Braking distance for motorbikes: too complex to be useful?
The Highway Code does not quote typical stopping distances for motorcycles. In fact, it does not differentiate between them and any other type of vehicle. This is because the factors affecting the braking capabilities of two-wheelers are so complex that they are pretty much meaningless. To find out why, it is a good idea to examine the facts.
In an article entitled ‘Does ABS make you stop faster?’ in the 4 January 2012 edition of Motorcycle News, four journalists tested a 1000cc Yamaha R1 (which could switch the ABS system on and off) by stopping as quickly as possible at 70 mph.
The experiment threw up some interesting results: there was a variation in the stopping distance between the riders of 11.63m with the ABS turned on, and 15.85m with it turned off. Two riders stopped more quickly with ABS turned on; the other two more slowly. This was the same bike, on the same day, using the same stretch of tarmac. This clearly demonstrates that the individual rider can make a huge difference to the way in which a bike stops. Add in other factors that would come into play in the real world when different bikes were being used in different conditions, and you have a recipe for such huge variations that any standardised stopping figures become meaningless.
What, then, are these factors, which can affect braking performance so much?
Reaction times test
None of us can react instantaneously: our brains first have to comprehend what is happening, work out what to do about it and then send signals to take the appropriate action. Tests carried out by Reading University show total reaction times of between 1.4 and 1.9 seconds, dependent on age. With these timings, a vehicle travelling at 60mph will be covering between 123.2 and 167.2 feet before any braking force comes into the equation. Add in other factors such as road surface and weather conditions, and those measurements will be even greater.
Maximum braking force is dependent on traction. This is friction between road surface and tyre. Introducing variables such as a wet road surface, or one that is slippery with diesel spill or ice, covered in gravel, sand or wet leaves or is bumpy, will again significantly alter outcome. With advances in today’s tyre and brake technology, it is not unreasonable to expect deceleration in excess of 1G. Test riders routinely achieve 60 to 0 stops on new motorcycles on perfect surfaces, over distances less than 120 feet.
Tyres also have a huge influence on traction: they may be brand new or part worn, under- or over-inflated, made of sticky race rubber or designed for touring and long life. Dual compound tyres are also common, as are tyres described as ‘knobblies’, i.e. those fitted to dual-purpose bikes that have off-road capability. Tyres also perform better when warm, so they will have less traction at the start of a journey, especially if it is a cold day.
The weight of a rider plus load can also affect traction. Adding weight to the rear of a bike will increase rear-wheel traction, adding it to the front will increase front-wheel traction. Braking in motorcycles is different from four-wheeled vehicles, as front and rear brakes are applied separately. As a bike pitches forward under braking and the front suspension comes under load, rear-tyre traction decreases and front-tyre traction increases.
Maximum traction – and therefore maximum braking efficiency – has been demonstrated when the rider transfers braking forces to the front wheel in a ratio of 50/50 rear/front when first braking then, as the weight transfers to the front wheel, 30/70 rear/front up to maybe 10/90, depending on the type of bike. It is not unusual for modern sports bikes to have a traction control device fitted: this will further affect stopping distances, as these sophisticated electronic devices control the spread of power to each wheel individually, reducing power to any wheel that is losing traction and therefore controlling a skid, which would again increase stopping distance. (Just to add further complication, different manufacturers produce different types of traction control devices, which work differently to each other.)
Although weight on a motorcycle is shared between two wheels, weight distribution between front and rear can change substantially, especially on a fully laden touring bike.
While adding weight to a motorbike will increase traction, it also increases the force required to turn, accelerate or stop the bike. Where you add or subtract the weight has significant effects: adding weight without adjusting the suspension will reduce cornering clearance. Adding weight over the rear wheel will increase rear-tyre traction and decrease front tyre traction, making the front wheel more prone to lifting under acceleration. Conversely, adding weight over the front wheel will decrease traction on the rear, making it light and more prone to a back-wheel skid. Decreasing fuel load also makes a difference (have you ever tried to lift six gallons of petrol?).
Bike test The experience of the individual motorcyclist is quite possibly the biggest factor of all. Imagine a brand new bike, tyres in perfect condition and properly inflated, perfect tarmac on a warm sunny day and an alert rider. Now, add in a car suddenly pulling out of a side road, a situation that requires emergency braking. Why will some riders fall off or collide with the car but others come to a complete stop or be able to scrub off enough speed to take action to avoid it?
The answer to the question is: confidence and experience. It takes a great deal of confidence to brake a two-wheeled vehicle as hard as you possibly can. At the forefront of your mind is the bike’s inherent instability and the amount of pain you’re going to be in if you fall off.
Confidence, meanwhile, comes from a mixture of experience and training. Any rider, experienced or novice, can and will benefit from additional formal training, and also from regular sessions of self-training. With every new bike you get, it is worth taking it to a bit of straight, empty road to practice braking.
To do this effectively, place a marker by the side of the road and approach it at a constant speed, brake as hard as you dare, and mark your stopping place. Repeat the process a few times, each time marking your stopping place. Within a few runs you will no doubt find that you can brake harder and more efficiently than you originally expected.
The exercise will reinforce the original statement, that quoting stopping distances for motorcycles is meaningless. If your own stopping distances can gradually be reduced under controlled conditions, who is to say which one would be accurate in any given circumstance?
At least once a year you should repeat the process, with each bike that you own, just to maximise the effect of the training: one day, it just might make the difference between stopping and crashing.
Posted on Jun 1st, 2012 by Lyons Davidson