Cohesive Friction vs. Abrasive Friction.
Braking Performance without the Brake Dust.
OE LOW METALLIC PADS vs. BENDIX GENERAL CT
Under normal Australian driving conditions using a BMW 330Ci, we trialled Bendix General CT against OE fitment low-met brake pads. Photos 1 and 2 (below) show the wheel rims after 1,300 kms. We also compared the wheel rims after automatic car washing. Photo 3 shows a noticeable residue still remaining on the wheel rims of the car fitted with the OE pads whilst the wheel rims of the car fitted with Bendix General CT show no residue in Photo 4.
Normal driving usage:
PHOTO 1 - LOW MET PAD
BMW 330Ci with OE fitment low-met brake pads after 1,300kms
PHOTO 2 - BENDIX GENERAL CT PAD
Same BMW fitted with General CT NAO brake pads after 1,300kms with general road grime but no brake dust
After auto car wash:
Photo 3 - LOW MET PAD
Wheel shown after automatic car wash – brake dust residue remains.
PHOTO 4 - BENDIX GENERAL CT PAD
Wheel shown after automatic car wash – no evidence of dust and road grime residue.
When it comes to brake dust, Bendix General CT out-performs European brake pads.
European car makers take brakes very seriously and brakes on European cars are excellent.
They need to be on the unlimited German autobahns, where cruising at 180km/h, 200km/h and even 250km/h is normal.
But continuous fast driving is hard on brakes. An emergency stop from 200km/h puts massive demands on a brake system and could be followed by a second (or third) high speed stop before the brakes have adequately cooled.
There’s nothing special about the brake calipers and rotors of European-built passenger cars or Toyotas and Hyundais sold in Europe. They are however fitted with high performance brake pads.
Most European cars imported into Australia are fitted with the same brake pads as the models sold in Europe, and are soon seen to have their wheels covered in dirty black brake dust.
With current friction material technology, high braking performance comes at the cost of high wear and dirty black wheels.
The pads and rotors are continuously transformed into a mess on the wheels. From a luxurious Mercedes Benz to an economical Holden Astra, the colour of the front wheels can be a better indication of a car’s origin than the badge on its grille. Owners of European cars are often surprised by the short life of their brake pads, and it’s rare for the rotors to survive beyond the second pad change.
How friction materials work
The brakes convert kinetic energy of a moving vehicle into heat. The simplest way for a brake system to absorb kinetic energy is to break chemical bonds in the rubbing surfaces of the brake pads and rotor. This is called “abrasive friction”, because the pads and rotor act as an abrasive, pulling each other apart, wearing, and turning the pad into dust.
A more sophisticated way to absorb kinetic energy is “cohesive friction” (or adhesive friction). Cohesive friction is used in modern non-asbestos organic (NAO) friction materials such as Bendix General CT.
In order to use cohesive friction, pads deposit a film of friction material on the surface of the rotor. As the rotor passes between the pads, the film and the pad surface heat up and become sticky. The pads and friction film bond to each other then break apart, absorbing energy. They bond and break apart continuously as the rotor passes between the pads.
No material loss from the pads and rotors means no brake dust.
Cohesive friction relies on the surface properties of the friction material and transfer film, which change with temperature. A material that is sticky at 30°C might be very slippery at 150°C. This means that an adhesive friction material must use a cocktail of adhesive ingredients to maintain the “sticky” properties over a range of temperatures.
Used under its design conditions, an adhesive friction material does not wear the rotor at all, as the rotor iron is protected by the friction film. The pads wear slowly, just enough to keep a supply of adhesive materials at the surface.
Cohesive friction is not possible at extreme temperatures, as the adhesives lose their effectiveness when they get too hot. The only friction available at high temperatures is abrasive, resulting in accelerated wear for pads that are not optimised for these conditions. In frequent or sustained high temperatures, a purpose-designed high-performance friction material should be used. In order to meet their high temperature operating requirements, high performance materials must use abrasive friction. More abrasion means more dust.
General CT – performance without the dust
Under Australian conditions, where speeds over 110km/h are all but impossible, high performance brake pads are overkill for most drivers.
Bendix General CT brake pads are not designed for repeated stops from 250km/h (drivers who reach these speeds on a race track might try one of the higher performance Bendix product lines).
For Australian road speeds, the dusty wheels and shortened component life of abrasive brake pads are an unnecessary compromise. With the latest in ceramic friction material technology, Bendix General CT provides ample braking power for any conditions likely to be experienced on Australian city streets or highways – without the brake dust.
Speed, energy and temperature
Kinetic energy is a property of all moving objects. The amount of kinetic energy is proportional to the mass of the object, and proportional to the speed squared. Doubling the mass will double the kinetic energy (a 3 tonne truck has twice the kinetic energy of a 1.5 tonne car at the same speed), doubling the speed will quadruple the kinetic energy (a car moving at 220km/h has four times the energy of an identical car moving at 110km/h).
When the brakes are applied, kinetic energy is converted into heat energy, causing the brakes to become hot. The temperature of the brakes depends on the amount of kinetic energy passing through them. This is why Autobahn speeds place such demands on a car’s brakes.
Stopping from 110km/h, a car’s front brake rotors may reach 200°C, the temperature of a warm oven. Stopping from 220km/h, the same car’s front rotors will reach 550°C, making the rotors glow red hot.
With many vehicles capable of reaching such speeds, and laws allowing them to do so, European car makers must specify friction formulations capable of operating reliably under extreme conditions – conditions that would only be seen on a race track in other parts of the world.
What is brake dust?
The dust produced by high performance brake pads is mostly rotor metal.
The friction performance is achieved by abrasion, breaking chemical bonds in the rotor, turning the iron into dust. Wear of the brake pads also releasing debris including polymer resins, which can combine with the rotor dust to make it more sticky on the wheels of the car.
An interesting experiment is to collect brake dust from the wheels of a car with adhesive tape. It is often possible to pick up the tape with a magnet, because the dust is mostly iron.
A set of rotors will usually be worn to minimum thickness and need to be replaced after two sets of high performance abrasive brake pads.
Water wise
Recent rains in Victoria and NSW have given some relief from the drought, but have not been enough to lift water restrictions in major cities. Washing the car at home is a distant memory for most drivers.
It’s not difficult to spend $10 and half an hour at a commercial car wash every second weekend, if your car’s wheels are black within days of being washed. The money adds up quickly, and everybody has better things to do with their weekends.
By eliminating brake dust and keeping the car’s wheels clean, Bendix General CT brake pads will pay for themselves in skipped car washes alone.
New car warranty
Fitting Bendix brake pads will not void a vehicle’s warranty.
The Australian Competition and Consumer Commission (ACCC) has advised that if the work is carried out by qualified staff who are capable of performing car servicing, according to the manufacturers specifications and using genuine or appropriate quality parts then regardless of where you choose to get your car serviced, your warranty will remain intact.
Waiting for the original pads to wear out before switching to Bendix General CT allows the rotors to wear more, and means putting up with brake dust for longer.