Brakes Part II

The Rear Brake: Later, we’ll think about the ‘control brake’ use of the brake on the back. For now, let’s consider rear-brake basics in minimum-distance stopping situations. We’ve considered that the front brake is capable of supplying anywhere from 70% to 100% of stopping power;
that means that the rear brake is capable of contributing about 30%, down to zero percent, to the shorter stopping distance, depending largely on the motorcycle itself. Sportbikes by nature have short wheelbases for turning agility, and are relatively lightweight. At the other end, large and heavy touring bikes, and many cruiser-style machines, are heavier, lower and longer.Why does that matter?

As a rider applies increasing front-braking pressure, and the composite weight is moving forward and down on the bottom of the front tire, the same weight is being progressively REMOVED from the rear tire’s contact patch, resulting in less and less available traction at the rear contact-patch. The simple physics of leverage and weight help us understand that short and light bikes will transfer more weight, and transfer it more quickly, as compared to longer, heavier and lower motorcycles. So, sportbike rider can expect very little additional stopping-power from the rear brake, while the cruiser and touring riders will have significant additional braking-potential available with proper rear-brake use. Remember, rider-weight, passengers and cargo will affect this dynamic on all styles of motorcycles. Should sportbike riders totally avoid the use of the rear brake for maximum stopping?

Many do. It’s a matter of personal choice, personal practice and confidence in one’s skill-level. Some do not. Of course, if the sportbike’s composite weight has transferred forward to the extent that the rear tire is actually off the ground, a not-uncommon event for sportbikers, the rear brake and tire can contribute nothing to increased deceleration G-forces. For the rest of us, emergency stopping should probably include skillful use of the rear brake. How does an accomplished rider properly operate the rear brake in a maximum-braking situation?

The rear brake and tire are at maximum effectiveness at the point the emergency stop begins, and the stopping-power progressively decreases, goes away, as the emergency stop is in progress. Of course, this is because there is less and less weight (weight = traction) on the bottom of the rear tire as weight-transfer is underway. Technically, one could begin by applying a moderate amount of pressure on the rear brake pedal at the onset of the emergency stop, and then progressively lessen the pressure while the bike is coming to a stop. Wow, that’s an awfully precise and delicate motor-muscle skill! If we goof that up, what are the possible consequences?

Too much braking pressure on too little contact-patch weight results in a sliding, or skidding tire. Such a skidding tire contributes nearly nothing to deceleration G-forces; the tire is ‘surfing’ instead of trying to lock the compliant rubber into those useful imperfections in the pavement surface, the ‘chain-and-sprocket’ effect. Worse, the tire is no longer spinning, generating the gyroscopic forces that add to the motorcycles stability. Even worse, the locked, stationary rear wheel and tire have no idea where they should be; ordinarily, they belong directly behind the front tire.

Since the portion of the motorcycle in front of the steering-stem is experiencing hard deceleration and the part of the bike behind the steering-stem can only receive braking force through that hinge-like steering-stem, and since the part of the bike behind the steering-stem accounts for as much as 80% of the composite weight – and it’s still full of forward-moving kinetic energy, the back of the bike is inclined to try to pass the front of the bike, rotating around the steering-stem. It fishtails; the back tire is almost certain to slide off to one side of the other while skidding. Okay. The rider is still usually able to balance, using the handlebars to keep the skid under control and the bike close to upright, but can no longer steer in a desired direction – unless the desired, safe direction is the same one the bike itself chooses. Slim chance of that, isn’t it!

In a tense emergency situation, should the rear tire lock and skid sideways, the rider feels the wiggly movement, or hears the tire screeching and releases the rear brake, a common involuntary reaction for many unskilled and unpracticed riders. Oops!

Some riders know about the next part; the potential for a painful high-side crash! Once the rear brake is released, the non-rolling tire is now once again suddenly rolling at road-speed, forces itself to try to get directly behind the front tire where it belongs and generates a substantial amount of unwanted oblique force into the chassis. The rider is often forcibly ejected forward and to one side, and then the motorcycle itself, now behind the rider, tries to chase the crashed rider down. Double jeopardy!

Rear-brake basics: Learn to use moderate-to-light pressure on the rear brake. You may be successful teaching yourself the ‘moderate-then-less-and-less-pressure’ rear-brake pedal technique in practice; however, it might be best to consider a ‘steady-very-moderate-pressure’ technique that has the potential to be more reliable and less risky under survival-pressure. But what if the rear brake locks? What do I do?

As a baseline technique, it is suggested that a rider leave the rear brake locked; in fact, ADD some pedal-pressure to keep it locked, and skid to a stop. Remember, you likely will still be able to balance and remain upright; however, you cannot change direction. If the rear wheel is almost totally aligned with the front wheel, a rider may be able to safely release the rear brake and retain control, although there is always the strong potential for a nasty ‘snap-wobble’. However, many riders’ perception of ‘almost totally aligned’ may be faulty, and a dreaded high-side is a substantial risk. Higher speeds reduce this ‘almost totally aligned’ cone to no more than a few degrees. Make good choices if you find yourself in this high-risk situation.

On non-pavement surfaces such as grass, dirt, and gravel, the skidding rear wheel may usually be released without nearly as much risk of a high-side. In fact, rear-wheel-skidding practice on dirt or gravel might be a good way to experience the ‘I can balance but not steer’ conundrum, and be better prepared. As with all skills-practice, there is always a risk.

An all too-common mumpsimus (look it up!) among unskilled riders is the proscription;

“Don’t’ touch the front brake! It’ll flip you over the handlebars!”

Where did that come from? Let’s speculate: First, from bicycling experience. Bicycles are short and light – the entire weight of the bicycle is usually a fraction of the riders’ weight – and it is entirely possible for a bicyclist to apply too much front braking and actually ‘flip over the handlebars.’ Motorcycles, however, are much longer than bicycles, and weight several times as much as the rider. Apart from sportbikes, who do sometimes lift the rear tire up and forward, albeit slowly, motorcycles will almost never ‘flip you over the handlebars’ from too much front brake pressure. So if it’s not the front brake that does it, what?...

Continuing to speculate, the unskilled and unpracticed casual rider finds himself or herself in an intense survival situation where maximum braking is called for. What is our learned and practiced response when in a similar situation while in our auto? STOMP on the brakes, right! With the same foot, the right one in most cases. Let’s assume the rider thinks about adding some front brake pressure, just about the time the rider’s involuntary response to the sideways skid and screeching tire-noise is to release the rear brake. Perhaps the rider was launched over the high side just about the time they remember reaching for the front brake. Perhaps the stalwart front brake is blamed for the rider’s incompetent use of the rear brake. Could this be possible? Not only possible, but quite likely. So, the front brake, our best friend when intense stopping is called for, is blamed for the sins of the rear brake. Oh my…

In summary, we squeeze the front brake progressively and firmly, as needed, and use the rear brake moderately in situations that call for hard or maximum stopping.

Braking Force Progression: The gold standard for stopping under braking has been recognized as one G-force, one times the force of gravity. Of course, gravity pulls stuff downward, and your motorcycle is moving perpendicular to the force of gravity, so 1G of braking would be the equivalent of the composite weight of the machine being resisted by the two miniscule tire contact-patches. Remember, your two tires cover less pavement area than two one-dollar bills! That’s less than thirty square inches – so you, you bike and any cargo, let’s say, weigh seven hundred pounds; 1G of braking would be seven hundred pounds being resisted by the coefficient of friction of thirty square inches, about 20% of one square foot  – probably less – of rubber against asphalt.

1G is usually recognized by professional test riders in the moto-magazines as a stopping distance of 120 feet at 60mph. Okay. So what happens if we slow down a bit? What would the same stopping force of 1G stopping distance be at 30mph? Sixty feet, right? Sounds right. But actually, braking distance changes at the square of speed; it is not linear. 1G of stopping force at 30mph would be THIRTY FEET, not sixty!

Why is that important to risk-averse riders? It helps us understand that modest speed reductions have a favorably disproportionate effect on actual stopping distances or… slowing down perhaps ten or twelve miles an hour when entering a higher-risk situation can cut actual braking distance in half, or even less. Important? You bet! So, think about getting rid of speed you don’t really need early, while it’s easy to do – trying to shed excess speed in tense and complicated situations is often the straw that breaks the camel’s back.

How Many Fingers? Here is another braking subject that generates intense and often needless controversy. Coaches in riding courses often insist that student-riders use all four fingers to operate the front brake, and oftentimes, student-riders feel more comfortable and fully competent when using one, two (usually), or three fingers to operate the front brake lever, while using the remaining digits to modulate the throttle. What’s the answer?

Recall that we observed that brakes are used for two distinct but related operations – slowing to a lower speed, or coming to a complete stop, either normal, planned stops like a traffic signal, or an emergency stop. Let’s consider the ‘stopping’ function first; when bring the bike to a stop, particularly a hard or emergency stop, precise modulation of the front brake lever is critical, in order to quickly but softly arrive at the level of threshold braking – the braking force that begins to smear the front tire but allows the tire to continue to roll at nearly road-speed. This allows the rider to maintain balance, steering ability and stability, while achieving maximum deceleration force. Consider that the muscles in all four fingers, working a less than maximum effort, can modulate force far more precisely than two (the most common split-finger configuration) fingers working at maximum effort.

On the other hand, when slowing to respond to road conditions, traffic events and the like, riders are often using the throttle at the same time as the front brake, and a split-finger technique assists smooth transitions between acceleration and deceleration, as a skilled and trained rider simultaneously overlaps throttle and brake use. So, which is best – two or four?

Let’s agree that this is a situational question. Riders might consider building the habit of using all four fingers – or covering the lever with all four - when stopping, or when approaching or entering higher-risk situations like complicated intersections, and use a split-finger technique when adjusting speeds for conditions – setting entry speed for an approaching corner would be an example. Riders who strive for a smooth ride often overlap brake and throttle use, and the split-finger technique facilitates skillful use of this technique. So, let’s use some common sense and put this needless controversy to rest.

What about the riding coach that insists on ‘all four fingers?’ If this is a beginning-rider training course, that’s what is called for; we learn and internalize the fundamentals first, and build the more advanced skills and techniques atop these fundamentals. Instructors and coaches in more advanced curricula are likely to accommodate competent split-fingers techniques and even coach these skills.

Finally, let’s consider the benefits of using the rear brake to precisely control speed, either in low-speed situations like parking lots and traffic jams, as well as increasing smoothness while cornering. First, in the low-speed setting a rider may become proficient, through practice in simultaneously holding the clutch in the friction-zone, the throttle held at a steady value and the rear brake then used – sometimes in conjunction with minute adjustments in the friction-zone – to control speed with far more precision than can be achieved with the throttle alone. Again, purposeful practice in this technique can add a high degree of confidence for riders, and can make walking-speed U-turns a breeze.

In addition, using the rear brake while cornering at roadway speeds can achieve a similar benefit - precise speed control and smoothness. This can be particularly useful for riders of heavier machines, and for those riding two-up. If one is using brakes during the first half of the corner – we know this technique as ‘trail-braking’ – use of the rear brake in addition to front-brake use can allow the rider to again precisely control speed near the apex of the turn without suffering traction risk. Remember, when using the rear brake against a steady throttle, the resistance between the braking and driving forces are not at the rear tire’s contact patch, but at the meeting of brake pad or lining, and disk or drum. The key to this technique is to continue to use a tiny amount of rear brake pressure AFTER fully releasing the front brake, usually at or near the apex of the turn.

Rider safe, ride often… think!