[arjithin]

Why ABS is provided in rear wheels? How does ABS in rear wheels help increase vehicle control/steering function.

I always see the skid marks in the road from the rear wheels of the big trucks and there breaks in the skid marks indicate longer stopping distance to me.

[libranof1987]

ABS is not "provided" to any wheel.

ABS ensures that, during braking, all the 4 wheels move at the same speed. If any wheel is found to be moving at a higher speed than the rest, the braking pressure for that wheel is increased so as to slow it down to the speed of the other 3. And of course, if one particular wheel is slower than the others, the pressure on that is reduced to get it on par with others.

So, ABS just ensures all 4 wheels are rotating at the same speed. This further ensures that the car remains maneuverable during braking on surfaces where traction is reduced.

Find more info here : http://en.wikipedia.org/wiki/Anti-lock_braking_system

[SS-Traveller]

Originally Posted by arjithin

Why ABS is provided in rear wheels? How does ABS in rear wheels help increase vehicle control/steering function.

Have you seen/heard of rear wheels locking up and causing a car to fishtail or spin out? Hope this explains why ABS exists for rear wheels.

[SDP]

Originally Posted by libranof1987

ABS ensures that, during braking, all the 4 wheels move at the same speed. If any wheel is found to be moving at a higher speed than the rest, the braking pressure for that wheel is increased so as to slow it down to the speed of the other 3. And of course, if one particular wheel is slower than the others, the pressure on that is reduced to get it on par with others.

So, ABS just ensures all 4 wheels are rotating at the same speed. This further ensures that the car remains maneuverable during braking on surfaces where traction is reduced.

Are you positive about that? I always thought that ABS has got to with preventing a wheel from locking and threby losing control.

Does the ABS work dramatically different in 2WD as against AWD drive vehicles?

[libranof1987]

Originally Posted by SDP

Are you positive about that? I always thought that ABS has got to with preventing a wheel from locking and threby losing control.

Once ABS ensures that all wheels are rotating at the same speed, it consequently means that wheels don't lock up, ain't it?

Wheels lock up when that wheel has received more braking pressure than others causing it to stop while others are rotating. So, ABS thru' its function doesn't allow that to happen.

[DerAlte]

Errr... which car has ABS in rear wheels only now?

[mayankjha1806]

ABS and EBD work in tandem to ensure that neither the wheels skid (i.e. to improve traction) and thereby improving control while braking. EBD on the other hand distributes right amount of braking to all wheels so as vehicle does not loose direction.

In slippery surface (like snow/ice) ABS takes longer (i.e. more distance) to stop as it releases brake and reapplies it so as wheels lock-rotate-lock-rotate cycle over then distance.

Needless to say they both work on all the four wheels as working on only some of them will not make sense.

[rangakishen]

Originally Posted by libranof1987

ABS ensures that, during braking, all the 4 wheels move at the same speed. If any wheel is found to be moving at a higher speed than the rest, the braking pressure for that wheel is increased so as to slow it down to the speed of the other 3.

Are you really sure about this? I think you've mistaken the functions of ABS and EBD. What you have described here is EBD where the system reads the relative speeds of the wheels and distributes the brake force accordingly.

ABS is simply Anti-Lock Braking System. It simply ensures that none of your wheels lock up.

So the first scenario that you have narrated would not hold good. If the wheel is moving faster when compared to the other wheels (if such a scenario arises, I dont know if it can), provided that the car only has ABS and no EBD, ABS will have no role to play.

[clevermax]

Originally Posted by libranof1987

ABS is not "provided" to any wheel.
ABS ensures that, during braking, all the 4 wheels move at the same speed. If any wheel is found to be moving at a higher speed than the rest, the braking pressure for that wheel is increased so as to slow it down to the speed of the other 3.

That is not how ABS works. Please read and understand how it works before posting such explanations. This will only confuse more people who want to understand what ABS is. You are probably talking about EBD.

Originally Posted by DerAlte

Errr... which car has ABS in rear wheels only now?

I also want to know! Wonder what triggered the question of this thread!

Originally Posted by arjithin

I always see the skid marks in the road from the rear wheels of the big trucks and there breaks in the skid marks indicate longer stopping distance to me.

Please explain this further?

[mayankk]

Originally Posted by libranof1987

Once ABS ensures that all wheels are rotating at the same speed, it consequently means that wheels don't lock up, ain't it?

Wheels lock up when that wheel has received more braking pressure than others causing it to stop while others are rotating. So, ABS thru' its function doesn't allow that to happen.

THAT would be defeated with hard braking with two wheels off the road on grass or dust.
My understanding is it maintains the status of the wheel to the point JUST before a skid.
still rolling, but highest amount of braking possible without locking up.
Is why youll see staggered skid marks on tarmac when an ABS equipped vehicle has a foot jamming the pedal.

[libranof1987]

Originally Posted by rangakishen

Are you really sure about this? I think you've mistaken the functions of ABS and EBD. What you have described here is EBD where the system reads the relative speeds of the wheels and distributes the brake force accordingly.

ABS is simply Anti-Lock Braking System. It simply ensures that none of your wheels lock up.

So the first scenario that you have narrated would not hold good. If the wheel is moving faster when compared to the other wheels (if such a scenario arises, I dont know if it can), provided that the car only has ABS and no EBD, ABS will have no role to play.

Originally Posted by clevermax

That is not how ABS works. Please read and understand how it works before posting such explanations. This will only confuse more people who want to understand what ABS is. You are probably talking about EBD.

Excerpt from Wikipedia :

"A typical ABS includes a central electronic control unit (ECU), four wheel speed sensors, and at least two hydraulic valves within the brake hydraulics. The ECU constantly monitors the rotational speed of each wheel; if it detects a wheel rotating significantly slower than the others, a condition indicative of impending wheel lock, it actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel; the wheel then turns faster. Conversely, if the ECU detects a wheel turning significantly faster than the others, brake hydraulic pressure to the wheel is increased so the braking force is reapplied, slowing down the wheel. "

"Electronic brakeforce distribution (EBD or EBFD), Electronic brakeforce limitation (EBL) is an automobile brake technology that automatically varies the amount of force applied to each of a vehicle's brakes, based on road conditions, speed, loading, etc. Always coupled with anti-lock braking systems, EBD can apply more or less braking pressure to each wheel in order to maximize stopping power whilst maintaining vehicular control. Typically, the front end carries the most weight and EBD distributes less braking pressure to the rear brakes so the rear brakes do not lock up and cause a skid. In some systems, EBD distributes more braking pressure at the rear brakes during initial brake application before the effects of weight transfer become apparent."

My bad if it caused any confusion.

The last line probably explains why the thread.

[SDP]

Originally Posted by libranof1987

Once ABS ensures that all wheels are rotating at the same speed, it consequently means that wheels don't lock up, ain't it?

With all due respect, have to disagree again.
Wheels do rotate at different speed foe genuine reasons like when the car is taking a turn. In fact differentials are designed for precisely that.

Completely OT, but posting a video that explains how a differential work (and the need for wheels spinning at different speeds).

How Differential Gear works (BEST Tutorial) - YouTube

[clevermax]

Originally Posted by libranof1987

Once ABS ensures that all wheels are rotating at the same speed, it consequently means that wheels don't lock up, ain't it?

Do you think all wheels will be rotating at the same speed during normal running conditions? What about going through a curve? A hard turn for example? While taking a turn, inner wheels travel shorter distance than outer wheels and thus they rotate at different speeds.

If ABS makes all wheels rotate at the same speed, its purpose is defeated - because ABS should allow one to hard brake while turning or swerving away from danger.

[getsriram]

I think ABS ensures that wheels which tend to lock up earlier than others are allowed to rotate at a slightly higher rate than the locking-point speed, to regain traction with the road. It does not ensure all the wheels rotate at the same speed.

On the other hand EBD, just identifies which wheels need more effort based on probably the weight distribution of the car and also during the time of braking. Again this also is not responsible for wheels rotating at same speeds.

And finally, the differential as correctly mentioned ensures and is responsible for the same speed rotation of the wheels.

So for instance, if you are braking suddenly and one of your front wheel is in oil which makes it vulnerable to lock earlier than other wheels, ABS kicks in to ensure it rotates at minimal speed and assuming the car has lots of weight in the rear section, the EBD ensures it supplies more pressure to the rear wheels as compared to the front ones. Together they ensure more effective and non-skid braking of the car.

But i am unsure if ABS would be present only for rear wheels, as the front wheels have more importance when it comes to braking.

Technical experts please correct me if my understanding is wrong.

Thanks

[SDP]

Originally Posted by getsriram

So for instance, if you are braking suddenly and one of your front wheel is in oil which makes it vulnerable to lock earlier than other wheels, ABS kicks in to ensure it rotates at minimal speed and assuming the car has lots of weight in the rear section, the EBD ensures it supplies more pressure to the rear wheels as compared to the front ones. Together they ensure more effective and non-skid braking of the car.
...

I am no expert, but why would a wheel be more vulnerable to lock when it is on oil? In my opinion, it would lose traction earlier (and a traction control system would be able to help). A wheel locking would solely depend upon the brake pad and the amount of force applied to the pedal. Whether the wheel is in oil or mud should not matter as far as locking the wheel is concerned.