How does suspension geometry work?

The suspension system keeps your car riding comfortably and handling predictably on the road. And a major contributor to achieving this is the suspension geometry.

The geometry of the suspension is all-important as it dictates how forces are transmitted from the road to the vehicle.

It defines how the wheels move up and down over bumps and it keeps the tyres in contact with the road at the ‘grippiest’ angle while cornering.

The geometry affects how precise your steering is, how comfortably your car takes a pothole and how safe your car is when you have to slam on the brakes on the edge of the road.

It’s one of those things that you feel all of the time, and rely on for your life only once in a blue moon. Well-designed suspension geometry can be a life-saver.

Suspension geometry is made up of two things: kinematics and compliance.

Kinematics is a fancy word describing the mechanics of what is going on. Imagine you made up a match-stick model of your suspension and were able to measure the various angles as your suspension moves up and down.

Now, imagine your match-sticks didn’t break when you applied forces, so you install a spring and put a weight on the chassis side and start measuring the forces.

There you have kinematics – a perfect-world mathematical model of everything that’s going on.

The word ‘compliance’ might make you think of how law abiding you are, but in this case it’s referring to how the suspension flexes when forces are applied.

Most of the flex comes from rubber bushes, which help isolate impact harshness, but some of it comes from the metal components, which flex a little as the forces are applied.

Mix these two together – the drawing-board perfect world of kinematics and the flexible world of compliance – and you have suspension geometry.

Suspension links either rotate or slide. Picture them as if you are looking down on a door from above – like a house plan. Wishbones and links as hinged doors, and struts as sliding doors. Like a plan, the doors are always drawn out showing the way they open.

The real key is to understand the size of the arc you are tracing. A small door gives a small arc, a farm gate gives a big arc.

Suspension design connects different rotating or sliding doors together, and mounts a wheel off the side of them. Connect two hinged doors together with a double-hinged joining piece and you have double wishbone suspension. Connect a hinged door to a sliding door that can pivot and you have a MacPherson strut.

Now you can picture the angles – they depend on where the doors are mounted, and the radius of the arc they trace.

Suspension geometry, like marriage, involves a lot of compromises. Given we are limited by rotating links, and the arcs they trace as they rotate, then adding in compliance of the rubber bushes, it’s fair to say that compromise will be the order of the day.

Figuring out the best compromise depends on what you are trying to achieve – and different suspension architectures have different ways of achieving those ends.

Supercars, like those from the McLaren and Ferrari stables, use double wishbone designs like their Formula 1 siblings. Mass-production cars such as hatchbacks often use MacPherson strut front and twist beam rear suspension. Old Toyota LandCruisers use leaf springs.

The type of suspension chosen for a particular model is typically a compromise between factors like suspension travel, weight, kinematic and compliance targets and production cost.

Once we’ve decided which suspension architecture we’re going with, we need to figure out the geometry. Or said differently, the sizes of the arcs and where they are mounted to get the required kinematics. Add in the rubber bushes and the compliance takes shape.

Engineers follow the rabbit hole down deeper and deeper layers. They look at how camber, castor and toe change as the wheel moves up and down, then technical things such as: roll centre height and migration; steering metrics like Ackermann, scrub radius and uniformity; anti-dive and anti-squat; longitudinal and lateral compliances – the list is long and the compromises plentiful. (See our glossary of terms below)

Specialised software packages are used to design then simulate the pros and cons of the different designs. Once you have a basic design, a prototype vehicle is built with the new suspension design and measured on a Kinematics and Compliance rig to make sure reality correlates with the simulation.

After the test driver’s first drive and despite the engineering teams’ best efforts, there are always problems that are uncovered.

Perhaps a rubber bush is too close to the exhaust and it becomes soft when it heats up, then hard when it ‘snubs out’ under cornering loads.

Perhaps the scrub radius is making the steering too lively when braking on the gravely road edge. Or perhaps the bump stop is making the car unstable when you hit a bump at full tilt around a corner.

Some of these problems can be ironed out with suspension tuning; others will need a complete redesign of the geometry.

Once the suspension is ‘there or thereabouts’, durability considerations come in.

Perhaps the ball joints are making the steering ‘sticky’ when they are new. Perhaps the rubber bushes’ stiffness is degrading, which changes the way the car pitches just enough to start making the occupants feel seasick. Or perhaps one of the ball joints is wearing out too early under high cornering loads.

It’s impossible to get everything right during the design phase in such a complex system, so trial and error comes into play.

So there you have it, suspension geometry 101. At first it was designed just to allow the wheels move up and down over bumps, but over time it has matured into an understanding of how forces from the tyre interact with the car, passengers and, ultimately, the driver.

Camber: The angle the wheels lean in at the top looking from the back. This angle coupled with how the angle increases with body roll (camber gain) means that the tyres have the best grip possible during cornering.

Castor: Like a shopping trolley wheel. This angle makes the front wheels self-centre even with your hands off the steering wheel. It’s a measure of the angle between the upper and lower ball joint looking from the side. This affects jacking, which, combined with Ackermann (below) gives some much-needed pressure on the outer front tyre during a snap oversteer!

Toe: Like a pigeon-toed person, cars often have a slight angle between the front and rear wheels looking from the top. This means the tyres are slightly fighting each other, either pushing each other, or pulling each other when rolling straight ahead. When the car leans into a corner, the outside tyre that will be doing most of the work is already tensed up ready to go. These angles change slightly as the wheel moves up and down (bump and roll steer) and shouldn’t make the car turn harder into the corner as it body rolls.

Roll centre: The pivot point in space the car rotates around during body roll, looking from the back. Important because the distance between this point and the centre of gravity affects how cornering forces affect body roll, which affects the stiffness of springs and anti-roll bars.

Ackermann: Discovered during the horse-and-cart era. During a tight turn, because of the width of the car, the inner front wheel is tracking a small radius than the outer wheel. So the inner wheel should turn tighter. It helps with the snap oversteer described above.

Scrub radius: You draw a line between the top and bottom ball joint all the way to the ground looking from the back. Then measure between the point it touches the ground and the centre of the tyre contact patch. Important, because braking forces push back from the centre of the tyre, and the car reacts from our line’s intersection with the ground. This rotating force (moment) pushes or pulls the steering. That is normally okay because both wheels resist each other, but if the car hits ice on the side of the road while braking, the tyre on the grippy side shouldn’t steer you off the road or into oncoming traffic.

Steering uniformity: The shaft from the steering wheel to the rack has to negotiate some angles so uses Hooke’s or universal joints. The steering velocity at the rack relative to the steering wheel is affected by these angles and the relative angles of the yokes of the joints… Who’d have thought!

Anti-dive and anti-squat: The antis are set by the angle of the wishbone inner mounting points looking from the side. When you accelerate or brake these jack up the front or the rear. Useful because it means you can run a softer spring and not have too much pitch.

Lateral compliance: How much the wheel centre moves in and out during cornering (both directions).

Longitudinal compliance: How much the wheel centre moves forwards and backwards during accelerating and braking.

Snubbing: When the rubber in a bush gets squashed between a rock and a hard place. In other words, the force curve of the bush goes exponential, so with enough cornering force the bush goes rock hard.

Related: How does a shock absorber work?