Give Me A Brake
Imagine that you’re driving along a typical suburban street. A movement to the side catches your eye and you spot somebody’s dog off the leash madly rushing full tilt down a driveway, barking madly at the cat across the road. Next thing you know, Doggo is rushing into the road. Without thinking, your right foot darts off the accelerator and hard onto the brake. Your car screeches to a standstill, stopping short of that lunatic of a dog and shoving you hard against your seatbelt. You growl something about idiotic animals and people who can’t control their dogs, then keep on going, barely thinking about the mechanical miracle that has just taken place.
Brakes. We take them for granted, especially after we’ve been driving for a few years. However, they are super-important for safety. Imagine what it would be like without them. You might have had a small taste of this sort of thing as a child if you screamed downhill on a scooter or skateboard (or, in my case, a bike with worn-out brake pads) – that feeling of being out of control and knowing that gravity will accelerate you faster and that there’s no way of checking or slowing that thing down.
Needless to say, brakes pre-date cars. Steam trains needed them and so did stagecoaches, and the basic principle behind all brakes is the same. The vehicle is moving because the wheels are turning, so to stop it moving or to slow that motion down, one needs to slow the wheels down. This is done by clamping something large and hard onto the wheel, which produces friction that soaks up the kinetic energy of the wheels. And this is the first and most important mechanical principle behind any brake: friction. The bigger the surface area applied to the turning wheels and the more force it’s applied with, the more quickly the turning stops.
In your car, the friction is applied by disc brakes, which have been around since Citroën put them on mass produced cars in 1955. Disc brakes consist of a metal disc that’s incorporated into the wheel. You can have a powerful pair of callipers that grab this disc as it spins and slow the turning that way. You can think of it as a more sophisticated adaptation of your old bike brakes: instead of grabbing the whole rim, it grabs something near the centre. The callipers are fitted with brake pads that are usually made of tough rubber, which absorbs heaps of force and can handle heat – and you need to make sure that you replace your brake pads on a regular basis, as they do wear out over time and you’re sunk without them. You’ve also got drum brakes (or disc and drum), where a stationary disk covered with an energy absorbing lining, known as a shoe, presses against the disc, applying the necessary friction.
A lot of kinetic energy and a lot of momentum are involved in a moving car. However, it takes the subtlest bit of pressure to slow a vehicle from, say, 100 km/h to 85 km/h as you approach a corner. If your average mid-sized sedan has a mass of 1600 kg and the equation for velocity is K = (m × v2)/2… you’ll have gone from 617.83 kilojoules to 445.98 kilojoules or a difference of 171 kJ. This is equivalent to roughly the energy expended by a petite woman doing slow dancing for quarter of an hour… and you sure didn’t apply that much with that little twitch of your foot. Obviously, something’s happened to amplify what your feet and legs did or the car wouldn’t have responded one iota.
The next mechanical principle that kicks in is the one discovered by Archimedes and I don’t mean the one that saw him running through the streets in the nude shouting “Eureka!” after his bathtub overflowed. I mean the “Give me a long enough lever and a firm place to stand, and I can move the world.” In other words, the lever principle. One tiny movement on the short end leads to a lot of movement on the long end. This is certainly at play in your brake system but amplification comes in the form of fluid courtesy of the principles of hydraulics. Don’t make me go into the equations for hydraulics, as that’s university-level physics and I didn’t study that.
Fluids can’t be squashed, which is how water pistols work. Actually, a water pistol is a good place to start understanding the principles of hydraulics. You couldn’t throw water with one finger very far or with much force, but by applying pressure to that water, you can get quite a bit of water going a fair distance, preferably onto your big brother’s face. The main force goes from your brake pedal to the master cylinder, which converts the force of your foot into hydraulic pressure, like your finger on the trigger of a water pistol. The brake fluid then exerts pressure on the slave cylinders (one for each wheel that has the brakes) and the slaves apply the brake drum or the callipers, and everything kicks in to slow the vehicle down.
There are a lot of moving parts involved and naturally, given the nature of things, the business end of the brake will wear out over time. And they will need to be replaced, so you really don’t want to try cheating or skipping on this important part of car maintenance.
If, for whatever reason, you’re in the scary situation where any of these systems fails, here’s what you do:
Level 2, 9 Help Street
Bill Nixon says:
I would add one final and extreme way to stop a car whose brakes have failed. After applying all the measures described in the video above, if you are still in danger of a serious accident and assuming you have an automatic transmission you are willing to sacrifice, you should attempt to move the transmission into reverse and then apply pressure to the accelerator. You will stop, real fast, but you will likely destroy the transmission. I know, my car did this one time accidentally, because the transmission was running low on fluid and it malfunctioned moving into reverse while the car was moving at 100 Km/h forward. The harder I pushed the gas pedal the faster the car decelerated.
July 18th, 2019 at 3:41 pm