How the non-ABS Braking System Works And Brake Test

How the non-ABS Braking System Works And Brake Test

Overview
The braking system fitted to non-ABS vehicles has an ‘X’ configuration. This ‘X’ layout incorporates a diagonal split. Two independent hydraulic circuits originate at the master cylinder; the first of these circuits operates one front brake and its diagonally opposite rear brake, and the second circuit operates the other two brakes. When the brake pedal is applied, a push rod connected to it moves forward a piston in the brake servo. This movement is amplified by the servo unit itself and is transmitted to a pushrod connected to the master cylinder. The mechanical force of the push rod is converted into hydraulic pressure in the master cylinder which contains two pistons and two pressure chambers. One of these pressure chambers is connected via hydraulic pipes to one of the diagonal split (i.e. to one front brake and its diagonally opposed rear brake, and the other chamber to the other two brakes.

The hydraulic pressure in both chambers is equal when the brake pedal is applied. The full driver’s braking effort will always be applied to the front brakes, but the pressure to the rear brakes is adjusted by a load sensing valve. This valve is fitted to the vehicle’s body and has a rod linkage attached to the rear axle which, in effect, measures the vertical distance from body to axle, and which will alter dependent on the load being carried. The load sensing valve contains a variable restrictor which adjusts as the load varies to allow an increase or reduction in hydraulic pressure to the rear brakes If there is a loss of hydraulic pressure in one of the ‘X’ system’s circuits the other will continue to operate, to ensure the driver retains braking via one front brake and its diagonally opposite rear brake, albeit with a longer pedal stroke. Handbrake operation is mechanical. When the handbrake is applied, cables to the two rear brakes applies the shoes. The rear brakes are adjusted automatically by operation of the handbrake.

When vehicle is road tested, pedal travel should be short and feel solid with no indication of ‘sponginess’. The vehicle should pull up squarely.

Apart from the brake test on the road, the best way to use the dynamometer equipment are Brake Tester. The data produced by this tester is very accurate to determine the braking performance of each vehicle. The accidents involving heavy vehicles occur frequently on the road in Malaysia is because there is no mandatory regulations requiring all types of heavy vehicles through a comprehensive brake testing, including through the vehicle during heavy load on a trailer behind the force of pressure generated by the fully loaded trailer trucks will decline over the next while braking.

Fig.1 : The data will be generated during braking tests with the dynamometer, the roller rotates automatically when the vehicle braking and producing brake force.

Fig.2 : Use as axle load simulator by tying down the chassis by chain/belts and lifting the lifting system of the roller brake tester to predetermined axle wiight.

The maximum brake force is given per wheel. Using the brake tester when the wheels of an axle are tested individually, the brake tester computer combines the brake force measured at the same pedal force or air pressure. When e.g. the left wheel locks at a pedal force 14kg and the right wheel locks at a pedal force a 17kg, then the brake forces that occur at 14kg for both left and right wheel. The brake imbalance is the difference in brake force expressed as a percentage. For most countries this imbalance is measured at the maximum brake force. If not, the values at which the imbalance is measured are generated next to the imbalance value. The imbalance is calculated using the same formula as the ovality.

    Fig.3 : Simulator for the brake air pressure transducer

    For those vehicles which have a greater difference in unladen and laden weight like heavy goods vehicles, trailers, etc. that have the brake data systems. This system measures the brake efficiency as tested normally unladen. The air pressure in the brake cylinder is measured at the locking point of the wheelbrakes. The axle weight is stored and the computer makes the calculation from the measured values still unladen condition of the vehicle. But need to calculation the brake efficiency in laden condition without having the vehicle laden.

    This calculation is made by the computer by extrapolation of the air pressure value mostly up to 6 BAR = 85 PSI. At this point calculate the relation between the air pressure at unladen weight in the brake cylinders at 6 BAR. The computer calculates the total available brake force divided by the gross vehicle weight. This is the total weight of the vehicle in fully condition. The outcome is the brake efficiency in percentage of the gross vehicle weight.

    Fig.4 : (Above) Normal brake testing like a standard brake tester (Below) The read-out simulation testing