Here in the first of a two part series IAM examiner Jon Taylor looks at the theory of braking and gives a reminder of why it’s one of the key riding skills we can never stop refining
Of all the skills needed whilst riding a motorcycle, when all else fails and the unexpected happens, your ability to get rid of speed and lots of it in as short a time span as possible, must rank up there with the best.
OK I hear you say, I did all that when I learnt to ride, I braked from 50kph/31mph in complete control making sure that neither wheel locked, it was no problem at all. Yes, but I bet you’ve never done that from high speed, that is to say 150kph/93mph or more, or when your bike is fully loaded and carrying a pillion, or on a slippery corner!
At the last one many of you will be thinking “Is he mad?... You’ll never catch me braking on a wet or slippery bend … that’s total madness!”
But hey, let’s think about it. Do emergencies always happen on flat road surfaces in dry conditions when we are riding alone?
No, of course they don’t and to truly know our, and our machine’s capabilities we need to not only practise, practise, and still more practise, but also understand the theory behind what is happening when we brake.
No matter how clever you are, your brain is just not capable of learning new very complicated tasks in the middle of a panic situation.
In recent safety campaigns it has been shown that in an impact speed with a vehicle at 70kph/43mph the likelihood of survival is 10%, yet at 50kph/31mph this rises to 50%, and at 30kph/19mph it is 90%. This is surely an incentive if ever there was one to practise our braking skills and that includes in ALL the circumstances you are likely to come across in your everyday riding.
When the need arises you won’t get the chance to practise a few times prior to it, virtually all your concentration will be taken up with the situation itself. It will be hard enough in the time given just to come to a decision as to what action to take, let alone reducing speed sufficiently to avoid serious injury, so unless your braking skills are so well rehearsed beforehand that you can do this instinctively, your chances of survival in the worst case scenario will be down to luck.
So first the theory, let’s have a look at just what exactly happens when we brake.
If you can picture yourself sitting on your machine, an average rider riding a medium sized machine will weigh together about 300kg distributed more or less 50/50 between the front and rear wheels so that each wheel has about 150kg of weight forcing down onto the road surface.
Now, as we apply pressure to the brake(s) we get what is known as weight transference, and it is this that is responsible for making the forks dive on machines fitted with telescopic forks. As the weight is transferred the springs compress thereby encouraging more weight transfer to take place. Weight transfer still occurs with machines fitted with front suspension systems that do not dive (BMW’s Telelever springs to mind) but it is slightly less due to the lack of fork dive.
So if the weight is being transferred to the front, what happens to the rear? Quite obviously the rear wheel will lose some of the weight that it carries, but why should that be a problem? Well, now imagine a canteen type metal and plastic chair. If we go up to that chair when it is empty and give it a push it will skate across the surface of the floor quite easily. Now place a 100kg person in it and try to push it and it’s a different story, it may just be possible to push it but it will take quite a considerable push to do so. But why?
Okay, some of that force will be needed to overcome the increase mass, but by far the larger amount will be due to the additional grip that is produced by the chair’s friction between it and the floor. The measurement of that grip between the chair and the floor is commonly known as the coefficient of friction. Now, how does this relate to our rider sitting on his machine? Well, as the rider starts braking they only have around 150kg of weight on that front tyre, but as the weight starts to transfer forward it soon rises to 200kg and more, and depending on the machine in question, may even approach 300kg.
Now thinking back to our chair with the 100kg person sitting in it, as the weight increases on the front tyre, so does the grip, and we can therefore apply yet more pressure to the lever and correspondingly achieve even more braking force. However, if we apply that same full braking force all at once by grabbing the lever as one might in an emergency, say sufficient to lock a wheel with 175kg of weight on it, before the weight transference has taken place, what will happen? Yes, with only 150kg on it at the time it will lock. So what we have to do is apply slightly less force initially, that is to say less than that needed to lock a wheel with 150kg of weight on it, and then progressively increase that pressure until we reach the point at which the wheel is on the point of locking, and as stated earlier this could almost be up to 300kg with 100% weight transference. This is sometimes referred to as “lemon-shaped braking”. Confused? Just look at the outline of a lemon, it starts with a shallow curve which progressively becomes steeper until it reaches a maximum and then tapers off as it comes to an end. That’s how your braking pressure should be.
In exactly the same way, the rider should aim to build the pressure PROGRESSIVELY until reaching a maximum and then tailing off as you come to a stop to prevent the forks extending suddenly as you finally come to rest.
For those of you who ever carry pillions, they will be very grateful for you using this progressive technique every time you brake. If you keep having helmets bashing together when braking, you are a prime candidate for practising this!
But what about the rear? Well, obviously if this extra weight is going to the front wheel it is coming from the rear, so in effect the pressure on the rear brake should be reducing as the front increases, but to consciously vary the pressure of the rear inversely proportional to the front is beyond most people’s capability even for the purposes of practicing, so what many people advocate is to just brake lightly on the rear and apply all your concentration to the front, which after all is the brake that’s going to be doing most of the work and is therefore crucial to get absolutely right.
Now let’s add in a few more variables such as carrying a pillion and possibly luggage as well, what effect would that have? Well, as we are starting off with more of the weight on the rear to begin with we can afford to use more rear brake, after all, the rear wheel has a lot more weight on it, possibly twice as much, yet at the front, before weight transfer takes place, the increase in load on the front tyre may be negligible depending on the wheelbase of the machine. However, once weight transfer does take place, a lot of that additional weight will still transfer forward to the front, especially considering that in the case of the pillion that weight is being carried quite high up, and the presssure required on the front lever can be quite considerable once transfer has taken place.
So, how much extra distance do we need for braking if we are carrying a pillion and luggage? Well the answer to that may surprise you. Providing your brakes are capable of stopping the combined load of the rider, pillion and luggage efficiently without fading, your stopping distance should be the same.
Why? Because it’s all down to that coefficient of friction again. More weight overall equals more weight on the tyres and therefore more grip on the road, so the speed at which you decelerate is almost entirely down to the level of grip of the road surface. When reconstructing accidents Police determine the initial speed of a vehicle from the length of the skid marks. It doesn’t matter what the size or weight of the vehicle is, if the wheels are locked they will all stop in the same distance, and that is determined by the coefficient of friction between the tyre and the road surface. (For the purposes of accident reconstruction though this is not used for motorcycles as one can never know what proportion of braking has been applied to each wheel, as unlike a car, it uses two separate braking systems but the physics are effectively the same.)
Now if you actually go out and practise that you may well find that you do in fact take longer to stop, and the most likely reason for this is that you are not compensating for the additional load sufficiently, by increasing the braking force. This only comes with practise!
So, getting back to our rider, they have to initially apply just sufficient braking force to ensure good initial weight transference and then progressively increase the force on the front to make full use of the additional grip given by the transference of weight. As we’ve seen above, this will vary with the load carried, it will also vary with the level of grip available between the tyre and the road.
Try using that 100kg of braking force on the front brake when riding on ice, or over spilt diesel, or on a wet worn surface, or when banking the machine! The result will be a locked wheel. When you factor all these variable in it’s a wonder we are ever able to stop the machine at all.
Another factor that affects weight transference, and therefore the braking performance of a machine, is its wheelbase.
A SuperSports lightweight 600cc machine will have totally different braking properties to a large touring machine or a Harley Davidson. The SuperSports 600 will have quite a short wheelbase whereas the large touring machine will have a much longer wheelbase to accommodate the passenger and luggage. Just have a look at the brake discs on the two different types of machine. Despite its light weight the sports bike will have two very large discs on the front and a very small one on the rear anticipating heavy braking from with speeds with a lot of weight transfer, whereas the tourer will probably have the same or smaller size brakes on the front (even allowing for its much heavier weight) yet at the rear will have a much larger brake.
The reason for this is all associated with its wheelbase. If you draw an imaginary line from the bottom of the front tyre to the bottom of the rear tyre, then up to the centre of gravity of the machine with the rider on board (usually somewhere around the rear of the petrol tank) and then back down to the bottom of the front tyre, you will see it makes a triangle. Do this for the sports bike and rider and then do it for the tourer and rider. What do you notice? The triangle for the sports bike has a much shorter base and is taller in relation to its base than that of the tourer. Which then is the most stable? Well, I hope you agree it is the tourer, and from that it’s quite easy to see that a sports bike can quite easily achieve 100% weight transfer, but this would be almost impossible on a tourer. Hands up who has seen a Gold Wing doing a stoppie. However, one look at the race bikes on television and you will see they regularly use 100% weight transfer, the rear wheel is in the air.
Getting back once more to our rider, not only do they have to factor all the road surface variables into the equation, but also the wheelbase, load carried and braking performance of his machine and one last thing mentioned earlier, bank angle.
In the same way as the co-efficient of friction of the surface and the degree of weight transference have an effect on the ultimate braking performance of a machine, so the bank angle does. Theoretically, if a machine is banked at 45 degrees, the amount of centrifugal force pushing the tyres out is equal to the amount of weight pushing the machine onto the road surface. Exceed that downwards force and the bike starts to slide.
In actual fact a machine, given a grippy road surface and sticky tyres can exceed that angle slightly as the tyres deform and form a small mechanical lock with the road surface which allows a little more than the theoretical 45 degrees. But the important concept to grasp is that as the bike banks over there is less and less grip available for other things such as steering and braking until at around 45 degrees, none is left. If the surface of the road is wet or icy, then maybe as little as a few degrees will start it sliding before any braking or steering is applied.
Next month Jon looks at putting the theory into practice
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