Brakes 35 ǁǁǁ͘ ĂŌ ĞƌŵĂƌŬĞƚŽŶůŝŶĞ͘ ŶĞƚ EŽǀĞŵďĞƌ ϮϬϮϲ a BEV. On vehicles like this, braking using the friction braking system is greatly reduced both in frequency of use and severity of use. It would be possible in a BEV with one-pedal operation to cover hundreds of miles on multiple journeys and effectively not use the friction braking system at all. So, here’s the scenario. You fit a brand-new set of pads and discs. As we’ve established, you’re a good technician, so you use the correct brake grease in the correct areas, you torque all the bolts to the correct setting, and you even ensure that the hub mating surface looks like new parts because you invested the time into cleaning the surfaces to prevent a return job for brake judder. Then when the customer collects their vehicle, you say those words. “Just take it easy for the next couple of hundred miles.” Then two or three weeks later the vehicle is returned with squealing brakes. What do you do? Phone your parts supplier and give them grief about the high-quality parts they sold you? Maybe you take the vehicle back in for free and inspect it; dust off the DTI Gauge and measure the runout. Maybe you replace them at your own cost to save hours of mucking around? But what really happened? The only braking the customer did that made the friction brake do anything was heavy braking. The first time the pads generated any heat, they were abused and became glazed. This can cause loud squealing noises, reduced braking capability, and, in severe cases, even slight brake judder. So, what does a great technician do on modern braking systems? Brake burnishing, or as it’s commonly known, bedding in the brakes, is now a crucial part of pad and disc replacement, with a number of manufacturers having prescribed procedures. A few things happen during a brake burnishing session. One is to heat cycle the friction material. The friction material’s friction coefficient is more stable and consistent once it has been heat cycled. Second is to conform and match the microstructure (peaks and troughs) of the friction material and disc friction surface so there is full contact. Third is to provide a uniform frictionmaterial transfer layer onto the disc friction surface. There are two main types of friction – adhesive and abrasive. Abrasive is like sandpaper, so you get the most wear/ dust. Adhesive friction occurs when a microscopic transfer layer of friction material deposits evenly onto the disc; the breaking and reforming of these bonds creates the friction coefficient. Pads typically operate between modes, but the transfer layer is vital for adherent friction. Abrasive friction is important to keep the discs in good condition and remove any surface corrosion. What’s involved So, how do you carry out a brake burnishing session? The routine manufacturers recommend may differ from one another and maybe even model to model, with factors such as vehicle weight, brake size and fuel type causing variation. However, there are some pretty standard points that most manufacturers will stick to. 1. While bedding your brakes, avoid braking down to zero. The lower speed isn’t important; the important part is NOT braking all the way to a standstill.
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