“Sometimes I think I would have enjoyed racing more in the days of the friction damper. Since you couldn’t do anything much to them or with them, I would have spent a lot less time being confused.”
Renowned racing engineer Carroll Smith had trouble understanding shock absorbers, so don’t feel bad if you’re in the same boat. When it comes to a car’s handling, shock absorbers are probably the least understood components in the system and this is still true today.
While their inner workings are hidden from view, their impact on performance can be huge. Fortunately, the basics actually aren’t that hard to grasp, but applying it to a race car has become a vast undertaking. Understanding all functions of a shock is an on going task and one that has not been done by most racers or their crew chiefs, don’t feel bad we learn everyday something new about what we though we understood . We’ve compiled several fundamentals of shock absorber design and operation that should help explain these engineering marvels.
Shock absorbers are misnamed; technically they don’t absorb shocks. These components actually damp the energy of the suspension’s movement by forcing oil through a special piston. Think of shock absorbers as timers that regulate how quickly the suspension moves through its travel, and you’re heading in the right direction. (This shock is a gas base valve shock and the base valve is in red)
Shock absorbers damp the suspension’s motions by converting the kinetic energy of the up and down movements into heat energy. If there were no shock absorbers, the springs would be allowed to oscillate up and down like bobble head dolls.
The shock absorber’s shaft and internal piston are designed to move in two directions: in and out. Compression—also known as bump—describes the shock absorber’s behavior when the suspension is compressed. The rebound phase—also known as jounce—occurs when the suspension extends.
Performance-oriented shock absorbers improve handling by better managing how a car’s weight is transferred. They are tuned to improve the timing of the suspension movements and give the driver more control. This is one of the things that we are seeing less of in our sport today or at less in the dirt track world. We are holding the car in posture more to reduce the poor job of damping that we are doing to the tire. Unlike other classes of racing we are not turning right and left which means we can favor the right side of our cars or over dampen them in which should improve the tires ability to make traction. In a dirt track world we have far less technology available to us and have more creative minds at work, which means even though we may not understand we still do a lot of things to help improve traction or try to. One of the things that is true in all forms of racing is; if you want to increase the tires ability to make traction you must protect the tire and mainly the side wall of the tire. This is why, I said we are doing a poor job of damping a car because we don’t understand how the tire is responding to the forces it is seeing on the track.
As a car corners, weight is transferred from the inside wheels to the outside wheels. A shock absorber with soft rebound will allow this weight transfer to happen too quickly. Since some off-the-shelf adjustable dampers only feature adjustable rebound, think about it this way: You’re stiffening the inboard shock absorbers’ rate of rebound, not the outboard shock absorbers’ rate of compression. This rebound control is what determines body roll speed. Important bit of take-home information: Compression holds the tires to the ground, while rebound manages the weight of the car. Both of these things have a huge effect on grip and how well the tire will stay stuck to the track. One of the problems on dirt is the ever changing grip level of the track surface and when coupled with weight transfer it becomes difficult to do, this is why we must focuses on the tire and the shocks job in damping that tire.
Most off the shelve shocks are non adjustable which applies to most rule package in today’s racing. Most non adjustable shocks don’t have the needed low speed to control the cars movement of weight transfer, both in compression and rebound. This is a huge factor in the grip change in the car, tuning the shock to handle weight transfer is one part to making grip in a non adjustable shock the other part is controlling the speed of it.
Single-adjustable shock absorbers are almost always rebound-only. Double-adjustable shock absorbers can vary both compression and rebound damping. Three- and four-way adjustable shock absorbers will allow you to change high- and low-speed damping characteristics for both rebound and compression.
A damper’s compression and rebound characteristics are determined by the internal piston passing through the damper’s oil. Holes drilled through the piston determine a damper’s low-speed characteristics. Flexible discs can control the size of these holes as well as when they’re open. (This is part of the black art of shock absorber tuning.)
There are a few different methods for controlling damping. Many units use flexible discs that cover the piston’s orifices to control both low- and high-speed damping. Very high-speed damping is often regulated by allowing maximum flow through the piston holes. For extra high-speed control, some units use remote reservoirs that add another set of pressure valves to the equation. Low-speed damping can be controlled by bleed valves that allow the oil to flow through the damper’s rod and bypass the piston, also through bleed shims or holes drill in the piston face. We refer to this as free bleed or bleed in a shock, it is the larges factor in making grip in a car. To much or to little will change everything in how the car performs.
High- and Low-Speed Damping
The terms high- and low-speed damping don’t refer to how fast the car is traveling, but rather how fast the shock absorber’s shaft is moving within the damper.
Low-speed damping occurs when the shock absorber shaft speed is less than a few inches per second. This typically occurs during slalom maneuvers and initial turn-in (corner entry).
The car might be moving quickly, but the suspension is moving up and down somewhat slowly. In dirt tack racing this is what gives a car attitude or holds the car in posture; it is also a large factor in grip and the level of grip. Low speed coupled with a balance of high speed is what makes a good shock package in today’s dirt track racing, this is why we keep working on the balance of a race car and the control over the tracks surface.
High-speed damping occurs when the shock absorber shaft speed is faster than approximately six inches per second. This is the damping that affects how a car reacts to surface irregularities—like expansion joints, potholes or the rough terrain of a rallycross or dirt track surface. High speed can be overlooked in the fact that it needs to be there to support the low speeds ability to work. High speed gives strength to low speed and the balance of the two together becomes more important as the grip level of the track goes down.
Which Shocks Should You Choose?
Look at the whole suspension as a package when choosing shock absorber valving. An off-the shelf, sport-oriented shock absorber can’t always handle a given spring rate. The final answer depends on a number of factors, including the suspension geometry, available grip, and weight of the car.
Generally speaking, we like a shock with digressive valving: the low-speed rebound damping is quite firm yet the high-speed damping is soft. This allows the car to corner well without losing traction over bumps. While this is very true on asphalt it becomes more difficult on dirt, low speed and high speed become more depended on each other because of the tires lack of grip. Highly digressive shocks on dirt seem to look good on the dyno but fail to perform week after week on the race track. Don’t be mistaken by one or two good finishes due to higher moisture levels, the shock needs to work in all conditions. Don’t forget that because it looks good on the dyno don’t mean it will race good, we still race a car and that car needs things to happen to make it work. Even though high digressive shock work for some applications we feel like it is not the best for a dirt car and is more suitable for asphalt and linear piston with a touch of low speed tend to work better when grip levels are low.
Compression damping is typically used to maximize the chassis’s grip and control the unsprung weight. Grippy surfaces can require more compression damping, while slick conditions call for very little. This is based on speed at most tracks but coupled with spring rates and the total amount of weight moving in the car compression becomes more important. The speed in which the weight is moved to that tire and the tires ability to hold the track surface will determine the amount of compression that car will need to see. This is often messed up when we add our lack of knowledge of rebound to spring rate and how it effects the car, we choose less spring because it is easier to understand than a shock is. So what I am trying to say is this in order to make speed we must have good spring rate in the car, in order to have good spring rate we must have rebound, but with out compression to time the weight transfer the tire will suffer and shear the contact patch.
We will never understand all there is to know about shocks and this is because we keep changing the rules in which we apply them. We will keep developing new ideas and ways to improve tire management, we will work with SRT piston development, mass damping, tire loads and damping in general to produce a better product. We are committed to your out come to performance and the desire to win, we want to go fast and we want you to go fast.
God Bless and GoFast