The detailed braking requirements are specified in ADR 31/02, which will come into effect for LEP vehicles on 1 November 2011. ADR 31/02 is based on UNECE R13-H.
The basic braking requirements are:
- service braking must be able to stop the car with a mean deceleration of 6.43 ms-2, and brake pedal force must be less than 500 N
- the service braking system must act on all wheels, and act equally on each side
- secondary braking must be able to stop the car with a mean deceleration of 2.44 ms-2
- the parking brake must hold the vehicle on a 20% gradient.
There are additional requirements related to performance from specific speeds and brake fade. The required braking performance must be met without using regenerative braking.
The UniSA prototype used go-kart hydraulic disk brakes with calipers that would retract to reduce brake drag when the brakes were released. These were not successful:
- the pads had to move too far to reach the disks, which required too much movement of the master cylinder and of the brake pedal
- the pads had to be shimmed to compensate for pad wear.
The brake system should not require vacuum assist; adding a vacuum unit will add to the mass and complexity of the car.
Wilwood make a self-adjusting, retracting Kart / Jr Dragster calliper that may be suitable. Wilwood also make cylinders and pedal assemblies.
The rear brakes could be hydraulic disk, drums, or perhaps even the Willwood Stealth perimeter brake.
Regenerative braking uses the motor as a generator. Current flows from the motor into the battery; the motor torque required to generate the current slows the car.
Regenerative braking is easy to implement with brushless motors, since the controllers already contain the power switching devices necessary for bi-directional current flow in each motor winding. With other types of motors, special controllers are required to support regenerative braking.
There are two main ways that regenerative power can be controlled by the driver:
- Accelerator pedal control
- Some electric cars have regenerative braking controlled by the accelerator pedal. Lifting the pedal completely can supply a small amount of regenerative braking, similar to engine braking in a conventional car. Alternatively, braking force can increase as the pedal is lifted to provide enough braking effort to almost stop the car, though this is disconcerting for drivers that are not expecting it. (Regenerative braking cannot completely stop the car because regenerative braking force decreases to zero as speed decreases to zero.) A gap between the pedal position where drive power starts and the pedal position where regenerative braking starts allows the driver to coast.
- Brake pedal control
- Alternatively, regenerative braking can be controlled by the brake pedal. The first part of the pedal movement will provide increasing amounts of regenerative braking. Pressing harder will give mechanical braking as well as full regenerative braking.
- Toyota's Prius uses a combined system where a small amount of regen is applied on throttle lift, with larger amounts applied based on master cylinder pressure. This brake pedal regen is then dropped to near zero under hard stops.
With Trev, there is not a great need to copy the driving experience of a conventional car.
The recommended configuration is to have regenerative braking controlled by the initial travel of the brake pedal, and to allow full regenerative braking. Separate control can be used to adjust the amount of "engine braking" applied when the accelerator is lifted.
Regenerative braking will not be available if the battery is fully charged, since the braking current cannot be applied to a full battery.
ABS and ESC
Most modern cars have an Anti-lock Braking System (ABS). Furthermore, Electronic Stability Control (ESC) is likely to become mandatory for mass-produced cars.
- Do we need to have ABS and ESC?
- Do we want to have ABS and ESC?
- How much mass, complexity and cost will these systems add?
- How well will these systems work on a three-wheeled vehicle?
The minimum requirement is that the park brake hold the car on a 20% gradient.
One of the complicating issues is that Trev has about a third of its mass over the rear wheel, whereas a conventional car has closer to half. If the park brake applies to the rear wheel only, the tyre may slide on a steep slope. It may be necessary to apply park braking to all three wheels.
One approach is to have brake discs or some other part of the wheel with circular pegholes around its rim, into one of which a parking brake lock pin can be clicked. It is a peg right through the wheel to stop it from moving. Something like it was done on the WW2 Land Rover and it was so suited to the needs of hill farmers that they kept it.