The Low-Down on Bike ABS, Linked and EVO Brakes

["Steve Makohin" <wateredg@xxxxxxxxxxxx>]

Hello,


I have written this article for people who are new to motorcycle ABS or to

other brake technologies (linked brakes and servo brakes), and for those who

may not be aware of some of the potential "gotchas" that are lurking with

bike ABS. This article is divided into sections to allow semi-interested

readers to skip parts they don't care about As always, the "Delete" button

is at your disposal.



The sections are:

 o Overview

 o Why ABS exists

 o How ABS works

 o ABS Reduces Stopping Distance (in most cases)

 o Motorcycle ABS Shines (in most cases)

 o The ABS Safety Net

 o When ABS Doesn't Shine

 o ABS is the Future of Motorcycling, Available Today

 o Linked or "Integral" Brakes

 o Combining ABS, Links, and EVO

 o Epilogue







The Low-Down on Bike ABS, Linked and EVO Brakes

(or "Why ABS Shines, and Sometimes Sucks")



Overview:





This article explains three separate brake technologies:

  1) ABS,

  2) Linked and Semi-Linked brakes, and

  3) EVO brakes, also known as servo-controlled brakes or

     "whizzy" brakes.

It includes a brief, non-technical overview of each technology, and it

focuses on the benefits and some of the negatives of these technologies.





Why ABS Exists:



Understanding why ABS exists and its purpose will help you understand what

to expect of ABS, as well as to appreciate some of its limitations. It will

also help you understand why ABS behaves as it does. Without this

information, you may be surprised to find yourself in a situation in which

you believe you "have no brakes" or your ABS "caused an accident".



ABS is an acronym for Anti-lock Brake System. The ABS systems on bikes, like

those on cars, are designed to do one thing, and one thing only: to help

prevent wheel lockup which could result in a skid and subsequent loss of

vehicular control. That is the one mission that ABS is designed to fulfill.



ABS on a motorcycle is arguably even more important than ABS on a car

because the consequences of loss of control on a motorcycle typically

results in the bike being "dropped", thereby causing damage to the machine,

and exposing the rider to the risk of personal injury.





How ABS Works:



ABS is a hydraulic brake system with a few extra components. Most motorcycle

ABS have independently functioning sensors on the front and rear wheels.

These sensors measure the speed at which each of the wheels are rotating,

and they report that information to an ABS computer. The ABS computer is

continuously running an application (software) that evaluates the wheel

rotation information and combines it with other information, and then

applies certain rules to determine if a wheel is at risk of locking up.

Different manufacturers, such as BMW and Honda, have different "logic", or

principles that are used to determine exactly what constitutes an "impending

lockup" condition at a specific wheel. Some highly simplified examples of

the logic that may be used are as follows:



 - IF front brake is ON AND front wheel rotation speed is less

      than rear wheel rotation speed THEN front wheel is at

      risk of lock-up



 - IF rear brake is ON AND rear wheel rotation speed is less

      than front wheel rotation speed THEN rear wheel is at

      risk of lock-up



Note that one of the conditions for sensing an "impending lockup" is always

"brake is on" -- ABS may activate only while the vehicle operator has the

brake applied.



The wheel rotation sensors and the ABS computer do their work very quickly,

on the order of making tens of evaluations per second, so they can detect an

"impending lock-up" condition and respond to it much more rapidly than any

human can. They can also respond to changes in conditions much quicker, and

much more precisely than humans.



When the ABS computer determines that a wheel is in a state of "impending

lock-up", it takes action to alleviate that condition. It does this by

reducing or releasing brake pressure at the affected wheel in order to give

the tire a chance to re-establish its grip with the road (i.e., no skid).

The ABS computer then reapplies the brake to that wheel to resume stopping

power. In resuming the application of brake force, the ABS computer may

still detect the "impending lock-up" condition, in which case it repeats the

cycle by temporarily reducing or releasing brake pressure at the affected

wheel, and then reapplying the brake force. This cycling of releasing and

applying the brakes happens very quickly, occurring several times per

second. Some systems are slow enough that the operator feels a vibration as

the ABS is engaged, while others may not give any tactile feedback at all.

It depends on the system.





ABS Reduces Stopping Distance (in most cases):



As mentioned earlier, ABS is designed to perform one function, and that is

to help prevent wheel lockup which could result in a skid and subsequent

loss of vehicular control. Even though ABS was designed to prevent loss of

control, it also affects stopping distances - shortening them under most

conditions, and sometimes lengthening them in others.



Maximum stopping power, that is the greatest rate of deceleration, can be

attained just *before* a wheel locks up. There are a variety of human

challenges, especially on a motorcycle, in trying to rapidly apply brakes to

quickly attain this threshold, and then to vary brake pressure as conditions

change throughout the stop. The dynamics start within a fraction of a second

of applying your brakes as your bike's momentum shifts the weight bias

forward, thereby lightening the downward force on the rear wheel (and in

doing so, making it more prone to lockup and skid) and increasing the

downward force on your front wheel (and in doing so, giving it more traction

and allowing you to apply still greater brake pressure before it locks up).

This means if you want to attain maximum stopping power, you must manually

and continuously vary the brake pressure independently at both wheels to

compensate for changing conditions. And that's not easy to do.



Extremely good riders, such as professional world-class racers, are

well-versed at threshold braking. Repeated testing with a variety of riders

and motorcycles, under a variety of conditions, have concluded that a

non-ABS bike can attain a shorter stopping distance than the identical

ABS-equipped bike, providing that all six of following criteria are met:



  1) The exercise takes place on a high-traction surface.

  2) The high-traction surface does not have any variables that vary the

     available traction.

  3) The rider knows the traction limits of the test bike on the test

     track (this requires multiple attempts to hone the perfect stop).

  4) The rider is mentally and physically readied for the braking exercise.

     (i.e., no surprises)

  5) The rider is well-trained at threshold braking, and well-practiced.

  6) No human error occurs during the execution of the exercise.



However, on the street, most of the time you will experience considerably

different conditions than those in the "perfect braking scenario" at the

track:



 o Tire temperatures vary to provide more or less grip.



 o Tires wear throughout their lifetime to provide different

   handling, so the maximum grip you got three months ago may

   not be the same as you get today.



 o Tire pressure may be a few pounds off from your previous

   threshold braking session.



 o Road surfaces vary as do the level of traction they offer.



 o Road surfaces may be further affected by debris such as

   sand or gravel, oil or gasoline, or even being slicked

   with rain.



Any or all of the items above make it difficult, if not impossible for a

human to quickly ascertain (without exceeding) the actual traction limits

that are available to them on a first attempt at threshold braking. To make

matters worse, the conditions may change suddenly and significantly during

the braking, such as starting the braking on smooth, dry, fresh asphalt

(superb grip), and then crossing onto older, glazed asphalt (reduced grip),

or encountering a little sandy patch (severely reduced grip). Worse still is

the fact that in an emergency situation, even the best-trained and

well-prepared human brains tend to simplify, and focus on a very few things.

This contrasts sharply with the multitasking that is required to execute

threshold breaking well.



Extensive research in the field of motorcycle braking, some of it dating

back to the Hurt Report (the most extensive study of motorcycle incidents to

date), tells us that in the vast majority of motorcycle incidents in which

emergency braking was required or in which it was a viable option, the rider

did not apply the brakes hard enough. For fear of locking up a wheel,

skidding, and losing control of the bike, riders increase their stopping

distances, in some cases dramatically, and as a result, they "ride into"

danger -- an obstacle, another vehicle, or off the road.



Motorcycle ABS performs extremely well in emergency braking situations by

allowing the rider to apply brakes hard and fast without worrying about

locking up a wheel and skidding. ABS will use up all the available traction

to provide maximum stopping power. And because ABS responds so quickly, it

continuously adjusts to account for changing tractions conditions, such as

running across a sandy patch or transitioning from grippy fresh asphalt to

much less grippy glazed asphalt or an unpaved shoulder.





Motorcycle ABS Shines (in most cases):



ABS performs so well on the street, that with very few exceptions, it

provides shorter stopping distances as compared to an identical non-ABS

bike. This reality has far-reaching consequences: Under the vast majority of

street motorcycling scenarios, an average rider with motorcycle ABS can

attain shorter stopping distances than the most skilled motorcyclists can

(e.g., a professional, world-class racer, etc.) on an identical, non-ABS

bike. This is true even when the pros are given multiple opportunities to

shorten their distances. Not surprisingly, even the pros can shorten their

stopping distances when they switch over to an ABS-equipped bike on the

street. Numerous tests by a variety of organizations have proven this, time

and again. Scenarios like the well-prepared, well anticipated, non-panic,

perfect threshold braking exercise on a high-traction non-variable track are

the rare exception when riding on the street.



Stopping distances are shortened with bike ABS for two distinctly different

reasons:



 1) The rider is able to apply the brakes hard and fast without the

    fear of a lockup and skid. Doing so lets you use significantly

    more of your brake's potential stopping capabilities, sooner.

    In other words, ABS lets you convert much more of your bike's

    potential stopping power into actual stopping power.



 2) ABS continuously monitors changing traction conditions, and it

    rapidly adjusts to compensate for those dynamic conditions. This

    helps to keep your brakes close to their limits of maximum

    stopping throughout the stop.





The ABS Safety Net:



Motorcycle ABS offers a considerably increased safety margin. However, if

you use up that safety margin, you may put yourself at even greater risk

than by not having ABS. For example, if you get into the habit of riding in

closer proximity to other vehicles, or at higher speeds because of the

belief that ABS will be able to rescue you when you call upon it, then the

additional safety margin offered by ABS may be insufficient to overcome the

higher risks you have introduced through your change in riding style.



When you ride an ABS-equipped bike, be aware when the ABS engages. When it

does, consider it a warning that you have already exceeded the limits of

your motorcycle and/or your riding abilities, and that it has prevented a

skid. Adjust your riding style to avoid repeated ABS engagements. And ride

as though you did not have the benefit of ABS available to you.





When ABS Doesn't Shine:



If your bike has ABS, then your motorcycle's owner manual will likely have a

section that duly warns you that ABS does not do some things well, or at

all. One of those things is circumventing the laws of physics. If you want

to get the most enjoyment out of your ABS-equipped bike, reduce risks, and

avoid potential pitfalls, it is important that your understand not only what

ABS does (as described earlier) but also what ABS does not do, and the

conditions under which ABS may not do what you want it to do.



In most cases, ABS can not be faulted for not doing something it was not

designed to do. In other cases, ABS has inherent characteristics that may

surprise unprepared riders.



The first group of issues relate to the limits of traction. Each motorcycle

has a finite traction limit, which is influenced by a variety of conditions.

The full amount of traction can be distributed between steering and

braking -- the greater the steering, the less the braking, and vice versa.

You can see this principle in action in a car without ABS that is in a full

wheel lockup. The full limit of traction is being expended on braking, and

the car will not steer under these conditions. Similarly, on a motorcycle,

when you are executing a turn at or near the limits of traction (all or

virtually all traction is being expended on turning), the application of

brakes may push your machine past its limits of traction resulting in a

skid. In this case, one or both wheels will slide out from under the bike to

produce what is commonly called a "low side." ABS simply cannot do anything

in this example because as soon as the skid is initiated, the bike is

already leaning beyond the point of recovery, so releasing the brakes has no

effect. In other words, an ABS-equipped bike fairs similarly to a non-ABS

bike.



The above example is an extreme one, because most street riders do no ride

at their motorcycle's performance limits in graceful sweepers. A more

realistic scenario is one in which the rider is executing a turn in which

they have a considerable traction reserve (e.g., 25%), and an unexpected

event is introduced (e.g., a deer crossing the road), and the rider responds

by applying brakes that exceed the traction limits. Once loss of traction

occurs and the bike is beyond the recoverable lean angle, a "low-side"

occurs, with ABS or without. For ABS riders, the important thing to note is

that the more vertical your bike, the greater the safety margin offered by

ABS.



Another group of issues arises in scenarios in which a very short

(transient) skid is preferred as compared to ABS's momentary release of

brake pressure. A real-world example is applying brakes, moderately on a

smooth, level, good-traction surface, and then encountering a recessed

man-hole cover. At the moment when the front wheel steps over the

high-traction road and momentarily descends an inch to a lower-traction

metal plate, the ABS system may detect the reduced wheel spin and trigger an

"impeding skid" condition, and pulse the front brake. This results in a

reduction of brake force and a lengthen stopping distance. Without ABS, the

wheel would very briefly reduce its spin rate, and then resume its effective

braking, likely without the rider even noticing a chirp from the tires.



A similar condition arises under moderate or hard braking on a road whose

surface is wavy. Such is the case with "frost heaves" in the northern United

States and in Canada, or on roads in a poor state of repair. Another example

is braking over a bump that throws the bike and rider upwards. In such

scenarios, the ABS may detect a momentary reduction in wheel spin, and pulse

the brakes. These conditions are exacerbated when riding down-hill, because

you have gravity adding to the forward momentum of your bike, so when ABS

engages, not only will it lengthen stopping distance as compared to a

non-ABS bike, the rider may interpret this as a "lunge forward", or the

brakes failing and then suddenly re-engaging.



By being aware of these characteristics, you can adjust your riding style

accordingly. In some extreme cases, where a rider spends much of their time

in these conditions, ABS may prove to be detrimental by doing the "wrong

thing" most of the time. Fortunately, some motorcycle models have switchable

ABS that allows you to turn ABS off when you know you will be spending a

fair bit under these conditions, and back on again when you're out of them.





ABS is the Future of Motorcycling, Available Today:



BMW was an early adopter of motorcycle ABS, and they have learned a lot from

the time they spent in the saddle. For example, the BMW GS series of

dual-sport bikes are available with switchable ABS to accommodate dirt

riding where deliberate rear-wheel skids are commonly induced (called "trail

braking"). BMW's newest R1200S sport boxer has available switchable ABS for

track days, to accommodate performance riding purists. All of BMW's

motorcycles either have ABS available, or standard.



But BMW is just a small manufacturer. Honda, on the other hand, is the

world's largest motorcycle manufacturer by a wide margin. In late 2005, they

announced that within a few years, every road-going Honda motorcycle will

have available ABS, and in many cases, ABS will be standard. This strong

endorsement tells us that motorcycle ABS is here to stay, and it will soon

be the norm rather than the exception. Other manufacturers are expected to

follow suit. Even the venerable Harley-Davidson is reported to be developing

their own ABS system, only to meet the demands for higher safety amongst

American police forces.





Linked or "Integral" Brakes:



Linked braking systems link both the front and rear brakes to the

motorcycle's brake controls such that both the front and the rear brakes are

applied when only the front brake lever or the rear brake pedal are actuated

by the rider. In the case of fully-linked brakes, both brakes are always

applied, whether you use your front brake (right hand lever) or rear brake

(right foot pedal). With semi-linked brakes, the hand lever affects both

brakes while the foot pedal affects only the rear brake.



In many cases, linked brakes has been proven to reduce stopping distances,

especially when teamed with ABS. For the shortest stopping distances, both

front and rear brakes should be applied up to the traction limits (i.e.,

threshold braking). This threshold varies depending on a number of factors

ranging from the rate at which the brakes are applied, to whether a pillion

is available to apply additional weight to the rear tire, and therefore

additional stopping power.



BMW's linked brakes, according to the manufacturer, are "intelligent" in

that they apply the "correct" proportions of front versus rear brake

pressure, depending on the circumstances. They are especially useful with

the cruiser crowd who frequently apply only the rear brake, which

coincidentally provides the least stopping power and is most prone to

lockup. With linked brakes, both brakes are applied when the cruiser rider

actuates the rear brake pedal, thereby providing much more stopping power.



Linked brakes do have their enemies, though. Authority motorcycles

frequently find themselves in low speed maneuvers, so it is desirable to

apply the rear brake alone ("trail braking") while also applying throttle.

Similarly, in performance riding it is useful to trail brake as you enter a

corner in order to elongate the chassis and stabilize the suspension. In

these cases, semi-linked brakes allow you to apply the rear brake alone, as

desired.



Linked brakes have proven to be a benefit in emergency stops, because a

rider does not always do the right thing in a panic scenario. Sometimes,

they apply only one brake, typically favoring the front brake (cruise riders

often favor the rear). In these scenarios, without linked brakes, the unused

brake ends up as unrealized potential stopping power, and stopping distances

increase unnecessarily.





Servo-Controlled Brakes:



The final attribute to BMW's brake technology is "EVO," which is most easily

understood as power-assisted brakes. EVO provides its assist only while the

ignition is in the ON position, and just like power brakes in a car, it

provides much more stopping power with less brake lever or brake pedal

effort.



Current motorcycles already have awesome brakes enabling a front wheel

lockup or a stoppie with just two fingers of braking force applied to the

right lever, so one may ask why bother boosting the braking power at all?

The real benefit, as claimed by the manufacturer, is in the EVO braking

computer, which senses the rapid application of brakes, thereby detects an

emergency braking condition and it can take appropriate action.



When the EVO computer senses this condition, BMW says that the brakes are

pumped up to full pressure about 1/10 of a second quicker than any human can

accomplish the same feat. This translates into a stopping distance from 60

mph to zero that is almost 9 feet shorter with EVO assistance than without.

Though 9 feet over a stopping distance of over 100 feet may not appear

meaningful, it may make the difference between connecting with an obstacle

or not. Or connecting at a lower speed, thereby reducing or eliminating

personal injuries.



Early releases of EVO brakes were criticized for being "grabby", or

providing much more stopping power than you would expect with little control

effort. Current BMW EVO-equipped bikes have been updated to be more

compliant to users' expectations, though they still may need some "getting

used to." Also, like a car, the power assistance disappears when the

ignition is switched to the OFF position, which surprises new EVO-equipped

bike owners when they roll their bike with the engine off only to discover

that they need to apply much more brake lever pressure than they expected to

stop the bike.



BMW bikes are not the first to take advantage of this "emergency stop"

program. Mercedes-Benz cars have had it for years, as do some BMW cars.





Combining ABS, Links, and EVO:



When you combine all three BMW brake technologies, you have the potential to

provide a much-enhanced braking experience. In a real-world scenario, a

rider is about to pass an SUV whose inattentive driver suddenly veers into

the rider's lane. From force of habit, the rider rapidly applies his rear

brake pedal. EVO brakes sense the rapid application of the brake pedal and

they determine an emergency braking maneuver has started. They pump up the

brakes to full pressure at least 1/10 of a second faster than the rider

could do so himself. Fully-linked brakes take the rear brake pedal control

signal and use it to apply both front and rear brakes. The linked brake

system senses the additional available traction at the rear wheel because a

pillion is on board, and distributes the brake force in appropriate

proportions across the front and rear brakes, using all the available

traction at both wheels and translating it into actual stopping power.

Sensing an impeding lock-up, ABS kicks in to prevent both wheel from

locking, thereby averting loss of control while providing rapid

deceleration. All this happens in the blink of an eye with

confidence-inspiring precision and control.





Epilogue:



No matter what brake technology your bike has, it behooves you to develop

superb riding and braking skills. These technologies, however, offer

dramatic gains in improved safety, especially in panic stops, and in the

case of ABS, when braking on streets whose traction is less than perfect, or

on which traction varies.



This is the real world, full of imperfections. And even the best riders with

the most highly developed braking skills can't come close to guaranteeing

that they will be able to meet all six criteria that are essential to

executing the perfect threshold braking maneuver, thereby outbrake a non-ABS

bike.





- -Steve Makohin

 '01 R1100S/ABS

 Oakville, Ontario, Canada

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