Originally Posted by Andyvh1959
I used to work directly with Bosch ABS systems development engineers on heavy trucks back in the late 80s early 90s. Although it was on trucks, the control principles of ABS are fairly uniform for railway, airplanes, cars, trucks and motorcycles. It all has to do with how much predetermined slip/wheel lock is programmed into the ABS controller, how qucikly the system can "cycle" the brake on any wheel on/off, and how quickly the wheel/tire being braked can "speed up" after the ABS brake control is released.
That last part is called hysterisis: the time delay for a slowed/stopped wheel to spin back up to vehicle speed before ABS control is applied again. A good comparison is a large truck wheel/tire assembly compared to a motorcycle wheel/tire assembly. Obviously, the truck wheel/tire has much higher hysterisis because of its higher mass.
In ABS control, on loose surfaces the system expects the wheel assembly to spin up faster than it actually does (like a tire plowing into sand or gravel, creatng a "dam" in front of the tire). But the tire does not spin up to speed, and ABS control then holds OFF brake pressure until the tire spins up. Like shown on that video, the bike had little or no braking down hill. In the case of loose surfaces that can "dam up" in front of a tire it is better to turn off the ABS control so the rider still has braking control. Generally speaking, on loose surfaces the vehicle speed is lower than on paved surfaces. That is why it is best to turn ABS off, when going off road.
ABS control on just the rear tire is also not a good idea on loose surfaces like sand, gravel etc. The downhil example on dirt is the best proof. Downhill on dirt is often best done by dragging the rear brake, sometimes to purposely lock the rear tire, to ride the dam of material ahead of the tire. Also, quite often on loose surfaces, its a good technique to lock the rear tire to bring the back end out or "back it in". With ABS control on the rear tire this can't be done either.
Also, like I said earlier, ABS control expects a wheel to speed back up once control is initiated. If the tire on ABS control dams up material, and the ABS cycles the brake off, but the wheel delays in spinning up because the "dam" is restricting it, the ABS may sense this as a fault condition and shut itself off and initiate the ABS warning lights. Again, to avoid a false system error, it is better to turn off the ABS for loose surface conditions.
To the comment "wire in a switch to turn it off", be careful how this is done. It "may" turn the system off. But it may also trigger an ABS fault mode. Then when back on the road, it would require resetting the system to clear the fault. Not as simple as just wiring in a switch on the ABS power lead.