All-terrain vehicle (ATV) is also known as quadricycle & is meant to handle a large variety of terrains. The suspension system of ATV is typically a double wishbone system which is suitable for achieving a lower center of gravity for different sports events viz., Baja SAE. The double wishbone is equipped with shock absorber which is either a coil over shocks or an Air shocks.
Coil over shocks uses helical coil springs to withstand the loads whereas air shocks use an enclosed air chamber which sustains the entire load coming on the wheels. The damping system is necessarily a hydraulic piston-cylinder arrangement with pressurized nitrogen in both these shock absorbers.
Coil over shocks constitutes helical coil spring of single or multiple rates fixed with a specified preload over the shock absorber. Coil over shocks also generally uses bump stopper to take the higher end loads to avoid metal to the metal touching condition. The spring wire size of coil springs depends on the amount of load on the shock absorber.
Air shocks constitute air chamber typically at a pressure in the range of 2-20 Bar. The air pressure in air shocks increases with the shock absorber stroke due to air compression & hence the load carrying capacity also increases.
Load characteristics: The typical load characteristic of coil over shocks & air shocks is shown below. The graph depicts that air shocks generate less load at small shock absorber stroke due to large air volume being compressed & much higher loads near the end of the stroke when the air is compressed to higher pressures in a small volume. Air shocks can be made to withstand higher loads at small shock absorber strokes but this will mean increasing the air chamber initial pressure which results in abnormally higher loads at full suspension travel. These abnormally higher loads at full suspension travel results in non-utilization of available suspension stroke which is not recommended for good suspension performance. The coil over shocks generate sufficiently higher loads at smaller shock absorber stroke & adequate end load at full stroke which is separately controlled by bump stoppers.
Pros of coil over shocks:
- Ride height/Rider sag can be adjusted independent of spring rate using spring preload adjustment
- Required spring stiffness is maintained by the coil spring which can be achieved with no compromise on ride height and end loads
- Complete shock absorber stroke is utilized for offering suspension function
- No change in spring stiffness due to higher damper temperatures in extreme endurance conditions
Cons of coil over shocks:
- Comparatively higher weight than air shocks
- Spring stiffness cannot be modified unless coil spring is replaced
- Bigger diameter than Air shocks
Pros of Air shocks:
- Less weight than coil over shocks
- Small diameter than coil over shocks which helps in compact packaging inside ATV
- Spring stiffness can be modified by simply changing air pressure which can be done even without removing shocks from ATV
Cons of Air shocks:
- Ride height/Rider sag cannot be adjusted independent of spring rate. For any change in ride height there is impact on spring rate
- Spring rate cannot be adjusted without impact on ride height.
- Typically for achieving 25-30% rider sag the end loads are achieved much higher resulting in non utilization of complete shock absorber stroke
- Under continuous driving conditions the temperature increases resulting in higher spring stiffness due to higher air pressures generated due to air expansion inside the air chamber.
The final choice between coil over and air shocks for ATV depends on the application of ATV & the expected performance level from ATV.