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.