In the seventh part of our handling series, Neil talks about brakes and how they aren’t the enemy of performance.
In the seventh part of our handling series, Neil discusses brakes and the evolution of Mini braking systems.
Brakes are the enemy of speed, right? Wrong – without brakes you can’t get the correct entry speed into corners. They are vital to a fast lap and of course, stopping at a crossing for pedestrians on the road.
Cars’ braking systems originally derived from horse-drawn carts with wooden blocks which pushed on wheels or leather straps pulled around axles. When you compare these to the latest Formula 1 components they still work on the same principle of using friction to convert the kinetic (motion) energy into heat. Back to those school lessons where we were taught that energy cannot be created or destroyed, just converted.
As cars got faster, a new solution was required which could handle more heat. This resulted in the drum brake, which performed brilliantly on a variety of land speed record and racing cars. The famous Le Mans Bentleys relied on drum brakes and they became a standard component on road cars.
When the Mini came along in 1959, drum brakes were the logical choice. They provided good, controllable braking in a compact package. The drums on the front had twin shoes arranged to give maximum efficiency against the forward direction of the car with twin hydraulic slave cylinders to operate them.
REAR BRAKE DESIGN
At the back, drum brakes were fitted, again with two shoes but this time one was more efficient when the vehicle was travelling backwards and there was one double-sided slave cylinder. Opposite that hydraulic cylinder was a cableoperated lever to be used for the handbrake. This rear brake design ran for the entire life of the Mini.
Everything was fine with the drum brakes, indeed they were factory fitted on all four wheels until the mid 1980s, until people started competing the little car. The regular, heavy braking required when racing and rallying generated too much heat for the drum brakes to dissipate. So when the Mini Cooper
“As cars got faster, a new solution was required which could handle more heat”
came along, a new factory solution was required.
The disc brake concept had been around for almost as long as the motor car but it wasn’t until newer materials and designs that it started to become more popular. What we’d recognise as the modern disc brake is more efficient at converting that kinetic energy into heat, better at dissipating that heat, and on the Mini also removes the need for adjustment. The drum brake does need less force to operate due to the way the pressure is applied.
The 997 and 998 Mini Coopers came with tiny seven-inch discs at the front under the 10-inch wheels. I say tiny, but when you compare them to the ~11.5-inch discs on the modern MINI you can see my point. The seven-inch discs and small pads on the 997cc Mini Cooper were not regarded to be much more effective than the drum brakes they replaced. Indeed, they are
considered to fade (overheat) faster than the drums. When the 998cc Mini Cooper came along, they were fitted with larger brake pads, which made them marginally more effective.
The Cooper S really brought disc brakes to the fore of Mini stopping power. The 7.5-inch discs fitted to this model were coupled with a brake servo which amplified the pedal pressure with the assistance of vacuum provided by the engine. So, not only did the brakes have a larger surface area between the pads and the disc, but also a greater disc area to dissipate heat and the servo to boost the amount of pressure the driver could apply to the pads and so squeezing the disc. To give you an idea of just how hot these brakes got on the twisty mountain passes of the Monte Carlo rally, there are tales of the discs glowing so much they became virtually transparent after particularly twisty alpine descents!
Minilite wheels improved the cooling of the discs when compared with the steel alternatives. Third-party alloy Minifin and Superfin drums are available these days which do help drum brake cooling and give much improved looks.
Finally, 12-inch-wheeled Minis came along on the 1275GT and 8.4-inch discs could be squeezed under these wheels, again giving a greater area to dissipate heat from and more pad surface area which could create more friction and a more controllable braking experience. Servos then started to appear on the 8.4inch disc models from the late 1980s.
As I mentioned, the rear drum brakes survived all of these changes. The braking force at the rear of a car is much less due to the way which the weight of the vehicle acts on the front wheels more when you are slowing down. If you have too much braking at the back, then it can get rather twitchy and ultimately lock the wheels up. The rear drums also provide a neat solution for the handbrake. If you have hot disc brakes and clamp them at a given pressure, when the disc cools, the metal contracts and the pressure releases. Major manufacturers have used this method, owners soon learned to leave their cars in gear after a spate of cars rolling off when parked with hot brakes.
Words and Photography
Rear drum brake assembly. Rear drum adjuster.
Rear drum slave cylinder. Superfin drum.
7.5-inch Cooper S disc. The 8.4-inch disc.