The common misconception is that the ABS system 'pulses' the brakes in the same way as a driver does when cadence braking. The reality is that modern systems (from Continental, Bosch and TRW) can modulate the brake pressure to each brake caliper individually, to maximise the available grip on each wheel. This is achieved by a set of valves that sit between the master cylinder and each of the 4 brake pipes, which are controlled by solenoids. The valves can be controlled in such a way that brake pressure to the caliper can be modulated and held at set pressures, not just on-off-on-off which seems to be the common misconception.
In an emergency braking event, the ABS system senses that such a scenario might be about to occur due to the sudden sharp release of the accelerator pedal. The system then pre-charges the brake system to minimise the pressure-lag of the brake circuit. When the driver then jumps on the brakes, the system monitors brake pressure up to a point where it starts to cap the rear axle pressure to ensure the rear axle does not lock up before the front (and therefore cause loss of control).
The system monitors each wheel speed carefully, and allows the wheels to slip relative to the road by around 10-12%. Maximum grip is attained at around these slip values, but it depends on the tyre and road conditions. The front wheels will start to lock-up (normally 1 before the other due to road surface conditions, steering wheel angle etc), momentarily exceeding the allowed slip, so the system reduces the brake pressure on that wheel (and can even increase engine torque to un-lock a locked wheel which can be necessary in some cases, for example on 4wd cars).
Once the front wheels are at their target slip level (10-12% to get maximum grip) the rear axle brake pressures are increased up to a point that the rear wheels meet their slip target. This ensures the rear wheels contribute their maximum to the deceleration, although depending on the car type, this can be as low as 15% due to the weight transfer to the front axle, but a normal value is around 35%. Non ABS cars have to cap the rear axle pressure much below their optimum slip %age to ensure the vehicle doesn't spin - this limits overall deceleration potential by quite a lot....
The system monitors wheels speed and target wheel slip on all 4 wheels around 50 times per second. Any road surface change, dip, crest, road friction change, brake friction change, tyre temp change due to heat etc will mean the brake force required to attain maximum grip needs to eb adjusted, and therefore the ABS system compensates by raising or lowering the brake pressure held at that wheel at that moment in time.
A non-ABS car, no matter how good the driver, cannot brake even in a straight line to within any closer than 110% of an ABS car, and it normally takes practice to know what the right pedal pressure should be on that road surface in that car - even on the worlds most theoretically perfect road that has a perfectly even road surface. Throw in real world situations in an emergency braking event and you are looking at 120 - 130% of the stopping distance of an modern-ABS equipped car.
Snow and ice can cause issues for ABS and traction control systems, but one must remember that the system is a 'blind monkey in the boot' and cannot actually see the road surface. Detecting what road surface the car is on by monitoring wheel slip vs brake pressures alone is a difficult job, and that is why it is often better to disable traction control to get out of a sticky situation. A car might lock it's front brakes with 60 bar of pressure on a dry tarmac road. On ice, this can be as low as 3-4 bar. I challenge anyone in a modern servo assisted car to modulate 3-4 bar - there's just no resistance built up yet at that level. If you lock the brakes you can sometimes get extra decel by snow packing, but on ice you'll be skidding at only 90% of the maximum deceleration, and with no steering control. I know i'd rather be using ABS.
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