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Big Lift Brake Bias
MICHAEL HAYES EXPLAINS HOW TO CORRECT YOUR BRAKE BIAS AFTER A SUSPENSION LIFT IN YOUR 4X4!
Brake bias is the term used to describe how the total braking force is distributed between the front and rear tyres. Many factors affect the amount of braking force that is generated. The most important one is the force (weight) pushing the tyre against the ground.
As your 4WD decelerates, weight is transferred from the rear to the front, which reduces the amount of braking force the rear tyres can produce. Brake too hard and the rear wheels will lock up causing them to lose traction and possibly swing around - not ideal.
As you go about modifying
and adding accessories to your 4WD, changes in the rig's ride height, weight distribution and centre of gravity will alter the brake bias.
What is needed is a Load Sensing Proportioning Valve (LSPV) that allows pressure to increase to the rear brakes as front brake pressure increases, but prevents the pressure reaching a level where the rear wheels will lock.
As the load on the rear axle increases, so does the amount of braking force that can be directed to the rear wheels. The LSPV automatically does this by varying the rear brake pressure in relation to vehicle ride height. A typical unit consists of a body, which is mounted to the chassis and contains a piston/valve assembly to control the flow of fluid to the rear brakes. An external lever, which is pivoted from the bottom of the body, has an adjusting bolt which contacts the end of the plunger.
On the other end of the lever is a spring linked to the rear suspension.
When the brake pedal is pushed, fluid enters the inlet port and travels through the valve at a reduced pressure out to the rear brakes. As the driver increases pedal pressure, the front of the vehicle dips, changing the valve's arm angle from horizontal, which allows increased brake fluid pressure to reach the rear hydraulic cylinders.
Because the LSPV's adjustment range is altered in relation to changes in vehicle ride height, there is always a setting given for the valve at standard ride height. This setting may take the form of a measurement between two points, or a pressure difference between the inlet and outlet of the valve. Consult your manufacturer's handbook for the correct adjustment procedure and setting range.
To aid in correct adjustment of the brakes
the use of a brake proportioning bracket extension is the best way to return the geometry after performing a lift. They are a simple DIY bolt-on bracket and are best purchased to suit specific lift heights.
A properly designed bracket also has some compensation in the length to counteract the raising of the centre of gravity. Once a bracket is fitted, it is recommended to pressure test the brake system for best results.
