Shock absorbers, or dampers, control the rate at which suspension springs compress and rebound. Unlike passenger-car shocks that have fixed damping rates, racing shocks have fully adjustable rates. As such they can tweak the suspension properties and fix a variety of handling problems. The idea isn't new. But sophisticated testing equipment now helps teams better "dial in" shocks for track conditions. Ultimately, properly adjusted shocks may slice tenths of a second off lap times, an eternity in the realm of NASCAR racing. So teams spend lots of time and big bucks to get the adjustments just right.
Shock absorbers convert energy stored in suspension springs to heat in a working fluid, in this case, lightweight oil. The oil dissipates energy as it passes through bleed holes and through orifices in a movable piston attached to the load. Bleed holes located in the piston shaft control flow at low piston velocities, about 2 ips or less. Piston orifice size, count, and entrance geometry set maximum flow rate.
In the NASCAR-type shocks, stacks of preloaded shims on either side of the piston act as valves that regulate or stop orifice flow based on piston velocity and direction. A piston moving upward in the compression direction forces shut the rebound shim stack on the piston top and bends open the compression shim stack on the piston bottom. Shims come in a variety of diameters and thicknesses from 0.002 to 0.020 in. to allow independent adjustment of compression and rebound damping rates. Thicker shims, for example, require higher pressure and piston speed to bend than thinner ones, raising damping rates.
Shock compression and rebound rates are basically set to match sprung (body) and unsprung (lower control arm, wheels, tires, steering spindle) mass. That is, a car with an unsprung mass equaling 25% of sprung mass, typical for a NASCAR vehicle, will have a compression-damping rate about 25% of rebound rate. The coil-spring suspension at each wheel can be modeled as a two-mass spring/damper system. In compression or bump, the unsprung mass moves and the body remains nearly stationary. In rebound, the heavier body follows the suspension movement.
Compression and rebound properties are further characterized as linear, regressive, progressive, or a combination of these. Linear damping, as the name implies, rises linearly with piston velocity. Regressive dampers have high rates of damping at low piston velocities. At a preset threshold velocity — controlled by shim thickness and stack preload — the shim stack "blows down," letting oil pass more freely through relatively large piston orifices. Damping force then "knees over" and remains nearly constant with rising piston velocity. Conversely, progressive dampers stiffen with higher piston velocity.
Overall damping rate is adjusted, in part, by pressurizing the aluminum shock bodies with nitrogen gas to between 20 and 300 psi. The gas is inert and prevents cavitation of the semisynthetic oil, as well. And it mitigates the effects of frictional and ambient heating. Front shocks on NASCAR vehicles during a race may reach temperatures of about 180 to 200°F, mostly due to radiant heating from brakes and, to a lesser degree, heat from damping. "Shock fade," a loss of damping force as oil becomes less viscous, can be held to about 2% at high temperature by use of a lighter-weight oil, say 2 wt instead of 5 wt. The trade-off is oil lifetime because thinner oils are less thermally stable than heavier ones.
Other wear items include piston seals and shims. Shims that bend open and shut will mechanically fatigue and lose their ability to hold back oil pressure, which then causes a loss of damping force. On the other hand, "The hard parts — pistons, shafts, and shock bodies — generally don't wear out, even after several seasons of use," says James Arenst, a mechanical engineer with Penske Racing Shocks, Reading, Pa., one of three shock suppliers to NASCAR.
Teams use tabletop shock dynamometers to check shock health, both in the shop and trackside. Shock dynamometers move a shock piston with an actuator (hydraulic or electromechanical) or an eccentric crank. Data from load cells, position, and speed sensors feed to software that maps the damping curve.
The ability to check shock operation is especially important in NASCAR racing-because "legal" shocks have only one external bleed adjustment. Typical road-race shocks, for comparison, have up to four external damping adjustments. Other changes to damping properties require that shocks be disassembled and subsequently tested. Damping curves that deviate from expected values may flag lack of gas pressure or improperly installed parts, for example. Shock dynamometers can as well quantify stiction and friction, both important metrics shock makers try to minimize.
But shock dynamometers are of limited use for car setup because they test only one shock at a time. Setup is the precise combination of steering and suspension adjustments and components that make a car handle properly for track conditions. "You don't set the car up around the shock package. You build the shock package based on the rest of the car setup," explains Penske's J.J. Furillo.
Furillo prepares shock "builds" for a number of NASCAR teams and typically suggests up to six shim/piston combinations for a given race. A team's crew chief and shock engineer may both have a similar number of shock setups. It is not uncommon for teams to own 100 shock absorbers fitted with different shims and pistons. Unfortunately, there is precious little track time to test them. In an effort to curb spiraling costs and level the playing field, NASCAR allows just seven testing sessions annually.
Lack of track time and the inherent limitations of shock dynamometers have a growing number of racers turning to sophisticated seven-post rigs such as the one at the Auto Research Center in Mooresville, N.C. Today, about half of NASCAR teams use the technology.
BRINGING THE TRACK TO THE LAB
Seven-post rigs at once test setup of an entire car including all four shocks. They incorporate seven hydraulic actuators: one under each wheel, one or two beneath the rear bumper area, and the rest under the front of the car. The actuators apply dynamic loads to the suspension and chassis to simulate the bumps, turns, banks, braking, and aerodynamic forces that a car would experience on a racetrack. A data-acquisition system and special software sample force-response data from 32 channels at 1 kHz. Data comes from sensors on the car and the rig itself including wheel-hub accelerometers,damper-displacement transducers, and load cells on each tire post. There are two basic tests.
The first, called a swept-sine test, drives each wheel post with equal lowfrequency/high-amplitude to high-frequency/low-amplitude signals. Sweptsinetests represent track input but in an ordered manner, which makes the data easy to work with.
The second type of test moves the actuators based on actual data gathered from a race car's onboard data-acquisition system during track testing. Telemetry including damper displacements, wheel-hub accelerations, speed, and lateral and longitudinal acceleration, are used to generate what is called a "drive file." The track and generated signals are compared to one another in both the time and frequency domains by an iterative process that takes about 2 hr. Drive files may also be generated from simulation data or with a random computer routine to isolate and emphasize certain input frequencies. A recent trend in seven-post testing called baselining optimizes shock setup after making changes to springs or other suspension components.
Together, the data help answer one important question: How much load is on each tire? Load is a direct indicator of grip, and grip determines how fast a car is able to go without skidding out. Testing and setup for a super speedway may emphasize aerodynamic-induced grip over dynamic or mechanical grip, for example.
Interestingly, seven-post tests apply only vertical forces to the wheels and chassis. Formula One cars, in contrast, may be tested on 11-post rigs that as well apply side loads to wheel posts for simulating cornering, lateral, and g-force. Some rigs spin-up wheels to quantify changes in tire spring rate with rotational speed, while others allow steering input. "Tire stiffness obviously changes with speed, but it's not a big factor for NASCAR vehicles," explains ARC Test Engineer Tom Sweetland. "NASCAR-type tires tend to have a much larger spring rate than the rest of the suspension. F1 car suspension, in contrast, can be as stiff as the tires so hubs are hooked directly to the wheel pans and tire flex is modeled in software."
Despite having fewer force inputs, "Seven-post tests are about a 90% predictor of on-track performance," says Sweetland. "Teams typically make the biggest gains in the first three tests and incrementally smaller ones thereafter because-the 'window'shrinks." However, any changes to tire design, common from year to year in NASCAR racing, wipe the setup slate clean and necessitate more seven-post tests — at a cost of about $5,000/day. But it's a bargain compared with track testing considering that a set of tires alone costs $1,600 and may last only 50 miles. A highly organized team with 40 sets of shocks built up can make 15 to 25 seven-post runs daily.
Still, seven-post tests are no substitute for track testing because the most important factor is absent — the driver. It's been said that shocks don't make the car faster; they make the driver faster by boosting confidence in the car. Teams consider driver input, but generally maintain focus on what makes a car go fastest, and expect drivers to adjust. In that regard seven-post tests have proven their worth. "We're talking 0.2 to 0.4 sec per lap, huge given that a typical NASCAR lap is only 20 to 30 sec," says Sweetland. "It would be extremely difficult to make that level of improvement based solely on horsepower gains ."
GET A GRIP
Race engineers responsible for car setup generally concentrate on what is called "grip," a measure of tire loading and available traction. The more grip, the faster the car is able to go without skidding out. There are two types of grip, mechanical and aero. Mechanical grip comes from the dynamic forces pitch, roll, and heave that act on a car in turns and bumps. Aero grip is the result of down force produced at speed by the front fascia/bumper and rear wing.
The addition of low-speed bleed to shocks, especially in rebound, promotes weight transfer and mechanical grip and lets a car better absorb bumps (heave), or what drivers refer to as "sit in" rather than "on top" of the track. An aggressive or stiff shock-damping rate, on the other hand, gives drivers a rough ride, but may be the "fast" setup because it immediately puts grip-inducing heat into the tires. The stiffer shocks make tire sidewalls flex more and provide proportionately greater low-speed damping.
As a rule, NASCAR racers run modest rates of linear compression damping for a smooth ride, and digressive rebound to improve handling. Digressive shocks typically operate below blow-off, the point at which damping force flattens with increasing piston speed. However, teams may build shocks to purposely blow-off on a particularly large bump on a track. For oval tracks — the type on which most NASCAR races take place — shock setup is biased for left turns. Here, cars run a lot more rebound damping on the left than the right side, and more compression damping on the right to counteract longitudinal roll and stabilize the car in turns.
A growing emphasis on aerodynamic downforce has sparked controversy on the role shocks play in controlling it. To pass tech, cars must sit at a certain ride height, which at the rear is measured from the ground to the top of the wing. Until recently, race engineers would use rear shocks to "tie down" the back of a race car and keep the rear wing out of the wind stream. This lowered drag and let the car go faster. Rear shocks were set up with excessive amounts of rebound damping, minimal bleed and compression damping, and low gas pressure. Once on the track, hitting even a small bump caused the rear to squat down and stay there. The setup effectively rendered the rear suspension inoperable and overloaded the rear tires, causing cars to wildly bounce around. After a race, the crew would lift up on the rear of the car to bring it back to legal ride height.
NASCAR officials eventually caught on and now mandate shocks can only dampen motion, not change ride height. At the super speedways, Daytona and Talladega, NASCAR further mandates teams use the same type and brand of rear shocks, in this case Penske. For these races, Penske builds several hundred shocks with identical piston and shim packages, and seals the units with tamperproof stickers. NASCAR officials then install lead seals on the gas valve. To prevent any shenanigans by teams, cars go through pre and postrace inspection. The shocks, when not installed on cars competing in qualifying or a race, remain in the possession of Penske or NASCAR officials who store them in trackside cages.
NASCAR officials also inspect a single shock from the first three qualifiers, the first three place finishers, and one random car. All teams can attend the shock teardown and many busily take notes about the different setups. Cameras are prohibited and no one can touch the components. But racers are pretty savvy and know by looking at shim stacks when shocks are more digressive or linear. The NASCAR Busch Series has what is called a shock board of legal parts. Penske, for example, makes 20 different pistons; Busch can use only four of them. There isn't as much money in this Series so NASCAR limits choices. Nextel Cup teams can use all 20 pistons, however. Teams can't alter shocks with custom-made parts and must use NASCAR-approved parts from the shock manufacturer. It's just another way NASCAR levels the playing field and keeps teams honest. Shock setups tend to vary widely among teams and no one "killer" setup exists.