The distance it takes a vehicle to come to a complete stop when the brakes are applied fully is called the stopping distance or braking distance. In an ideal world you will have plenty of time to check your mirrors before you stop, but in the case of an emergency, full braking will be applied and if the vehicle has anti-lock brakes (ABS), it will come to rest in its braking distance. Note that this does not include the thinking distance while your brain is deciding you should stop! We can get into the pure mathematics of a perfect situation (which is usually what is tried to be achieved when a manufacturer tests its vehicles), but in reality it is affected by a number of factors:
Original speed of the vehicle
The faster the vehicle is travelling, the more energy that has to be dissipated to bring the vehicle to a stop.
The coefficient of friction between the tyres and the road surface affects how soon the wheels will begin to skid. A skidding wheel has less friction on the road. Softer tyres and coarser roads tend to have greater friction than harder tyres and slick roads. Wet roads have much less friction than dry roads.
The coefficient of friction between the brake pads and rotors affects how much energy can be transferred from rotational motion to heat, thus slowing the car down. Most mathematical systems assume that the brakes are capable of locking the wheels – check our article on brake fade, though, as this isn’t always the case.
Consistency of the surfaces exposed to friction is important. If one tyre has much less air pressure it will have different braking characteristics. Slightly deflated tyres have more rubber in contact with the road, for example, but at a point, greater deflation can result in the tyre slipping on the rim.
If one tyre is on a coarse part of the road and another tyre is on a slick part of the road such as a white line, less friction there means that that that tyre won’t be able to perform as much braking as the other tyres.
In a perfect world all the brakes would be balanced and provide equal force. In an even more perfect world, all brakes would dynamically adjust the level of braking as individual wheels began to skid, and this is what happens with anti-lock braking systems with electronic brakeforce distribution. If two of your wheels are on a very slippery surface, such as a wet grass verge, and the other two are on tarmac, the system will apply as much force as possible to each wheel individually.
Most cars have a low coefficient of drag and the braking effect of the air is not significant. However, some cars, such as the Bugatti Veyron, have an air brake that is deployed to give extra stopping power. In the case of the Veyron, where it is deployed at high speeds, it can provide 0.6g of braking power in addition to the 1.3g that the Veyron will already achieve. This video shows a Veyron braking and you can see the spoiler angles upwards to form the air brake.
There is always some energy lost through the rotation of the tyres – it is the force that resists motion and is partly caused by energy dissipated as the tyre is deformed where it touches the road.
A really good driver will react to a situation in 0.75 seconds. Even in drag racing a perfect reaction time (when the drivers know the lights are going to go out) is 0.5 seconds. On the road your reaction time is likely to be more like 1.5-2 seconds. This is one reason why using the two second rule is important when you are following another vehicle.
What should you take away from this as a driver?
- If you are following a car and you are only 1 second behind it, if it stops abruptly (i.e. immediately) you will have hit it before you even have time to get your foot from the accelerator to the brake
- Your tyres should all be the correct pressure and have the correct tread depth
- Your brakes should be functioning properly
- You should buy a Bugatti Veyron (kidding).
Calculating the stopping distance
This YouTube vehicle explains the maths behind calculating stopping distances.