Part 5: Damper Fundamentals
The last article in our series covered suspension geometry, from its effect on car handling to tuning aspects. In this installment, we'll delve into the topic of shock absorbers, a subject that everyone is aware of but few understand. Shock absorbers can influence the handling of a car more than any other suspension component. Unfortunately, shock tuning is one of the least understood and most often incorrectly tuned aspects of chassis set up. Even the name "shock absorber" or shocks in automotive slang is technically not a correct term. Shock absorbers do not absorb shock; the suspension's springs do that. The term that engineers and racers use for shocks is dampers, a more accurate description of the device. So from now, free to free to slap anyone upside the head when they use the term "shocks."
A Simplistic View of What a Damper DoesWhen a car hits a bump, the suspension's springs compress to absorb the force of the bump. This is good, as the absorption of the bump helps ride comfort, keeps parts from breaking off of the car and most importantly, helps the tires stay on the road so they can do their job. The force of the bump is momentarily stored in the compressed spring as potential energy. As the bump passes, the spring rebounds and returns most of this energy to the suspension. If this compression and rebound is not controlled, a car would simply continue to bounce up and down like a pogo stick for many cycles after hitting a bump-much like your grandma's Oldsmobile. This isn't the best thing for comfort or traction, as tires like steady downward force on them so they can stay in contact with the ground and provide grip. If the bouncing is severe enough, the tire could even hop off the road surface-and tires in the air provide zero grip.
A damper helps dissipate spring rebound energy by resisting suspension movement. When properly damped, the spring and the car natural oscillations are attenuated (subdued) within one up and down cycle. This greatly improves ride comfort, the tire's ability to maintain traction and the driver's control of the car. This is a rather simplistic explanation of what a damper does. For all of the simplicity of its function, a damper has a huge effect on a cars handling-perhaps more than any other single suspension component. We'll get more in-depth into how a damper affects handling in future editions of this series.
How Does a Damper Work?
A modern damper is basically an oil-filled cylinder with a piston attached to a rod in it. One end of the damper attaches to the body of the car and the other end attaches to the suspension. Simply put, a damper is a hydraulic device that resists motion. The damper contains many valves and orifices to meter the flow of oil through the piston and the body of the damper to control the motion with a resisting force otherwise known as damping. As the piston and rod go up and down in the body of the damper when the suspension moves, the oil being displaced has to move from one side of the piston to the other. The only way for the oil to get to the other side of the piston is by forcing it through the orifices, which results in resistance in both compression and rebound movement of the suspension.
This resistance damps out a suspension's natural oscillations caused by the springs. The damper converts boinging energy into heat energy, which is dissipated into the air.
Generally, a damper has more resistance in rebound than compression, as its primary job is to reduce rebound oscillations in the suspension, or more simply, prevent the suspension from springing back with as much force as when it compressed.
The resistance in compression, or compression damping, helps assist the spring to prevent the car from harshly bottoming out if a large bump is hit. The ratio of compression to rebound damping force for a car is usually about 70-percent rebound and 30-percent compression, although it can range from 50/50 to 90/10, depending on the application.
To understand dampers fully, it's important to know about the different sorts of dampers and damper terminology. Lets start with perhaps the most common term thrown around in the performance suspension world, the gas shock.
The Gas Shock
You have probably heard of the term "gas shock" before. Most people think a gas shock is a shock filled with a pressurized gas like the ones that prop up your rear deck hatch or hood. Guess again. A gas shock is simply a damper where the oil is kept pressurized with a gas, as opposed to a regular hydraulic shock where the interior of the shock is at ambient atmospheric pressure.
There's a simple and cool reason for using gas pressurization, when the pressure of a fluid is increased; its boiling temperature goes up. When it's decreased, the boiling point can drop greatly, even to below room temperature. This is why water boils faster when you're camping on top of a mountain, and why room-temperature water will boil off in a vacuum.
As the suspension moves, oil is forced through a shock's orifices and valves at high speed. The oil pressure on the entry side of the valves metering orifice increases while it drops greatly on the exit side. The pressure will drop so much that localized boiling of the oil will occur, causing tiny gas bubbles to form. The localized boiling is called cavitation, which can become so severe that all of the oil inside the shock will start to foam from the bubbles. When the foamy oil passes through the damper's metering valves, the damping force becomes wildly inconsistent and the amount of damping force is greatly reduced. This phenomenon is called shock fade. Shock fade can easily happen any time a car is driven hard. Now you can smack me for using the word "shock."
To help prevent fade, damper designers pressurize the oil internally with an inert gas like nitrogen. Much like the way a pressure cooker speeds up cooking by raising the water's boiling point in foods, a gas-pressurized shock greatly resists cavitation and fade by using high internal pressures within the shock body to raise the boiling point of the oil.
There are basically two types of dampers found in the high performance aftermarket, the twin-tube and the mono-tube. Both twin-tube and mono-tube shocks can be gas-pressurized dampers. We will discuss the operational details of the twin-tube damper in this month's installment.