Any vehicle which is accelerating or braking will experience a longitudinal force at its centre of gravity. This force will case the vehicle to pitch and is mainly reacted by the suspension springs and dampers. The layout of the links in a suspension system can also be used to modify the amount pitch a vehicle experiences for a given braking or traction force. This mechanism is referred to as the suspension anti-properties. Suspension anti-properties is an umbrella term for the following key performance indicators:
- Front suspension Anti-Dive under braking
- Front suspension Anti-Lift under traction
- Rear suspension Anti-Lift under braking
- Rear suspension Anti-Squat under traction
This post will focus on front suspension Anti-Dive and Anti-Lift, but the same principles apply to the rear suspension.
Consider a vehicle in a steady state braking scenario. The tyres apply a rearward force on the vehicle at the contact patch and the vehicle body experiences an inertia force which acts through its centre of gravity. This force at the centre of gravity causes a load transfer from the rear suspension to the front suspension of the vehicle. Therefore, the effective weight of the front axle increases, and the weight of the rear axle decreases. For a suspension with no anti-properties, this load transfer is reacted only by the springs and will cause the body of the vehicle to pitch forward or ‘Dive’.
A front suspension with anti-properties (i,e, anti-dive in this case) does not change the amount of load transfer under braking (The load transfer is a function of braking force, centre of gravity height and wheelbase). What anti-dive does is change the amount of load going through the springs and as a result, the pitch of the vehicle. It does this by translating some of the longitudinal braking force into a positive vertical force in the suspension. This vertical force component reduces the amount of the weight transfer which needs to be reacted by the springs.
The same is true for steady state acceleration and front suspension anti-lift. In this case the acceleration force on the vehicle centre of gravity is towards the rear of the vehicle and causes a front to rear weight transfer. The anti-lift in the suspension generates a negative vertical force on the suspension which helps to reduce the front suspension lift and as a result the vehicle pitch.
The amount of Anti-Dive or Anti-Lift in a suspension system is controlled by the position of the castor pole relative to wheel centre (see the post on bump castor for an explanation of the castor pole). The anti-dive and anti-lift angles are shown for a front suspension below. In RACE the anti-properties are reported in Degrees and also as Newtons of vertical force per Newton of longitudinal force, N/N.
How much anti-properties a vehicle requires very much depends on the mass properties, wheelbase and tune of the vehicle. Consider the case of a production vehicle, tuned for ride comfort (so low wheel rates), with a high centre of gravity, and short wheelbase (i.e. Compact SUVs). Anti-properties in such a vehicle are important to control the pitch of the vehicle under acceleration and braking. For this type of vehicle, the weight transfer is large and if this is coupled with soft wheel rates, the vehicle will see a lot of pitch before the reaction force from the springs is large enough to balance the weight transfer. This would not be comfortable or reassuring for the vehicle passengers. Vehicles of this type would have somewhere between 5-10 Deg anti-dive on the front axle and 15-30 Deg anti-lift on the rear axle. Production saloon cars would be closer to the lower end of these ranges.
At the other end of the scale, consider a stiffly sprung, low centre of gravity racing car. In this scenario the weight transfer will be much lower than the SUV case due to the low centre of gravity and any weight transfer that does happen will be easily reacted by the stiff wheel rates with little pitching. For this reason, most race cars would have little or no anti-properties in the suspension design (i.e. close to 0 Degrees).
Another less obvious element of the pitch behaviour of a vehicle is the driver perception of braking. Some level of dive under braking is pleasing to a driver. It gives a level of feedback that the brakes are doing their job. A vehicle with zero pitch under braking (i.e. 100% anti-dive) may feel unusual to a driver.
As with many suspension KPIs, the levels of anti-properties in a suspension system must be balanced versus the impact on linked KPIs. Linked KPIs are KPIs which are difficult to tune independently. In the case of Anti-Properties, they are strongly linked with bump castor and kinematic wheel centre recession. All of these KPIs depend on the position of the castor pole relative to wheel centre/tyre contact patch. This topic will be covered in the ‘Positioning the Castor Pole’ article in the Suspension Design Library Tuning Series.