Springs and dampers
Suspension works by absorbing energy from various inputs. On a bike this is bumps from the ground and movement from the rider.
To understand suspension fully, firstly we need to look at the different types of inputs and the terminologies used to describe them.
Velocity refers to how fast the suspension unit travels when it is compressed. This usually depends on how fast the bike is travelling or how hard the impact from the bump was. If a bike travelling at 10 mph hits a bump of 1 inch you can appreciate the velocity of the suspension is going to be lower compared to if the bike where travelling at 40 mph.
Velocity is split into high speed and low speed inputs. A high speed input is generally agreed in the suspenion world as being 4" per second and is generally caused by bumps and hits from terrain.
Low speed is generally considerd to be the acting force of the unsprung mass including the rider so any inputs from the weight of the bike and movement of the rider.
We can then split high and low speed inputs into four catagories, High and low speed compresion and high and low speed rebound.
High and low speed compression is effected by bumps and rider input. High and low speed rebound is effected by unsprung mass, spring rate and travel postion. (how far the suspension has moved through its stroke)
We have already covered high speed and low speed compression but high and low speed rebound is slightly harder to figure out.
A high speed rebound input is casued by the spring being sufficatnly compressed so that it rebounds with a speed over 4" per second. This will generally only happen if the shock has recived a high speed compresion input.
Low speed rebound is generally cased by pedal induced forces or a weight shift of the rider.
A Histogram like the graph above shows us how long a suspension system stays in a high or low speed input and allows the user to tune in or out suspension characteristics through steup and tuning. Without data like this its very difficault to get shock tune corect for every rider or frame leverage.
Weight and mass
On a full suspension bike we have weight that is described as sprung mass and weight that is described as unsprung mass.
Sprung mass can be defined as any weight supported by the springs. So the front triangle, saddle, handlebars, rider etc etc.
Un-sprung mass can be defined as any weight not supported by the springs. The rear triangle, the fork lowers, wheels, tyres, disc brakes etc etc..
When designing bikes manufacturers take into account how much sprung mass is on the bike as this ultimately translates to how the bike feels to ride and why some bikes feel heavier than others even though on paper they are the same weight.
Springs can be split into two groups. Air and coil.
Air springs are simply a volume of air trapped in a sealed unit. This unit is then able to be compressed squashing the trapped air inside.
There are many advantages that make air springs very useful on mountain bikes. One is that they are very light compromising of only air, pistons and seals.
Also air springs are naturally progressive. Meaning you need more and more force to compress them as your run through the springs travel.
In fact if you half the volume of an air spring you double the pressure and there for increase the force required to compress it further half creating a "rampy" feel.
With this in mind you can appreciate that if we had a shock that has 3” of travel (technically named stroke) with a pressure of 275psi in it, every time we half the volume the pressure increases so by the time the air spring is “bottomed out” the pressure would be at 1100psi. This causes massive stresses on the seals and is one of the only down sides to air springs.
For this reason it is very difficult to get an air shock to work properly on a bike with allot of travel. Over 7” of travel air shocks start to struggle with pressures and will “blow” seals and “swap air” and generally cause you all sorts of woes.
To counteract this we can simply increase the overall volume of the shock. This means we can run a lower starting pressure meaning a lower finishing pressure when the shock is at full travel meaning less stress on all the components involved. To explain fully its worth taking a look at monster trucks! who doesn't like monster trucks?
A monster truck tyre has a huge volume and are generally pumped up with a very low psi yet they feel incredibly hard and support the weight of the truck and driver yet a road bike tyre that has a very low volume has to have a much higher pressure to carry the weight of the bike and rider.
Another issue that can affect an air shock with a large stroke is heat. The more active the shock is the hotter it will get and the more everything will expand. This has repercussions not only for holding the air in but also against the consistency of the damping as the viscosity of the fluid can change and components can become tight. We get around this by using heat sinks and “piggy backs” to increase the surface area of the shock which allows the heat to dissipate away and keep the shock cool.
The terms negative air and positive air refer to separate air chambers being used to support the compression and rebound strokes of the suspension unit. This can be found on Rockshox dual air suspension systems and is more common place in forks rather than rear shocks due to the extra space needed to make the system work.
A solo air system simply uses a piston that either checks sideways or has a small hole in it to regulate pressures in the spring for compression or rebound strokes.
Coil springs are wound lengths of steel or titanium and are used on many different types of bikes because of their range of properties and relative in-expense.
If we looked at a bike coil spring we would find two sets of numbers printed on it. These are important as they ultimately tell us weather that spring will fit our bike or not.
An example of these markings would be;
450lb x 2.75 350lb x 2.50
The first number is rated in lb and is the spring weight. The second number is the shock stroke.
The way the spring weight is measured is by calculating how much force is needed to compress that spring by 1”. So our example spring needs 350lbs of pressure to compress it 1”, the higher the number the stiffer the spring.
Coil springs are what we would term as a linier spring. This means that the amount of force needed to compress the spring by a further inch would be another 350lb and would remain at 350lb increments throughout the stroke of the shock Until the spring is “coil bound”.
“Coil bound” is term used to explain when the coils of the spring touch each other and the spring can no longer compress. Damage can occur to the spring when it becomes coil bound and in bicycle terms you will feel a harsh bottom out through the bike.
Progressively wound coil springs are made from coiling a tapered rod into a spring shape so that you have different size coils through the length of the spring. They work on the principle of “active coils”. The smaller coils become coil bound before the bigger ones so they become inactive quicker making the spring feel progressive.
The second number on our spring refers to the shock stroke.
The shock stroke is the distance that the shock piston travels from its starting position to its bottomed out position. This simply measured as per the picture below.
This measurement is important as it determines how far apart the coils are and also how thick the wire used to make the spring is. Ideally we want the shock to bottom out before the spring becomes coil bound as the shock is damped and you can damage the coil if this is wrong.
This has all been taken into account when the manufacturer winds the spring.
As titanium is a lighter metal than steel we find we have to use more of it in springs to make the coils strong enough to cope with the intended forces. This means using thicker coils. The thicker the wire used the further apart you have to wind the coils to prevent coil binding. This is why ti springs has wider coil windings.
Further factors to take into account are deflection and spring tension.
Springs will naturally want to deflect outwards and this is more prevalent with longer springs, and in forks you will find that the coils run a small rubber boot around them so when it does deflect outwards it doesn’t tap on the inside of the station. These rubber boots are called ISO boots.
A spring, like a thread or a helix is a spiral and will naturally want to twist when compressed. Some spring manufacturers are now producing bearings that fit either side of the spring to reduce any tension that may build up in it as its compressed. This can give a much plusher, smoother feeling spring.
Front fork springs are incredibly specific and generally come specked to be used in one fork only and are usually available in soft, medium, hard etc.
Suspension dampers are used to control the speed of either the compression or the rebound stroke of the unit.
They use the laws of hydraulics and displacement to achieve this.
A damper is basically a piston that runs through fluid. The greater the surface area of that piston the bigger the effect will be on displacing the fluid it’s running through.
If there is no way for the fluid to flow through the piston you will simply gain a hydraulic lock effect and the piston won’t move “locking out” the system.
Here is a picture of a basic suspension piston. The cut outs determine how the oil flows through the piston.
If you cover the holes in the piston you increase the surface area, which makes it harder for the fluid to flow through it. A simple way of doing this is by having an offset rotatable disc that sits under the piston. By rotating the disc it closes more and more of the holes and eventually covers them all locking the system out completely. This is the basis of lock out in most suspension systems.
This however is a very basic way of controlling fluid. To get a more fine tuned system we use shims to cover the holes. Shims are very thin discs of various sizes that flex out of the way of the oil when under pressure.
Here is a picture of a piston with a very basic “shim stack” on top.
We can arrange these shims to either be very stiff giving a very slow feeling system or we can arrange them to flex a lot giving us a fast system. They are usually arranged in stacks and are very difficult to build and tune properly.
Velocity sensitive damping refers to a system that reacts to different rates of input. Sometimes refered to as dual flow damping. These are piston systems that have both a needle and shim control system giving the system the ability t control both high speed and low speed inputs on the same piston.
The blow off valve shim stack is set so that if it is given a high velocity impact of a set force a spring that sits ontop of the shim stack will compress allowing the shims to come away from the piston head and allowing the oil to flow quicker. The amount of force needed to do this is set by a” pre load” adjuster which is in most cases your high speed adjuster.
You can see these springs in place in the picture below:
Another type of damping system commonly used in forks and shocks is needle and orifice.
Mainly used for rebound dampers, it is simply a port hole in the piston tube where the fluid flows in and out of the top of the piston head. In the middle of the piston tube is an adjustable needle that can be adjusted via the rebound dial to cover more and more of the hole as its adjusted. The more the port hole is obstructed by the needle the firmer the damping will be.
Cavitation is the process of small bubbles forming in a fluid due to the fluid being exposed to a drop in pressure. This happens mostly around the damper piston as it runs through the fluid at high speed "working" the fluid as it goes and creating low pressure areas where tiny bubles are able to expand and colate into larger bubbles casuing inconsitant damper characteristcs.
Cavitation is the main cause of damper performance loss. Old fluid looses its ability to cope with cavitation and fluid should be repalced frequently and should be veiwed as a perishable item.
Some manafactures have pioneered systems where cavitaion doesn’t occur or is at least reduced. They have done this by re designing the dampers so there is no air in them to begin with meaning there is les chance of the fluid to cavitate as the pressures near the surface of a fluid are much lower. Get rid of the surface of the fluid and you get rid of 90% of the cvavitation. Comine this with pressure applied to the fluid and you further reduce the chance of bubbles forming, hence the need for nitrogen pressure in rear shocks.
These systems are called cartridge or closed damper systems
Having a closed system however does come with some issues in that at high velocity the fluid cant get out of the way quick enough so a compensator system is put in place. In mst systems this is seen as a bladder that can expand as the damper displaces fluid throughout its stroke.Rear shocks use an IFP (internal floating piston) to create the compensaition.
There are two other types of damping system, “open bath” and “semi open bath”.
An “open bath damper is simply a damper that “smashes into a volume of oil usually stored in the fork lowers, and requires a gap of air above the fluid for the fork to move up and down.
“Semi open bath” basically store the oil and the damper inside the “stanchions” of the fork but still require an air gap for the fork to operate.
Both of these systems are susceptible to hydrodynamic surface cavitation.