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Shock Absorbers Aren't
A detailed discussion of what they really do

By: James R. Davis



If the readers here do not mind, I would like to post an article that talks of shock absorbers in an effort to remove any mystery about what they do and how they work. Safety issues are often merely technology issues - once you understand the technology.

To begin with, they do not absorb shocks - your springs do that.

When your bike hits a bump in the road your wheels can do nothing but follow the curve of that bump. Your tires compress fractionally, but not enough to make a meaningful difference in the effects that bump will have on the rest of the bike, and you. If the wheels of your bike were connected directly to the frame, without springs and shocks, the bike would rise at least as high as the bump, almost instantly. The effect, of course, is that, if severe enough, when the bike came back down you would be left in the air. Your hands would probably not be jerked off the grips, so they would be pulled forward with the rest of the bike while the rest of you was still in the air - and then, worse, you would come down.

Obviously, the fix to that problem is to keep as much of the bike other than the wheels from rising in reaction to that bump (i.e., make as much of the bike as possible 'sprung weight'.) There is a tremendous amount of kinetic energy imparted to the wheels when they hit that bump. That energy must be captured before it is transferred to the bike's frame. And that is exactly what the springs do. By compressing, the springs absorb the energy from the wheels.

Remember pogo sticks? If all you had between the wheels and the frame of your bike were springs, then the only difference the springs would make would be a short delay before the bike was tossed into the air after hitting the bump. That is, once compressed the only thing the springs can do is decompress (that's the law). The energy the springs will exert during decompression is almost equal to the energy that went into compressing them in the first place. (A token amount of the kinetic energy will be converted to heat to make up the difference.)

Now we can understand what the shocks do. They DRAMATICALLY slow down the decompression of your springs (and in the process they convert much more than a token of the total kinetic energy stored in those springs into heat.)

A shock absorber consists of a tube filled with oil, which acts as a hydraulic fluid, and a piston (which is not physically connected to any part of the tube) that slides up and down within that tube, pushing its way through the oil. The piston is connected to one end of the shock absorber via a steel rod, the tube is connected to the other. One end of the shock absorber is connected to the frame of the bike while the other end is connected to the wheel hub (or to a swing arm that is connected to the hub.) Thus, when the wheel moves up towards the rest of the bike the piston is pushed thru the oil. The oil provides resistance to the movement of the piston which slows it down. In the process kinetic energy is converted to heat. (This is why you must change your shock absorber oil regularly - the heat breaks it down.) The oil in these tubes would totally stop the movement of the piston were it not for the existence of a valve in the piston that allowed the fluid to pass thru it. This is because, like water, the oil cannot itself be compressed. That valve can be made to allow fluids to flow faster in one direction than the other. For example, you would probably want your springs to compress faster than they are allowed to decompress. Without that valve your springs would not compress at all, leaving you as bad off as if the wheels were directly connected to the frame. Similarly, if the springs are too strong for the load they are carrying, too much of the kinetic energy will be conveyed directly to the frame of the bike, because they will compress too slowly, if at all.

But just as slowing the compression rate of the springs too much results in ineffective control of bumps, allowing their decompression to happen too quickly is just as bad. Were that to happen you would have 'pogo stick' reactions to bumps. So, it is essential that the design of the springs and shocks on your bike take into account how heavy the bike is and what kind of riding you do. But all such designs are compromises, and you can do things to totally frustrate the designers intentions - and end up hurt or worse as a result.

For example, when you put a passenger or heavy luggage on your bike you should increase the tension of the springs surrounding your shocks. Failing to do that can overload the system and get you close to the 'pogo stick' level of responses from them. Taking a street machine into the country, off road, and pretending it's a motocross machine can do the same.

But even assuming you don't do anything that extreme you will find that the design of your shocks is not perfect. (If it was, you would never feel a bump in the road.) The fact is, sometimes the road surface changes from perfectly level to bumpy. And some of those bumps (and potholes) can be awesome. This is where a few dollars can make a difference. You can replace the springs that come stock on your bike with a set that are called 'progressives'. These provide a normal soft ride until they are confronted with an unusually severe bump, at which point they get harder and harder to compress. And while the oil in the shocks cannot be compressed, air can be. So some shocks are 'air assisted' - in addition to the oil they have a small amount of air in the tubes. These 'air assisted' shock systems are sometimes attached to an onboard compressor that can be used to increase or decrease the pressure of the air, thus making the shocks either harder or softer without having to change the compression of the springs when your load weight or the road surface changes substantially. (Also, of course, you can increase the weight of the oil in the shocks to slow them down.)

The shock absorber 'system' on your rear wheel tends to have larger springs and have them mounted on the outside of the hydraulic tubes while the one on your front wheel have the springs within the tubes. The ones in the front are contained within the 'forks'. If you take a close look at your shocks you will find that the ones in the rear are typically angled forward from the wheel to the frame of the bike while the ones in the front are angled backwards. These angles tend to be directly in-line with weight shifts resulting from acceleration and braking.

The angle of the front shocks (forks), usually called the bike's 'rake', is essential to maintain! It establishes, along with the front-end 'offset', the bike's 'trail' which determines the bikes handling and steering control. The more extreme the rake is on your bike, the 'slower' your steering will be. (Except at extremely slow speeds - where extreme rakes often result in the wheel 'flopping' over and dumping the bike if you do not have your hands firmly in control of the grips.) If you were, for example, to lower your bike by shortening the front and back shocks, the wheelbase would also be shortened (the distance between the front and back tires). Since your front wheel would touch the ground closer to directly under your handlebars, your steering would 'quicken' as a result. In fact, even shortening the shocks by only one inch could result in steering that was so fast that your steering damper (another small shock absorber) could not safely handle it. The result, known as a 'tank slapper', would be violent swings of the wheel from side to side, and with high probability a dumped bike. (That is an overstatement. If you absorb some of the oscillation into your arms and avoid transferring that into the rest of the bike (through your contact with the seat), or use some braking caused weight transfer to the front of the bike, you can abort the 'harmonic' and probably avoid dumping it.)

In short, your shock absorbers are designed to help keep your tires on the ground regardless of surface imperfections so that they can do work for you.

Your shock absorber systems make your bike controllable. Make sure they receive factory recommended oil changes, do not modify them, adjust them for major changes in the weight of your vehicle or expected road conditions, and they will do their jobs reliably.

Copyright © 1992 - 2021 by The Master Strategy Group, all rights reserved.
http://www.msgroup.org

(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

     
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