Let’s dig into dirt late model shocks
I first should preface this post by saying that I am not an engineer; I didn’t even stay at a Holiday Inn Express recently. The knowledge I bring to these posts are things I’ve picked up over the years of working on race cars, books I’ve read, or seminars I’ve attended. I’m going to put together some sort of virtual Rolodex in the future consisting of links to tools, education, and services that I use; so keep dropping by to check in with the progress with the site.
Sock numbering system
This post I will probably break into several parts because my outline seems to have gotten pretty big. A recent post on an internet forum made me realize there might be a need to try and explain what is going on in the shock absorber market nowadays. It seems within the last few years there has been an explosion in shock absorber technology in the dirt track market. (late models, modifieds, street stocks, etc) I break it up into two eras; the first is the twin tube era. Everyone was on twin tube shocks because the technology behind mono tube stuff hadn’t trickled down to our market from some of the higher end divisions. The numbering systems on the twin tube shocks were always two valve, three valve, four valve, etc. Everyone learned that system and everyone talked to each other about shocks using that system. Now, I believe, we have begun a new era in shock technology. Shock technology has evolved to the point that mono tube shocks prevail in popularity over twin tube. Don’t get me wrong there is nothing wrong with good twin tube shocks, but I bet if you go to any local late model show in the country, eight out of ten cars will have mono tube shocks on them. I’m sure those percentages are less in less expensive classes, but the number is increasing every year. The way people are discussing shocks nowadays has also changed. Shock people define shocks in pounds of force at a given velocity, usually inches per second of travel. There is too much disparity between shock builds to define a shock with a single digit. Terms like bleed, linear, digressive, shim stack, and gas pressure get thrown around do describe a single setup in a shock or a valve code. Everyone has opinions on what valving or type of shock is the best, so I won’t get into that. This article is simply meant to be a primer into the new world of shock technology. So, sit back, grab an ice cold Coke, and enjoy.
How to read a shock dyno graph
The first area i think I will tackle is understanding shock dynoing and understanding the data that comes out of the dynoing process. Many people might think I should be going through how a shock is built and how the internal mechanisms work, that will come later. Right now you have these things bolted on the car that you got with these cool looking graphs and you don’t know what all that information means. The first graph I will be showing you is called a Force vs. Displacement graph.
The Force vs. Displacement plot is divided into four quadrants. We’ll label these quadrants A, B, C, and D. Quadrants A and B represent the compression side of the of the shock and quadrants C and D represent the rebound side of the shock. A shock dyno operates on the same principle as a crankshaft in an engine. When it is at top dead center there is a fraction of a second the shock is at complete rest or zero velocity. As it rotates, even at a continuous motion, the vertical motion of the shock shaft accelerates until it reaches it’s peak velocity at ninety degrees from top dead center. The velocity it accelerates to is set by the speed of the dyno. Some dynos have speed adjustable dials, and some are preset by a belt configuration. As the crank rounds the ninety degree mark and heads toward the one hundred and eighty degree mark, the shock shaft starts decelerating until it comes to a complete stop at the on hundred and eighty degree mark. The horizontal axis on the graph represents the amount of force the shock is exerting. The same thing then happens in the rebound side where quadrant C is acceleration and quadrant D is deceleration.
There is a lot of data that can be pulled from the graph, but we will start with simply the force numbers at the peak velocities at ninety and two hundred and seventy degrees. In many cases is not the peak force because of something called hysteresis, but for simplification right now we will consider peak force at these points. Most racing shock builders consider low speed to be in the one to three inch per second range for velocity. This usually is the point in shock terms what many call the bleed area, although this is not always the case for every shock, because a soft shim stack will start to open early causing dampening through the piston. Bleed in a shock is basically a free flowing path that lets the oil flow from the rebound side of the piston to the compression side of the piston without any restriction. On the high end adjustable shocks the amount of bleed is usually what is adjustable. On canister style shocks the dial on the canister side will affect the compression side and the adjuster on the shaft will adjust the rebound side. Of course there are always exceptions to the rule. Check valves can be added or removed to make the adjuster on the shaft either compression or rebound adjustable or affect bleed on both sides.
Shock shaft speeds
The mid speed area is usually the three to ten inches per second area. I think the six inch per second number is becoming the industry standard when people talk shocks, although there is no hard fast rule. Always ask what speed people are dynoing at if they are talking generally about force numbers. The mid range and high speed ranges are all controlled by piston construction or the shim arrangement that is on a piston. There is some other technology out there like high speed adjustable shocks or the Ohlins TTX shocks, but these are out of the scope of what we commonly see in the dirt track community. At least for now. In the next post we will go into a little more depth on reading the dyno chart and what adjustments to the shock will generally adjust what parts of the dyno graph.
Till next time, race hard and be safe.