Safe Speed Forums

Purpose
Prevent excess wheel slip/lock under braking, control wheel slip at optimum level to maintain braking AND steering authority.

Basic Function
Reference vehicle speed calculated from wheel speeds.
Individual wheel speeds monitored against vehicle speed to give wheel slip.
Wheel slip controlled within programmed threshold by holding, reducing & adding pressure on individual wheels.

Hardware (minimum)
Wheel speed sensors
Hydraulic Valve block + pump (integrated)
ECU

Links
ABS Suppliers
Bosch ABS
TRW Automotive (navigate to Braking Systems > Product Information > Advanced Vehicle Control Systems > Anti-lock Braking Systems)
Continental
Other sites
Rospa description

For further info & how to comment on this post please refer to active vehicle systems

I would like to just complicate this slightly, as I did some work with some people that make ABS Control Units.

An ABS Control Unit is really two separate parts:

A hydraulic Control Unit that is placed between the Master Brake Cylinder (where all the braking force comes from) and the individual brake cylinders at each wheel.

And the electronic controller, which originally was independent of the engine management system. This is connected to the ABS Sensors (which detect the speed of rotation of each wheel), and the the hydraulic control unit, which it tells what to do.


The basic logic is: that if a single wheel is stationary while the others are moving, then a wheel is locked, and the system should unlock it by releasing the hydraulic pressure (a valve momentarily diverts the fluid back to the reservoir). There are some tolerances built in, so for example when all wheels are moving very slowly then don't bother.

The original systems were very, very basic, and worked on all wheels at the same time - which was convenient, because there was only a single brake circuit anyway. But a single locked wheel releases the pressure on all brakes at the same time, which is bad news if you have one wheel on ice while the rest are on tarmac - it is worse than not having ABS at all. This is why in the enthusiasts web sites you will often find references to disabling the ABS in winter by removing a fuse.

Later versions, and the associated improvements in brake circuitry upped the channels to 2, and then 4, which is what most new cars will have. In this scenario each brake circuit has it's own independent line, and the release of brake pressure only applies to the wheel that locked. There is almost never any reason to disable such a system.

For many cars these days, the ABS components have been upgraded substantially - the electronic controller is integrated into the Engine Management Unit (the brain of the car), and the hydraulic controller has in many cases gained the ability to also apply braking pressure to any wheel without the brake pedal being pressed. This means that the EMU gets the ABS sensor inputs, and can work out what the car is doing at all times - like detecting under-steer, and can it by applying or releasing braking pressure. This is Stability Control which is known by many TLAs.

can you think of an example of this ??

usually engine management & braking ECUs are completely different systems & suppliers.



yes yes... i'm getting there.
this is just a description of ABS.. which is usually listed alongside all the other TLAs that a stability system also allows.

i have to confess not having been around for much of the history i don't know it so well.

a single channel system doesn't really bare thinking about these days! there were also some doomed attempts to do it all mechanically.

the number of channels an ABS system has refers to how many hydraulci circuits it can independantly control.

so single channel reduces the pressure to all wheels.
2 channel systems act on just the front wheels, controlling each independantly to prevent excess slip & maintain steering control.
3 channel systems add a rear channel to the two fronts, controlling both rears at the same pressure. less common now in europe but still fairly common in the US with their live axle SUVs & pickups etc.
4 channel the latest and most common variant can control each wheel independantly.

as a side note the downside to controlling wheels individually across an axle (i.e. left to right) is that if one has more grip than the other the ABS will allow more pressure and hence more braking force on it. this means an unbalanced force from left to right which causes unexpected yaw. various strategies to ensure this occurs in a controlable manner are usually implemented in the software.

Links
ABS Suppliers
Bosch ABS
TRW Automotive (navigate to Braking Systems > Product Information > Advanced Vehicle Control Systems > Anti-lock Braking Systems)
Continental

Rospa description

My E-reg Accord (1987 I think) was different again. It had 3 circuits, but it was the backs that were independent and the front that worked as a pair. It allowed one front to lock without any brake release taking place. If either back locked the pedal kick-back was small. If one front locked, nothing. If both fronts locked, substantial kick-back on the pedal.

I actually flatspotted a tyre once in that car when I came upon a very slow moving vehicle on a bend going my way. It was my first ABS equipped car and I did as I was told in the manual - kept the brake planted.

wacky

the pedal feedback is related to the amount of fluid being shuttled about, obviously one circuit & 2 brakes requires more than a single circuit & a single brake.

with 3 channel systems there are also variations on the number of sensors, 4 obviously one on each wheel, allows better control of the joint circuit by monitoring both wheels. or just 3 sensors with one mounted on the diff (usually).

i think you'd be hard pushed to buy a new car in europe with a 3 channel system these days (i may be wrong).

Well it's odd you should mention that! The first car I ever drove with ABS was an Accord. I was a student working for a major manufacturer at the time (late 80s) and we were "benchmarking" it. I borrowed it for a weekend and tried standing on the pedal while steering. It didn't "steer" much better (if at all) than a non-ABS car and for quite a while after that I lived under the impression that ABS wasn't all it was cracked up to be!

Suddenly, it all makes sense!

My current car (early 90s) has (I'm told) a three channel with 4 sensors but only one channel for the rears. I'd always wondered why it had a brake compensator valve working off the rear suspension AS WELL AS ABS. I read somewhere that three channel systems had to adopt a "select high" or "select low" strategy when a wheel locked. They could either choose to maintain the rear line pressure at the level where only one wheel was locked ("select high") or lower the line pressure to both wheels until the locked wheel started rotating again ("select low"). In the former case, there was the chance of greater braking but (I guess) it needed a compensator valve to limit the maximum pressure and in the latter case I gues they could have dispensed with the compensator but would have had to put up with limiting the braking on the back to the wheel with the least grip. Does that sound plausible anyone?

the valve may have just been for rear proportioning (see DRP) and if its linked to the rear suspension it will be adjusting based on load (more common on estate models) something DRP may struggle to do.

i googled this definition for select low, primarily to see how public domain this strategy was.

so yes your definition is pretty much correct (according to my understanding) and ensures stability on the rear axle.

on the 4 channel system this strategy can be applied also for the same reason, especially where independant control can cause unbalanced brake force & yaw as mentioned above.

Yes, it definitely is for rear proportioning. It's a front wheel drive saloon car but the valve is connected to the centre of the rear anti-roll bar so it only senses double-wheel bump (or, effectively, changes in axle load).

For ages I was puzzled as to why it should have been fitted with one if it also had ABS. Surely, I thought, both the ABS and the valve were there to serve the same purpose - stopping the rear wheels from locking?!

I then read somewhere that the car only had a three channel system so I'm now thinking that the ABS selects the rear wheel with the most grip rather than the one with the least grip when lockig occurs. The car then relies on the proportioning valve to ensure that it won't spin when there isn't much difference between the grip at the high speed wheel and the grip at the low speed one. Does that seem reasonable?

I always thought that the rear compensator is present to take account of ABS failure. Without it, if the ABS fails (fuse blows, any of the wheel sensors goes intermittent causing it to shut off eg), yoiu'll lose it when you plant the pedal.

I work for a company that converts cars for (among other things) for disabled use. This often involves cutting the floor out at the back to lower it for a wheelchair. About 10 years ago, one of the biggest headaches with this sort of conversion what what to do with the rear compensator valve (it usually attached to the bit we cut out). These days, none of the vehicles we modify actually have such a valve. I'm therefore less inclined to believe it is there as a fail-safe and more inclined to think that it was only there before 4 channel systems were common. - Unless, of course, it was just because 10 years ago, people didn't trust the ABS systems as much as they do today?

Possibly - or possibly it was actually cheaper to leave it in than take it out when ABS was only available on TOTR cars.

Out of interest, in your converted cars did you re-attach the compensator somehow, bypass it or just cut the hydraulic feed to the back and have done with it?

the valve did come from before ABS, its a pre-emptive passive device that limits the rear axle pressure (sometimes in proportion to axle load). you can still lock the rears on a poor surface (although if i recall corrently legislation requires that the front axle locks first).

i guess these days all ABS systems incorporate the DRP function that replaces the valve but on earlier systems this may not have been the case so it would be logical to have ABS to react to wheel locking & the valve try and prevent the rears reaching ABS in the first place.

In the Halcyon days of the brake compensator, locking up was invariably the fronts. With the ABS, the kick-offs I guess are mostly from the rears if that's the case (unless of course the design of the brakes is such that the rears get next to nothing anyhow).

I wonder just how many ABS fuses or connections have failed and launched an unsuspecting driver off the tarmac arse end first.

No Roger, we didn't cut the rear brake lines and have done with it! It would have upset the Vehicle Certification Agency (and maybe even a few customers!)

We re-located the valves and re-calibrated them. On many (if not all) light vans and van-derived cars of the period, there was a little graph stuck on one of the door pillars or under the bonnet showing pressure-in vs pressure-out for several different rear axle loads. We had to attach pressure gauges to each side of the valve and then load the back of the car up on a weighbridge and check the pressure on each side of the valve at each of the specified loads.

Interestingly, as far as the other point is concerned, I think ABS system failures are very rare and I'm sure the manufacturers size the components to minimise the disruptive effects anyway. Besides, during the type approval tests, you need to carry out several braking runs under all sorts of load conditions and even on split-friction surfaces both with and without the ABS working. Although it's not actually a "fail" if the car swaps ends with the ABS fuse pulled out, it still has to demonstrate "reasonable" stability under braking. That's not to say you COULDN'T make one spin but it would probably still require a fair bit of provocation!

My wife's car has ABS and electrinic stability control. It has no rear brake compensator and I get the feeling the rear brakes do a lot more work because when you brake hard, you can almost feel the tail squat as well as the nose diving (i.e. the car seems to drop vertically on all its springs a little).

And finally, yes, it is a legal requirement for the fronts to lock before the rears under all normal operating circumstances!

i seem to recall top gear (or similar) managed to get the rear wheels on a matiz to lock first

...then I expect the manufacturer might have a "conformity of production" issue to address!

Oh how I remember those glory days of ever-so-slightly elliptical rear drums, that ensured that all emergency stops were performed with one or both rear wheels spragged solid!

It's all so boring and predictable these days...

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At the risk of diverting this off topic, I remember my father's very rotten 1961 mini in the mid seventies. When you braked hard, the handbrake cable tightened on one of the rears and locked the wheel hard on, as the part of the rear subframe holding the back wheel pulled away from the rest of the car (restrained by that cable and not a lot else!) - until the car had come to a complete stop when the wheel sprang back into place and the car resumed normal gait. It's scary what we used to get away with in those days!

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