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1 Roehrig Engineering, Inc. Home Contact Us Roehrig News New Products Products Software Downloads Technical Info Forums What Is a Shock Dynamometer? by Paul Haney, Sept. 9, 2004 Racers are beginning to realize that shock absorbers (dampers, really, but we ll use both terms interchangeably) are probably the most critical adjustment on a racecar and they re working hard to understand them better. The best tool a racer can use to learn more about shocks is a shock dynamometer. Most serious race teams have a dyno at the shop and another one in the transporter. Most big teams now have a shock guy who s either a full-time employee or an outside consultant present at tests and race events. Why Does a Race Team Need a Dyno? Dampers control the movement of the wheels with respect to the road surface and this determines some of the force on the tire contact patches and the amount of grip the tire generates. Dampers, as well as the springs and anti-roll bars, also control the movement of the chassis with respect to the wheels as the chassis rolls. So shocks have a critical influence on the ultimate grip generated by the car and also on how the car feels to the driver. Since dampers are a critical component of a racecar, they should be tested periodically to make sure they re working correctly. Modern racecar dampers are almost infinitely adjustable so, when a race engineer finds a damper set-up that makes the car faster under certain conditions at a certain racetrack, that engineer will want to have dampers set up the same way the next time the car runs on that or a similar track. The only way to do that is to run the shocks on a dyno and document their characteristics. Cost cutting in racing is responsible for a trend toward race series with specified components-shocks for instance-so the big-budget teams can t go out and pay a lot of money for special components. Race series administrators try to limit team budgets and level the playing field by specifying critical components. Some race series now specify the shocks that can be used in their races. But it s not clear to me that spec shocks save the team any money.

2 In the first place the spec shocks are generally less expensive and therefore of lower quality than precision racing shocks. These inexpensive shocks probably need to be tested more often to make sure they re working correctly. Inexpensive shocks vary more from unit to unit which is another reason they need to be tested. You don t want to have the shocks on the front of your Formula Mazda adjusted to the same settings but giving you 50 lbs more damping on one side than the other. Race teams that have several drivers running cars with spec shocks, buy extra sets of shocks, dyno them all carefully, and then match sets of shocks on each car according to driver preference and ability. This is one reason REI developed a low-cost shock dyno in Our Model 2VS sells for only $8,000. With good credit a team or car owner can be using a dyno by making a leaseto-own payment of as little as $200 a month. We now sell more 2VS dynos than any other model. What Is a Shock Dyno? The force vs. shaft speed graphs racers use to characterize shock performance (right) come from data generated by testing a damper in what is generally known as a damper dynamometer or shock dyno. This is a machine that compresses and expands a damper at known speeds and measures the forces produced. I ll start out by describing the simplest form of shock dyno. Even the least expensive REI dyno is much more sophisticated than this simple example. This schematic shows a frame holding an electric motor with a drive belt and pulleys that spin a crank plate attached to the damper shaft through a linear bearing. As the motor spins the crank, the damper piston moves up and down just like the piston in a cylinder of an engine. Bolt holes in the crank plate allow several different stroke lengths. Different pulley diameters give different crank-plate rotation speeds. The load cell measures the force generated by the damper. REI shock dynos also use a velocity sensor to read dampershaft speed and a displacement sensor to provide information on the precise vertical position of the damper shaft. We all know that the speed of a piston connected to a crank varies continuously as the crank rotates. You might remember from high school math or physics that this type of motion is called sinusoidal because it varies with the sine of the crank angle. The piston comes to a stop at top dead center, accelerates to a maximum speed halfway down the cylinder, and slows to a stop again at the bottom. If you have a damper attached to a crank, its piston does the same,

3 have a damper attached to a crank, its piston does the same, and the force generated also varies continuously. We know, however, that the maximum speed of the piston happens only once per stroke, when the piston is halfway between top and bottom, and that s also when the damper generates maximum force. Dampers are speed-sensitive devices, they generate more force when the shaft moves faster. To say that another way it takes a large force to move the damper shaft at higher speeds. With our simple shock dyno shown in the schematic we change the crank stroke to vary the maximum shaft speed of the shock and/or we use drive pulleys of different sizes. REI dynos use computer-controlled variable-speed motors. Here s How a Dyno Works With our simple shock dyno, you mount the damper, choose a stroke and pulley, and turn on the motor. The crank turns and the damper shaft moves up and down until you turn off the motor. If you know the motor rpm, the pulley ratio, and the stroke, you can calculate the maximum damper shaft speed. For example, let s say the motor turns 1,000 rpm, you ve got pulleys that reduce the motor speed so the plate turns 100 rpm, and the stroke is 1 inch. 100 rpm is 1.67 revolutions per second and the length of 1 revolution is the circumference of the circle traveled by the crank bolt or Pi times the stroke x 3.14 x 1 inches is about 5 inches per second. This is the maximum speed of the damper piston, and it happens twice each revolution of the crank, once with the piston going up in compression and once again with the piston going down in rebound. If we keep this example really simple and connect the damper directly to a weighing scale with a circular dial, we can stand there and read the scale pointer directly. What we ll see is the pointer cycling from 0 to some maximum rebound force as the shock expands, returns to 0, and then peaks out again at the max compression force as the piston goes back up. The needle on our scale goes from plus some number to minus some number as the damper cycles from compression to rebound and back. We can just write down the numbers on the scale when the needle peaks. REI shock dynos use a computer to read the load cell as well as the other sensors and store the data so it can be graphed easily. Most dampers are set up to give more force in rebound than compression so, as our simple machine cranks away, we might see the scale peak at 190 pounds in compression and 250 pounds in rebound. We know that, at a shaft speed of 5 inches per second, the damper produces 190 pounds in compression (or bump) and 250 pounds in rebound. We d like several data points so we can draw a curve. If we change our drive belt to pulleys that give us different maximum shaft speeds we can get enough data points to draw that curve. After we make these runs and read the scale we can make a table like this: Max. Speed Bump Force Rebound Force in./sec. lbs. lbs

4 If we present the data in this table as a force vs. shaft speed graph, it looks like this. We generated this data by running our constant-speed motor (100 rpm) and changing the drive belt among the pulleys to give us 5 maximum piston speeds, and we read the bump and rebound forces at those maximum speeds. Then we made a graph by connecting the dots. This is called a Peak Velocity Pickoff plot or PVP. The graph shows us that the shock we tested has a pretty steep rebound curve while the compression curve starts low, rises quickly, and then levels off. The real benefit of a machine like this comes when you test all four shocks off your racecar and find out that they all give different readings even though they re supposed to have the same valving and they are all set to the same external adjustments. Some small difference in readings is OK, but the closer together the better. If you ve got the tools and experience, you can take your shocks apart, change the internal valving, and test them again. Maybe you ll find contaminated oil, bad seals, or worn parts. Shocks wear out like any other mechanism and need to be rebuilt periodically. A shock dyno also allows you to see the effects of external adjustments. If the data above represents settings in the middle of the range of adjustments, varying them in increments from full-hard to full-soft will give you a set of curves that show the effect of those changes. That will happen if your dampers produce changes big enough to be seen by your machine. If you really are just reading a scale by eye you might miss some fine points. Reliable, high-resolution data acquisition and easy-to-use analysis software are important. That s why people buy REI dynos!. Data from an Entire Cycle You can get more information and more detailed information from a damper by taking data over a complete cycle of compression and rebound and graphing that data. This is called a Continuous Velocity Plot (CVP), and REI s SHOCK data analysis software quickly provides this type of test.

5 provides this type of test. Here s another look at the damper dyno schematic. Notice the notations around the crank plate for Top Dead Center (TDC), 90 degrees, Bottom Dead Center (BDC), and 270 degrees. When the crank pin is at TDC the damper is compressed in bump. As the crank plate rotates clockwise (it s expanding so that s the rebound direction) from TDC, the damper piston accelerates in rebound from a stop to maximum speed at 90 degrees and then slows to a stop again at BDC. Rotation continues and the piston accelerates in compression to maximum speed at 270 degrees and slows to a stop again at TDC. The length of stroke and rotation speed depends on how you ve set up this simple machine. REI dynos have computer-controlled motors that allow continuously variable speeds. This graph shows force data taken continuously during one revolution of the crank plate and, therefore, one full compression/rebound cycle. We ve chosen the standard notation of shaft speed in the rebound direction as negative and compression force as positive. This standard generates a damper curve with compression in the upper left quadrant and rebound at the lower right. We ll follow the shock cycle of compression and rebound starting with the crank at top dead center, (TDC) and the shock fully compressed. The crank pin is rotating but is stopped momentarily at BDC so the damper piston is stopped also. This is the point on the graph where it crosses the shaft speed line at zero and the force line at zero. As the crank pin rotates through 90 degrees the shock piston accelerates from a stop to maximum speed. If we re taking data for a Constant Velocity Plot we get that part of the curve that goes from zero force and zero speed up to a peak at maximum force and shaft speed.

6 After the point of peak velocity the piston begins to slow down but there is still some damper force at zero shaft speed. The bottom part of the curve shows shaft speed and negative force increasing as the crank plate goes to 270 degrees and then decreasing as the curve goes back up toward zero speed and force. As rotation continues, speed goes negative (compression) and force increases to a maximum again at 270 and back to 0 at TDC. This stuff is complicated and you might have to look at the schematic and the graph a while before it becomes clear. The important point is the force increases with piston speed. On the lower section of the curve the piston is accelerating where the curve is headed down and slowing down as the curve swings back up. It s the same on the top part. The piston speed and damping force increase to a maximum and then decrease again. This is a lot more data than we had when we just changed the stroke and looked at the damper force at maximum piston speed. So why doesn t the damper develop the same force when it s slowing down as it did when it speeded up? I m not certain, myself, but remember you ve got a bunch of oil moving through the washer stacks and bypass paths, and it has some mass and momentum. And I don t think those washer valves necessarily close the same way they open. Also, the fact that you re compressing the oil in the shock and the piston is always accelerating, slowing down or speeding up, may have something to do with the shape of this curve. Graph Formats Actually there are several ways to show this data. The S-shaped curve is the most precise way to present the data of a complete cycle of a damper. This graph would, however, get even more complicated if you overlaid data that showed the effect of internal adjustments or valving changes. To make thing simpler and easier to read you ll often see data presented with bump and rebound curves in the same quadrant, as in the graph below, and everybody understands that some forces and movements are in different directions. When showing bump and rebound data that has adjustment effects, however, it makes sense to use two quadrants, as in the graph below.

7 A good shock dyno generates data that you can use to test and set up dampers. More and more racers are buying shock dynos and these people say they are learning how a shock works and that helps them make their racecar quicker. webmaster@roehrigengineering.com with questions or comments about this web site Copyright Roehrig Engineering, Inc. Last modified: Tuesday, May 17, 2005