the batteries from overcharging. The motor controller controls the desired motor power. Many controllers allow regenerative braking, i.e. power is fed back into the battery during deceleration. Some solar cars have complex data acquisition systems that monitor the whole electrical system, while basic cars show battery voltage and motor current. In order to judge the range available with varying solar production and motive consumption, an ampere-hour meter multiplies battery current and rate, thus providing the remaining vehicle range at each moment in the given conditions. A wide variety of motor types have been used. The most efficient motors exceed 98% efficiency. These are brushless three-"phase" DC, electronically commutated, wheel motors, with a Halbach array configuration for the neodymium-iron-boron magnets, and Litz wire for the windings. Cheaper alternatives are asynchronous AC or brushed DC motors. Mechanical systems The mechanical systems are designed to keep friction and weight to a minimum while maintaining strength and stiffness. Designers normally use aluminium, titanium and composites to provide a structure that meets strength and stiffness requirements whilst being fairly light. Steel is used for some suspension parts on many cars. Solar cars usually have three wheels, but some have four. Three-wheelers usually have two front wheels and one rear wheel: the front wheels steer and the rear wheel follows. Four-wheel vehicles are set up like normal cars or similarly to three-wheeled vehicles with the two rear wheels close together. Solar cars have a wide range of suspensions because of varying bodies and chassis. The most common front suspension is the double wishbone suspension. The rear suspension is often a trailing-arm suspension as found in motorcycles. Solar cars are required to meet rigorous standards for brakes. Disc brakes are the most commonly used due to their good braking ability and ability to adjust. Mechanical and hydraulic brakes are both widely used. The brake pads or shoes are typically designed to retract to minimize brake drag, on leading cars. Steering systems for solar cars also vary. The major design factors for steering systems are efficiency, reliability and precision alignment to minimize tire wear and power loss. The popularity of solar car racing has led to some tire manufacturers designing tires for solar vehicles. This has increased overall safety and performance. All the top teams now use wheel motors, eliminating belt or chain drives. Testing is essential to demonstrating vehicle reliability prior to a race. It is easy