Bendix advises on brake pad measuring and technical points.
Brake pad identification is a crucial step in any brake job, particularly now, when some vehicles can be fitted with several different brake pad options and vehicle owners have greater access to aftermarket brake upgrade kits.
The brake fitter is now required to carefully confirm that the new pads are the same as the old pads. Sometimes a visual inspection, or back to back comparison of the pads is enough to identify a difference, other times when the pads shape is the same, a couple of simple measurements will verify the part.
Bendix advise that pad wear checking is the most useful tool, not just to show when to change the pads, but to pick indicators of a problem throughout the braking system.
MEASURING FOR PAD IDENTIFICATION
The key features when identifying a brake pad are its Shape, Length, Width and Thickness. For a positive identification the Bendix website and latest printed Bendix catalogue will provide the information and confirmation required.
The Pad Length is the total length of the backing plate and friction material excluding any overhanging clips or a protruding Wear Sensor. The Pad Width is the total width of the backing plate and friction material also excluding any overhanging clips such as anti-rattle clips. On some pads however Bendix advise that the shim on the backing plate wraps around the ends of the pad increasing its effective length and in these cases the Pad Length is the total length of the backing plate including the shim.
To assist the trade brake fitter, both the Bendix website and Bendix catalogue include sections which shows what the disc pad consists of along with measurements of Length x Width x Thickness. Where there are different size pads in the set, both will be illustrated.
MEASURING PAD WEAR
During the braking process the friction material and brake rotor absorb high levels of heat and the friction material acts as an insulating barrier to slow the heat transfer to the brake calliper and other components. As the pads wear there is less friction material so more heat is transferred to other components. To avoid this it is recommended that brake pads are replaced when there is less than 3mm of friction material remaining.
When measuring it is important to measure at several points along its length and width to identify tapered or uneven wear. There is normally a small taper along the length of the pad as the rotor tries to drag the pad in the direction it is turning. Some high performance multi-piston callipers now use different diameter pistons to counteract this affect.
Differences in thickness in a single pad of more than 0.5mm can indicate a problem with the calliper, requiring service, reconditioning or replacement. Bendix advise that even if the pads have worn evenly with no taper they can still tell a story with the differences between pads indicating calliper problems.
Outer pads worn more than inner pads in a single piston floating calliper can indicate the floating calliper is binding on its sides either through excessive wear or lack of lubrication.
so after a brake application the piston retracts but the outer pad is held in contact with the disc. To prevent binding it is recommended that the calliper sides be always cleaned with Bendix Brake Cleaner and lubricated with special purpose Bendix Brake Lubricant.
Wear on both pads on one side of the vehicle could indicate the piston is unable to retract, possibly due to corrosion or a damaged rubber piston boot. In this situation both pads are left in contact with the disc after each brake application causing rapid wear and the driver may experience the vehicle pulling to one side after the brakes are released.
In extreme cases of piston binding, the piston may not be able to move at all causing the opposite wheel to do much more braking, wearing the pads prematurely and causing the vehicle to pull to one side during braking. A further recommendation from Bendix is made to help prevent corrosion by bleeding the old heat affected brake fluid during the service and topping up with quality Bendix Brake Fluid.