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-   -   Need for ABS in the rear wheel? (https://www.team-bhp.com/forum/technical-stuff/112379-need-abs-rear-wheel-2.html)

Again, not the same speed of rotation - during turns a differential ensures power delivery in correct proportions to both wheels rotating at different speeds without having to slip one of the wheels.

Small confusion here - where has the OP (or anyone else) stated that ABS systems exist with only the rear wheels being included? If I understand correctly, the OP's query was regarding why have ABS sensors on the rear wheels at all, when the front wheels are the more important ones as far as controlling the car is concerned.

@DerAlte: Maybe the OP is a budding automobile designer, looking to shave a few $$$ from the manufacturing costs by providing 2-channel ABS for the front wheels only, instead of the 3- or 4-channel systems used nowadays? :Shockked: :p

Losing traction while the brake is applied results in wheel locking.:)

What getsriram tried to say was that when brakes are applied and one of the wheels is on a slippery surface, that wheel will lock up. It is absolutely right and ABS will function over here to make sure that the wheel doesn't lock by taking brake force off of it. Traction control only comes into play under acceleration (mild or hard).

ABS - Functions by releasing-applying (alternatively and quickly) brake pressure!!
Traction Control - Functions by limiting power transmission.

It depends on the action of the vehicle when these systems come into play.

ABS in essence is opposite to traction control. Traction control makes sure your driving wheels never slip (SPINS). ABS makes sure no wheel locks up (stops spinning)

I am sorry, but I am still not able to understand the part highlighted in bold. As per my understanding, wheel locking is to with friction between the brake pad and the drum/disk and has nothing to do with the friction between the tyre and the road-surface (e.g. in the case of oil on road).

incorrect.
A wheel on oil will lock up with less force on brakes than say on clean tarmac.
It takes less effort for the clasping of the disc to stop the wheel from rotating.

+1 to Mayankk

Its like this.
Consider all four wheels to be on same surface and brakes are applied.
No wheels lock up. Friction b/w brake pads and disc only slows the vehicle since this friction is lesser than the friction between road and tyre.

Its more like a competition between friction between brake pad and disc (say Fb) and friction between tyre and road (say Fs).

Now if Fs>Fb (normal scenario; ie all wheels on same grippy surface), no locking will take place as the brakes are only slowing the vehicle but they are not enough to lock up the wheels.

Now if Fb>Fs (scenario when one of the wheels is on a slippery surface hence Fs reduces BUT Fb REMAINS SAME), the wheel will lock up as the brake pad and disc win the friction competition.

Couple of real life examples will also help.
When on a bicycle, if you wanted to skid the rear wheel you would apply the rear brake hard on some sandy part of the road. The above analysis explains it. The amount of brake you are applying (Fb) is more than the grip the sandy road is offering (Fs) hence you skid (wheel locks up).
Same when you apply brake under rainy conditions!!

Hopefully I was able to convey the message.:)

Can't one ask an independent question? Now the you have the made the old man the forget the why. Egad!

The one decent reason I remember for maximizing rear wheel braking (hence the provision of ABS for the rear wheels) - as someone has pointed out here - is to gain the max braking out of the rear wheels before weight transfer takes place.

Even if one reduces channels to two, it will still make sense to ensure max braking on rear wheels in the initial braking.

If so, we should all be proud if he is able to do that - that is an extremely creditable achievement in the industry. Usually accompanied by a small but not insignificant part of the savings given as recognition!!! :D

Well, one advantage of ABS which is usually marketed is the steerability while braking.

But, ABS also minimizes stopping distances on most(normal) surfaces. Also, preventing rear wheel locking up means, less probability of unexpected over-steer or fish tailing.

Hence it only makes sense to have ABS in all four wheels.

Also, in slippery conditions, when you have to brake while cornering, it's better to have a more forward brake bias as with weight transfer, cornering and the slippery conditions, the rear could easily lock leading to a spin or even worse.
So, an ABS should be working on all four corners. I've always seen ABS modules with four channels. There perhaps were early versions with just two channels.

No, Nitrous-anna, even the oldest Bendix was a 3-channel one as far back as 1971. Didn't reckon with cornering, since both the rears were on 1 channel.

Not quite sure about this. From the same Wikipedia article - "In 1970, Ford added a antilock braking system called "Sure-track" to the rear wheels of Lincoln Continentals as an option(it became standard in 1971)h".
Reference - Directory Index: Lincoln/1971_Lincoln/1971_Lincoln_Continental_Brochure
From the above brochure - "This computer-controlled system improves braking stability by helping prevent sustained rear-wheel lock-up on ice, snow, or wet pavements". So, this cannot be a three channel ABS, right?

It would be, if the rear had only 1 wheel RPM sensor and the front had 2. I think it was a Bendix system, and it was analog/TTL controlled. The Bosch system, which debuted IIRC in 1985-86, used an 8-bit micro-controller.

Although I have expressed views contrary to what you have mentioned, a little bit of goggling threw up a Mazda video which actually talks about the ABS monitoring "relative" speed of wheels and helping in low-grip scenarios.

Here's the link
Mazda Anti-Lock Braking System ABS. - YouTube

Sorry for the confusion. I understand your point now. Thanks!:thumbs up

DerAlte, from what I understood from the brochure, the car did not have ABS on the front wheels. It only talks about the rear wheel lock-up. From the same Wikipedia article - "In 1971, General Motors introduced the "Trackmaster" rear-wheel only ABS as an option on their Rear-wheel drive Cadillac models". So, there were systems where they provided ABS only on the rear wheels and these could not be three channeled ones.

In a 3-channel, I know that the 2 channels are one each for the front tires and the third for both the rear tires.