Tuning air suspension

One of the outstanding advantages of air suspension is that the suspension is easily configurable to achieve different outcomes in both ride and handling. This article looks both at on-the-go changes and also suspension set up.

Maintaining constant ride height with varying loads

Maintaining a constant ride height is achieved by adding air to the springs when the load is greater, and removing air from springs when the load is reduced. It is most easily achieved by automatic self-levelling using suspension height sensors and an electronic control system.

Altering spring rate

Altering spring rate can be achieved by using a solenoid valve to connect the spring to an extra volume (an accumulator). Having switchable extra volumes allows the spring rate to be altered to suit the driving conditions. For example, to stiffen the spring when driving fast, the solenoid disconnects the extra volume. When a softer spring rate is required, the solenoid opens.

Another way of altering spring rate is to interconnect the springs at one end of the car. Because most road bumps are one-wheel bumps, taking this approach softens the ride because when one wheel rises over a bump, the air can flow to the other spring’s volume. For this to work, the air must again be able to flow freely – that is, the interconnecting line must be sufficiently large. Note that in taking this approach, body roll will increase (sometimes dramatically so!) and so a solenoid may be used that blocks this interconnection when cornering or when spirited driving is being undertaken.

Altering ride height

Sometimes it can be useful to be able to manually (or automatically) change ride height to suit the conditions. For example, if the driver has a manual ride height control, they can choose to lower the car when driving fast on smooth surfaces, so lowering the centre of gravity and improving handling. (Less bump travel is needed on smooth surfaces, so this lowering doesn’t cause an issue.) If best fuel economy is desired, lowering the car will reduce total aerodynamic drag. Conversely, if entering a steep driveway or passing over rough ground, the driver may wish to increase ride height.

Changing handling balance

Handling balance is most dramatically altered by changing roll stiffness at one end of the car. For example, if the car understeers (the front runs wide), increasing roll stiffness at the rear will normally reduce it. If the car oversteers (the rear runs wide), increasing roll stiffness at the front will again usually reduce it.

In air suspension systems, the roll stiffness of the car can be changed in two ways. Firstly, you can stiffen the spring rate by changing the effective volume of the spring, as described above. If the spring rate is stiffened at only one end of the car, it will increase roll stiffness at that end. This will have a similar effect to stiffening the anti-roll bar at that end of the car. Secondly, if you inter-connect the springs at one end of the car, roll stiffness at that end will be reduced.

You might ask when you would actually want to alter handling balance – surely, you’d just set the suspension up for the best outcome and leave it configured in that way? One very good application is when the road is slippery e.g. it is wet or a loose surface. For best stability, traction and grip in these conditions, the suspension works better if roll stiffness is reduced. So having a solenoid-controlled inter-connection laterally between the springs can help achieve this change – all at the flick of a switch.

Ride and handling overview

It can be easy to get a little lost when setting up suspension – especially air suspension that has more variables than conventional suspension. Here’s an overview guide to setting up an air suspension car to have good ride and handling.

Excessive Oversteer

• Change rear tyres – wider, softer, lower profile, higher tyre pressures
• Increase rear toe-in
• Increase rear negative camber
• Stiffer front anti-roll bar
• Stiffer front springs – smaller volume
• Stiffer front shock absorbers, especially in bump
• Soften or remove* rear anti-roll bar (* if the suspension design allows this)
• Soften rear springs – add extra volume that works with spring
• Interconnect rear springs
• Soften rear shock absorbers, especially in bump
• Change driving style

Excessive Understeer

• Change front tyres – wider, softer, lower profile, higher tyre pressures
• Increase front negative camber
• Increase front positive castor
• Increase rear toe-out
• Stiffer rear anti-roll bar
• Stiffer rear springs – smaller volume
• Stiffer rear shock absorbers, especially in bump
• Soften or remove* front anti-roll bar (* if the suspension design allows this)
• Soften front springs - add extra volume that works with spring
• Interconnect front springs
• Soften front shock absorbers, especially in bump

Poor ride

If too soft:

• Stiffen dampers (especially in rebound)
• Reduce spring volumes
• Increase tyre pressures

If too hard:

• Soften dampers, especially in bump
• Increase spring volume – add extra volume that works with spring
• Interconnect springs (e.g. at the front, or at the back)
• Raise ride height to ensure bump stops not being impacted
• Reduce tyre pressures

Too much pitch:

• Ensure rear springs are higher in natural frequency than front springs
• Increase damping

Sideways head jerks:

• Soften or remove* anti-roll bars (* if the suspension design allows this)

This article is extracted from my book Custom Air Suspension.