Chassis Tuning With Dampers – A hard look at shock absorbers and their effects on handling

When the suspension moves, the shock absorber is designed to control movements of the suspension and chassis, working in conjunction with the springs. The springs actually absorb shocks over bumps and help control body roll. The shocks control the oscillations of the springs, determining how fast the spring compresses or extends. Stiffer shock rates slow spring movements while a softer shock rate allows the spring to move faster. A shock is far too soft if it allows the springs to oscillate, or bounce, more than one full cycle. A shock is far too stiff if it limits suspension travel. A shock that is too stiff can cause the tire contact patch to bounce off the road surface over bumps or jack weight in the car after body roll occurs, even pulling the tire off the track surface for an instant.

BUMPS

The first job of a shock is to control the tire contact patch over bumps. On the track, a shock that is too soft allows the chassis to bounce after hitting a bump or rut. The car feels like it wallows. If the shock is too stiff, when a bump is encountered the tire contact patch can be pulled off the track surface, or at least the load on that tire is reduced significantly. This affects handling balance and jacks weight around the chassis, making the car feel unpredictable. The loss of traction is considerable and the car is difficult for the driver to read. Over bumps in corners, the car feels like it skates.

A shock dampens vibrations by creating friction. Racing shocks all use hydraulic fluid in a tube with a piston. The piston pushes the fluid through a series of valves, and bleeds, controlling the “rate” of the shock. The valves and bleeds can be varied to change the rate. Different valves and bleeds are used for rebound and compression. The valves for either rebound or compression can be changed, together or independently to change how the shock valving works over bumps, during body roll, and during pitch under braking and acceleration.

There are several types of shock designs, but all have the same effect on the chassis. Some shocks are gas charged and some are not. Quality, wear characteristics, rebuilding and performance as heat builds up inside the shock are the primary factors when comparing shock designs.

BUMP VS. REBOUND

Bump, or compression, occurs when the shock shaft is being moved into the body. This occurs on the front of a bump, the back of a rut, the right side when turning left, the left side shocks when exiting a left turn, the front under braking and the rear under acceleration.

Rebound, or extension, occurs when the shaft is being pulled from the body. This occurs on backside of a bump, the front of a rut, the left side shocks in a left turn, the right side shocks exiting a left turn, the front under acceleration and the rear under braking.

SHAFT SPEEDS

How fast the shaft pushes and pulls the piston inside the shock body affects the rate of the shock in bump and rebound. Shock rates change as shaft speed changes, making shock absorbers shaft speed sensitive. The faster the piston or shaft speed, the stiffer the shock is. Shocks work mostly within a range of about 3 inches per second to about 20 inches per second. The lower speeds come into play during weight transfer when tire loads are changing. The higher speeds come into play over bumps and ruts. A shock manufacturer can alter low, medium and high speed valving to control what the shock does in different situations. Low and medium speed valving are used to control how the shock influences handling. By altering shock valving for different shaft speed ranges, a shock builder or engineer can create a shock that affects tire contact patch control over bumps and ruts and also affects the weight transfer characteristics so that handling can be fine-tuned for a variety of conditions. Bigger open-wheel teams have a wide variety of shocks in the trailer to accommodate a variety of track conditions from a tire-contact-patch-control perspective (medium to high speed valving) and a handling perspective (low to medium speed valving). The first factor affects overall tire traction. The second affects handling balance at different spots on the racetrack.

There are four components to how shocks affect weight transfer:

HOW MUCH WEIGHT IS TRANSFERRED

The weight of the car, its track width/wheelbase, the center of gravity height and cornering force are factors.

WHERE WEIGHT IS TRANSFERRED

The spring rates and antiroll bar rates acting at the tire contact patch, along with geometric effects from components like lateral locating devices (Watts link, panhard bar, track bar), control this.

WHEN WEIGHT IS TRANSFERRED

This is controlled by when the driver uses the steering, the brakes and accelerator.

HOW FAST WEIGHT IS TRANSFERRED

Shock rates and how abruptly the driver uses the controls affects the speed of weight transfer.

SPLIT VALVE AND ADJUSTABLE SHOCKS

Split valve shocks are built with different valving in either bump or rebound from what is normal for a specific shock. This chassis tuning allows specific handling problems to be cured by changing one or more shocks. Adjustable shocks can offer adjustments in rebound only or for bump and rebound (double adjustable). Tuning can be accomplished by adjusting the shock absorber, most often while still on the car. In any case, changing the valving of shocks overall, in bump only or in rebound only, can change the handling of the car and improve lap times. For the most part, tuning with shocks is considered a fine-tuning adjustment once the chassis is setup and tuned.

SHOCKS BOTTOMING OUT

If a shock bottoms or reaches full extension under load, handling can change dramatically and damage to the shock can occur. Bump stops on the shaft of the shocks reduce this and some chassis builders use rebound travel limiters to keep the shock from reaching full extension. Full extension is usually less of a problem.

COOLING

Shocks dampen by using friction, which causes heat. Heat buildup can affect the rate of the shock, always softening it. Dissipating heat always helps shock performance. Bumpy tracks create more heat than smooth tracks. It is best not to cover shocks, and even to duct cool air to the shocks. Aluminum shocks dissipate heat faster than steel bodies. For coil-overs, threaded body shocks cool better than smooth body shocks with thread spring perches over the body for the coil-over adjusters.

TUNING HANDLING WITH SHOCKS

In the middle of a race, you are running just behind the race leader. He is pulling away slightly, and the place on the racetrack where he seems to gain is going into the corners under braking. You’re driving in just as deep as you can, but the leader is able to drive in half a car length deeper. If you go in that hard, you pick up a small push. The car is great everywhere else and you can run with the leader. Your cars are identical and on the same tires, so where could his advantage be?

The key to this situation is probably a shock absorber. Shocks really don’t cure problems, but they can be great for making small improvements in handling in specific parts of the track or in a corner. Similarly, shocks will not cure a big handling problem, though they can cause handling problems if they are bent, bind or too stiff or soft.

HOW SHOCKS AFFECT HANDLING

The shock controls how fast weight is transferred. The valving for low shaft speeds is the primary controlling factor for weight transfer. This affects the load on a tire and can change the handling balance while weight is being transferred. Once all weight has been transferred, the shock no longer influences handling. Let’s review.

In general, rebound damping controls how fast weight leaves a tire while bump controls how fast weight goes onto a tire. Stiffer valving causes a shock to react more quickly. Softer valving slows the reaction of the shock. Stiffer valving gets the load to change more quickly. Stiffer rebound valving gets the load off a tire more quickly and onto an opposite tire faster. Stiffer bump valving gets the load onto that tire faster.

When going into a corner, as long as the driver is moving the steering wheel or the brake pedal, the shock has an influence on tire loading. Braking causes weight to transfer forward, compressing the front suspension and shocks, extending the rear suspension and shocks. When cornering, the weight transfers from the inside to the outside, extending the inside suspension and shocks while compressing the outside suspension and shocks. When both braking and cornering take place, as they nearly always do going into a turn, both effects occur. In a left turn, the right front, which is compressing from roll and pitch, and the left rear, which is extending from both factors, are moving the most and will have the biggest influence. The left front and right rear are receiving opposite movements from roll and pitch, reducing their movement and therefore their influence.

In midcorner, where braking and acceleration forces are small, and lateral forces are highest, all of the shocks have an influence, but shock travel is very small, especially with solid axles, which have high roll centers and little body roll. This reduces the influence of the shock on handling balance.

At the exit, we basically undo what happened going into a corner. Under acceleration, the rear shocks go into compression and the fronts into rebound. As cornering is reduced, the inside shocks go into compression and the outside shocks into rebound. While both forces occur in the exit phase of the corner, the left rear and right front move the most since those shocks have forces working in the same direction while the left front and right rear have opposing forces. Don’t take this to mean that the left front and right rear shocks do not influence handling. The influence is just a little less.

A slight push going into the corners, like in our example, keeps the driver from going in as deep the other car. Keep in mind, this is a very slight push. The car has a very good basic setup and is fast. The car ahead is about .1 seconds a lap faster. Shocks can be helpful in this situation.

A push means the front tires are exceeding optimum traction limits. We need a little more traction on the front, and little less on the rear. A stiffer rebound shock on the left rear will help the entry push problem. Let’s say we go one number or click stiffer in both bump and rebound. We want the stiffer rebound, but what will the stiffer bump valving on the left rear do to the handling? The stiffer bump on the left rear could cause the car to loosen up at the exit. If it helps going in it will likely loosen the car on the exit. Since the exit is more important than the entry for faster lap times, this may not be such a good change. Of course on a road course, changes must be made to both the left and right side shocks at the front or rear of the car. If only one corner is modified, gains in turns in one direction will be negated in turns of the opposite direction. And the loss will be greater than the gains in most cases.

In most cases where the car is really good except for one spot in a corner, the best change is using an adjustable shock where at least rebound settings can be changed, or use a split valve shock. You also could have a shock manufacturer custom-valve a shock for you. In our example, increasing the rebound valving one number with the bump valving remaining the same cures the entry push without changing the balance in midturn or on the exit. This is a really good thing. But if the push were bigger, this change may help a little, but would not cure the problem. Something else is causing the problem in this case, most likely springs and antiroll bar rates being off.

Let’s look at one more example. In this case, the car is loose on the exit. Again a shock change will only help if the car is really close to begin with. In this case we could decrease the rear bump rate or increase the front rebound. Decreasing the front rebound allows the weight to get off the rear faster. Again, adjustable shocks or split valve shocks will help here.

BASELINE SHOCKS

In 95 percent of all cases, the baseline shocks should be the ones recommended by your chassis builder for the track you’re running. Extremely bumpy tracks may require a change to softer shocks if the car skates over the bumps or feels unstable. Many racers make the mistake of going too far away from the baseline setup and end up with an unworkable setup on the car.

Many years ago I had the opportunity to work with a shock company tuning shocks on an oval-track car. We spent the first day with baseline shocks and worked on the setup to get consistently fast lap times. The track was short and lap times in the 15-second range. The next day, we worked on finding the last few hundredths of a second by tuning only with shock absorbers. This was an interesting experience and should help put into perspective how to approach tuning handling with shocks.

The first run was a baseline. We would run a warm up lap, five timed laps and a cool-off lap. We dropped the fast and slow laps and averaged the middle three. After the first run, the engineer asked me what the car would do if I drove 10 feet deeper into the turn before braking. My immediate answer was the car would understeer. We made a single shock change to get weight onto the right front more quickly. On the next run, the average lap time was .05 seconds faster. We repeated this process about eight more times, and each time we gain a few hundredths of a second. By the end, we were about .4 seconds faster than the morning baseline run. And the day warmed up and the track surface was hotter, so we gained even more. The point is that if you start with a setup that is quick and balanced, you can find the last few tenths of a second with shocks.

WHEN TO TUNE WITH SHOCKS

Here are some important criteria for when tuning with shocks could help make your car faster:

• Each tire contact patch must be optimized. Camber, caster, tire pressure and toe must be right before tuning with shocks. If these aren’t right, you’re chasing your tail and wasting time.
• Static weights and crossweight percentages must be very close to optimum.
• Make sure that there are absolutely no binds in the suspension.
• Define were the problem occurs. Often a corner-exit problem is a corner entry problem not recognized by the driver. A driver can easily overcompensate for a corner entry push causing a loose condition on the exit.
• The handling problem must be small. The car should already be fast. Don’t expect more than .05- to .10-second improvement in lap times.
• It takes a skilled driver who is consistent and sensitive to changes to really tune with shocks. New drivers should spend a test day making shock changes to the car to see what they do. This experience is extremely important to wring the last bit of performance out of the car.
• Make small changes. Going up or down two numbers or two clicks on adjustable shocks is a big change. And only change one corner at a time when tuning with shocks.

Shocks are clearly a valuable tuning tool. When you are seeking accurate information when striving for the last couple of tenths on the track, choose your sources wisely. It is easy to be misled. Understanding what a shock can do is important. Getting the desired results takes effort and skill. Don’t expect too much. Get a good basic setup on your car, then small shock changes can pay off.