Braking, or stopping distance, is the distance a vehicle travels from the full application of its brakes until it has stopped. Vehicle manufacturers try to shorten stopping distances because the shorter the distance, the more likely the driver is to be able to avoid an obstacle or a nose-to-tail accident. Often you’ll get a statistic which gives braking performance from 100-0kph in seconds and/or distance covered.
Modern vehicle technology has shortened braking distances. Better suspension and tyres are more efficient. Electronic brakeforce distribution ensures that maximum braking power is applied dynamically to each wheel, and anti-lock brakes prevent skidding. This video shows a comparison between locking the wheels and using anti-lock brakes. Skip through to 2:20 to get past the boring stuff.
A number of factors act on a vehicle to slow it down
Brake friction
Brakes use friction to slow down the car and dissipate the heat created through their components. There are different types of brakes, such as disc, carbon ceramic and drum. They have different braking characteristics and can be affected by heat, wear, pad glazing, brake fluid temperature and whether the brakes are dry. Check our article about brake fade to read more about that.
Brakes should be balanced so that they provide a consistent force on either side of the car. There will be more braking effort expended at the front of the car because as you brake, weight shifts to the front and the front wheels provide most of the stopping power. This is especially important on a motorbike as overcoming the friction of the front wheel can cause it to wash out from under the rider, and overcoming the friction on the rear wheel can cause the bike to fishtail.
Road friction
The tyres on the road are slowed down by the braking action, and their friction against the road’s surface slows the vehicle down. If too much force is applied, the brake friction overcomes the tyre friction and the wheels lock, which reduces the friction on the road. For best braking performance all tyres should be correctly inflated.
Road friction is influenced by the type of surface, its temperature and how wet it is. Tyre friction is influenced by the contact patch (how much tyre is in contact with the road surface, which is affected by the inflation of the tyre, and the tread pattern), block movement (how much the tyre rubber moves), and how soft the rubber compound is.
Rolling resistance
As the tyre rotates it deforms against the surface of the road and this takes some of the energy out of the rotation. At its simplest level, this is rolling resistance.
Air resistance
Most cars are designed to be aerodynamic, and therefore air resistance doesn’t play a huge part in the braking performance of a car, unless there’s a strong headwind. Some hypercars, such as the Bugatti Veyron and the McLaren MP4-12C, have air brakes that are deployed under heavy braking. You can see in this briefly in this video at around 5:35
Reaction time
While reaction time isn’t a mechanical force acting on your car, it does contribute to the overall distance that it takes you to stop. A racing driver’s reactions could be as quick as 0.5 seconds because they are expecting danger; in our complacent state of driving our reaction times are more consistently 1.5-2 seconds. That means if you are only leaving one second between you and the car in front you are dramatically increasing your chances of running into the back of it if it brakes heavily.
If you want to know the equations for how you work out stopping distances, this guy with impossibly solid hair explains
Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists
