When pulling together all the components for a kit car, a buyer has to make myriad decisions on the quality versus the price of each part. This is the only way to build a car that has the owner/builder's stamp on it, restricted, perhaps, by part availability and price versus the overall impact that part will have on the finished project. This process holds true with regard to shock absorbers--it is a choice made depending on price, quality, and what you want it to accomplish vis-a-vis what you want your car to do.
For the last five years, the shock absorber has grabbed the headlines as the hottest way to tune a car's suspension. From all the attention the shock is getting, you would think that it is the most important part on the car when it comes to handling. It is not.
Our English friends call a shock absorber a dampener, which may be a better name, because its job is to dampen the oscillation of the spring. The spring controls and absorbs road shock and vehicle weight transfer, not the shocks. The spring controls the up and down movement of the tire as it goes over undulations in the road. The spring also controls the movement of body roll or weight transfer as the car makes transitions from left to right turns and from front to rear braking and acceleration. The shock absorber controls the speed that weight is transferred in a car, not the amount of weight transferred. It also controls how quickly the wheel settles back on the road after hitting a pothole.
Hydraulic Pressure
A shock uses hydraulic pressure to dampen the oscillation of the spring in a controlled and timely manner. The shock resists the spring's movement by forcing oil through a series of valves, orifices, and blow-off plates in the shock's piston. How much and at what velocity the oil is allowed to pass through the piston and valves determines the dampening characteristics of the shock.
A shock absorber must dampen the movement of the wheel and the chassis at different speeds. When driving a vehicle on a smooth stretch of pavement, there is very little up-and-down movement of the wheel. Therefore, the piston inside the shock has very little velocity or movement. As the speed of the car increases and potholes or speed bumps are encountered, the piston shaft must move quicker. Shock manufacturers must develop shocks that have the correct valving to handle both extremes of shock travel. Every shock made is a compromise between ride comfort and performance. Some shocks are valved for more comfort and offer a Cadillac ride. Other shocks have firmer valving and give a more performance feel.
Valving Characteristics
A shock absorber uses different valves and springs to adjust the dampening force of that particular shock. There are three types of valving characteristics that dictate how the shock will perform. They are progressive, linear, and digressive. Progressive shocks start out soft and get stiffer quickly as the shaft speed increases. Linear shocks uniformly increase stiffness as the piston speed increases. It is similar to the progressive valving but does not make dramatic changes at higher speeds. Digressive is just the opposite of progressive. It starts out stiff, then it tapers off.
A progressively valved shock generally will produce a better ride quality, because the shock is softer at slower piston speeds. A digressive shock offers a harsher ride because it is stiffer at slower speeds. The advantage to a digressively valved shock is vehicle control. It offers more stability because it takes a lot of the body roll out of the car, which enhances the corning ability of the vehicle. Linear valving is predictable. It starts off soft and gradually gets stiffer as the piston speed increases. The ride quality and car control will be average.
Several of the shock absorber manufacturers offer adjustable shocks. The customer can adjust the rebound and compression dampening qualities on a double-adjustable shock. While on a single-adjustable shock, normally just the rebound is adjustable. An adjustable shock offers a number of benefits. First, if the spring sages or loses rate, the shock's compression can be increased to add stiffness to the ride and make up for lost spring rate. Secondly, if you install stiffer springs, there is no need to buy a new shock. The rebound dampening ability of the existing adjustable shock can be tuned to control the extra energy of the new spring. As the spring rate goes up, you need more rebound dampening to control the greater capability of the higher rate spring. The third reason for adjustable shocks is the ability to tune the suspension. You can change the ride and performance characteristics of the vehicle by turning the adjustor on the shock. Remember the spring rate is the most important thing in changing a suspension's handling characteristics. The shock is a fine-tuning tool.
Mono- vs. Twin-Tube
There are three different types of shock absorbers: mono-tube, high-pressure gas; twin-tube, low-pressure gas; and straight-hydraulic, twin-tube. Each has advantages and disadvantages. It is impossible to say that one design is the best in all applications. A properly valved mono-tube shock will work as well as a properly valved twin-tube shock. The bottom line is the spring does not know what style shock is controlling it.
Another area of personal preference is the use of coilover shocks. The major advantage to coilovers is the ability to easily change the car's ride height. With larger 5 1/2 -inch springs, you must use spacers or buy different-size springs to vary the ride height. Never cut a spring to lower the car. It will change the spring rate to an unknown value and complicates the tuning procedure.
Because of its small size and lighter weight, a coilover may be a benefit for high-performance vehicles. You also have a better choice of spring rates available with a coilover. With big 5 1/2-inch springs, generally the spring rates are in 50- and 100-pound increments. The spring increments for coilovers are generally 25 pounds. Coilover shocks are available in twin-tube and mono-tube designs.
Stock Shocks
Before talking about changing out the stock absorbers that came with your kit, it would be prudent to examine what some of the top builders include with their cars. Shell Valley primarily uses its own patented shock absorber called the "Silver Bullet." They CNC the outer billet aluminum housing and insert a Monroe gas shock absorber. They picked a little stiffer shock to match the performance they wanted in the car. The ride height is adjustable, but the valving is not adjustable. Rich Anderson, the owner of Shell Valley, told us, "We can make our own shock cheaper than can we can buy a coilover. It is a lower-cost alternative to some of the adjustable shocks and still gives us the performance we wanted. If someone wants another alterative, we supply an Aldan coilover shock with the adjustable valving on the bearing end. It is adjustable for compression only. The Aldan shock link matched our shock link and the spring we use works on that shock. We use the same shock on all our kits because the chassis is basically the same. We either just lengthen or lower the middle of the frame."
For tuning and ride purposes, Shell Valley has different spring packages available. They are all 10-inch springs. In the front, you can have 350-, 450- or 550-pound springs depending on the size of your engine. In the rear, it will be either a 250- or 300-pound spring. The weight difference between a big-block and a small-block will dictate the basic spring package. If you are going to run on a road course and be turning hard, you want a stiffer spring. For the Cadillac ride, you will want the softer spring.
At Factory 5, they use Pro shocks on their spec racers and are now switching over to Bilstein shocks on the street cars. Pro is a very good, inexpensive racing shock used primarily in circle track racing, so you know they are durable.
Jim Schenck, head engineer at Factory 5 Racing, said they switched to Bilstein because, "They are the best mono-tube shock we could get at a reasonable price. The Bilstein mono-tube shock is on the same level as a Koni or other high-end shock. A big thing with our kit is the price. It is one of our selling points. If we can offer better parts for the same price, we are going to do it. I have driven the car with the Bilstein shocks and there is a handling improvement without a detriment in ride quality. The Bilstein shock is not adjustable. They are making a shock specifically for us. We didn't want to change the geometry of our suspension and they didn't really have anything that fit our car that was compact enough in size to fit into the chassis and had enough travel."
Three Spring Rate
s Schenck said, "We use different spring rates based on engine choice and whether it is a street car or a spec car. The standard front spring for a small-block is 450 pounds, for the big-block it is 500 pounds, and the spec racer is a 600-pound spring in the front. We don't change the valving of the shock for the small-block or big-block, but we do change both rebound and compression for the spec racer.
"We didn't go with an adjustable shock because of the cost. Most people use the spring rates we offer and wouldn't be able to tune the shock to the springs and get a combination that works well. For most people, it is easier for us to do the work for them rather than have them figure out what adjustments to put on the shock. We work with shock companies and compare shocks back-to-back in order to see what valving works the best."
Gary Stowe, technical director at Street Beasts, told us, "Like a lot of the other manufacturers, our kits are relatively generic in the sense that they are engineered to use readily available OEM parts. If a customer chooses to use a version of the Mustang II front suspension or an 8.8-inch Ford rearend out of a Mustang, our shocks and springs are pretty much equivalent to a heavy-duty or upgraded OEM suspension component.
"For the person who wants to build a Pro Street car out of one of our kits, we developed some very specific suspension packages. We use a special frontend and an Alston Fab 9 housing for the rearend that we finish in-house that will accept up to a 20-inch-wide rear tire. It is a narrowed rearend in a four-link setup with Carrera coilover shocks. With those specific packages, I have been working very closely with Ben Abernathy at Carrera. We have developed with them some very specific-length and special-valving shocks for our application."
As with most kit car companies, Street Beasts changes its spring and shock package depending on the engine combination used. A smaller block in their '34 coupe would use a 425- or 450-pound-per-inch spring, while a big-block would use a 525- or 550-pounder. The shock is subject to 200 pounds of extra weight over the front axle because of the big-block engine.
How To
Koni Shocks are known worldwide for their racing double-adjustable shocks. They also make high-performance single-adjustable shocks for street use. Koni Sales Manager Lee Grimes told us how someone would go about upgrading the shocks on his car. He said, "It is really quite easy. The first thing we will ask is how he uses the car. Is it a daily driver, a Sunday car, a cruise car, or is it a lapping day car? Cobras and other kits are getting more popular for going out on racetracks. The next question is: What is his budget and is he looking for a coilover or non-coilover shock? From that point, we talk about dimensional requirements. If he already has a car on the road, he can use the front and rear dampeners he is using as a point of reference. We ask him to take some measurements on the car. We will find out what the attachment points are.
"Are they spherical bearings, rubber eyes, or pin-style mounts? We will have them take a measurement of the shock on the car from the lower mount to the upper mount at static ride height. Then we will check his current shock and see where it is in relationship to its maximum extension and compression. For example, if the car sits at 16 inches of static ride height, then we want to make sure that is about the middle of the overall stroke range. If the shock's stroke turns out to be 22 inches extended and 15 inches compressed, obviously he is almost bottomed out. We suggest a shock length that puts your static ride height needs toward the middle of the shock's overall range. Then once we have something that will physically bolt to the car and be in the right stroke ballpark, we will talk about what spring rates he running. Then we can select the valving for his use."
Everything Koni makes is either single- or double-adjustable. We asked Grimes if a kit car builder needs an adjustable shock when most probably won't crawl under the car and play with the adjustment. He responded, "I disagree a little bit. You have a guy who has a Manx dune buggy. Yes, he is going to be in a budget range and not going to be adjusting his shocks. On the other hand, there are a number of Cobras and other cars that go to the track. For these guys, an adjustable shock is important."
An adjustable shock could be a benefit if your car serves a dual purpose of being a weekend plaything and a twice-a-year race car. Being able to stiffen the shocks for the on-track days and soften it for daily use would be valuable.
New Idea
Edelbrock has come up with a different way to valve their Performer IAS (Inertia Active System) shock absorber. Their shock engineer Dave Shirley explained how it works. "This is the first shock that differentiates the wheel motion and chassis motion on rebound. We have come up with a way to break up the two different motions. You can extend the shock two ways. The wheel can go down and the chassis can go up. That is two very different requirements. When the chassis is going up, you have 4,000 or 5,000 pounds trying to pull up in a slow motion. You need lots of dampening load. When the wheel wants to go down, you have a 100-pound item trying to accelerate 10 times the speed of the chassis. We have a shock that can tell the difference. As the wheel pulls on the shock, it opens up some bypasses, get softer, and allows the wheel to move at the speed the wheel wants to move. Because we can do that, we can increase the dampening to control chassis motion. The shock is firmer and defaults soft if it hits wheel motion. I am talking about rebound only, forget about compression. At slow speeds, the inertia valve will stay closed. At higher wheel speeds the valve will open to make a softer shock.
"A conventional shock has a valve stack on rebound and a valve stack on compression. We have a valve stack on compression, and on rebound, we have two valve stacks and a means to bypass one of them. The inertia valve is opening and closing ports allowing the oil to free-flow as the wheel accelerates down."
The Right Shock
You know you have enough rebound dampening in a performance street vehicle when the car goes through one complete suspension cycle and settles. For instance, when you hit a bump, the shock compresses, then extends, then comes back to ride height. That is one cycle. If the car continues to bounce or goes through another cycle, you need to add more rebound dampening to the shock.
Most street rods or kit cars are over-sprung. All the manufacturers are building more rebound than compression dampening into their shocks, because as the spring compresses, it gets stiffer. The shock does not or cannot control that movement effectively. It is more important to control all of the compressed energy as the spring rebounds. The theory is to let the spring do the compression work and let the shock take care of the transition from bump to rebound.
Tech expert Pat LaValle at QA1 told us, "Shocks are often sprung incorrectly because 90 percent of the street rod builders have never taken the time to get the car on scales to figure out weight and where the car's weight bias is. Is it 60 percent up front when you think it is 40 percent? You need to spring correctly front-to-back and use a four-corner scale to be accurate. Choosing a shock depends on which valving you'd like and which use you have designated. Is your car being shocked for road racing or the street. It is best to consult your shock guy or the manufacturer's tech line."
It's your car, your shock, your usage, and your choice...but that's part of the fun of the project, isn't it?
