The tale of a rear brake

General belief suggests that the rear brake in a two-wheel vehicle is mostly a decoration and doesn’t do much. Though there is a grain of truth in that, the reality is much more nuanced.

Backstory

Every field of human activity has its own rulebook. Though the naive part of me wished all its content was evidence-based, more often than not, it’s a motley collection of truths, half-truths, superstitions, and myths. You know, something like drinking an alcoholic beverage while out in the cold supposedly keeps your body warmer, while in reality, it does the exact oposite. Or what many a grandmother in my home country (mine included) would say about dough mixing – it can only be done clockwise or the cake will not bake properly.

The reasoning behind such guidelines is usually tactfully left undiscussed. If so many people share this belief then it surely must be true, right? Well, not exactly, as was the case with my grandma’s dough superstition – when I was a few years old I sneakily added a healthy dose of counter-clockwise movements to my baking routine, and somehow never affected the end result. Though the young me was quite surprised at that, the adult me was not – after all, why should a particular rotation direction be preferred by nature?¹As it turns out, nature actually does distinguish between clockwise and counter-clockwise rotations, but as far as we know only in the world of elementary particles and not macroscopic object such as unbaked dough.

Motorcycling has its own lore of beliefs. One of them concerns the usage of the rear brake. Some say it’s completely superfluous and is there only to pass EU conformity laws. Others acknowledge its usefulness in cruisers but diminish its value in naked or sport bikes. Yet another say the only use for it is to initiate a slide and back the rear on a racetrack. So who’s in the right here?

Mechanics of braking

To get a basic understanding of the role the rear brake plays let’s take a look at the forces a motorcycle is subject to during braking in a straight line. In this case, all sideways forces cancel out and we can safely focus only on forces acting in the plane of symmetry of the motorcycle – take a look at the picture below for a schematic²Braking in a turn is a much more complicated affair, but sonner or later I’ll write a piece on that as well. I hope.. If we disregard the aerodynamic drag, there are only five forces we should be concerned with. They are the two normal forces acting on the wheels at the tires’ contact patches ( and ), the two friction forces slowing the motorcycle ( and ), and the force of gravity (). Moreover, the friction forces are proportional to the normal forces. The constant of proportionality is called the friction coefficient and can differ between the front and rear tires.

We can now write and solve Newton equations of motion for a decelerating motorcycle to find relationships between all forces. If you are interested in mathematical details, expand the box below. If not, simply skip it and continue reading.

The biggest takeaway is that braking always increases the force acting on the front wheel, at the same time diminishing the force on the rear. This phenomenon is known as load transfer and is precisely what causes a motorcycle to dive under braking³There are front suspension designs that limit this behaviour, but front fork-equipped motorcycles – a vast majority – will always dive under braking.. Crucially, it doesn’t matter if you brake with the front, rear, or any combination of the brakes – load transfer will always occur.

What does it mean for a rider? Because tire friction coefficients are not infinite, decreasing the load on the rear wheel will result in a smaller peak braking force. On the contrary, increasing the load on the front means that the braking force can be higher. Under the right conditions, load transfer can lead to lifting of the rear wheel, rendering the rear brake useless. This is what leads some people to believe that the front brake alone is enough to stop a motorcycle under all circumstances. But the reality is more complicated than that.

The devil in the details

Think of a normal braking scenario. You approach a corner at speed, and as you do so you apply the brakes to slow down. How hard you brake is completely up to you and depends on your speed, road conditions, weather, skills, and so on. The important thing is, though, that it is completely under your control. However, you can get the exact same result with varying usage of front and rear brakes, i.e. altering the brake balance.

Take a look at the graphs below, showing the effect of different brake balances (thick black lines) and tire friction coefficients (graph axes) on deceleration (thin grey lines) expressed in terms of Earth’s gravity acceleration (i.e. g) for a sport motorcycle:

The first thing that we see is that the bigger the share of the front brake in the overall braking force the higher deceleration can be achieved. Ultimately, by using the front brake only we can go up to 1g, at which point the rear wheel starts to lift. Now let’s take a look at a similar graph, but for a cruiser motorcycle:

Again we can see that the more the brake balance is shifted to the front, the quicker you brake. However, because of a longer wheelbase and lower center of gravity, the rear wheel is still loaded even at the point at which the front tire loses grip (dashed line in the graph). Therefore maximum deceleration – and thus the shortest stopping distance – can be obtained only if you use both brakes. The same applies to riding with a pillion, as a passenger shifts the position of the center of gravity backward and up.

Low grip conditions

When you’re riding on a track on a warm, sunny day, you’re probably fine with testing the limits of grip to shave that last tenth of a second from your lap time. However, in real life, the conditions are not always as good as on the Côte d’Azur in the middle of summer.

For instance, a motorcycle tire friction coefficient can reach 1.1-1.3 on a good, dry road, but drops to about 0.4 in the wet. That is much too low to fully unload the rear tire before the front starts to slide. Therefore applying the rear brake on top of the front will stop the motorcycle quicker. This can be clearly seen in the graphs above – with the coefficient of friction of 0.4 the sport motorcycle can only brake at about 0.26 g with only the front brake applied, and at about 0.36 g with both brakes. That’s a whopping 38% more stopping power coming from the rear, which can make a difference between making it or breaking it in an emergency situation.

Furthermore, in most real-life scenarios you probably want to leave a safety margin to your riding. As cool as pulling a stoppie at the traffic light is, sooner or later you’re not gonna stop in time. Thus, normal braking occurs at somehow lower friction coefficients, and the rear brake can help you stop quicker. If the available grip is low to begin with, e.g. due to rain, then the graphs above suggest a brake balance of about 60/40 front to rear for both types of motorcycles.

Interestingly enough, the presence of a pillion can actually influence this figure so much that a 40/60 balance is better. Unfortunately, it’s hard to give a hard number without more data – here experimenting on your own can be your best shot.

Parkings and garages

Not all riding is done at high speed. Going through a car park, zigzagging through a row of cars in a traffic jam – all that is done at low speeds. Low-speed maneuvering usually necessitates turning the handlebars by a significant amount.

Take a look at the picture below. It shows schematically motorcycle wheels when the handlebar is turned by a large amount. The blue arrows represent the direction of braking forces that would act on both wheels if you applied the brakes.

So far so good, but what does it mean for us, riders? Well, if you tried to stop using your front brake, the braking force will have a big component perpendicular to the center plane of the motorcycle. This component will try to tip you over and might just succeed – it certainly did when I was learning how to ride. On the other hand, a braking force applied at the rear wheel will have a much smaller tipping component, which most riders can deal with.

Keep in mind, though, that this rule shall only be applied at low speed and lean angles. Doing so at speed and high lean can easily lead to a spectacular crash.

Grande finale

I hope that this article increased your understanding of the role a rear brake plays in a motorcycle. Though it is true that the front brake is a more powerful one, by no means does it imply that the rear brake is useless. Mastering its usage is definitely a valuable skill, not only on a racetrack, but also in real life. I therefore strongly encourage you to practice using the rear brake the next time you ride. Perhaps one day this skill will save your life.

Footnotes

• 1
  As it turns out, nature actually does distinguish between clockwise and counter-clockwise rotations, but as far as we know only in the world of elementary particles and not macroscopic object such as unbaked dough.
• 2
  Braking in a turn is a much more complicated affair, but sonner or later I’ll write a piece on that as well. I hope.
• 3
  There are front suspension designs that limit this behaviour, but front fork-equipped motorcycles – a vast majority – will always dive under braking.