Suspensions play a vital role in improving ride quality and ride comfort. It connects the vehicle’s body to the tires and transfers all the forces between the vehicle’s body and the road.
The suspension system of a vehicle refers to the group of mechanical components that connect the wheels to the frame or body which include a system of springs, shock absorbers and linkages. Suspensions serve a dual purpose in a motorcycle i.e. contributing to the bike’s handling and braking. It also protects the vehicle and the luggage inside it from damage and wear.
- Suspension systems can be characterized into 3 groups: passive, semi-active and active.
The passive suspension system limits the motion of the body and wheel by limiting their relative velocities to a rate that gives the required ride comfort. This is achieved by using some type of damping element placed between the body and the wheels of the vehicle, such as hydraulic shock absorber. Their performance is limited and can only be changed by changing the characteristics of dampers and springs.
Semi-active systems can only vary the viscous damping co-efficient of shock absorber and do not add energy to the suspension system. They require less energy to function and are less expensive. These systems are designed to dissipate energy in a controlled manner by changing the damping, but there is no way to add energy to the system with this suspension technique.
An active suspension system consists of springs, dampers and force actuators which dissipate or add energy to the system in a controlled way. These systems provide better cooperation road handling and ride quality in which the sensor continuously monitors the operating conditions.
Purpose of suspension system
The suspension system of a motorcycle primarily has two aims: to handle the stability and keep the balance during braking and secondly, to offer comfort to the rider and pillion during bumps and uneven surfaces.
- Basically there are 3 main parameters that affect the suspension performance.
- Sprung and unsprung mass
For our purposes the most important characteristic of a spring is its “rate”. This is a measure of its stiffness and is determined by measuring the extra force needed to compress (or extend in some cases) the spring by a given small amount. This can be expressed in N/mm (kN/m gives the same numerical values).
Types of Springs
- Coil spring
- Leaf spring
- Air spring
- Torsion bar
- Little to no internal damping
- Coil springs in steel are the most common by a long way
- They may be evenly wound (constant pitch) to give a linear rate, or they may be wound with a varying pitch to give a progressive rate. In this case as the spring is gradually compressed, the closer wound coils become “coil-bound” (i.e. touch each other and act as a solid mass) and so the rate rises.
- Low cost
- Compact Size and used in many suspension types
- Used in many early applications
- Internal friction provides damping
- Provides Lateral location for the axle
- Prone to weaken overtime
AIR SPRING/GAS SPRING
- Compressor pumps the air into a flexible bellows
- Uses pressurized air
- Air pressure inflates the bellows
- Air shocks have the advantage of easy adjustment to compensate for different loads on the bike. If a passenger and luggage for a trip, doubles the load on the back end, then just double the initial gas pressure.
- The bar resists twisting action an acts like a conventional spring
- One end is attached to the frame and the other to the lower control arm.
- Up and down of the suspension system twists the torsion bar.
- It will then try to return to its original shape, moving the control arm to its original place.
- Light in weight.
- Less space occupies and ride comfort is more.
A damper (shock absorber) is simply an energy absorber. It keeps the suspension from continuing to bounce after spring compression and extension. Limits spring compression-extension movements to smooth the vehicle’s ride.
Imagine that a large bump has fully compressed a suspension strut, at that instant, energy is stored in the spring as potential energy. As the spring returns to its static length it gives up this energy, which if there were no damping, would be transferred entirely to the mass of the bike in the form of kinetic energy (energy of motion).Without dampers(shock absorbers), the vehicle would continue to bounce up and down long after striking dip or hump in the road.
Sprung and Unsprung mass
Sprung mass is the mass that is sitting on top of the suspension. For a motorcycle this is essentially all of the bike’s mass less wheels, brakes and part of the suspension components.
The unsprung mass is therefore the whole mass minus the sprung mass, and so comprises mainly of the wheels, tire, brake discs, calipers, axle, and lower fork sliders.
Pre – load adjustment
Motorcycle suspensions are designed so that the springs are always under compression, even when fully extended. Pre-load is used to adjust the initial position of the suspension with the weight of the motorcycle and rider acting on it.
The difference between the fully extended length of the suspension and the length compressed by the weight of the motorcycle and rider is called total sag. Total sag is set to optimize the initial position of the suspension to avoid bottoming out or topping out under normal riding conditions. “Bottoming out” occurs when the suspension is compressed to the point where it mechanically cannot compress any more. Topping out occurs when the suspension extends fully and cannot mechanically extend any more. Increasing pre-load increases the initial force on the spring thereby reducing total sag. Decreasing pre-load decreases the initial force in the spring thereby increasing total sag.
The total travel of a suspension system is the distance the suspension travels between total compression and total extension. The travel distance on off-road and dual-purpose bikes tends to be very high; the rear suspension travel on cruisers tends to be relatively little.
Fork Dive/Brake Dive
When you apply the brakes on a moving motorcycle, the weight transfers to the front wheel, thus compressing the suspension. When stopping, a motorcycle equipped with telescopic forks adds weight to the front wheel, transmitted through the fork. This transfer of weight to the front wheel compresses the fork, changing its length. This shortening of the fork causes the front end of the bike to move lower, and this is called fork dive or brake dive.