The basics of suspension - terms and general info
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[edit] Foreword
This is a basic introduction to suspension. It does not cover servicing or setup of specific brand/model forks/shocks, it is simply to help people understand why suspension is the way it is, the basic concepts of suspension, and why certain things should be adjusted to suit the requirements placed on a suspension system.
[edit] Mass, "centres of mass", and "sprung mass"
Mass is a property of matter, which determines the amount of inertia (resistance to acceleration) that it has. A "centre of mass" is a point (for the purpose of explanation and calculation only, ie it is not a physical object) about which a body's mass is evenly distributed in all directions (that is, you could balance the body on a pin head if you could get the centre of mass precisely over it). "Sprung mass" is the part(s) of a body separated from the ground by suspension - obviously "unsprung mass" comprises the parts that are not suspended (wheels etc).
[edit] What is suspension?
Suspension, by definition, is any system that allows a body to be deliberately isolated from another (so that they are not rigidly joined) whilst remaining in indirect contact (eg via a spring or a liquid or a magnetic field). Obviously, we are dealing with it in the context of mountain bikes, where its main purposes are to insulate the main mass of the bicycle/rider from rough terrain and thereby help increase traction, and to absorb large/sudden shocks that could otherwise be dangerous to rider or bike. Traction is best maintained when a wheel can stay on the ground with as constant normal reaction force (that is the force perpendicular to the two surfaces that are against each other, ie the ground and the tyre) as possible. Suspension aids this by allowing the wheel to track the ground whilst moving up and down at a rate that would not be possible by a rigid body using only gravity to hold it down. Obviously, if a rigid body gets pushed upwards hard enough, it will leave the ground, and it can only accelerate downwards at a fixed rate (due to gravity) to come back to the ground. A suspended wheel can both move upwards without greatly affecting the centre of mass of the body, and be pushed downwards (by the spring) at a greater rate of acceleration than gravity can provide (and again, without greatly affecting the vertical position of the centre of mass). Obviously, this means that the wheel can stay on the ground, which translates to the bike having more traction.
Shock absorption is provided in the same manner; that the wheel can move upwards to roll over an obstacle (bump) without affecting the height of the centre of mass anywhere near as much. This means that the sprung mass can continue moving in a relatively straight line, notably without the rider's centre of mass being bumped upwards significantly. This translates to a smoother-feeling ride. Suspension relies on the inertia of the sprung mass being significant enough to allow the springs to compress without shoving the centre of sprung mass off-line too far. For this reason, reduction of unsprung mass is desirable, and in theory the softer the spring the better (purely for bump absorption that is - overly soft springs are bad for other reasons which will be mentioned later).
[edit] What makes up suspension?
Most suspension systems consist of a few basic things:
- sprung mass: this is usually the rider and front triangle of a bike
- unsprung mass: this is usually the wheels and so forth that are between the spring/damper and the ground
- a spring of some sort, to allow the unsprung mass to move relative to the sprung mass, and to return it to a position where the suspension can be compressed again next time it sustains a compressive force
- a damper.
Dampers are there for a few reasons - they stop the suspension oscillating almost infinitely once it is set in motion, and they serve to control the motion of the suspension for various reasons, such as to prevent bottom out, to control the rate of suspension extension [ie "rebound"], to help maintain the bike's attitude/geometry, and also (in some cases) to aid with pedaling efficiency.
[edit] The difference between damping resistance and spring resistance
First of all, spring resistance is POSITION sensitive only. That is, the more you compress it, the harder it pushes back. A simple damper, however, is SPEED sensitive. That is, the FASTER you try to compress (or extend) it, the more resistance it has. The relationships between speed and resistance are not always linear, sometimes resistance increases exponentially with speed (which can cause what's known as "spiking", which is when a sudden hit causes the damping resistance to increase far too much and make the fork/shock feel like it's bottomed out mid-stroke). However, a good damper will be designed so that resistance increases at a linear or more likely, DECREASING rate. That is, if you double the speed, the resistance will be less than double. This allows for high-speed hits to use more of the travel without spiking. It is also what allows for a difference between high-speed and low-speed compression adjustments.
Note however, that some dampers these days are position sensitive too - this is done to allow the suspension to move more freely in the early regions of travel (to absorb bumps easily) whilst resisting bottom out.
[edit] Low/High speed damping
High and low speed are usually mentioned in conjunction with compression damping, where low speed is usually taken to mean compressions due to things like pedalling motion, g-outs, and sometimes landing jumps/drops etc where the compression speed (of the damper shaft) isn't very high. High speed refers to the speeds the damper shaft attains when it takes a sharp hit (travelling over high speed stutter bumps for example, or hitting a square-edged bump whilst traveling at high speed). Note that the low and high speed terms do not refer to how fast the bike is being ridden - they are related to the shaft speed of the damper.
[edit] Spring rate
Obviously you want to try and choose the correct spring rate to suit your weight, frame/fork, riding style, suspension preferences etc. Softer springs give more comfort and traction (to a point - too soft can make this worse), but you may lose out on bottoming resistance, cornering/jumping ability (same as running slower rebound in that respect), and so forth. Generally you want to choose a spring that gives you ~20-35% sag in the fork, and ~30-45% sag at the rear (for a DH bike). Obviously these are rough figures only, and if you like a stiffer or softer than average setup then you should adjust accordingly. For other applications (trailriding, XC, jumping, whatever) you will need to work out how you want the suspension to work and where you prioritise, and work from there.
[edit] Compression damping
Compression damping is, obviously, a speed-sensitive resistance to compression. In theory, ideally you want minimum high-speed compression damping for bump absorption, but a fair amount of LSC (low speed compression damping) to help the bike maintain its geometry during braking and cornering etc. However, in reality more high-speed compression may be necessary to stop the suspension blowing through its travel on hard hits.
[edit] Progressiveness
Most suspension systems have some form of progression built-in, mainly for the purpose of preventing bottom-out. Progression/progressiveness is simply a measure of the how the OVERALL resistance to compression (ie the sum of spring force and damper resistance etc) increases as the suspension gets further into its travel. The progressiveness can be provided in the form of a progressive spring (such as a variable-wind coil, or an air spring - air springs also include the air pocket in most open-bath forks which decreases in volume and thus increases in pressure when the fork compresses), a progressive linkage (generally only found with rear/frame suspension, not forks) which alters the geometry and thus the force on the shock with respect to position, or position-sensitive damping. Position-sensitivity of damping should, for the purposes of discussion, be kept separate from the speed sensitivity of damping.
[edit] Rebound damping
Rebound damping is the most critical part of a suspension setup. Rebound damping helps absorb the ENERGY of a hit to the suspension. The reason for this is, when the suspension is compressed by a sudden hit, the energy of the hit is stored in the spring (minus whatever work is done by the damper during the compression stroke). When the spring extends again, it releases this energy. The rebound damper absorbs some of this energy (well, more accurately it converts the mechanical energy into thermal energy ie heat), which means that the suspension extends at a slower rate, which means that less energy is transferred to the rider/sprung mass. For this reason, rebound damping (for off-road riding) must be run adequately slow (if you were to only consider this, especially for single bumps rather than repeated stutters etc, the slower the better, however in reality that's not entirely the case). Rebound damping also affects how a bike jumps, springs out of corners, and obviously extends to allow the suspension to react to upcoming bumps. A compromise must be reached between these factors when setting a bike's rebound. Realistically, for bump absorption over rough ground at higher speeds, the rebound does need to be run at a speed which will allow it to extend as much as possible after each bump without bucking the rider or becoming harsh/uncontrollable. Keep in mind that all rebound dampers are, in a sense, position sensitive because the suspension will ALWAYS push hardest at the end/bottom of its stroke (because that's where the most spring force occurs obviously, and spring force is the only thing causing the suspension to try and extend), so the notion of a fork/shock "packing up" (ie not extending enough to allow the next bump to be absorbed, and bogging down further into the travel each time it takes a hit) if the rebound is run too slow is not necessarily reached at a certain "threshold", it gets better or worse on a sliding scale - this means that there isn't necessarily an optimum speed. Obviously rebound should be tailored to its application, for example on a dirtjump bike you would generally want fast rebound for more kick off the lips, whereas on a DH bike you may prefer to make use of a somewhat more stable ride by slowing the rebound down, and trading off the jumping/bunnyhopping ability somewhat. It should also be noted that generally, for safety reasons, it is best to run the rebound of a fork somewhat faster than that of the rear shock - otherwise the bike can have a tendency to try and buck the rider over the bars.
