How Motorcycle Suspension Dampers Work

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

There was a fellow in my high school who drove to class most days in a last-legs 1949 Ford whose dampers/shocks had lost all their oil. One morning he was behind the school bus and we could see that his car never stopped nodding. Each bump he hit would be repeated as five or 10 nods, and by then he’d have encountered another bump, so the motion continued.

What we were seeing was a mechanical oscillator: a mass on a spring. Deflect the spring and let go and the mass bounces up and down on the flexible spring. A clock pendulum is another example—in its case the “spring” is earth’s gravity, which tends to return the pendulum to pointing straight down at the center of the earth. Pull it to one side and let go and it swings back toward its rest position. But when it arrives, it has momentum that keeps it going, past the rest position, finally reversing direction once its kinetic energy of motion has been 100 percent converted into potential energy (because at the end of its swing the mass is higher than in its rest position.

Unless powered by some mechanism, the pendulum swings back and forth until whatever friction is present reduces its energy to zero. That friction in this oscillating system is called damping. Not dampening, which is what a person ironing clothes does with a little sprinkler bottle. Just damping.

The clapped ‘49 Ford we were watching kept nodding because its suspension dampers had leaked away all their fluid. They produced zero damping force.

Why Suspension Needs Damping

Around 1903 in the early days of motor racing, Renault in France decided the wild bouncing and leaping of racing cars on dirt roads had killed enough drivers and their mecaniciens. They began to attach friction-producing devices (dampers) to suspension to take away the excess suspension energy that could build up from nodding to bouncing and finally to complete loss of control.

The Era of Dry Friction Dampers

By the end of WWI in 1918, the usual form of damper was a sort of scissors with a miniature clutch at their pivot. One lever of the scissors attached to the vehicle’s frame and the other to the axle close to a wheel. A large wing nut allowed the pressure on the little friction clutch to be adjusted. These were called Hartford or Andre dampers.

In-town driving at low speed feeds less energy into suspension than does fast driving on the highway, so passenger comfort required a softer damper setting in-town and firmer on the road. It became tiresome to have to stop at the city limits and take a quarter-turn off those wing nuts to adjust the damping. Ritzy cars soon offered cable-controlled friction dampers that the chauffeur could adjust from the driver’s seat.

Friction damper on a 1933 Velocette fork. (Yesterdays Antique Motorcycles en Classic Motorcycle Archive, CC BY-SA 3.0, via Wikimedia Commons/)

Dry friction dampers have stiction: it takes more force to set them in motion than it does to keep them moving. This produced a small but perceptible extra shock each time a damper moved—the extra force needed to break friction’s grip and get the little clutch sliding. We could oil the friction material but that attracts road grit.

The Smoothness of Fluid-Based Damping

In seeking smooth motion the mind naturally turns to fluids. What if we could make suspension motion operate a little oil pump? Then we could adjust the damping by restricting the pump’s output with a small orifice. Some early hydraulic dampers used up-and-down suspension motion to rock a center-pivoted lever, each end of which moved a piston in a cylinder. As the lever rocked, it pushed one piston down and lifted the other up. Fluid being pumped from one piston to the other could be restricted by an orifice, producing a smooth rather than stick-slip damping force. Such a fluid damper could be made adjustable by placing a tapered needle in the orifice, its position adjustable by an external knob.

Modern dampers, aka shocks, use fluid moved through orifices. (KTM/)

Yet even though this fluid damper was much smoother than the earlier dry friction damper, it was still necessary to proportion the damping according to the speed of the vehicle—the higher the speed, the more energy wheel motion put into suspension, and the greater the damping force needed to remove that energy so it did not result in nodding or leaping.

Applying Most of the Damping Force on Rebound

Having damping force act equally on compression and rebound made the compression stroke increasingly harsh. It is compression, after all, that throws the wheels up, possibly off the pavement, causing loss of grip and possibly of control. But with fluid-based damping it was easy to put a one-way valve in the damping circuit, such that most or all of the damping would occur on rebound. That made a big improvement.

Variable Orifice Technology

Also a problem was the behavior of fluids being forced through an orifice. As the speed of suspension movement increases, the damping force increases not in proportion to speed, but exponentially. This means that a damper that gave a comfy ride in-town quickly became unpleasantly harsh as speed rose on the highway. This condition—of the damping force of a simple orifice approaching infinity—is called “orifice limitation.” Over the years it has been responsible for a lot of harshness.

A way to soften this was to provide one or more extra orifices, normally covered by a spring-backed plate washer or a ball. As the vehicle sped up on the highway and bumps were encountered at higher speed, instead of the damping force increasing so fast that it became harsh or even rigid, the extra pressure would push open the plate-valve or ball to uncover one or more extra flow orifices, slowing the rate of increased of damping force.

Tubular Dampers

After about 1950 tubular dampers came into wide use (probably because they could be fabricated from low-cost steel tubing rather than from expensively machined castings). The problem with them was that something had to give as the damper rod slid into the cylinder on compression. That led to two-tube construction. The inner tube was the damper cylinder with damper rod and piston. The outer tube, connected to the inner only at the bottom, was partly filled with damping oil, partly with air. Because air is compressible, this allowed the damper rod to enter the cylinder without having to compress the oil. Millions and millions of such simple dampers have been built for vehicles – the “tube shocks” on a car I drove in the mid-1960s came to the manufacturer at just over two bucks apiece—the miracle of mass production.

Very simple damping systems—largely based on a combination of fixed orifices and one-way valves—handled motorcycle suspension damping tasks for many years. Remaining problems were many, and grew more troublesome:

1. Relying on fixed orifices, plain or covered by blow-off valves, exposes suspension action to orifice limitation: harshness that borders on rigidity. This was why riders at Daytona in this era timed an upshift to coincide with the thump of hitting the banking on exit from Turn 5. A moment without power softened the grip-destroying harshness of that bump.
2. As the damper piston moved back and forth in its oil-filled cylinder, sudden motion could pull the oil apart or cavitate as orifice flow was unable to keep up with piston movement. The shock when such cavitation collapsed made problems for tire grip. Fancier dampers might have a “recuperator valve” that would open to ease this process.
3. In city and highway riding, tire grip is seldom challenged, but in racing, winning lap times depend upon being able to operate continuously near the grip limit. That magnifies any inconsistencies in suspension response. It was not possible to piece together smooth damping curves from fixed orifices and clacking one-way valves.
4. Suspension engineers had underestimated the actual speed of damper piston movement, so the testing machines (“shock dynos”) of that time stroked with elephantine slowness, preventing them from revealing certain problems. Fork dampers were especially crude in action because it was so easy for them to mix compressible air into the damping oil, making it into a spring. Many dampers ejected oil upward inside the fork tube, where its return was slowed by its clinging to the suspension springs then being increasingly located inside the fork tubes. The result was uncovering of the damper, which then sucked air. Adding more oil just stiffened the fork, as the pressure of the air above the higher oil level now rose faster on compression.

Pressurized Dampers

A number of inventions have brought us to the present sophisticated state of suspension damping. To prevent cavitation, dampers were pressurized by adding to them an accumulator containing either a sliding piston or a flexible diaphragm, separating the gas pressure from the damping oil.

The pressurized portion of a modern damper. Nitrogen is held in the small “piggyback” tank and pressurizes the space between the top and the slider that separates that space from the oil. (KTM/)

Varying Orifice Area by Bending Washers

To escape from the harshness of orifice-controlled compression damping, a ring of many orifices was covered by a flexible washer restrained at either its ID or OD. Rapid compression movements caused such washer to flex, creating a variable orifice. Suitable combinations of washers of various spring constants were able to generate quite smooth curves of damper velocity versus damping force. Washers and spacers were combined to produce almost any desired curve. During the period in which this art was being learned, suspension specialists could be seen trotting around race paddocks carrying (or wearing around their necks) wire loops on which someone’s washer stack was stored.

Naturally, complete disassembly of the damper was required in order to alter the washer stacks.

In early days it was usual to apply the large flow from damper piston motion to control rebound, while compression operated on the smaller flow caused by the oil displaced by the damper rod itself.

Because it’s generally easier to control large flows than small ones, this separation is now often replaced by a simple damper piston carrying no washer stack, pushing fluid in one direction through an external compression stack, and in the other direction through an external rebound stack. This not only improves control during compression, it simplifies service because the whole damper no longer needs to come apart for changes to the washer stacks.

The damping section of a modern off-road motorcycle shock. (KTM/)

Low-speed damping has been controlled by what we often call a clicker which is just a tapered needle on a screw thread with a spring detent (hence the clicking) to hold it in the desired setting.

It is now possible to employ electronics to control variable orifices, offering the potential to alter damping curves at a keystroke, TFT screen on your motorcycle, or automatically according to changing conditions.

There is a penalty for the sophisticated suspension damping we now enjoy. Its cost has risen somewhat from the “two bucks” mentioned above.

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