Dirt Late Model Shocks Understood

Digging back into dirt late model shocks

Welcome back, this is the second installment in the series of articles to try and bridge the gap in understanding between the way shock absorbers used to be numbered and current numbering and understanding. Shock technology moves faster, I think, than computer technology. Shocks that were run for a season should probably need to be sent in for a good rebuilding and maybe a little tweaking. Shock guys, or gurus if you like, are continually experimenting and tweaking there valving set ups to try and make the car faster. Lets go back to our shock chart and dig a little further into understanding what that graph tells us.

Linear dyno graph









Notice how the lines at each speed are fairly equidistant from each other. That shows that this shock is considered linear. Speed increases directly proportional to the resistance increase. Although anything can be achieved, most shock companies build their shocks with linear pistons, so as the shaft velocities increase the resistance pressure produced by the shock increases. The other main piston type out on the market now is called digressive.


Digressive dyno graph for a Bilstein

dyno graphBilstein36mmhogan








The above graph is an example of a digressive shock piston. Shock resistance increases, not always directly proportional to shaft velocity, to a certain point where the shock blows off and will not create anymore resistance as the shaft velocity increases. Notice as the lines get out away from the center line of the graph they actually get closer together. There will reach a point where the lines will run on top of each other showing that no more resistance can be reached as the speed increases. Now this case is actually in a perfect world, because there will always reach a point where not enough oil can pass through the piston fast enough to get it to completely blow off. There will reach a point where the shock will again start to build pressure. This can all be tailored on how the piston is built and the amount of flow through the piston as to what speed this will happen.

All of the different shock manufacturers each have their own design of pistons which in turn gives their particular shock a certain ‘feel’. That is why, I believe, certain people like particular shocks and others do not. Some shock manufacturers believe in using less gas pressure in their shocks which gives them a more twin tube shock feel. Twin tube shocks are not pressurized and do not exhibit any rod pressure, as it is called in the industry. Rod pressure, simply put, is the amount of force the shaft exhibits when the shock is at steady state. —–OK. —–OK, well a simpler explanation is this, push a gas pressure shock down and hold it with your hand, the pressure pushing back on your hand when you hold it steady is called rod pressure. Like I said before some shock manufactures believe this gas pressure is bad for handling in the slick because it takes a certain amount of feel out of the car. Other shock manufactures believe that a certain amount of gas pressure is necessary to create a perfect balance to a shock, which in turn makes a shock more responsive. I believe the latter. I’ve had it explained to me in great detail at a seminar I attended and this makes the most sense.

Crank shaft dynos

I will try my best to explain it. A crank dyno operates on the principle of a sine wave and measures a shock as a sine wave. A sine wave will¬† graph acceleration of the shock shaft, but cannot measure jerk. There are some shock dynos that will somewhat measure jerk, like an EMA, but that is for a different article. The fact is this. The dynos most shock builders use, except manufactures, will not measure jerk. Drivers can most definitely feel jerk. If using gas pressure to balance a shock against the base valve and on piston valving increases responsiveness that is good in my book. Some dirt late model shock people say reducing gas pressure will help grip and get the car to dig into the track when a gas pressure shock¬† won’t. I think in this case there is usually something wrong with the shock valving, not the gas pressure.

Well I think I’m going to wrap up this edition of the shock talk. There is so much to talk about with shocks; I’m wondering where to go next.

Til next time, for those of you still racing, stay warm and be fast.


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