ABS systems may seem complex, but they are based on simple physics.
Today, almost every car sold (actually, I think every car) has ABS as a standard feature. What is it? Why is it? And how does it work? And what is traction control, automatic stability control, and trailer sway control - and why are they related?
Well, as you may remember from driver's ed, if you get into a skid, the idea is to steer in the direction of the skid and then pump your brakes, so as to not lock up the wheels. As you may remember from Physics class. the coefficient of static friction is much higher than the coefficient of dynamic friction. Of course, "you don't need to know this in the real world" - right? So people sleep through driver's ed and through Physics, and their world is a painful mystery to them.
Simply stated, when something is in solid contact with another object, the amount of force needed to move it is higher than the amount of force once it is moving. If you've ever slid a heavy piece of furniture across a floor, you understand this - you have to give it a "heave" before it gets going, and once it is going, it slides much easier. Same with a toboggan. You may be "stuck" on a hill, but once you get started, the dynamic friction is much less and the toboggan keeps going.
With wheels and tires it is a little less intuitive. If your car tire is turning at the same speed as the road is passing by, the rubber is in solid contact with the road and the forces of static friction come into play. However, once you skid (or slide sideways or "drift") the coefficient of dynamic friction is much less, so the car actually travels further when you skid, before stopping. That's why hammering on the brakes is often the worst idea - it will take longer to stop in a skid than with the wheels rolling and the brakes properly applied.
But it gets worse. Once your wheels are locked and you are skidding, you give up all directional control - the car will go where it wants to go - where momentum will take it - and you have little to nothing in the way of steering ability. That's why they recommended pumping the brakes back in driver's ed (before ABS of course!) as you would likely stop faster and could steer away from obstructions and avoid an accident (hopefully).
Of course, this is easy to say, hard to do. Human instinct, when a collision is imminent, is to yam on the brakes, leaving behind skid marks which lead right up to the point of impact.
With a car it is bad enough - imagine having 18 wheels to lock up. Back in the 1970's, NHTSA proposed that all 18-wheelers have ABS. Problem was, back in the 1970's, even the smallest of computers were the size of suitcases and not very reliable. Experiments were tried, but eventually, the highway administration had to relent, realizing that while the technology was theoretically feasible, it wasn't practical to apply - yet. Compounding the problem was the nature of truck brakes, which work on air pressure - or the lack thereof. It was just a bad idea for the time, much as the brief period in 1974 when they mandated that all cars have an interlock so they would not start unless your seatbelt was fastened. It was such a fiasco, it was rescinded within months.
But over time, the technology improves - computers will fit in the palm of your hand, or actually in a chip the size of your fingernail (or less!). Not injectable just yet - conspiracy nuts take note! So it seemed like a good idea to mandate this for cars. It first appeared as an option, but pretty soon, it was brought out across the board.
How does it work? It is pitifully simple. On each hub of each wheel is a toothed portion made of ferrous metal (cast iron or steel). A small sensing coil mounted near this counts the teeth as they pass by. By counting the pulses, you can determine how fast the wheel is rotating or whether it is rotating at all. These tiny signals (which are pretty weak) are fed to a small microprocessor which is programmed to monitor the wheel speed of each wheel and then determine whether one or more of those wheels are locked in a skid or not. If three wheels are reading as turning and one is reading stopped, well, you have a skid condition.
Note that if all four wheels are locked, the system may not detect a skid - or at least older systems had trouble with this. Note also that these tiny sensors on each wheel make a very low-voltage pulse signal. They are usually connected to the car's wiring harness with a plug located right in the wheel-well, where it is hit with rocks and debris and salt and water - what could possibly go wrong? Often when a diagnostic code reads "wheel sensor" it doesn't mean the sensor has gone bad (indeed, it is just a coil - how can that fail?) but that the connector is corroded. A little emery board on the corroded contacts and some dielectric grease goes a long way - but the same could be said for most electrical connectors on a car.
The microprocessor then sends a command to an actuator, which is located in-line with the hydraulic brake lines. It will then pulse the brake signal to one or all wheels. Early versions, I believe, pulsed all wheels, and often made the pedal pulse as well ("to show you it is working!"). People unfamiliar with how the system worked often freaked out at the pedal pulsing and took their foot off the brake pedal and thus collided with a tree.
ABS pulses the brakes for you, so pumping the brake pedal with your foot is now less than helpful - it may confuse the ABS system and not give you optimal braking.
But the big key is, ABS, when it works right, allows you to keep steering, so you can yam on the brakes and still steer around an obstacle and hopefully avoid a collision. Problem is, many people panic (a normal human reaction) and don't try to steer. They just put on the brakes and pray.
Will ABS allow your car to stop in a shorter distance? Yes and no. Yes, it will stop faster that it would if you were skidding, on dry pavement, or on wet pavement, snow or ice. But if you are not skidding, the stopping distance should be about the same, with or without ABS.
Once every car on the road had ABS systems, other features are possible, given the hardware already installed. Traction control is one of these features. Before traction control was available, we had to rely on posi-traction or what is generically known as a limited-slip differential. An "open" differential (the plain kind) will allow one wheel to slip if it gets on a slippery surface and then stop transmitting power entirely to the other wheel. The same effect that allows you to go around corners without tearing the rubber off your tires, makes driving in snow or ice a problem, particularly if you get stuck.
So limited-slip differentials were devised, which used clutch packs or other devices to "lock" the differential if slippage was detected (mechanically). And of course, hot-rodders loved them because with a "posi" you could leave two burnout marks on the road, instead of just a lame one.
But that wasn't why limited-slip differentials (LSDs)were created - well, at least not officially. It didn't take long, however, for Engineers to realize that the same hardware used for ABS could be used for traction control as well, simply by reprogramming the microprocessor. Some early systems would detect wheel spin on the drive axle(s) and then suppress engine power to reduce the amount of torque. Such as system, when used in conjunction with LSD would help get you unstuck from a ditch, but of course, would spoil the fun for hot-rodders who want to "lay rubber". So turning off the traction control became de rigeur for the performance crowd, and we have a plethora of YouTube videos of Mustangs and Corvettes spinning out after leaving a car show.
We had the same problem with the M Roadster - it was so tail-happy with its wide "staggered" set of tires that with the traction control off, it would swap ends on a moment's notice, a particular victim of what is called lift-throttle oversteer. I learned very quickly that the car was basically undrivable without the traction control engaged. In fact, a lot of these cars today with jackass-wide tires owe their existence to "all those darn computers" as they are basically undrivable without electronic aids. There is a reason why we didn't have series 40 tires on street cars in the 1960.
Other types of traction control will sense wheel spin and apply braking pressure to the wheel that is spinning, thus transferring power to the other wheel. These are more advanced and they work a lot better, I think. I recounted before how we drove to Central New York in April with our old E36 cabriolet that had older, nearly bald, summer tires. I thought I would get one more trip out of them and then re-shod it when we got to New York. A late season snowstorm had other ideas. But even with bald summer tires, the car went uphill, from a standing start, in the snow. The traction control clanked and banged as it applied the brakes to the rear wheels to prevent wheelspin. But once it got going, we were set. Not that I recommend driving on bald tires in the snow! Far from it.
Some manufacturers, such as BMW call this combined ABS and traction control, "Automatic Stability Control" or some such nonsense. They claim that these computer aids, programmed in tandem, can stabilize a vehicle, such as an ungainly SUV - and once again, we have vehicles that, by themselves, are dynamically unstable. But with "fly-by-wire" controls, can be made driveable. The "SMART" car (Mercedes) when it first came out, wasn't very stable, and thanks to stability control, such a short-wheelbase, tall vehicle, which has the stability characteristics of a kitchen stool, can be made to handle like a roadster.
Kind of crazy when you think about it.
But that's not all. The same hardware has been used to provide "trailer sway control" for towing. If a trailer becomes "tail-happy" with too much weight behind the rear wheels, it will sway out of control, often overturning and causing a wreck - sometimes with fatal consequences. It is not unlike airplanes, which are basically uncontrollable if the center of gravity is too far aft. Any control input the pilot makes, only makes things worse. A crash is inevitable. But thanks again to computers, planes like the F-117 "stealth fighter" can fly, even though they are dynamically unstable. Fly-by-wire allows the plane to remain stable by making minute control adjustments many times per second - faster than a pilot can react.
In a similar fashion, trailer sway control can modulate the brakes on the tow vehicle in such a manner as to counteract trailer sway and dampen it before it gets out of control. It seems like black magic, but again, it is largely the same hardware used in traction control and ABS, just with slightly different programming, which can be enabled, when the vehicle detects a trailer is present.
There are other applications as well. Some have tried to use average wheel speed to indicate whether a tire is low on air pressure (as a low tire will have a small effective diameter). But with the popularity of low-profile tires, I think such a system would have limited use and not be able to provide immediate warning of low pressure, as the more conventional tire pressure sensors do.
It is kind of interesting how one technology leads to another. Once you have the wheel sensors installed, you can do all sorts of interesting things - like get rid of the "speedometer cable" of the good old days. Why have a mechanical cable when you have four electronic sensors instead? The list goes on and on.
Of course, this means more complex technology and people get nervous about that. "Suppose the computer goes haywire and sends the car off a cliff?" OK, Boomer, that could happen, right? The main thing people are worried about is repair costs - these systems are complex and expensive to repair - right?
Well, yes and no. They rarely need repair, particularly as the technology improves. The processor either works or it doesn't - it will be most likely to fail within days or weeks of first use, what we call "infant mortality" in the electronics business. Ditto for the ABS actuator - I have rarely heard of one failing, even on a high-mileage car. The big problem, as I noted before, is in the sensor connectors or the sensors themselves, which are not hard to replace. However, you do need a code-reader and reset tool (or laptop interface) to read error codes and then reset them, once the repair is made.
Difficult? Not really, just a different skill set than adjusting the jetting on a carburetor or seeing the gap on your "points" ignition. And some people aren't interested in learning new skills.
One thing is certain: These systems are here to stay, if only because people are worse drivers than ever before and need as many electronic aids as possible to keep them out of trouble. Moreover, our insistence on driving overwrought and top-heavy trucks and SUVs means that stability control will be a necessary feature. Of course, even with this, rollover accidents are still on the rise. You can't revoke the laws of Physics with electronic controls.
But it's not like they aren't trying.....
