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Welcome to my Motorcycle Suspension Terms Glossary.
I have been thinking a lot recently about swapping out the stock front-end of “Cal” for a springer front-end. You know when sometimes you see a custom-bike on the road or even a factory-bike and you just like the “look.” Well, that is what the springer front-end has always been for me – a “look” that I just inherently like and have always wanted.
When I built my first Harley Davidson Shovelhead a good 20 years ago, I didn’t have the money to do everything I wanted. I am not saying that I am rolling in it now by any means; but I am working A LOT smarter not harder during my restorations.
For example, I do a lot of my own rust removal and custom painting – really all my blog posts are geared at the DIY motorcycle repair person who is on a budget. The first shovelhead was simply a chrome monster – I can admit that now, while “Cal” is more the product of elbow-grease and love.
Anyways, total digression there, but the point is that I thought I would share some lingo regarding motorcycle suspension. There are a lot of terms that go along with the simple process of dampening bumps in the road, and it is fun to know all the terminology. The sortable and searchable table below is a combinations of resources I have found around the web. Check the references below the table for original sources.
|A formula that calculates the relation between the drive sprocket, rear tire contact patch, swinging arm pivot height, and the chain force lines. In order to determine the rear suspension’s characteristic to squatting under acceleration.
|The spin axis of a wheel.
|The compression piston and valving that fits onto the compression bolt assembly.
|A closed-end, thick rubber, cylindrical shaped piece that contains the nitrogen gas in a rear shock. The bladder works like an extra cushion on HSC.
|BOTTOMING or BOTTOMING OUT
|A riding situation whereby all the suspension travel is utilized.
|A taper shaped dense foam piece that fits on the shock shaft.
|A bronze or plastic ring used as a load bearing surface in forks or shocks.
|CENTER OF GRAVITY/MASS CENTER
|The center point of the motorcycle’s mass. Normally located somewhere behind the cylinder and below the carburetor of a dirt bike.
|The frame, swing arm, suspension, and wheels of a motorcycle.
|A fork shaped piece of aluminum used as the bottom mount for most shocks.
|The knobs or screws that control the LSC & LCR circuits of the forks or shock.
|COMPRESSION BOLT ASSEMBLY
|A large diameter bolt that houses the low speed compression adjusting screw and the compression valve assembly.
|The damping circuit that absorbs the energy of compression forces on the damper.
|When the rider applies steering pressure in the opposite direction of the turn.
|A fluid chamber with a means of regulating the fluid flow to restrain the speed of the moving end of the damper during the compression or rebound strokes. A set of forks and a rear shock are considered dampers.
|The parts of a shock comprised of the clevis, shaft, bumper, piston, and shims.
|The large diameter aluminum tube in the lower leg of telescopic forks.
|The relative speed in which the moving end of a damper compresses or rebounds. The two different speeds are high and low.
|The process of absorbing the energy of impacts transmitted through the forks or rear shock on the compression stroke, and the process of absorbing the energy of the spring on the rebound stroke.
|There are normally four damping circuits which affect the damper’s speed. There is both a low and high speed circuit for the compression and rebound strokes.
|DAMPING ROD FORK
|A simple type of fork that utilizes a tube with holes in it to create compression and rebound damping, delivering an extremely progressive damping curve.
|During heavy braking the front fork will compress due to the weight transfer. The term “dive” refers to the horizontal movement of the bike.
|The action of putting the bike into a full lean position quickly.
|FORK OIL LEVEL
|The level of oil within the fork as measured when fully compressed without the spring installed. It is used in tuning the amount of air contained inside the fork.
|The amount the bike settles under its own weight.
|FRONT END DIVING
|This is what happens when the front forks compress quickly. It usually occurs when braking for turns.
|The term geometry refers here to a motorcycles overall geometry. A motorcycles geometry is a combination of rake, trail, etc.
|The quality of response from the chassis of a motorcycle, while riding through a variety of obstacles like turns, jumps, hills, whoops and bumps.
|A word used to describe the quality of the damping.
|A term that describes the high speed oscillation of the forks when braking for a bend at the end of a fast straight-away. Every motorcycle has a certain frequency band when it oscillates. This frequency can be tuned to a higher vehicle speed with a sacrifice in the bike’s ability to turn.
|A term that describes what happens when a bike falls to the outside of a turn.
|Damping to control fast vertical movements of suspension components caused by road characteristics such as square-edged bumps
|Wheel hopping is when the tire bounces up off the ground due to a reaction from a bump.
|High Speed Compression damping circuit is affected most when riding fast over square-edged bumps.
|High Speed Rebound damping is affected in the same riding circumstances as HSC.
|A word often used to describe both “Pogoing” and “Packing”.
|A term that describes what happens when a motorcycle falls to the inside of a turn.
|Damping to control slow vertical suspension movements such as those caused by ripples in pavement.
|Low Speed Compression damping circuit is affected most when riding through turns.
|Low Speed Rebound damping circuit is affected in the same riding circumstances as LSC.
|MID TURN WOBBLE
|When the bike wobbles or weaves near the apex of a turn.
|An inert gas used to pressurize the bladder or reservoir of shocks. Some also fill tires with this due to its stability at varying heats.
|When the rear shock is compressed by the wheel hitting one bump and cannot rebound quickly enough to absorb the impact of the second or third bump.
|A cylindrical shaped piece of steel with several ports arranged around the periphery so as to direct oil towards the face of shocks.
|A ring that fits around the piston and prevents oil from by-passing the piston and shims.
|A small diameter steel rod that fits into the upper legs of cartridge forks. It fastens to the fork cap on one end and holds the rebound piston and shims on the other end.
|A motion fore or aft, when the front end dives or when the rear end squats.
|A fixed point at which a lever rotates. Example: swinging arm or suspension linkage.
|When the rear shock rebounds so quickly that the rear wheel leaves the ground.
|Pre-load is applied to the fork and shock springs in order to bring the bike to the proper ride height or race sag dimension. The pre-load can be biased to change the bike’s steering geometry. High pre-load/less sag in the front forks, will make the steering heavy/slow and more stable at high speed.
|Material used to adjust a fork’s preload internally. Typically, thin-walled aluminum or PVC tubing is used
|This term refers to number of millimeters that the forks or shock sag with the rider on the bike in full riding gear. This is essential to proper suspension tuning but is often overlooked or adjusted incorrectly.
|The angle between the steering axis and a vertical line.
|REAR END SQUATTING
|Squatting occurs when you accelerate the motorcycle. The chain forces push down on the rear wheel. The resultant forces are transferred up the swinging arm into the main frame causing a lifting force which extends the front end causing a weight shift backwards.
|The damping circuit that affects the stored energy release of the compressed spring in order to reduce the rebounding speed of the damper.
|A cylindrical shaped device that contains oil and nitrogen gas.
|A term used to describe a fine-tuning service for altering the compression and rebound shims in order to affect a certain damping characteristic that keeps the motorcycle’s wheels following the terrain in many riding situations
|Suspension adjustments (raising or lowering the fork or lengthening or shortening the shock) to alter the chassis attitude of the motorcycle.
|A motion where the motorcycle leans left or right from straight-up riding.
|The angle of rotational motion about the swinging arm pivot axis.
|S.A. PIVOT AXIS
|The point where the swinging arm mounts to the frame and rotates.
|The amount the front or rear of the bike compresses between fully topped out and fully loaded with a rider (and all of his riding gear) on board in the riding position
|A rubber or plastic cylindrical shaped piece that prevents oil from being lost from the damper.
|The chrome rod on the rear shock that has a clevis on one end and the piston and shims fastened to the other end.
|A thin, steel, round, flat washer used to exert resistance on the oil flow through a piston. A series of shims (valve stack or valving) with varying outer diameters and thicknesses are arranged in sequence to provide a damping affect.
|The aluminum cylinder which contains the damper assembly.
|A machine that cycles a shock absorber at different damper speeds and measures the resistance posed by the four damping circuits.
|A condition that occurs when the shock oilbecomes so hot that it loses it’s transmitability. The damping affect is reduced and the shock compresses easily and rebounds quickly.
|When a motorcycle wavers back and forth rapidly at high speeds.
|A word used to describe how the forks work when the damping is too stiff/slow. This is also associated with “Arm Pump”. The feeling in your arms when your forks aren’t absorbing the energy of impacts to the wheel but instead transfer them to your arms.
|A steel wire that is wound into a coil shape and tempered in order to provide resistance to compression forces and store energy for release to the extended position.
|A motorcycle suspension springs stiffness, expressed in kg/mm, N/mm or lb/in.
|Compression of the rear suspension as a reaction of weight transfer and/or chain pull. Squat occurs especially usually under rapid acceleration.
|The angle of the handle bars as you rotate them left or right about the steering axis.
|The axis where the forks rotate in the frame.
|A combination of the words static and friction. This word is used to describe the tension exerted on the moving damper parts by the stationary parts like the bushings, seals, and wipers. Low stiction is desirable because it has less of an affect on the damping.
|These words are used to describe the damping quality of the forks or shock. With regard to the “Clickers”, these words refer to the direction of rotation that you will turn the clickers in order to improve the damping. Turning the clickers clockwise will make the damping stiff/slow. Turning the clickers counter-clockwise will make the damping soft/fast.
|Used inside a shock absorber to create damping when forced through orifices or valving. The fluid is also used for lubrication and should be incompressible.
|When the rear end of the bike pivots around from side to side very quickly.
|The rear fork that connects the rear wheel to the frame.
|When the forks rotate from stop to stop rapidly and your arms and body slap back and forth against the motorcycle’s gas tank.
|Occurs when the suspension extends to its limit. A shock with a spring of the proper rate mounted should have just enough force to top out without a rider on board.
|On the front end, the horizontal distance between the steering axis at the road surface, to the tire contact point. Generally forks with off-set axles have more trail than forks with straight through axles.
|These are shims with very small outer diameters that are used to separate the normal shims of the low and high speed valve stacks.
|This term refers to the suspension oil’s ability to transmit shock loads. As the oil’s temperature rises, the transmitability falls. Example: With every increase in temperature of 18 degrees Fahrenheit the transmitability of the oil falls 50%.
|TRAPPED AIR SPACE
|The height of the air space that forms in the top of the fork tube between the fork cap and the oil.
|Includes the steering stem bottom clamp, and top clamp. The triple clamp assembly connects the forks to the frame.
|The distance from the center of the fork tubes to the steering stem center. The greater the offset, the smaller the trail dimension.
|The number of millimeters that the bike sags under it’s own weight without a rider.
|The unsprung weight of the motorcycle are parts like the wheels, brakes, swingarm and suspension linkage, and the lower front fork legs. The sprung weight is all the parts of the motorcycle that are supported by the suspension.
|A term that refers to a series of shims either for the compression or the rebound damping.
|A rating system for oils that measures the oil’s flow rate through a fixed orifice at a certain temperature. Also known as the oil’s weight. Example: SAE 30 Wt.
|The flow rate characteristic of the oil over a range of temperatures. The VI rating of an oil is directly linked to the oil’s transmitability. Cartridge fork oil has a VI# of 115. Shock oil normally has a much higher average operating temperature so its VI# is 300.
|A term used to describe what happens when the bike and rider fall to the inside of a turn.
|Instability of the rear of the motorcycle, a side ways movement of the rear end from one side to the other.
|Also called weight distribution. The amount of weight on each wheel of the motorcycle.
|The distance between the front and rear axle centers.
|A word used to describe a motorcycle in motion with the front wheel off the ground.
|Instability of the front end of the motorcycle, a very fast oscillation from side to side of the front end.
|A motion that veers left or right from the motorcycle’s heading angle.
References: This table is a combination of various internet resources I have found covering the issue, including three primary sources. The URLs for these sites are here – (1) http://www.twowheelforum.com/37-technical-help-maintenance-tuning/11584-suspension-definitions-other-terminology.html#/topics/11584 (2) http://www.sportrider.com/suspension-tuning-guide-learning-lingo#page-8 (3) http://www.lustracing.co.uk/suspension/motorcycle-set-up-glossary.html
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