My apologies for the length of this, but hopefully it is useful to many.
i thought abs did the pumping for you maybe you could have 2 wheels on abs either front or back who knows
Maximum braking is achieved when the wheels are just about to lock. Very few can achieve that, especially in a real emergency. Practising all day on an airfield or track to prove someone can outbrake an ABS equipped car may be fun, but is of no value in the real world.
When any wheel locks, braking is reduced on that wheel. If a rear wheel locks up, the car tends to try to pivot around, with the rear trying to overtake the front. Just like we've probably all experienced on our bicycles in the past. With one or both front wheels locked, steering is ineffective.
Cars without ABS should have a pressure reducing valve to the rear wheels to prevent locking. Sometimes this is load sensitive, especially on larger cars or vans. If a wheel locks, the rule used to be to release the brakes, then reapply, cadence braking. It is doubtful anyone can do this more than 2-3 times per second, and this releases all wheels. In most emergency situations it is impossible to release the brakes as you slide towards a crash. We just press harder.
So the clever bunnies at Bosch invented ABS. With a speed sensor on each wheel the control module knows when one or more wheels lock and releases that wheel only, reapplying the brake as soon as the wheel turns again. If I remember correctly, this can happen up to 15 times per second, on individual wheels. Whilst operating you will feel a pulsing under the brake pedal as the pump reapplies the pressure.
Stopping distances with ABS will be longer than maximum braking without a skid, but shorter than a skidding car. The main advantage of ABS is steering control. You can steer around obstructions and as the wheels lose grip, the ABS releases the appropriate brake(s) and allows the car to steer. The steering takes off a lot of speed.
ABS always operates on all four wheels, never only half the car. Early systems had individual circuits for each front wheel, but a single circuit for the rear as a pair. Rear drums can still be a pair, but rear discs are usually individual. With now mandatory adoption of anti-skid systems, rear brakes are now all individual circuits.
When ABS operates, it is usually best to keep the pressure on and let the car do its stuff. It is incredibly good at it.
If your car skidded and tried to turn around, there are several possible causes.
A problem with the pressure reducing valve(s) to the rear.
Different tyres with significantly different grip levels.
changing road surfaces giving differing levels of grip to each wheel.
Faults within the brakes, such as worn linings, fluid leaks onto the linings, seized wheel cylinders, etc.
What about engine braking Is it a viable method of slowing down the vehicle id break slightly from fourth gear to Third to Second. instead of braking heavily from 4th to second
Try a little test.
On a straight and quiet road, at about 30mph, sway the car gently right and left. Then repeat with the clutch down and feel how the car is more floaty and takes more steering to give the same amount of turn. This demonstrates how the car is less stable with the clutch down.
Changing down through the gears as you slow is unnecessary, and each time the clutch goes down, the car control is compromised. Recommended method of slowing is to brake smoothly in whatever gear you were in, putting the clutch down just before the 'stall' point, then selecting the next required gear. (5th to 1st if appropriate)
Many higher geared cars, especially diesels, will fight early, so an intermediate gear may be necessary, so slowing to a stop might involve two changes, 6th to 3rd, then 3rd to 1st.
However, in ice and snow, the ABS gets confused and very flustered. A very heavy stop from 20mph on snow can have the ABS sounding like a machine gun and stopping distances enormous. Gently braking in these conditions will get a better result.
Better than ABS is looking further ahead, beyond the vehicle in front and keeping more space, looking around and anticipating events, so reducing the need to brake as a reaction. Try to remove the 'suddenly' from driving.
New cars now have a stability program. This uses the ABS system to apply individual brakes lightly to keep the car in a straight line when braking. (I often get the feeling that the Fiesta is trying to show off, by twitching unnecessarily. Or maybe just not as clever as it should be.)
Emergency Brake Assist.
More clever bunnies in Germany, Mercedes, Bosch and a university, did some research and found that in a real emergency, most people only apply about 85% of the available brake force. More clever programming and if the brake is pressed faster or more heavily than set thresholds, the ABS system will apply full effort. The early systems on Passats, around 1998, were a bit over-enthusiastic and stopping gently was difficult. This is sorted and EBA-equipped cars can stop very quickly, and very violently. Ask any recent learner using a Corsa D.
Hopefully this answers the general queries, and I've not missed anything important.
