It all looks very complicated, and...it is. The piston pump and control motor need to make multiple adjustments each second to keep the tires gripping properly. The extra weight of the abs system is well worth the sacrifice for street riders.
Up until now, if your bike had 'ABS' written on the side you probably weren't an SSB reader. Ever since the first anti-lock brakes appeared on a crusty old BMW K100LT tourer in 1988, they've largely been the province of dull, overweight, underpowered touring machinery. It's no surprise really, considering the characteristics of the early systems. They added a load of weight to the bike, needed a huge battery and massive alternator to provide the extra electrical power needed, and didn't actually perform all that well anyway. Over the course of the next twenty years, the systems got lighter and better, and the bikes using them also improved. But up until last year at least, nothing that could reasonably call itself a high performance bike came with ABS (though some might argue that the Honda VFR could squeak in under that description).
On the face of it, ABS sounds like a fantastic idea. You brake as hard as you want and a computer stops the wheel from locking up and planting you on your face. Brilliant. What's not to like?
The basic parts of an ABS system are pretty straightforward. You need a sensor on each wheel to measure how fast it's turning. These sensors are connected to a small computerized ECU, which in turn operates a hydraulic pump/valve unit. This main hydraulic unit is connected by brake hoses from the brake master cylinders to the calipers, and the pressure generated from pulling the lever is transferred through the valves and hydraulic circuits in the unit.
When you press the brake lever, the brake light switch tells the ECU that you're braking and it analyzes how the wheels are behaving. If it 'thinks' the tires are about to lock up, it sends a signal to the hydraulic unit causing it to reduce the pressure to the caliper(s) on that wheel. The brakes are released slightly, the wheel doesn't lock up, and you don't lose grip and crash. The pump then repressurizes the brake system, operating the caliper(s) again, and the brakes come back on.
The system can repeat this cycle several times a second, constantly releasing and reapplying the brake pressure until you stop braking or your tire's grip on the road improves. On most systems, you can feel a pulsing at the lever as the pressure falls and rises (though not on the new CBR600RR).
That all sounds pretty simple, eh? So what's all the fuss about? Well, the hard bit is working out when the wheel is going to lock. And it's especially difficult to work out on a motorcycle. With a car, all four wheels rotate at pretty much the same speed (in a corner, the inner wheels are rotating slightly slower, since they're covering less distance). But by and large, automotive ABS systems can just look at the speed of each wheel, and if one is slowing down much more than the others, then chances are it's about to lock.
But when a bike brakes, the weight transfer to the front compresses the front tire, making the rolling radius smaller. At the same time, the weight coming off the back tire de-compresses the rear tire, increasing its rolling radius. These changes in the effective radius of the tires affects the rate of rotation, making the job of the ABS ECU much harder. Is the change of speed of the wheel because the tire is compressing or expanding, or is it because you've lost grip?
Additionally, when the bike leans over, since the back tire is wider than the front, it experiences a greater change in rolling radius, making it rotate slightly faster than the front. You can hear this yourself in a corner-when you lean over, the revs rise as the effective gearing ratio lowers. The effect is less than that from braking, but it's still there.
What the ECU does, therefore, is employ a much more complex algorithm that also analyses the rate of deceleration of the wheel's rotating speed. The ECU 'knows' how quickly the wheel can decelerate under normal braking, and if the wheel exceeds this known value, then there's a good chance it's about to lock up. The system still analyses both wheel speeds though, which is why you get a warning light if you wheelie an ABS-equipped bike. The computer doesn't know that you just hoisted an almighty half-mile monster-it just sees the front wheel slow down while the rear is still spinning fast, and it gets confused.
Once you have a reasonable control algorithm for the system, the next problem is cycling the hydraulic system on and off quickly enough. You need to be able to open a valve to drop the caliper pressure quickly enough to prevent the wheel locking, yet not so fast that the brakes go off too much. Similarly, you then need a pump unit that can very quickly re-pressurize the fluid to the caliper, without braking too hard. And you need this combined valve and pump setup to operate as quickly as possible, going through several on/off cycles a second. Too slow, and the brakes simply feel as if they've turned off-which is very disconcerting for the rider. But making valves and pumps operate this fast takes a lot of electrical energy and adds weight and size to the componentry, battery and generator.
Should we expect that all high performance bikes will come fitted with an ABS system in years to come? Presently we have the option, because under race conditions the experienced rider can still outperform the computer. That will almost certainly change one day, but for the time being enjoy your options and don't neglect weighing them. For the average street rider, a sportbike equipped with an ABS system could be the difference between crashing or staying safe and upright.
BMW has been at the forefront of ABS development on bikes, and with the launch of its HP2 Sport last year, it claimed to have the first sportbike fitted with ABS.
Sportbikes offer special problems for ABS systems. First of all, they're less able to handle the extra weight and electrical power demands of the systems. Adding twenty pounds to a 375-pound, cutting-edge supersport machine is much more significant than adding twenty pounds to a 600-pound sport-tourer.
But the geometry and tire performance of a supersport bike also provides significant challenges. Basically, locking the front wheel is not the biggest problem when you're braking very hard on a sportbike with sport tires on a dry road. There's enough grip to lift the back wheel off the ground a long time before the front wheel will lock, so stability is the bigger problem.
Honda claims its new Combined-ABS system addresses these problems more effectively. It does this by combining the front and rear brake systems, with a sophisticated ECU controlling the balance of the braking efforts as well as preventing the wheels locking. Stomp hard on the rear brake lever, and the system applies the front brakes - stopping the bike in a controlled, smooth fashion. And if you're hard on the front brakes, the system also applies the rear - adding to the total braking effort and improving stability.
But on Honda's CBR600RR, the new system adds almost twenty pounds to the mass of the bike - making it amongst the heaviest of the 600 supersport class. It also adds a hefty chunk to the price of the bike: it costs $1000 more than the non-ABS version.