Trends in automotive braking

The key component of any car braking system is the disc brake pad. Stopping a 1300 kg car travelling at 100 km/hour in 5 seconds generates over 100 kW of heat in an area of only 50 cm2. That heat is dissipated into the brake pads. Within seconds, temperatures can reach 600°C and in extreme cases even higher. Despite that, braking must be safe and smooth. With an increasing focus on HSE (Health, Safety, Environment) these days, disc brake pad manufacturers (compounders) are having to look for alternatives to traditional raw materials like copper and other materials, which can give rise to health and safety issues.

New friction materials must still perform well even under extreme braking conditions.

Brake pads must give the same pedal feel and deceleration in repeated city braking as well as sudden emergency stops down steep mountain roads. That must happen whether conditions are wet or dry, muddy or icy, acid or alkaline, and in temperatures from -40 to +50°C.

Braking trends are increasingly affecting not just compounders but also car manufacturers. Standards are on the way to ensure friction materials reduce emissions. Fine particulate emissions are becoming increasingly unwelcome in cities. And even electric vehicles, which use regenerative braking that reduces wear and emissions, place their own demands on braking systems.

Nothing must compromise braking performance. Safety is the priority, but brakes also have to work without NVH (noise, vibration and harshness) such as judder, groan or squeal. Doing all this thousands of times without fail needs leading-edge design, a consistent manufacturing process and extremely consistent base material. Compounders and car manufacturers alike must have confidence that the brake pads won’t disintegrate when they’re needed most – in an emergency. If there’s one part in a car that must not fail, it’s the brakes.

Stricter environmental regulations have reduced exhaust emissions from new cars to a level where emissions from tyres and brakes are becoming increasingly significant. Although tyres can have around 10 times the emissions of brakes, brake emissions are primarily airborne. Non-exhaust emissions now account for over 90% of PM10 particulate matter emissions from traffic, and up to half of those come from brake wear. EU regulations on fine dust emissions in cities have even led Volkswagen to investigate vacuum pumps as an option for removing dust from a brake pad.

Friction materials consist of a complex mix of different raw materials to ensure they can cope with the extreme conditions under which they have to operate. A friction material can have up to 20 different raw materials, divided into different groups:

• Abrasives increase the friction coefficient, while also increasing the rate of wear of the counter face material. They remove iron oxides from the counter friction material, as well as other undesirable surface films formed during braking.
• Reinforcing fibres provide mechanical strength to the friction material. They contribute to the plateau formation for a stable friction film under all braking conditions.
• Lubricants decrease the friction coefficient and wear rates during braking. They stabilize the friction level, particularly at high temperatures.
• Binders maintain the structural integrity of a brake pad when it is subjected to mechanical and thermal stresses. They hold the components of the brake pad together, preventing its constituent parts from crumbling and coming apart.
• Fillers improve a brake pad’s manufacturability as well as reducing the overall cost of the brake pad. They also play an important role in modifying certain characteristics of brake friction material.

Besides the obvious benefit of extending brake life, decreased wear also reduces fine dust emissions. Wear debris is generated by the high shear forces during braking that stress the friction material surface (Figure 2). Consistency in manufacturing is again critical.

Although Lapinus’ mineral fibres are 100% safe, other materials may not be. Compounders are under pressure to reduce hazardous materials while improving wear characteristics. A European Working Group is also looking into various aspects of brake emissions, with updated legislation likely to follow.

Sustainability of vehicle use is also a major environmental issue, which has helped fuel the rise in new car sales of electric vehicles.

Despite this, vehicle access is increasingly being restricted in large cities, and young people in particular are turning away from owning cars altogether. The convenience of hailing cars, using public transport and even finding bikes via apps is already reducing the appeal of car ownership, and this is likely to decline even more in the future as autonomous driving becomes a reality.

Depending on how the electricity is generated, electric vehicles promise much more sustainable transport with lower CO₂, NOx and particulate emissions. Although EVs are still a small percentage of new-vehicle sales, the share of cars containing an electric engine is expected to rise to 50% by 2030.

Electric and hybrid vehicles are equipped with regenerative braking. This means that the electric motor acts as a generator when braking which charges the battery. Conventional brakes are much less used so drivers need replace brake pads only rarely, even for city driving. Brake pad wear for autonomous driving cars is even lower because of better prediction of emergencies. McKinsey predicts that fully autonomous driving with human override (level 4 autonomy) could account for 60-70% of all miles driven in the US before 2030. Brake pads for those cars could last the whole life of the car. All this will also greatly reduce the brake aftermarket. 

Corrosion, wear and noise

Longer brake pad life brings its own problem, though, since corrosion on rarely used brake pads and rotors can affect brake performance. This should be addressed within a couple of years with corrosion-resistant friction materials and rotors.

Quiet operation is particularly important for electric vehicles because of their virtually silent running. Noise (including high-frequency squealing), Vibration and Harshness all become unacceptable in terms of driver and passenger comfort. Currently brake pads on EVs have to be much larger than on conventional cars due to the additional weight of the car caused by the batteries.

So, although electric vehicles promise a breakthrough solution to urban pollution from traffic congestion, brake wear and the resultant emissions must fall, too. There is talk of zero-emissions cities in the future. Safe driving will then above all need advanced design and highly consistent, quality-controlled production of the brake pad materials.

Brake compounders and raw materials suppliers have to work together closely to improve the brake friction, lubrication and wear (tribology).