TEMPERATURE EFFECTS ON THE DAMPER

Transcription

1 TEMPERATURE EFFECTS ON THE DAMPER Temperature affects the oil, the gas pressure, and the seals in the shock. It can also increase and/or decrease certain tolerance fits inside the damper itself. Aluminum shocks may be affected differently than steel, as well as aluminum pistons compared to steel pistons. During the course of your race, the damper changes. Do you know what is happening to it from the beginning to the end? Is it important? Test: Tested the same shock on a 2 stroke at 10 in/sec using a Roehrig Engineering 5VS crank type dynamometer. We collected data using the Roehrig Engineering s Incremental Temperature Test to get data at 100, 120, 140, 160, 180 and 200 deg F. For the test, we zeroed the load cell with the shock hanging from the upper crossbar and pre-loaded it 2 for the test. We ran a standard Roehrig Engineering gas test, stopping at mid-stroke for 2 sec. In the second test, we reduced the amount of preload to ¼ so that at 100 deg F the shock would cavitate and as the temperature increased, which in turn increased the gas pressure, the cavitation went away. One feature of Roehrig Engineering s (REI) Shock software is the ability to collect temperature based data. Because temperature affects the damper, it is important to know at what temperature the test is preformed so that a proper comparison can be made change to change or shock to shock. By warming all your dampers to a standard temperature and then collecting data, they are all being compared at a standard set of parameters. Why does Roehrig feel that it is important to warm your dampers? If you collect all your data in the shop at 72 deg F and then you go to the track, you will likely be at a different ambient temperature. And even if you have a nice air conditioned trailer, you may need to dyno a shock off the car so it will be warmer than the shop ambient of 72 deg F. By having all you data at say 100 F, you will be much closer to the wide variety of temperatures you will encounter. What does my shock do during the course of a race or fuel stop? Most people pressure their shocks off the car and fully extended. They usually pressure them at room temperature. They put them on the car and during the practice or qualifying or race, the shocks get hot. How hot they get is based on many things; the valving, the bleed amount, the surface of the track or whatever you may be racing on, what they are close to on the car (exhaust, brakes), what kind of cooling air they get and how long they are running. Does the shock change from its collected temperature to its on the car operating temperature? Most likely it does. Temperature thins the fluid to some degree. Temperature increases the gas pressure in the gas chamber and as a result increase the gas force (or spring) of the damper. Different shock materials grow at different rates and tolerances may increase in some areas and decrease in 1

2 others. So it is easy to see why at some point, you have to take into account the operating temperature of the damper and see what it does at the various temperatures. Figure #1: A Series of Tests from 100F to 200F Purple: 100F, Blue 140F, Olive Green 200F Figure #1 shows the same shock at three different temperatures. The actual test started at 100 deg F and ran in 20 deg F increments until it reached 200 F. We used the REI incremental temperature step test. You can see, as the shock gets hotter, the force begins to fade. You can also see that it affects all areas of the shock, the low speed bleed, the bleed to valve stack interaction and the high speed. So if you start the race with this shock on the front of the car, say right front, then as it gets hotter it will begin to react differently from when you started. Maybe your shocks do not get very hot or your races are very short, but if you shocks are on the front of say a late model or ARCA car, then you will see a change brought on by temperature. 2

3 Since one of the things a damper does is influence the interface between the sprung and unsprung weight of the car, as you burn fuel off, the front percentage of nose weight increases and your front shocks get softer, what do you think will happen? You have all heard my car is good later in the run or the longer we ran, the worse we got. Well, this has something to do with that. Many, many, many other factors are involved, but you have to tackle them one at a time and the more you understand about one, the better. Figure #2: Temperature Affects on the Comp Open/ RB Closed Cycle Purple: 100F, Blue 140F, Olive Green 200F Figure #2 shows one side of the shock cycle; the compression open / rebound close side. By looking at the live cursor, using the Roehrig Shock Software, you can see the force of this damper reduce. At 5 in/sec, the rebound goes from -296 lbs to -282 lbs to -269 lbs at 200 F. That is a drop of 27 pounds. The same thing happens on compression, not by as much force, but perhaps the same percentage drop. It starts at 119 lbs then 114 and finally 108 lbs in the end. 3

4 You can also examine the gas force increase, which can be equated to additional spring force developed by the damper being heated. Using your Roehrig Report section you can see the gas force and resulting calculated gas pressure from each temperature. At 100 F, the gas force on the shaft is 28.6 lbs and this results in about 90 psi of pressure in the gas chamber. At 200 F the gas force has grown to lbs and 132 psi in the chamber. That means the front of your car could be gaining spring rate the longer you run and the hotter the shocks get. Do you want more spring rate the longer you run? Now, if we had started with more pressure, then the affects would have been even greater. Or, if you compress the shocks to put them on the front of the car, you have already started to increase the gas pressure from where you set it. So, now you have data from your car during the race. You bought temperature strips from McMaster Carr or some other catalog, you put them on the car and now you know your front shocks get to 180F and the rears only get to 140F. What to do now. You go back to the shop and run your shocks through a temperature test to see how they fade during a run. Maybe certain shocks fade more than others, maybe there are better fluids or oil to buy and maybe some pistons fade less than others. At least now you can begin to understand what is happening. What if you started with more rebound in the front of the car knowing that it would fade during the race. What if you prove that the rears do not change much at all and you could stand to use more gas pressure to help build rear spring rate during a run? What if now instead of guessing, you could now ask questions and develop a test to find answers? I have a thought experiment for you that will lead into a future paper. If your front shocks gain temperature during a run, say they get to 180 F. By gaining temperature, we now know the gas force is building in the front shocks. If you started with 150 psi in your front shocks fully extended, you then compressed them 3 inches to get them on the car, now you are at 175 psi. Then the race starts and your fronts heat up to 180 deg F after 10 laps, what do you think the pressure is now? 200 psi, maybe more. That means you have gained 50 psi and you have gained front spring rate. This added spring rate from the shocks raises the front of your car 3/16. Do you know what happens next? The driver complains the car is getting tighter and tighter. Is there anything you can do to reduce this pressure build up? You could start with less gas pressure and thereby reduce the overall addition of gas force. Figure #3 is an extreme example but it is interesting to see. I reduced the preload on the shock for doing the test, instead of 2 inches of preload, I went to ¼. I ran the shock through the same temperature step test to see how it would change over the course of the race. 4

5 Figure #3: Temperature Affects on Cavitation Red 100F, Blue 200F The red trace is the run at 100 deg F. It is cavitating, it does not have enough gas pressure to prevent cavitation. The blue trace is the run at 200 deg F, cavitation is gone! What happened? The shock got hotter, the gas chamber got hotter and the gas pressure increased. When it increased enough, the cavitation went away. So from the start of the run to the end, enough temperature and enough pressure was increased to change the damper completely. If you do not think this works, how low do you start your tires for a race? You do not start them at the pressure you want to run at, you start them low knowing they will build up pressure during the race. Shocks are no different. I can also tell you, people have begun races with shocks cavitating, knowing that after a few laps they heat and pressure will be enough to make it go away. I know I have done this. What is cavitation? 5

6 cav i ta tion (kăv'ĭ-tā'shən), n. 1. The sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces, such as those resulting from rotation of a marine propeller. 2. The pitting of a solid surface. Huh? Well, we will leave it for the next paper. Keep watching the Technical info section of the website: for future papers to help and maybe confuse you more. Any suggestions for technical info can be submitted to Michael@roehrigengineering.com. 6