Anti-squat, anti-dive and anti-lift geometry which will be referred to as “anti-geometry” when discussing them as a whole is a form of geometry at the front and rear wheels that alters and controls the amount that a car will compress the springs due to acceleration, deceleration or braking conditions. A common misconception is that anti-geometry controls or affects wheel loading where more squat on accelerating will give more rear wheel loading. In reality the wheel loading remains the same regardless of anti-geometry as the total longitudinal load transfer under steady state acceleration or braking is a function of the wheel base, braking or acceleration force and centre of gravity height. What anti-geometry does achieve is the amount of load going through the springs and the pitch attitude of the car.
Before describing what each in particular anti-geometry is an understanding of how a car moves and rotates under acceleration or braking conditions must be achieved. All forces acting upon a car act through the centre of gravity. The centre of gravity is therefore the centre of rotation for any acceleration or braking inputs. When braking, due to linear inertia changes, the car will rotate forwards around the centre of gravity, therefore lifting behind the centre of gravity towards the rear and lowering the car in the area in front of the centre of gravity. The opposite occurs for acceleration. The diagrams below show the direction of rotation about the centre of gravity for braking and accelerating conditions.
In order to prevent this rotation from happening or to limit the amount of rotation an opposing force needs to be applied to the centre of gravity. This opposing force comes in the form of anti-geometry. In order to measure your anti geometry, the side view swing arm for your front and rear suspension systems must first be established. The article below will focus on cars with the much more common outboard brakes set up where the brakes are located with the wheels as opposed to be mounting inboard of the axle. The diagrams below show how to measure the side view swing arm for a double wishbone/ A arm set up and for a Macpherson strut set up.
Double Wishbone and A- Arm Suspension
The double wishbone and A-arm set up is the most common suspension layout on racing cars. In order to generate the side view swing arm a few steps must be taken. First of all a line must be drawn through the two mounting points of the front upper wishbone towards the back of the car. Next another line needs drawing that passes through the front lower wishbone mounting points towards the back of the car. These two lines can be seen in blue on the below diagram. Where these two lines intersect is the instant centre (shown by the red circle below). Finally a line needs to be drawn from the front instant centre to the centre of the front wheel contact patch (shown by the red line below). This is the side view swing arm for the front suspension. The same needs to be repeated for the rear suspension passing through the rear mounting points towards the front of the car until you have your front and rear side view swing arms.
MacPherson Strut Suspension
The Macpherson strut set up is more common for the front suspension on road cars that have turned into race cars. The side view swing arms are created slightly differently than on a wishbone set up. First of all a line needs tracing from the centre of the damper top mount at an angle perpendicular to the damper body towards the centre of the car. This can be done for the front and rear as shown below. Next, a line needs tracing through the chassis mounting points for the lower arm towards the centre of the car. These lines are shown below in blue. Their point of intersection is the instant centre (shown by the red circle). Finally a line needs drawing from the front instant centre to the centre point of the front wheel contact patch. A line also needs drawing from the rear instant centre to the centre of the rear wheel contact patch. These lines are shown in red below and make the front and rear side view swing arms.
Anti-squat geometry features on the rear wheels as they are the only wheels that will want to squat during acceleration due to the rotation of the car. Anti-squat limits the amount of compression or vertical displacement of the rear wheels due to the acceleration of the car. Anti-squat geometry is a function of the control arm geometry at the rear wheels. The amount of anti squat present is determined by a few factors such as the side view swing arm length and height, wheelbase and centre of gravity height. The diagram below shows the required dimensions on a car that has had its side view swing arm drawn.
If the car has 100% anti-squat then no compression of the rear suspension due to acceleration forces will occur at all. If the percentage is lower than 100% then some compression will occur due to acceleration forces and will increase as the percentage gets smaller. The percentage of anti-squat can be calculated using the below equation:
Anti-dive geometry is effectively anti-squat for the front wheels under braking conditions. When a car is under braking conditions, the braking force acts directly through the centre of gravity and causes the car to rotate. Anti-dive geometry prevents the car from diving on the brakes and stops the front wheels from deflecting vertically or going into bump due to braking conditions. Similarly to anti-squat geometry, the percentage of anti-dive that the car has is based upon side view swing arm length and height, wheelbase and centre of gravity height. However, anti-dive is also based on the percentage of front braking effort also known as the brake bias. This is often a 60/40 split or something similar on racing cars with 60% braking effort going to the front wheels. The diagram below shows the required dimensions on a car that has had its side view swing arm drawn.
If the car has 100% anti-dive then no compression of the front suspension due to braking forces will occur at all. If the percentage is lower than 100% then some compression will occur due to braking forces and will increase as the percentage gets smaller. The percentage of anti-dive can be calculated using the below equation:
Anti-lift geometry is used mainly on front wheel drive cars and four wheel drive cars. It works when an acceleration force is acting through the centre of gravity causing the front wheels to raise as the rear tries to squat due to the acceleration force acting on the centre of gravity causing the car to pivot. The effect of lifting the front wheels under acceleration in a front wheel drive or four wheel drive car is that traction will be lost at the front wheels which could cause the front wheels to spin instead of applying full traction to the surface. Therefore on power understeer, such as at corner exit, is reduced.
Anti-lift geometry is in the front suspension and also uses the side view swing arm to calculate the amount of anti lift installed in the system. The side view swing arm for the anti-lift in the front is different to the above diagrams. For anti lift the side view swing arm must be drawn from the instant centre to the centre of the front wheel. Also, less input data is required as once the side view swing arm has been drawn, its angle from horizontal must be determined in degrees along with the forward thrust being put through the front wheels due to acceleration and the anti lift equation can be fulfilled. The diagram below shows the measurements required.
In order to calculate the percentage amount of anti-lift that the front suspension geometry has, the below equation can be used:
Anti-lift is also used on the rear wheels for braking conditions to prevent the rear of the car from rising under heavy braking, reducing the amount of braking effort that the rear wheels would be able to provide. The diagram for rear anti lift geometry is more complex than for the front and requires a few more inputs. It is based upon side view swing arm length and height, wheelbase and centre of gravity height from the ground. Alternatively to the front, the percentage of rear braking must be used for this equation. So you if you have a 60/40 split the decimal percentage of 0.4 will be used in the below equation instead. The diagram below shows all the required measurements.
In order to calculate the percentage amount of anti-lift that the rear suspension geometry has, the below equation can be used:
The Benefits of Anti-Geometry
All of the anti-geometries are most at home being used on aero cars; particularly aero cars that utilise underbody aerodynamics such as venturi cars. This is because the angle of the underbody was very carefully designed to produce maximum downforce at the set angle. Therefore any changes in the height of the floor pan front to rear would disrupt downforce and therefore massively reduce the grip of the car. With 100% anti-geometry installed it meant that on throttle or brakes the bottom of the car would not pivot and therefore maximum aerodynamic grip was available from the car.
A reason it is useful on non-aerodynamic cars is where racing cars have been made from what was originally a road car. This is due to road car geometry often containing a lot of camber gain in bump. Therefore, when power is applied if the rear of the car goes into compression, the rear camber will increase causing contact patch and grip levels to reduce. This is where anti-squat geometry helps to prevent camber on throttle providing more even levels of grip when on power. Camber gain is often present at the front wheels as well so anti-dive geometry works in the same favour under braking conditions to keep the contact patch as flat as possible and maximise braking effort.
Another reason anti-geometry is useful on all racing cars is due to the fact that most race cars have a very low ride height and have an increased chance of bottoming out on circuit. If a car bottoms out on circuit then the suspension suddenly becomes void, the car is slowed down by the increased friction and damage can occur to the underside of the car and any underbody aero. This is where anti-squat and anti-dive geometry come in. With some anti-squat and anti-dive geometry installed it provides a limiting factor to how much the car can physically dive or squat in accelerating or braking conditions making it very difficult for the car to bottom out.
A benefit of anti-lift geometry is that it will keep the front suspension down under acceleration conditions which are beneficial for a front wheel drive or four wheel drive racing car. It is well known for its use on the Subaru rally cars leading to them having improved corner exit speed over their competitors. This is due to when power is applied after the apex, the anti-lift geometry will keep the nose down on the car, keeping the front driven wheels in contact with the ground allowing them to apply more power without causing the front wheels to spin. Therefore, corner exit understeer is reducing allowing the car to apply the power earlier on corner exit.
The Negatives of Anti-Geometry
The main drawback to anti-geometry is driver feedback. When a driver applies the brakes in a car they expect the front of the car to dip down. The more severe the dip, the harder they are on the brakes. When they apply the throttle they would also expect the back of the car to sit down slightly. The more the back sits down, the harder they are accelerating. Anti-geometry removes this sensation and provides the driver will very little dynamic feedback under these conditions leaving them feeling only the G forces from their actions. This can be difficult to drive and also difficult to anticipate reaching the limits of grip during braking and acceleration causing an unsettled car.
Therefore it is often best to design some squat and some dive back into the suspension system and not run 100% anti-geometry unless the aero dynamics dictate so. Putting closer to 80% in the car will still provide minimal movement to acquire most of the benefits of having the geometry but will also provide the driver with the feedback they need to drive the car to its full potential.
What if I Have Over 100% Anti-Geometry?
Having over 100% anti dive geometry can cause the opposite effect to happen and can cause jacking. A common feature of having over 100% anti-dive is wheel hopping during braking conditions. Likewise, having over 100% anti-squat can cause the rear to lift instead of squat during acceleration which feels very odd to the driver and can cause poor driving and unloading of wheels at the wrong times resulting in a very poor handing car.
Some cars that have a lot of aerodynamic downforce have over 100% anti squat when static. This is because once the car is up to operating speed, the downforce being provided by the aero has deflected the geometry to a new position where the anti-squat and anti-dive percentages have moved below 100% and if they were not set above to begin with then the car would not have enough anti-geometry installed when the car was moving.