheric pressure, you incorporate air into the oil. Very tiny bubbles form, which change the oil’s viscosity. To ensure that you only have oil in the area where the shaft moves, the floating piston separates the oil. To keep it pushing on the oil, nitrogen gas fills the space above the floating piston. The gas is usually filled to a pressure above atmospheric pressure, which pressurizes the oil and creates a force pushing the shock shaft out. If you compress a shock and then let go of it, the shaft will extend all by itself. Not quickly, but over a few minutes. The higher the pressure, the harder it is to compress and the faster it extends.
NASCAR sets a maximum shock pressure. A closed container under high pressure is called a bomb. You’re just asking for a seal to blow, a piston to fail and you’ve got a catastrophic failure. Second, if you let the teams pressurize the shocks as much as they wanted, the rear shocks would essentially be non-moving. That’s not a great situation: it puts additional stress on the tires, makes for a very rough ride, and makes the car more challenge to keep under control when it hits a bump. An over-pressured shock will keep the spoiler up on the air, giving the car an aerodynamic and a mechanical advantage over the other cars.
At New Hampshire, a couple of cars didn’t pass the height sticks in post-race tech inspection. The teams were allowed to push the cars back around the garage and then the cars passed. Shocks respond differently when they get hot than they do when they are cold. If a team doesn’t pass something like the height sticks the first time, NASCAR lets them wait awhile for the car to cool down and settle. It’s much like how the recommended pressure of the tires on your car is specified when the car hasn’t been driven for awhile – the tires need to be cool. If you fill to the recommended pressure when the tires are how, the tires are going to be underinflated.
It’s very easy to overpressure a shock – gauges may differ, or the shock specialist might be rushing to respond to a last-minute request from the crew chief and miss the target pressure.