Braking Time and Distance – Wet vs Dry

Stopping distances for cars when driving is a calculation based on the driver’s thinking distance; the distance the car has travelled before the driver reacts to a hazard and the braking distance, which is how long the car takes to stop once the brakes have been applied.

What affects stopping distance? Thinking distance and Braking distance affect overall stopping distance and both the thinking distance and braking distance can be affected depending on various circumstances.

It’s these circumstances which affect stopping distances of a car. What affects the overall stopping distance of a car is as follows:

Thinking Distance

Thinking distances is dependent on the driver and other than the speed at which the vehicle is traveling, can be increased (taking longer to react) by:

  • Tiredness,
  • Drink or drug (legal or illegal).
  • Age – an aging driver may also have an increased thinking distance
  • Lack of concentration. Such as the use of mobile phones whilst driving
  • Bad vision can decrease reaction distance

Braking Distance Wet vs Dry

Other than the speed at which a vehicle is traveling, braking distance is affected and can be increased by:

  • The weight of the car
  • Condition of the brakes. Worn discs and/or pads can take longer to reduce a cars speed.
  • Condition of tyres. Highly worn tyres can increase braking distance, especially during wet weather.
  • Wet or icy roads will increase braking distance.
  • Road surface condition. General condition of the road surface and how well maintained it is. Gravel, dirt and mud will increase braking distance.

But what about the efficacy of your brakes to improve your braking time? Here are some vehicle-related aspects to consider when calculating braking time and distance:

  • Braking with locked wheels

On dry roads, the stopping distances which can be achieved when braking with locked wheels vs threshold braking are relatively minor. However in the wet, if you lock up the wheels there is a much more dramatic reduction in friction which can lead to you ploughing ahead on entry to a corner. If you do lock up wheels you’ll suddenly wish you had ABS, but if not now is a good opportunity to practice your threshold, cadence or avoidance braking techniques.

  • Tyre pressures and grip

Lower tyre pressures will increase the size of the contact patch, so letting some air out (while adhering to manufacturers guidelines) will allow you to gain a small grip increase in wet conditions. This will only help up to a point though!

  • Tread depth and braking distance

Recent research at MIRA (UK) measured the stopping distances at 80 km/hin conditions that represented moderately heavy rain (0.5mm to 1.5mm water depth). The research discovered that tyres with a 3mm tread had a 25% better performance than those at 1.6mm. This represents an extra 8 metres added to the stopping distance in wet conditions. In one test, when a tyre with a tread depth of 8mm was compared to one with only 1.6mm, the stopping distance increased by 13 metres.

So the moral here is if you are considering a separate set of wet tyres for track use, try not to use them for everyday driving and make sure the tread is over 3mm deep. As for brakes, if they’re not 100% working (and working well), your braking time will drastically increase and may even lead to brake failure.

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