How do Anti-Lock Brakes (ABS) work?
Authored by: GarySheehan and DW10+BE4/5L
Submitted by: Johnf514
Here is an example of a tire's mu slip curve. The Y axis is the tires tractive capability and the X axis is the amount of slip between the tire and the road surface.
The curve relating to straight line braking is the Longitudinal Mu Slip Curve.
The far left of the curve is zero slip. So at zero slip, the tire has no traction. A tire needs to be in slip to generate traction.
The peak of the curve is the tire's maximum grip level, which occurs at a given amount of slip. A very good racecar driver is capable of modulating the brakes to keep the tire operating at or very near the peak of this curve. This is called threshold braking.
Once you exceed the peak of the mu slip curve, traction becomes unstable and the wheel rapidly slows down relative to roadspeed and locks up. This happens very quickly. It is not possible to modulate the brakes to have the tire stay in a given location past the peak of the curve. The tendency will always be towards lockup.
As you can see, as the tire approaches lock-up (towards the right of the graph) the tire's traction decreases. So a locked wheel/tire is much less efficient than a tire operating at the peak of its mu slip curve.
ABS operates in the range of slightly before the peak to slightly past the peak of the mu slip curve. When the ABS computer senses excessive slip (by comparing a given wheel speed to the wheel speed of the other tires), the ABS system releases brakeline pressure to the wheel with excessive slip to get it rolling near road speed again. The tire is now operating to the left of the peak. It then reapplies brake pressure until it senses excessive slip again and repeats the cycle. Many times per second. So ABS keeps the tire operating around the peak of the mu slip curve, but does not maintain at the very peak of the slip curve.
The more sophisticated the ABS system, the closer it is able to operate at the peak of the mu slip curve.
The slip-mu curve, as in friction coefficient as a function of slip, depends heavily on the surface. On loose gravel and snow the greatest friction, or longitudinal force, is reached when the wheel is locked. This is because the tire deforms, i.e. digs into, the surface. IIRC, on ice the best grip is at low slip rates. Concrete and asphalt are somewhere in between.
To achieve the best braking force the ABS system should know the slip-mu curve of the surface which it of course doesn't. The ABS compromises over maneuverability and braking force.
The software of an ABS system is a closely guarded secret of its manufacturer (Bosch, Teves, Denso to name but a few). Even car manufacturers don't know what goes on in those black boxes which they just plug in. Writing such software is a tricky business because you also have to consider what happens when the friction level at the left side differs from that at right (the car might spin rapidly).
My guess is that depending on the ABS system and on how (un)lucky you happen to be, it's possible to stall the engine. In my experience (don't know whose ABS and which generation) the ABS first uses a slower cycle. It probably somehow measures the friction during the slow cycles. So if you disengage the clutch during this time the engine just might stall presuming both driven wheels are locked at the same time.
Braking with a 4WD car is more complicated because the three differentials may distribute the braking moments in many more ways than is possible with one differential.
The best way to emergency brake is IMHO to floor the brake pedal first and then worry about the clutch. If you first depress the clutch you will lose precious time that you could use to start braking. If the engine stalls you will lose power steering if it's hydraulic. I'm not sure if you will also lose brake boost during ABS braking. It depends on the system I think. In most cases there is an electrically driven hydraulic pump that restores the brake pressure during each ABS cycle, so I guess it doesn't lose the vacuum used to boost the brake pressure even if the engine is stalled and isn't generating any suction.
The best way to avoid an accident depends on the situation of course. It might be best to hit the clutch to give the best side force for steering. Or it may be best to brake as hard as you can.
By hitting the clutch at the same time as the brakes, you will lose your engine braking (though only a slight amount anway) plus your drive wheels are no longer active and you can lose traction.
The braking torque of the engine is negligible compared to the torque of the brakes. The brakes always win even in 1st gear. And you are trying to slow down so I wouldn't worry about traction. The best thing about depressing the clutch is that the car handles better because the engine isn't braking and thus consuming precious side force of the tires or producing yawing moments. People, including me, tend to instinctively let go of the accelerator when their car starts to slip sideways. Usually this just makes the slipping worse.
I think that the ABS is at its best when a n00b like me notices too late that he should have taken that turn to the right, steers to the right, hits the brakes because he's going too fast, locks the wheels and slides to the snow bank in the corner but luckily doesn't hit any of the traffic control devices and avoids any damage at all. Been there, done that. In a van without ABS.