Stock Car Science

Juan Pablo Montoya’s pole run last Friday at Kansas was disqualified when his shock absorbers failed tech inspection. The shocks and springs are important components of the supension. A car without a suspension would bounce all over the track. When you hit a bump, the springs compress. When you go over the bump the springs extend back, which keeps the wheels in contact with the track.

The problem is that springs are, well, springy. When you compress a spring and let it go, it extends, then compresses, and just keeps bouncing up and down. Hence the need for shock absorbers.

The shock absorber (like the spring) connects the wheel to the chassis. When a spring compresses, it stores energy. That energy is what is dissipated by friction when the car bounces up and down. You’d like to damp out the spring’s oscillations more quickly than friction allows.

A monotube shock absorber (the type used in NASCAR) has a piston (a disc with holes in it, shown below) moving up and down in oil. The holes in the piston are very small, which means it requires quite a bit of force to move the piston. Punch tiny holes in a piece of plastic and then try moving it through pancake syrup. This movement of an object through oil introduces an interesting way to control force.

The force exerted by a spring is proportional to how far you compress or extend it. (That’s Hooke’s law). In a shock, the force exerted by the shock is proportional to the speed at which the piston moves through the oil. So the force depends on how fast you move, not how far you move. You pick the shock to match the spring and the two work together.

A spring can compress and extend. So can a shock. When the shaft is pushed into the shock, it’s called compression. When the shaft is pulled out of the shock, it’s called rebound. You’d like to be able to adjust the rebound and the compression independently. One way you do this is that the two sides of the piston have different hole geometries. A hole may be large on one side and small on the other, which affects how the oil flows through the piston.

Another way you can tailor the compression and rebound response is by changing the shims (very thin washers) that bend when the piston moves up and down. The shims bend more when the piston moves faster, uncovering more of the holes and allowing more oil to move from one side of the piston to the other. (At very low speeds, a valve in the shaft allows direct flow.)

The key to how a shock works is the resistance to the piston’s motion. The resistance is provided by the oil. Most shock oils are between 2-5 wt. More viscous oil provides more resistance to motion.

Look back to the first picture of the shock. There’s an area there marked ‘gas’. Ignore that for a moment and assume you just screwed the two pieces of the shock together without doing anything special. If you want to try this experiment at home, get some cooking oil and put it in your blender. Turn the blender on - that will mimic what happens when the piston moves up and down. (If you have a French press coffee maker, that would be a more accurate analogy, but a blender has the same effect and is less likely to engender spousal irritation.)

Whirl the oil in the blender for a few seconds and you’ll notice that you have foam. Foam is gas bubbles trapped in a liquid or solid. Stryfoam, for example, is air bubbles in a polymer. The foamy mess you have in your blender was created when the whirling motion incorporated air bubbles into the oil. (When you have bubbles of one liquid trapped in another, that’s called an emulsion, but it’s the same idea.)

There’s a problem when shock oil foams. The principle on which a shock works is that the oil provides resistance to the piston’s motion. Air trapped in the oil makes it much easier for the piston to move. You use oil in a shock because it is incompressible, which means that when you press on it, it doesn’t change volume. When the oil foams, you push on it and the air that’s dissolved in the oil doesn’t provide much resistance. A marshmallow (which is a foam) is a combination of sugar and air pockets. When you press on a marshmallow, the first thing that happens is you press all of the air out of the air pockets and it’s pretty easy to do that. Only after you’ve squished the air out do you start to compress the sugar that forms the rest of the marshmallow. A shock has to be filled with an incompressible fluid for it to work. Foam isn’t incompressible.

You have to pressurize a shock. One reason is because oil sloshing around the inside of the shock won’t provide much resistance for the piston. If we fill the top part of the shock with an overpressure of air, the pressure above the oil will prevent the oil from sloshing. Air is about 21% oxygen, with the rest primarily nitrogen. Oxygen is more soluble in oil than nitrogen, meaning that it is easier to dissove oxygen in the oil than it is nitrogen. Depending on the type of oil, the difference can be a factor of two. This is why nitrogen is used to pressurize shocks. Nitrogen gas is less likely to create foam than air. In fact, if you press down hard enough on the oil, you can actually decrease how much gas is dissolved in the oil. You literally press the dissolved gas out of the oil.

NASCAR allows the rear shocks to have nitrogen pressures between 25 psi and 75 psi. Apparently, the rear shocks on Montoya’s car had a pressure of 85 psi. There was an interesting discussion on NASCAR Now with John Darby in which he pointed out that after the initial overpressure was discovered, they allowed the shocks to cool to ambient temperature before re-measuring the pressure. Why? The ideal gas law in action. When gas gets warm, the gas molecules in the tires move more rapidly, and that increases the volume and the pressure. The same thing happens in your tires.

NASCAR wanted to give the team the benefit of the doubt: perhaps the shocks had gotten so warm that the pressure had increased beyond the allowed value. I made a quick calculation (remembering that the 75 psi/85 psi are gauge pressures, so you have to add 15 psi of atmosphere to those numbers, converting degrees Fahrenheit into kelvin, and making the assuming that the change in volume is negligible) and I estimate that the temperature of the shock would have to rise about 60 degrees Fahrenheit to create a 10 psi change in pressure. It’s not at all unreasonable for the shocks to reach that temperature during two laps of qualifying, which is why NASCAR waited until the temperature of the shocks had come down before making the measurement.

What difference would an overpressure make? When teams realized the importance of the car’s attitude in terms of aerodynamics, the primary job of the shocks became keeping the rear end of the car up in the air to get maximum downforce. Higher pressure in the rear shocks could be used to keep the tail end of the car in the air longer. The upper pressure limit used to be 175 psi. That was changed after the 2005 fall Dover race, where two Hendrick cars finished 1-2. The Hendrick cars were set up so that the rear of the car didn’t come back down very quickly after a bump. Both the cars failed the post-race max height inspection after half an hour of waiting. The language in the rule book now requires the shocks to return to their normal position after compression within "a reasonable time", and a maximum value for the nitrogen pressure. A really high nitrogen pressure prevents the shaft from returning quickly.

There’s another reason for a maximum pressure limit. A closed container with a very high interior pressure is also commonly known as a bomb. If there were a failure in the threads or (more likely), the seals on the ends of the shocks, you could have shock parts flying out without warning.