Mike Cummings, an oval-track...
Mike Cummings, an oval-track racing shock specialist with Bilstein runs a rebuildable shock through its paces on a shock dyno.
These are the parts of a rebuildable...
These are the parts of a rebuildable Bilstein shock with two disk stacks (right) to control valving. The vertical stack to the left is for a linear shock, the stack on the right is for a digressive shock. You can see that either way its a fairly simple assembly, and many shock manufacturers actually encourage their customers to learn how to rebuild their own shocks to help their racing program. Other shock manufacturers use different devices to control compression and rebound characteristics, but if you can read a shock dyno sheet the end result is all the same.
At every level of racing shock tuning is becoming increasingly important. These days its the rare NASCAR Winston Cup team that doesnt have a man devoted solely to shock absorbers. Rusty Wallace once called the job of the shock tuners a black art. Given the many variables available between the car, track and shocks themselves, thats understandable, but its not necessary for a Saturday-night level racer. Understanding how to pull information from a shock dyno sheet and apply it to your car can send you a long way down the road to shock-tuning bliss.
Its An Energy Absorber
First, a little background information so were all on the same page. A shock absorbers job is to smooth out the reaction of the springs, thats it. It does its job by transforming energy from the movement of the cars suspension into heat, which is radiated from the shock body. The spring rates you choose control how far your car rolls when you turn, or dives when you brake. Shock valving controls how long it takes for the car to roll or dive forward. Rebound damping in your shock controls how long it takes the spring to return your car to ride height after the suspension is compressed. The larger the valving number, the more the shock resists the springs expansion. Conversely, compression damping controls the opposite movement by the same means.
There are three major categories of shock absorbers, which are based on the way they react to force. (Note: This is not a discussion of shock absorber design, meaning twin-tube versus mono-tube.) The first shock absorbers were progressive, meaning that as shaft speed increases so does the damping rate, or the energy required to move the shaft at that speed. A progressive shock is the simplest design, and also the least useful in practical terms. Its simply a perforated disk moving through a resevoir of oil. At slow speeds the oil passes through the holes in the disk easily. At higher speeds more energy is required to move the oil through the holes in the perforated disk.
Eventually this type of shock reaches hydraulic lockno matter how much force is applied to the shaft of the shock, travel speed cannot be increased. A progressive shock is old technology, and its performance is unacceptable to the modern racer. Low, continuous forces like body roll through a turn are too poorly damped, and on the other end of the spectrum the shock is unable to absorb sharp, high energy forces: holes, ruts and otherwise rough tracks.
Linear shocks were the next big advance in shock technology. As shaft velocity increases, damping forces increase at a linear rate. On a shock dyno graph (Diagram B) both compression and rebound damping appear as approximately straight lines diverging from each other as shaft velocity (or force exerted on the shock) increases. This style of shock has been a boon for racers because it allows increased control at lower shaft speeds, which is vital for controlling a race car through the corners.
The third type of shock is a fairly recent development. Digressive shocks are essentially the oppostive of progressives. As shaft speed increases, damping forces increase at a decreasing rate. Diagram C is a shock dyno graph of a typical digressive shock dyno curve. Digressive shocks provide low-speed damping control without being unreasonably harsh on rough racetracks. Both modern linear and digressive shocks allow separate damping and rebound rates. So far manufacturers have not been able to combine, say, digressive rebound with linear compression in one shock yet.
The Dyno Sheet
Most racing shock manufacturers adhere to a simple numbering system to describe their valvings, but dont be fooled into thinking that you are comparing apples to apples. For example, Bilstein measures its valvings in newtons per meter while Penske uses American pounds and inches. The only way to know how shocks from different manufacturers will compare on the track is to put them on a shock dyno. Also, the dyno will tell you how a shock will behave at every velocity you are likely to see on the racetrack. Normally, shock movements associated with body roll (roll, squat, dive) in short-track racing range between zero and four inches per second. Ruts, holes and other harsh track conditions generally fall into the six- to 12-inch range. This makes it easier to diagnose what is happening where on the dyno sheet.
Most shock dyno sheets graph shaft velocity along an axis of force exerted in both compression and rebound. The more force it takes to move the shaft a given distance, the more the shock will resist movement of the suspension either up or down. Shaft position does not affect damping. This is important to remember since a common misconception is the farther the shaft travels into the shock body the harder it is to compress. This is almost universally untrue.
Most racing shocks are easily tunable by a qualified technician and sometimes by the racer himself. This allows the racer to easily tune for a given track and even individually tailor compression and rebound curves. In a circle-track environment, competitors normally use less compression damping than rebound.