No Fault Braking
A Real-World Comparison of ABS Systems

By Michael Kneebone
September 1992

Reprinted with permission of Motorcycle Consumer News.
For subscription information, call (800)365-4421, or visit www.mcnews.com

On a good day, with wide handle-bars, smooth pavement and some luck, it is possible to lock a wheel on your motorcycle during a panic stop and stay up. Unfortunately, you don't usually get the chance to pick the time and place for a panic stop. More likely, someone else will pick it for you. It's during those unplanned stops that having an ABS system on board can pay for itself.

But before we discuss ABS, it's important to understand why a locked wheel can cause a crash. The best way to do that is by imagining our way through a front-wheel-locked stop.

Why We Need It
Our crash course in braking begins at about 40 mph when you apply a handful of front brake. Unexpectedly, you ride over a small patch of gravel and the front wheel loses traction. With the wheel skidding, you've lost control, yet the motorcycle is still continuing straight. All is well-so far.

Unfortunately, a sliding tire is free to go any direction it wants. But with a firm grip on the bars and good balance, you've managed to stay upright to this point. The correct procedure from here is to release the brakes enough to get the wheels spinning again, and then re-apply them.

That's a lot easier said than done as you watch Grandma Jones' Buick hang a left in front of you. With your adrenaline pumping, you have a pretty healthy grip on the brakes, and easing off isn't part of your brain's game plan. Now there's a whole list of things that can go wrong.

Your biggest worry is the back half of the motorcycle-it's probably trying to pass the front. With the rear wheel partially unweighted and the steering neck turned into a huge hinge, it doesn't take a lot of effort for the rear to pivot and swing to the side. As if that's not enough to deal with, with the front wheel stopped you've lost the stabilizing gyroscopic force that smoothes out and slows steering inputs. With spinning wheels you can hit a six-inch pothole and the motorcycle will just about steer its own way out. With a locked wheel, any disturbance, no matter how small, can be disastrous.

Perhaps your undoing will be by way of some antifreeze or a small rock, or simply a tiny imperfection in the pavement. Whatever the cause, the wheel turns sideways (for discussion, let's assume the wheel turns to the left). If you're really quick, this is your last chance to release the brakes and regain control of the motorcycle. But more than likely you're past the point of no return. Once the tire changes direction, so does the contact patch. With a side thrust acting against the tire, the wheel suddenly turns full-lock against the steering stop violent enough in many cases to damage the metal stops.

As the right handgrip is ripped from your hand, the left grip is pushing your arm towards you. You react on instinct, reaching out for the missing handlebar, unknowingly shifting your weight outside and to the right. Now you're not only sliding, you're leaning to the right with the front wheel turned to the left. Congratulations. You just got tossed off your motorcycle.

From start to finish, our entire crash (not including sliding down the road) took about three seconds. Unfortunately, even in a controlled environment the idea of stopping by easing off the brakes is just not natural. The only way to really learn the technique is to practice (a risk in itself) until it becomes an automatic reaction. Then practice it again, and again.

Where ABS Comes In
Of course, not everyone will want to explore the potentially hazardous area of expert braking, and that's where an anti-lock braking system takes over. With its sophisticated computers and complex hydraulic systems, ABS will allow even the most timid to venture into the nether regions of full-compression braking without having to risk life, limb and motorcycle in the process.

For now, ABS is available on a limited number of motorcycles; five BMWs, the Honda ST 1100 and the Yamaha FJl200. Each of these manufacturers use computer-managed hydraulic systems for their ABS packages. (For complete technical reviews of the ABS systems and their operation, refer to the August 1991 and January 1992 issues of RR.)

Like our rider, if a wheel starts sliding, ABS's job is to ease off the brakes and then re-apply them. However, ABS goes one step further. Instead of waiting for a wheel to slide before taking action, modern ABS systems can actually predict wheel lock-up before it happens and begin to ease off the brakes.

This predictive quality is thanks to a relatively new concept in computer programming called "fuzzy logic." Although this is, at best, a crude description of a very complex computer system, ABS "thinks" along these lines: "Something is up! Only 9 milliseconds ago my front wheel was traveling 59.4mph and the rear 59.6. Now the front is only turning 26.3 and the rear 31.3 mph. I know it takes at least 20 milliseconds (on a perfect braking surface with sticky tires) to slow either wheel by a single mph. I'd better look into this a bit more deeply." Long before the rider realizes anything is wrong, the computer has figured that we are losing traction at a rate faster than that conducive to a controlled stop even under the best conditions.

From here, it's a simple matter of rechecking the data from the wheel sensors, running some internal diagnostics to make sure the computer and wheel sensors are okay, asking the backup computer to confirm our diagnosis, then issuing the command to the hydraulic systems to ease off the brakes. The entire process takes less than one second.

Once the wheels spin back up to speed, the computer re-applies the brakes and the process starts over again.

BMW's ABS checks wheel speed approximately 330 times per second and can modulate braking force seven times per second (the mechanics of fluid management are the limiting factor - not the computer). The Japanese, with a three-year-newer design and faster hydraulic Systems, have upped the modulation rate to 10 times per second. That's three times faster than even the best rider can ever hope for.

Unfortunately, ABS cannot scan the pavement ahead and make minor adjustments to braking pressure to optimize stopping power. Today's systems leave that job to the rider. The ability to look ahead and decide to reduce braking pressure, or to increase it, explains why a skilled rider can often out-stop someone who just squeezes the brakes as hard as possible and lets ABS take over. While ABS will save your butt, it is as yet not totally efficient (in terms of stopping distances) in modulating the brake pressure, no matter how fast the computer and hydraulics work.

The current crop of ABS bikes work best if you're willing to work with them to find the fine line between ultimate stopping power and a locked wheel. From a technical standpoint, with a relatively simple software modification and a few plumbing changes, ABS could take over all braking decisions, controlling both the front and rear brakes from either the hand or foot levers. Squeeze either lever for an easy, medium, hard or panic stop and the computer could manage the rest. But the controversy over integrated braking would pale in comparison to this level of automation. Besides, the legal complications involved with a completely automatic system will probably keep that idea hidden on a secret prototype forever.

The Tests
In June of 1991 RR contacted 10 ABS- equipped BMW owners and asked them each to carry a notebook. Each rider had a simple but time-consuming job. Each time their ABS activated, they recorded the following: Road surface, weather conditions, type of braking (i.e., easy, panic, etc.) and, in their opinion, whether the ABS helped or interfered with bringing their motorcycle to a safe stop. (See Figure 1)

Figure 1. BMW Owners Anti-Lock Braking Report
Based on 9 (of 10 that started) riders over 8 months, totaling 26,709 miles

Fast stops due to close calls
 ABS not neededABS assistedTotal
Call pulled in front347

ABS Activation: Non-Panic Stops
Wet pavement (rain)22
Oil or Grease14
Gravel or dirt11
Changing light9
Other causes17

Other panic stops (defined as those stops when riders let ABS take complete control

Has ABS saved you from an accident? Eight out of nine said "yes"

In April of 1992, with our RR test group data in hand, we turned our attention to a comparative test of non-ABS vs. ABS-equipped motorcycles. Representing ABS was a 1992 BMW K100RS-ABS and a 1992 Yamaha FJ 1200-ABS. Unfortunately, the ABS-equipped Honda ST1100 came a bit late to the party and had to be tested separately in June.

While we could have disabled the anti-lock braking systems (it's a simple matter of throwing the computers into a fault mode or removing a fuse) and run those same bikes for the non-ABS test, we chose instead to line up one each of those same models without ABS.

Why? To see how the additional ABS plumbing affected brake feel at the lever, and to compare just how a motorcycle with an ABS failure stacked up against the standard model.

Representing the non-ABS group was a 1991 BMW K100RS, a 1991 Honda ST1100 and a 1991 Yamaha FJ1200. However, more important than the motorcycles were the testers themselves. While a professional racer would probably generate valuable input in terms of stopping power, we wanted a broad range of experience among our riders. Anything less wouldn't give us the real-world comparison we were looking for.

Our testers consisted of a closed-circuit endurance racer, a long-distance endurance rider, a RR staffer, a touring rider with 70,000 miles of experience, a motorcycle mechanic with a 37,000 mile riding history, and a relative novice with less than one year and 4200 miles of riding under his belt. We gave our riders the following labels: Racer, Mileman, RRider, Tourer, Mech and Newguy.

Although the bikes were heavily insured, money can't repair testers' bodies. We toyed with the idea of getting an outrigger outfit to protect riders from a high-side, but decided to pass. Besides, the security of the outriggers might have gotten our test group to brake harder than they would on the street, and that again ran counter to the idea of a real-world test.

Additionally, since it was impossible to safely "scare" our riders to emulate a true "panic braking" experience, we simply instructed each rider to stop as hard as he felt his skills would allow without taking unnecessary chances. A stern lecture before each set of braking runs had to substitute for outriggers.

For the test area, we found an industrial park that was under construction in a Chicago suburb. Besides being deserted on the weekends, it provided us with most of the surfaces our pilot group of BMW riders had reported ABS kicking-in on:

Despite our quest for safety, there was still some danger involved. While we had an open area nearly a mile long, the wide street had curbs on both sides-some thing we made a point of constantly reminding the riders of.

Since four of our six testers had never ridden an ABS-equipped motorcycle, we planned a day of practice just to get everyone up to speed. Additionally, while the BMW, Honda and Yamaha are similar in design, they each have a unique ABS "feel" that takes some getting used to. Your first hard stop on an ABS bike can be quite an eye-opener. Like our imaginary non-ABS crash, the best way to describe the feel of ABS is to imagine riding each of the three brands through a typical quick stop with an obstacle thrown in. In this case, we'll be hard on the brakes crossing a metal sewer cover at 50 mph.

With the front suspension compressed and the nose down from braking pressure, the BMW's front tire contacts the slick metal cover and ABS kicks in, releasing the brakes. The motorcycle reacts about the same way it would if you manually eased off the brakes. The suspension unloads, bringing the nose of the BMW up. Meanwhile, you get the "feeling" that you are accelerating (really, you're just not slowing down for a split-second). In an instant. the Beemer is back on the brakes, compressing the suspension again and throwing you forward against the bars. Until it finds traction. the BMW unit will continue to perform this ritual seven times per second.

After a ride on the BMW, you would expect the Honda's faster system (around ten times per second) to provide even more up-and-down bouncing. but the exact opposite is true. With the system working faster, the suspension and your body have less time to feel and react to the on-again-off-again dance. Like the BMW, the seat-of-your pants lets you know ABS is working. In addition, the Honda goes one step further so that when ABS engages, the hydraulic system causes the lever to pulsate (or brake pedal if the rear wheel's ABS is engaged).

Like the Honda, the Yamaha cycles ten times per second and provides feedback through both levers. At speed, the Yamaha system was the slickest of the three. On both the BMW and Honda, riders clearly felt the stopping-freewheeling-stopping action of ABS. On the Yamaha, ABS-aided braking from high speed felt closest to a dry pavement, non-ABS stop. The Yamaha was clearly the favorite among our test group.

There is one catch to relying on ABS to manage braking. At some point, the computers have to let the wheels "stop," and return braking control to the rider. That's about 3 mph on this crop of motorcycles. While this is walking speed, if you're not paying attention on a slick surface (like oil-covered concrete at gas pumps), the wheels will lock and the bike will slide out from underneath you. These same conditions are common near stop signs (slick paint strips or oil). ABS-equipped or not, you still have to use your head.

The Real Truth
Although we set out to perform a three-way ABS shoot-out, in less than ten braking runs it became obvious that comparing the stopping distances of the Yamaha and the BMW (and later the Honda) was a waste of time. Under equal conditions, the three ABS bikes will virtually always stop within a few feet of each other. Generally speaking, in a "squeeze the brakes as hard as possible and let ABS take over" kind of stop, the Yamaha and the Honda, with their faster cycling times, will out-stop the BMW by several feet. But that was also true of the non-ABS models (Refer to Figure 2).

Figure 2. Comparative Braking Distances by Motorcycle
(Average of Five Passes on Dry Pavement)
Expert rider, all distances from 60 mph
BMW (non-ABS model)153 ft
Full ABS Control162 ft
ABS disabled
155 ft
Honda (non-ABS model)149 ft
Full ABS Control156 ft
ABS disabled
150 ft
Yamaha (non-ABS model)148 ft
Full ABS Control152 ft
ABS disabled148 ft

It was clear from our first set of runs that an ABS-equipped motorcycle with a "failure" would stop as quickly as the non-ABS models (the difference in numbers was insignificant). Although ABS bikes have several more feet of plumbing, which could theoretically make for mushy or less reactive brakes, except for our Racer (who insisted all the bikes needed braided steel brake lines), our testers felt the ABS bikes equaled the feel of the stock machines.

On the advice of our testers, we also decided to make a last-minute change to our testing procedures. On ABS-aided stops, our original plan had each rider stomping on the brakes as hard as possible, letting ABS take over complete control from start to finish. While that information proved useful, we were more interested in finding out how ABS would react when encountering a poor braking surface on a typical street ride. A ride that did not include trying to set braking records with each stop. With that goal in mind, like our non-ABS tests, riders were instructed to stop as quickly as they felt they could, leaving ABS to manage the poor traction obstacles.

For our comparisons we added the category "Full ABS Control" - and it means just that. At the beginning braking marker, riders jumped on the brakes as hard as possible, relinquishing control to the computers. The "/ABS" indicates those passes where riders stopped as hard as they felt comfortable (the equivalent of a non-emergency, but forceful stop), with ABS only kicking in over the bad stuff. While we measured stopping performance over sand, dirt, loose gravel, oil and water, it was our over-the-sewer-cover test that seemed most representative of the comparative stopping power of ABS when encountering a less-than-perfect surface. (Refer to Figure 3).

Figure 3. Dry Pavement Braking over Sewer Cover *
(All measurements recorded with Yamaha FJ1200) **
Racernon-ABS158 ft
w/ ABS163 ft
Full ABS Control166 ft
Milemannon-ABS181 ft
w/ ABS172 ft
Full ABS Control167 ft
Road Ridernon-ABS160 ft
w/ ABS165 ft
Full ABS Control162 ft
Tourernon-ABS186 ft
w/ ABS167 ft
Full ABS Control166 ft
Mechanicnon-ABS199 ft
w/ ABS171 ft
Full ABS Control169 ft
w/ ABS168 ft
Full ABS Control166 ft

NOTE: The surface used for this test varied slightly from that used in Figure #2 to compare ultimate stopping distances. Stopping distances between Figure #2 and Figure #3 are not comparative.

* All braking distances expressed in feet from 60 mph
** Both the ABS BMW and ABS Honda ST ran using this same test and achieved similar results.

Our non-ABS stopping distances varied widely from rider to rider. With the exception of Racer and RRider, as riders approached the slick metal sewer cover they released the brakes approximately 6-7 feet before reaching the cover and did not get back on them for several feet after they were back on the pavement.

That seems easy enough to correct, but as riders pointed out, at 50mph (that's 73 feet per second, or the approximate speed they crossed the cover), it appeared to them that they were braking about the time they reached the cover. They were incorrect in their assumptions.

Racer and RRider took a different approach to this problem. Knowing that the pavement was clear and sticky just past the metal cover, they skidded over it before easing off the brakes to get the wheels rolling again. Tricky, impressive and short-but hardly safe. We were also surprised to see our novice (Newguy) out-gunning more experienced riders in the dry pavement test. It turns out our novice commutes in the confines of Chicago's traffic-clogged streets where, as he put it, "You know how to brake, or you die." Meanwhile, our suburban road riders had obviously let their braking skills slide good reason for a refresher course in braking.

The data in Figure 3 reflects what we were watching out on the range. The ABS machines were consistent, with rider after rider stopping near the same marker. As the ABS motorcycles crossed onto the sewer cover, each machine released the brakes and got back on them immediately after the rider was back on pavement.

The most stunning performance of all was Mech's. His best non-ABS run was 197 feet (199 average of three runs). But with the confidence and help of ABS, he managed to finish only 11 feet behind Racer's 158 foot stop! With no crashes or close calls and everyone worn out, we adjourned for a week of rest.

Doin It In The Water
Our next set of tests involved water. The acceleration area (approximately 1400 feet long) was dry. As riders entered the 400 foot-long speed adjustment area (an area reserved to get the speed to an even 60 mph) they encountered water provided courtesy of an industrial hose. About 125 feet farther down the range a second hose kept the last half of our braking area, including the manhole cover, completely soaked. In all, approximately 380 feet of pavement (350 of which we could use for measurements) was thoroughly soaked.

No one in our test group actively practices wet-weather, maximum braking, which is about as average as you can get. Like the other non-ABS tests, each rider was instructed to brake as hard as they felt comfortable and to avoid crashing at all cost. (Refer to Figure 4)

Figure 4. Wet Pavement Braking over Sewer Cover *
(All measurements recorded with Yamaha FJ1200) **
Racernon-ABS298 ft
w/ ABS211 ft
Full ABS Control193 ft
Milemannon-ABS326 ft
w/ ABS240 ft
Full ABS Control201 ft
Road Ridernon-ABS278 ft
w/ ABS202 ft
Full ABS Control197 ft
Tourernon-ABS350+ ft
w/ ABS236 ft
Full ABS Control210 ft
Mechanicnon-ABS350+ ft
w/ ABS252 ft
Full ABS Control209 ft
Newguynon-ABS350+ ft
w/ ABS215 ft
Full ABS Control210 ft

*All braking distances expressed in feet from 60 mph
** Both the ABS BMW and ABS Honda ST were run using this same test and achieved similar results.

The results from our non-ABS stops were shocking. Three riders rode through all 350 feet of water-soaked pavement before stopping on a dry portion of road at the other end. Even Racer and RRider, the daredevils who skidded over the sewer cover during the dry pavement tests, got off the brakes long before crossing the wet sewer cover.

The improvements in stopping distances when riders counted on ABS were almost unbelievable group averaged 120-foot shorter stops! Not one of our riders on a non-ABS motorcycle could outstop an ABS machine. None were even close! Riders that had never been on an ABS motorcycle were amazed at the amount of traction available for braking before the ABS needed to take over.

With that information in hand, we ran a second non-ABS test but found that without the ABS safety, net, stopping distances were, on average, only 11 feet shorter than the first set of non-ABS tests. Every tester agreed that with knowing the ABS was there to catch them, pushing the braking envelope to the outer edge was not a problem for them.

In A Turn
Unfortunately, anti-lock brakes are not very effective while cornering. Ask an engineer and he'll get this far-away, glassy look and start talking about tire slip ratios, lean angles and traction coefficients. Roughly translated, it means this: The farther you lean over, the less traction is available for braking (or acceleration). Get on the brakes hard in a corner and long before either wheel lock, you'll find yourself sliding.

When a tire loses its grip, the slide is toward the outside of the turn. Getting off the brakes (or the throttle) may not be enough to regain directional control (the motorcycle is trying to go sideways, not forward, you'll need to help the process along by steering with the bars. It's a pretty intense maneuver.

So, if ABS can prevent wheel lock-ups, what's the problem in turns? Unfortunately, today's generation of ABS, though quite complex, does not account for lean angles. The same computer logic for straight-line braking (i.e., calibrated to allow near tire-smoking stops) does not apply when tilted over. Someday faster computers with more capacity to handle increasingly complex ABS programs, combined with faster hydraulics with the ability to regulate (not just modulate) brake line pressures, may be able to solve the problem.

Still, the ultimate answer, as one source hinted, may be the addition of lean-angle sensors and computers that completely take over the full-tilt braking chores. A solution that will take years of R & D to sort through, and even then will only happen if riders are willing to accept an increasing level of automation.

ABS Operation Over Other Hazards
Although we tested ABS on a variety of surfaces, braking problems could be broken down into two basic groups: Limited distance hazards (such as metal gratings, tar strips, a patch of sand, gravel, or dirt), and long-distance hazards (wet pavement and dirt roads).

Oil and grease created a unique problem. During one of our tests (run at 30 mph due to the location), while crossing a 13-inch patch of oil mixed with some grease and dirt, the tires picked up enough gunk to be a problem as the same spot came around on the next rotation, six feet down the road. After our Racer nearly lost it on a non-ABS Yamaha, we limited the test to ABS only. ABS didn't bat an eye at this challenge. It simply went through the gunk and cycled two to five times until the garbage scrubbed off the tires.

We also incorporated another test not on our original list. One of our BMW riders reported missing a stop sign on a rainy night. Not entirely unusual, except that the road ended (Illinois farm roads tend to end in T's quite often-a real problem if you're not alert) and he found himself on a farmer's lawn going 40 to 50 mph. To verify his claim of a "a very long, but controlled stop," we ran the ABS Yamaha and ABS BMW over a newly sodded, water-soaked lawn. While a series of passes would have been nice, no one was willing to let us destroy their lawn. We were limited to three test passes.

Next to ice, wet grass is about as slippery as it gets. Our reader was correct. Both the BMW and Yamaha models with ABS came to a nice, controlled stop in a reasonable distance. And yes, ABS does work on ice, too. Given that we didn't get our hands on the Yamaha until the end of March and the Honda until June, a comparison was not possible (besides, no one was really anxious to try it anyway). However, we did book some winter miles on a BMW K100LT-ABS. Winter riding also brought out another problem. In temperatures lower than -5 degrees F, the drag from the wheel bearing grease and the brake pads rubbing against the discs keeps the wheels from turning after ABS releases the brakes (on ice). That's not an engineering problem related to ABS, but rather a cold weather problem you should be aware of.

On dirt roads, ABS wasn't even flexing its muscles. Like the wet pavement tests, riders were amazed at how much traction was actually available. Gravel was the same story. Without the fear of a slide-out, you can actually feel the tires gripping against the tiny, loose rocks as they move around under the wheels. Now that is impressive!

In Conclusion
If we are to try to pick a winner of this "contest," based on both the stopping data and the testers' comments, it would have to be the Yamaha FJ1200 ABS. Though all the ABS systems worked extremely well, it was unanimously felt that the Yamaha system worked the smoothest and was the easiest to handle. The lever and pedal "feedback" were the least harsh and obtrusive on the FJ, and therefore probably less likely to disturb or frighten the rider into over-reacting during a high-stress, panic stop situation.

In the end, everyone could hardly believe just how good ABS really is. It's certainly not a sales gimmick. This is clearly the biggest advance in braking safety since the advent of the disc brake. Our group of testers had just one complaint: Why is ABS not available on more motorcycles?

At least part of the answer to that question lies in our own resistance to change and, in particular, to bikers' reluctance to abdicate any amount of control of their mounts to "technology." We're a fairly independent lot, and take a certain amount of pride in the fact that we regularly rely on our own abilities and talents to get ourselves out of trouble.

Still another part of the answer comes with the rather large cost of ABS. High-tech braking has a price, and it's one that is high enough to make most of us stop and think twice about purchasing it. BMW's system adds $1000 and 20 pounds to the equivalent non-ABS models. Honda's system weighs 11 pounds and raises the ST's price tag by $1900 (though to be fair, that also includes the Traction Control System, which we will test in a future issue). The Yamaha is the light-weight king at 9 pounds, adding $1200 to the FJ's retail price.

I guess in the end the only real question is not whether or not you can afford ABS, but can you afford to ride without it?

Reprinted with permission of Motorcycle Consumer News.
For subscription information, call (800)365-4421, or visit www.mcnews.com

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