The dyno, as shown here, is...
The dyno, as shown here, is ready to operate. A dial caliper, a calculator and a notepad complete the package. You can build this dyno for about $100. It checks shocks in both compression and rebound.
Welding the hammer, T-bar...
Welding the hammer, T-bar and main frame. Construction is simple: The hammer is made from rollcage tubing, the rest is one-inch square tubing.
This is the T-bar. The top...
This is the T-bar. The top has mounts for the shocks, one end for compression, the other for rebound. The T-bar pivots on the frame crossmembers. The hammer strikes the bottom of the T.
The creep clip has nothing...
The creep clip has nothing to do with your nerdy cousin. It is slipped over the crossmember of the main frame, keeping the weight or gas pressure of the shock from moving the T-bar before the hammer strike.
The hammer provides the force...
The hammer provides the force to move the shock. It pivots at the top of the main frame. Its extension fits under a lock bar to hold the hammer until release. A total weight of about 14 pounds provides adequate movement for accurate measuring. A rubber snubber is welded to the impact end.
The hammer latch is necessary....
The hammer latch is necessary. It allows the hammer stroke to be the same every time.
The assembled unit ready to...
The assembled unit ready to fire a stroke. The hammer is locked in the up position. A QA1 No. 5 valve shock is bolted in place for testing on the compression stroke side.
The hammer has just impacted...
The hammer has just impacted the T-bar. Its movement from level is noticeable. In this test the QA1 shock is mounted to the other side of the T-bar to check rebound, or extension, movement. The creep clip kept the T-bar in place before impact.
A measurement must be taken...
A measurement must be taken before the hammer is cycled. Notice the top of the T-bar is level. The dial caliper is used for precise measurements.
After the hammer stroke, the...
After the hammer stroke, the T-bar is now tilted; another measurement is taken using the same points. The difference in these reflects the amount of movement. This is the basis for comparison between shocks. With just a little practice you will get accurate results.
The average racer these days desperately needs a shock absorberdynamometer. Why, you ask? Because what you buy might not be what youneed. Due to many factors, design, production methods, etc., a shockabsorber may not have the rate of compression and rebound the labelsays. Trouble is, you will never know unless the shock is run on a shockdynamometer. There are several shock dynos on the market that can cyclea shock at various speeds, control temperatures and give a computerizedreadout. The readout provides much useful information.
Roehrigmakes one of the best commercial shock dynos, but the problem for manyof us is its $22,000 price tag. [EDITOR'S NOTE: Since this articleoriginally published in 2002 Roehrig has introduced its low-cost 2VSdyno. The dyno comes complete for $8000 and has the same structure andsoftware as all their other dynos]ND Tech has a very good air-powered dyno, which provides a lot ofinformation and sells for about $11,000. Powerhouse Products makes agood air-powered dyno, too. Winston Cup teams typically have one ofthese types of dyno in the shop and another in the transporter. If youare a Saturday-night racer with that kind of budget, purchase one now.It will tell you a lot about your shocks. The problem is, most of usdon't have that kind of money in our race cars, let alone thediagnostic equipment to tune it. Some shock manufacturers will dyno ashock for you and give you a printout for less than $10 per shock.
Why dyno-test shocks Some shocks, particularly those on the lessexpensive side, do not always conform to the labeled rate. This isusually because of production tolerances. A .001-inch difference in ahole or plate can mean big changes in shock control. Racers often say,"Well, I changed a shock and it didn't make anydifference." It could be, his No. 5 shock might have thecharacteristics of a No. 4 or a No. 6 valve could be closer to a No. 5valve. This could lead to a situation where you think you are putting ona lighter/ heavier shock when in fact you didn't make a change atall. Worse yet, you might be going the wrong way. But all is not lost aslong as you know what your shock is doing.
Some of yourolder or less-expensive shocks might be quite good. They can do theirjob and last a long time. But for proper handling, the shocks need to bechecked for specific compression and rebound values. Only then can theybe used on a car with confidence. One other thing to think about is theneed to check shocks occasionally. Even expensive shocks can lose gaspressure, wear or be internally damaged. Also, if you have rebuildableshocks, such as QA1, a dyno can confirm your changes and adjustments.
How the dyno operates This racer's dyno does notconnect to a computer and it doesn't cycle a shock at varyingspeeds while controlling the temperature. It is not the equal of theexpensive, purpose-built units, but it will give you a repeatableindication of the valve ratings of your shocks at an affordable price.This dyno can do that for about $100.
The dyno in this case is a slightly sophisticated hammer. The hammer drops from a set height each time. It engages the bottom leg of a T-shaped bar, which has a pivot at the intersection of the T. The shock connects to the end of one branch of the T. After the hammer is released and impacts the T, the shock will move a given amount in relation to the shock valve. Measure the amount of movement and compare it to other shocks or a known value.
QA1 representatives provided us with several shocks with different valve rates. All of them came with dyno sheets. As a suggestion, get at least one of these with a dyno sheet to use as a standard to measure the others. QA1 has a very good Roehrig Shock Dynamometer and can furnish a dyno sheet with each shock. QA1 shocks with No. 3, No. 4, No. 5 and No. 6 valves were used to establish a baseline for our racers dyno.
Dont be surprised when the dyno tells you a 50/50 shock isnt necessarily a 50/50 by the numbers. Discussions about this can go on way into the night. Suffice it to say the racetrack makes a stronger bump in compression than the spring in rebound. Thus the manufacturer adjusts the valves to allow the car to see the same amount of action both ways. On rebuildable shocks like the QA1 you can change this around to your hearts content if you have a dyno. On the QA1 shocks there was a close correlation from one valve number to another when comparing our readings to their dyno sheet. Our readings, as follow, were taken on the compression stroke.
No. 3 valve 1.265 inch (shock shaft moves this amount)
No. 4 valve 1.180 inch
No. 5 valve .960 inch
No. 6 valve .800 inch
The higher-number valve has less movement while the lower-number valve has more movement. Notice the differences in movements do not follow a linear path as anticipated. However, this difference can also be found in QA1s dyno sheets.
The numbers themselves are not the same: Our racers dyno readings are for measured movement only when the same force is applied. A real dyno reads out in speed and pounds of resistance. Still, when using this dyno you can determine one valve rating from another. Please understand, the numbers here will not be the same as yours, only relative. This is the result of using a hammer of a slightly different weight.
Each manufacturer has its own standards as to what a No. 4, No. 5 or other valve might be. In this case, you should establish which are the lightest and heaviest shocks in your inventory. Note the manufacturers designation. Use this to divide out the other valve numbers. Actually, the valve numbers are just numbers someone pulled out of a hat many years ago. They seem to have no direct meaning. They only divide up a range of force and speed movements. So a Carrera No. 4 may not be the same as a QA1 No. 4. It wont make any difference as long as you know how to compare them.
This little dyno rates shocks against each other in both compression and rebound at one speed. It wont tell you everything about a shock, but it will give you a method of comparison. It can catch those out-of-tolerance shocks.
How To Build It
Nearly all construction is 1-x11-inch gauge square tubing. It will take 40 feet of this material. In addition, get the following from A&A Manufacturing: two AA-027-B seat tabs, eight AA-064-A bushings, one set of AA-160-A shackles, and two AA-021-B gussets. This, plus 24 inches of half-inch round steel rod and a few bolts are all the parts you need.
The basic framework of one-inch square tubing is a rectangle with outside dimensions of 36x16x8 inches. The two main frame crossmembers are welded in at 15 inches from the bottom. With this assembly welded together, it is time to make the T-bar. The T-bar is made from two 13-inch pieces of one-inch-square tubing welded to form a T. Note the legs of the T are offset. This allows the hammer to clear the rebound end of the T. Center a five and three-quarters-inch piece and weld at a 90-degree angle on the top of the T. Weld AA-027-A plates at each end of the T. This is where the shock will mount to the T.
The hammer is cut from a 13-inch piece of .095 tubing or one and one-quarter-inch pipe. Weld on a 31-inch handle in the center. The upper end of the handle gets a five and three-quarters-inch crosspiece. Remember, it is offset to clear the T. The weight of the hammer is important. The total weight, handle and all, should be 14-16 pounds. If it is much lighter, it is harder to measure movements. If it is heavier, it has more force than the shock will see on the car. The added weight can be anything. For this purpose, a torched-off piece of two-inch steel plate worked just fine. An old Ford nine-inch pinion is about the right weight too. On the striking end of the hammer, a rubber lower control arm snubber from a 78 T-bird was attached. You will find it as NAPA PN 265-4002. This keeps the valve from vibrating with the impact. Weld a six-inch extension to the top of the handle to lock under the release device.
Make a creep clip to keep the shaft from creeping down under its own weight. It is absolutely necessary when testing a gas pressure shock, which extends under its own pressure. Also, regular shocks bolted in with the shaft up require the clip to hold the starting position. In the preceding cases, travel measurements must be taken immediately after impact.
The hammer release is merely a piece of one-inch square tubing with a hole in one end. A bolt is welded to the top of the main frame. The handle is placed over this bolt and rotated over the extension of the hammer. When the handle is pulled back the hammer drops. The hammer drop must be exactly the same every time.
Main frame feet must extend 24 inches on the hammer end and six inches on the other. They can be welded in place, although ours are removable since the dyno is transported frequently.
As built, the dyno is made to use rod end shocks. Brackets for other end types can be made from some steel scrap and bolted to the original brackets.
How To Use It
Bolt the shock, shaft end down, to the T and bolt the other end bolts to the plate where at least several inches of shaft are showing. Raise the hammer and lock it into place. Level the T. Release the hammer. Do this several times before taking measurements; it adds to repeatability.
Now level the T and set the hammer again. A dial caliper will be necessary to measure the distance moved. Measure from a place on the rod end to a place on the shock body. This measurement needs to be precise. Trip the hammer and measure the shock travel. This is the distance that will tell you how this shock relates to others. In use, at least three cycles are necessary to be sure of a consistent reading. Often three readings within .010 inch are possible. Dont get frustrated too quickly if you havent used a dial caliper before. Explain to your friends that both the dyno and the dial caliper have to be broken in.
If you started with the shock on the compression stroke, now place it where it will be on rebound. Do not expect the rebound and compression readings to be the same even with a 50/50 shock. But then again this is what a shock dyno is for, isnt it?
In addition to the dial caliper, have a calculator handy. This will help with the math involved in determining the measurement differences. Make a chart for each shock, recording both rebound and compression numbers. Then place these numbers in order. If you have a shock that has been dynoed, use it as a standard. Fit it into place in the results of your measurements. Whether it is labeled a No. 4 or a No. 7 makes no difference. It becomes the standard by which your other shocks will be measured.
Dyno placement is necessary for repeatability. The feet on the end opposite the hammer should be placed against something heavy and immovable. A good wall will work.
As with many things, a bit of practice goes a long way. Heres hoping you have a shocking experience.
Contact Sleepy Gomez at email@example.com