| The Shock Dyno |
| - by Mark Whitney |
| It seems that whenever the word “dyno” is attached to another word in motorsports, great confusion and mystery soon follow. This could be a chassis dyno, engine dyno, inertia dyno, water pump dyno, brake dyno or, in this case, a shock dyno. But, if you were to take any of these pieces of equipment and look at them for the simple functions that they perform, it would be quite easy to understand what they are doing in each case. |
| The shock absorber dynamometer is a piece of equipment that measures the resistive forces developed by the shock when moved through various velocities. Different shock dynos will work in different ways, but the dyno we will focus on here is the Brinn/Millican unit. This unit was selected because it is a very versatile unit as well as being an affordable unit for racers and local speed shops. |
| The first component to consider in the dyno is the motor. This is the part that provides the motion for the shock to resist. The motor also controls the speed the shock will be tested. Many shocks have different levels of valving. Low and high speed valving are the most commonly talked about types. A shock dyno should be able to test in all of the ranges of valving that the shock has. The Brinn/Millican dyno has two speeds from which to test shocks. Other dynos will have a variable speed motor to test shocks at a full range of speeds, but for the purposes this dyno was designed, two speeds is enough. |
| Since the shocks need to be tested at speed for extended periods of time to come up to operating temperatures, the motor moves the shock in cyclic fashion. The motor compresses and extends the shock repeatedly. Since the force value at a particular velocity is how shocks are measured, it was important for the designers to match the force value to the exact velocity value. This may seem quite easy until you realize that, for every cycle the shock travels, it goes from zero to maximum velocity and back to zero compression and zero to max to zero velocity rebound. Without high-speed costly electronics, it would be impossible to read each velocity. The people at Brinn/Millican chose to work off the “peak hold” method of measuring the forces. This brings us to the force measuring devices. |
| The resistive forces of the shocks are measured by devices called “load cells”. These are electronic sensors which change resistance as force is applied to them. These load cells are attached to resistance-measuring devices with digital readouts. As the shock is cycling, the readout changes too quickly to be read. The peak hold feature on the readout stores the maximum load recorded and displays that on-screen. Since the maximum force is going to be produced at the point when the shock is at maximum velocity, the readout on the display will correspond to the maximum velocity for most all shocks. The only cases it would not correspond is in the case where a shock is bent or has a bind. In this case, the maximum force will correspond to the force required to overcome the bind. |
| Now that we understand the principles of the shock dyno, we should cover its use and uses. The first procedure in using the dyno is to load a shock into the fixture. The pins slide out from the collars and allow the bearing ends of the shock to fit into the bracket. The pin is then slid back into place. With gas pressurized shocks, it may be necessary to lock one end into place and push against that end to collapse the shock into the other bracket. Next, select the speed you wish to run the shock. This is simply done by pulling a pin and selecting the correct hole in the machine’s crankshaft. With the machine on, it is time to zero the readouts. This will clear the memory from the last run or remove any forces it received while you were mounting the shock into the brackets. The readouts are zeroed by holding the peak button and pressing reset. |
| With the readouts zeroed, you can start the test run. Normally, you would start the test in low speed to check the low speed characteristics of the shock. The low speed value for this dyno is 4.77” per second. |
| The shock needs to reach operating temperatures before you should start taking readings. A shock’s operating temperature should be between 100 and 180 degrees. Maintaining consistent temperatures from run to run and shock to shock will improve the accuracy of the tests. Many dynos will have a clamp-on temperature recorder. This will display the temperature of the shock while the run is going on. For dynos without a clamp-on temperature gauge, an IR tire pyrometer will work very well. |
| It is important to understand where to get the shock temp from. On monotube shocks like a Penske or Bilstein, the shock temp should be taken in the center of the body. This is where the temp will be most accurate to the overall working temp of the shock. On dualtube shocks like a Pro, Carrera or AFCO, the temp should be taken on the end caps. This is because the piston rides in the inner tube and the oil only contacts the outer tube, to transfer heat, at the end caps. |
| Since the readouts will hold a peak reading, they will need to be zeroed again after the shock reaches operating temps. This will delete high readings produced from the first couple of cycles with the cool oil. |
| Once you have recorded the low speed readings, it is time to switch the dyno into high speed mode. A pull-pin on the crankshaft is pulled out and placed into the long stroke position. This may be difficult with gas pressurized shocks because the shock will want to extend as soon as the pin is pulled. Once you have the pin in the new position, you are almost ready to test at high speed. The item to remember with pressurized shocks is that there will be an initial value on the readout. This is because the gas pressure is actually producing a spring rate in the shock. Most commonly, operators will not re-zero the readouts because this is an additional force that will be seen at the car’s tire. This may not seem important, but when diagnosing shock problems like bad valvings or shaft problems, every measurement is critical. |
| After the readouts are zeroed, the shocks can be run on high speed. The high speed value on this dyno is 13.3” per second. Once again, if the shock has cooled off, it will again need to be brought to operating temps before recording the final readings. This again means deleting the readouts before recording the final numbers. Now you have both sets of numbers for the shock, low and high speed compression and rebound. What can you do with them? |
| Before getting into what you can do, we should probably discuss what you can’t do. The one thing you can’t do is make a direct correlation between shocks of different manufacturers. This is an important point to emphasize, using a non-acceleration dyno and only recording two points, any attempt to make across-the-board statements between shock manufacturers is a mistake. Differences in valving packages, low and high speed bleeds, methods of achieving dampening, etc. will lead to errors and assumptions that, in some cases, will have you read the misleading values to what the car will actually experience on the track. Comparing shocks of different manufacturers should be performed on dynos that run the shocks through many acceleration modes and have sophisticated data acquisition to the shock values at different velocities and accelerations. These are the same conditions the shocks will see on the track when traded one for the other. |
| Now that we know what the dyno can’t do, we can look at all of the comparisons that it can do. One of the first teams to use the dyno, when Trackside tested it, was Jim Mavlouganes, who drives a Pro Stock at Stafford Speedway. He runs Pro Stocks and was having a problem with the car being inconsistent. He pulled all the shocks from the car and all of his spares and headed for the dyno. In total, he brought 16 shocks and spent 2-1/2 hours on the dyno. When he was done, he had found two bent shafts and one shock that he questioned the valving on. He sent those three shocks to Pro to be rebuilt and picked his best spares and put them on the car. In the five weeks since he made the change, Mavlouganes has finished at least three spots better in the feature events than previously in the season and is currently second in Stafford points. |
| The reason Mavlouganes was able to find the questionable shock was from his close working relationship to his shock company. As you have seen before, we strongly recommend racers work with companies that make the most technical information possible available to their customers. The companies listed in this article were very generous with the information volume they supplied. |
| The next test for the shock dyno was a valving change. Pete Mariconda and Tim Smith, who each work on Pro Stocks at Riverside Park Speedway, came to test their shocks. Mariconda was having difficulty with the car dumping on the right rear. Both teams run Penske Racing Shocks. All of the shocks were run on the dyno and the results recorded. Since the Penske’s are re-buildable shocks, the two rebuilt their shocks on-site to see the differences. The shocks were re-pressurized and run on the dyno again. After the results were compared, it seemed the right rear shock was a little low in value. Again, the shock was disassembled and the valve stack was taken apart and measured. The measurement came out exactly to what it was marked. The only other area to check was the low speed bleed jet. The jet was replaced and measured. Instead of a .072 jet, someone had mistakenly built this shock with an .080 jet. The jet was replaced and the shocks were put back on the car. In final analysis, this virtually eliminated the dumping problem and tightened the car. |
| Gas pressure or the gas cell is very important to the total operation of the shock. A loss of gas pressure or corruption of the gas cell will cause improper values to be displayed on the shock dyno. On gas cell (dual tube) shocks, a simple test to determine if the cell burst is to fully extend the shock, then invert the shock and slowly compress it. If while compressing the shock a “dead spot” is noticed, then it is a very good possibility that you have a gas cell problem. |
| We have covered a few reasons why a racer should own this shock dyno, but what about a speed shop? What benefit could be in it for them to own one? Well, for the duration of this test, the dyno was housed at Agawam Speed Shop in Agawam, Massachusetts. Victor Mari, the owner of Agawam Speed, feels the dyno is a way that he can better serve his customers. By having the dyno on-site, his customers can bring their shocks and determine if there are any problems. While they are in the shop, if they need to replace any of their shocks, they can buy them right there. “It makes my business more service-oriented,” said Mari. “You can show the racer right there that they have a bad shock, then they will want to buy one. It’s like a couple of years ago when speed shops were buying spring raters. It was a sales tool for springs. Now, we have one for shocks.” |
| We would like to thank the Brinn/Millican people for allowing Trackside use of the dyno this season. We look forward to the next generation of dynos which should be available very quickly. |
| Mark Whitney is a Technical Staff Writer for Trackside Magazine. This article is reprinted from the September 5-18, 1997 issue of Trackside. |
