To pick up where we left of with Fisher's Single Pivot Technology and talk about Jack, 'Brake Jack' - or rear suspension activation caused by applying the rear brake. And the common myth of its results.
Some manufactures have a belief that single pivot designs are less sensitive to bump absorption or 'lock-up' , and that single pivots actually extend or 'jack' the rear shock when the rear brake is applied. So Gary being the genius that he is, decide to look into this and quantify 'brake jack' and its affect on different full suspension designs. And what Gary found was that in the real world, on a properly set-up single pivot, VPP or For Bar full suspension design, the braking forces actually compress the rear suspension, not extend it.
And this is very good, especially when your flying down a descent heading into a tight corner, go ride
Vulture's Knob (and you appreciate the result), as you hit that turn at a high rate of speed and began to apply your rear brake your body weight is going to shift. The compressing of the rear shock helps keep your bike and you in a neutral riding position and not taking a header over the handle bars. And
Fisher's Single Pivot Technology provides the right balance of suspension compression due to the rear brake.
Trek provides us with some excellent full suspension platforms as well, a single pivot with linkages. And their new thinking in full suspension technology,
R1 Tunded Suspension. R1 optimizes each Trek full suspension bike for its intended riding style: race, trail, all mountain or gravity.
Here's some general suspension terms :
Telescoping fork: One part of the fork collapses into the other. This is the most common type of suspension fork used today.
Linkage Fork: Linkage is used to collapse the fork. (Examples: Girvin, Lawwill Leader)
Conventional Fork: The part of the fork attached to the axle is a larger diameter than the upper part of the fork allowing the upper part to enter the lower part during compression.
Inverted Fork (upside down fork): The upper part of the fork is a larger diameter than the lower part allowing the lower part to enter the larger upper part during compression.
Position Sensitive: The amount of damping is controlled by the how deep the suspension unit is in it’s stroke.
Speed Sensitive: The amount of damping is controlled by the speed at which the damping is being moved.
Stroke (AKA travel): The entire distance the suspension can be compressed. On rear suspension stroke refers to the distance, which the shock’s shaft can travel, not the distance the wheel can travel.
Sag: The distance the suspension compresses under the weight of the rider and bicycle with no other forces acting upon the suspension.
Spring: The spring is what holds the fork up. It can be a metal coil spring, air, microcellular urethane or elastomer.
Spring Rate: The amount of force that a spring produces when it is compressed one inch. If you placed a 300-pound spring on a bathroom scale and compressed it one inch, the scale would read 300-pounds – compressing it two inches would read 600 pounds. MBA
Pre-Load: The act of mechanically compressing the spring in order to raise the starting point of the spring curve.
Spring Curve: The rate at which a spring will compress in relation to the force applied.
Coil bind: When a spring coil contacts the next coil.
Negative Spring: A spring that directly opposes the positive spring to lessen the amount of force required to initially compress the spring. A negative spring can also be used to help lessen the forces of top-out.
Top-out: When the suspension returns to full extension.
Bottom-out: When the suspension is fully compressed.
Damping: The act of slowing the spring’s movement.
Emulsion: A dispersion of tiny air or gas bubbles throughout a liquid. A new compound is formed because the gas cannot be removed from the liquid.
Aeration: The condition when gas bubbles are present in a liquid. The gas bubbles will try and separate themselves from the liquid.
Cavitation: The act of air passing through a damping circuit, creating a gap in damping performance.
Rebound: The act of suspension returning from a compressed state.
Compression: The act of collapsing the suspension.
Brake Dive: A condition when the fork compresses under heavy braking loads.
Blow-off valve: Any type of hydraulic valve designed to resist the flow of fluid until it reaches a predetermined pressure threshold.
Free-bleed circuit: A hole or port that allows shock fluid to bypass the shock’s main damper piston during both compression and rebound.
Damper Piston (AKA piston): a perforated piece of material that fluid passes through. One-way valves (or circuits) meter the flow of fluid to create different flow rates depending on the direction the fluid is passed through the piston.
Orifice: A passage of, exact and pre-determined size for controlling oil flow.
Progressive rate: Any configuration that amplifies the effects of the spring and shock as the suspension reaches full compression. MBA
Falling rate: Opposite of progressive rate suspension.
Straight rate: A suspension system that causes the spring and damping forces to increase in direct proportion to its rate of travel. MBA
Leverage ratio: Wheel travel divided by the shock travel. Front forks have a 1:1 ratio, the wheel moves an inch the fork moves an inch. Rear suspension designs differ greatly in leverage ratio depending on the design. The most common is 2:1, in which the rear wheel moves 2 inches for every inch of shaft travel at the rear shock.
Unsprung mass: The weight of the wheel, tire, brakes and suspension parts. SW
Spiking: A condition in which the forces exceed the oil’s ability to flow through the damping circuits creating a hydraulic lock.
Hydraulic Lock: A condition in which a suspension unit’s movement is limited by fluid.
Oil Viscosity: The thickness of the oil.
Fade: A reduction in damping control caused by heat
Fork Components:
Steerer: The center tube of a fork that rotates on the headset bearings inside the frame’s head tube.
Crown: The bridge that connects the steerer tube to the legs.
Upper Tubes (AKA “Uppers” or “Stanchions”): On all RockShox forks the upper tubes enter the lower tubes during compression.
Lower Tubes (AKA “Lowers” or “Castings” or “Sliders”): On all RockShox forks the lower tubes are one piece cast aluminum or Magnesium alloy.
Brake Boss: The steel or titanium studs that brakes can be attached to.
Bossless Casting: A lower tube that can only be used with disc brakes.
Bushings: Metal or Plastic sleeves that another part rotates or slides on. RockShox uses aluminum backed plastic bushings pressed into the fork lowers for the uppertubes to ride on. RockShox rear shocks have steel and plastic bushings for different purposes.
Rear Shock Components:
Coil/over shock: A type of rear shock that utilizes a metal coil spring mounted externally over the shock body.
Eyelet: The holes at either end of the shock that are used to mount the shock to the bicycle.
Shaft: The narrow rod that enters and exits the shock during actuation.
Shock Body: The larger diameter part of the shock, which the shaft enters.
Reservoir: The part of the shock that contains the IFP (Defined in damping section) and controls the displacement compensation for the damper.
Piggyback reservoir: A reservoir that is connected and placed parallel to the shock body.
Remote reservoir: A reservoir that can be placed in another location from the damper connected by tubing or piping.
Shaft Eyelet: The eyelet that is connected to the shock shaft.
Body Eyelet: The eyelet that is connected to the shock’s body.
Eye-to-eye: The distance between the shaft and body eyelet at the shock’s full extension.
Spring Collar: The removable collar that is used to contain the spring between the eyelets.
Threaded Collar: The collar that is attached to the shock body via threads. The collar threads up and down to control preload and difference in spring height.
Air Can: The orifice on a rear shock that contains the air spring. Traditionally it is mounted outside of the damper.
Spring Types:
Coil Spring: A metal rod wrapped into a coil to form a spring.
Progressively Wound Spring: A spring with its coils spaced closer together on one end to create increase in spring rate as the spring is compressed.
Air Spring: This is a sealed volume of compressed air that is used as a spring medium.
MCU: Micro Cellular Urethane, used as a spring medium, but also has damping characteristics.
Elastomer: An inexpensive form of bumper compound that is used as a spring medium.
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Damping:
Damper: A friction device used to control the speed at which the spring moves. Most dampers use oil that passes through holes to create the friction that transfers the energy into heat. The oil is also used to dissipate the heat.
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Dampening: Is not a word. If you find yourself wanting to use this term use “damping” instead.
Damper Rods: This is a very simple and cost-effective method of damping that originated in ‘70’s motorcycles. This open bath system utilized orifices to control the flow of oil. RockShox’s HydraCoil and HydraAir systems are both damper rods.
Cartridge: This damper is a fully sealed unit that utilizes a piston and shims to control oil flow. RockShox introduced the Judy cartridge in 1994. The lack of compensation and non-serviceability plagued the RockShox cartridge for it’s entire life cycle (which ended 2001).
Open Cartridge: This is a cartridge that is not sealed and allows for greater heat compensation. Both Marzzochi, Manitou and Fox use this system.
Air Damping: When air is used to control the damping of the fork. This system is used by the Elglund air cartridges and the Cane Creek rear shock.
De Carbon Damping: Named after it’s inventor Dr. De Carbon, it’s a type of damping that uses a piston backed by high pressure nitrogen called an IFP (Internal Floating Piston).
IFP: Internal Floating Piston. A piston that is used to compensate for displacement created by the damping shaft entering the damper. The IFP is backed by a spring (either gas or mechanical) to place constant pressure on the damper as to not allow air to enter. Along with displacement compensation, the IFP can also compensate for heat expansion.
Compression Damping: The controlling of the speed of the suspension as it compresses.
Rebound Damping: The controlling of the speed at which the suspension returns.
Low (or slow) Speed Damping: The controlling of the spring speed when the shaft speed is slow.
Shaft Speed: The speed at which the damping shaft enters and exits the damper. Shaft speed is not directly related to bike speed.
High Speed Damping: The controlling of the springs compression and rebound at fast shaft speeds.
Check: A device inside the damper that only allows oil to flow through an orifice in one direction. Used to eliminate the effect of adjustment to compression or rebound on the opposite by creating separate circuits for both.
Blown: A non-descriptive term for a suspension malfunction.
And I owe props to Demo Bob, a technical specialist at Trek for passing along the suspension terms to me. You can check him out at Trek's
Demo Tour.
