[phynix123]

If ABS maintains the braking power just enough to avoid locking on the lower mu side, it'll automatically prevent locking on the higher mu side. Because, the sliping force is never reached that side.

[im_srini]

Originally Posted by Sutripta

Steering control is still with you. You have to use it to maintain directional control.
ABS (not ESP) will not steer for you.

Oks, thanks for the clarification Sutripta.
So, in the video clip I'd cross-posted, just having an ABS wouldn't have saved the driver from a spin.
He would've had to actively counter it, so on a split-µ surface ABS doesn't eliminate a spin - just reduces the probability.

Originally Posted by phynix123

If ABS maintains the braking power just enough to avoid locking on the lower mu side, it'll automatically prevent locking on the higher mu side.
Because, the sliping force is never reached that side.

Hello Phynix,
So the ABS would, on detecting impending wheel lock, reduce braking pressure on all wheels ?
My understanding was that with the whole 4-channel, 3-channel ABS thing, you've got individual wheel control.

[rr_zen]

@sudev: A very useful thread with lots of insights indeed. It not only stresses the need to have ABS but more importantly educates drivers on using it effectively.

[bravo6]

Originally Posted by Rehaan

Yes, he's talking about the rotational inertia of the engine.

Breaking it up, the brakes have to do two things when the clutch (and gears) are still engaged :
1) Reduce the forward inertia of the vehicle
2) (Possibly) Reduce the rotational inertia of the engine

At last someone who shares my point of view!

Higher the engine RPM, higher would be the engine braking. So unless you are shifting down gears sequentially to keep the RPM fairly high (think powerband), while braking, while steering to avoid any obstacles, you would only be using very little engine braking till the RPM drops to a level where it becomes insignificant. Once the RPM reaches idle, the engine braking is negative, ie you are being pushed forward, not slowed down by the engine.

Answer this: on a non-ABS car, when do you feel it is easier to lock up your wheels while braking hard? When the clutch is depressed or when it is not?

If it is the former, does it not imply that the engine was using up some of the braking force to slow itself down? Sure it makes you feel "better" using engine braking as well, but at the end of the day, it is another mouth to feed. When we can learn not to stomp on the throttle and cause wheel-spin every time we start off, it isn't really that hard to control the brake pressure, even in an emergency; all you need is practise!

Originally Posted by Jeroen

What effectively happens is that the wheel is slowed down by the brake pads onto the rotor, more braking power is provided to the wheels, through the driveshaft from the engine (engine braking). It means the brakes can work a fraction less, hence less wear.

True. Only if the engine was off to begin with.

If the wheels are not sliding (locked up or spinning faster, for a particular vehicle speed, there can be only one particular engine RPM (in the same gear of course). If the brakes are slowing down the wheels, they in turn have to slow down the moving engine parts. If you stomp on the brakes of a non-ABS car without pressing the clutch, it will stall when the wheels lock up (may restart if you ease off the brakes while the car is still moving)

Since the engine is following the speed of the wheels, it is wasting some braking for reducing RPM as well.

• Rev the engine with the transmission in neutral and hold just below redline for a second
• Release the throttle sharply
• Note the rate at which the RPM falls to idle

This will be the fastest rate at which the RPM can drop naturally on it's own. If it were in gear and the vehicle is moving, it will take a much longer time to drop to idle speed.

So, for any event requiring a greater rate of reduction in RPM (i.e. moderate to hard braking) the brakes need to dissipate not only the vehicle's speed, but also the engine's as well.

It might be easier to understand if you imagine trying to stop a rotating ceiling fan. You still need a certain amount of effort to stop it right? So some amount of braking force is wasted in countering the rotational inertia of the engine components, which could have helped in stopping the vehicle sooner.


Advantages of using engine braking:

1. Makes locking up wheels a bit harder - useful since most people would never have practised panic braking and would have no idea where the transition from maximum braking to wheel lock up occurs (which again varies depending on road surface, tyre characteristics, etc.) If you don't have ABS or were using older vehicles with less effective brakes and tyres, this may seem good, but in reality can increase stopping distance.
2. Finer control of vehicle speed - you don't need to use the brake to reduce speed slightly. E.g.: An automatic sedan doing 80 can take 600-800m to coast down to 40kmph. A manual can slow down much faster without touching the brake.
3. Lesser brake usage - going downhill in an automatic can be a bit scary since the car's speed continues to increase till you brake. But if you can select a lower gear, it is as safe as a manual
4. Driving on twisty roads - easier to modulate speed; you can drop a gear to slow down instead of having to brake for a corner

...in order to get the maximum braking out of the engine you need to downshift more, thus more wear and tear on the clutch

Agreed. Brake pads are cheaper than clutch plates.

As a matter of interest, on planes they do use engine braking mostly to reduce brake wear and break component temperature. Most planes are perfectly capable of stopping with their brakes only (and most have pretty advanced ABS as well). But by deploying thrust reverser, the brakes need to work less. A lot of commercial planes have auto brake systems which provide for a specific setting a specific deceleration. (e.g. 4 m/s2). Even when deploying the thrust reverser the auto brake system keeps this deceleration to its chosen setting, it just uses the brakes less, thus less wear and tear.

Jeroen

Engine braking on a road vehicle is completely different from thrust reversers on a jet plane. The key thing to note is that the throttle is closed and RPM drops in a car whereas in planes after touching down, the thrust direction is reversed and the turbine RPM increased for more "braking". The speed would continue to reduce till the plane stops, and then would move in reverse. Something that would never happen with regular brakes. Small parachutes coming out of the rear a fighter plane to slow it down after landing would have been a more comparable analogy.

Ships need to use their "reverse gear" to slow down as well, since they don't have brakes.

Originally Posted by Jeroen

once ABS kicks in you get as near as maximum braking power as you can get

With practise you can out brake the ABS version in straight line on a clean surface. But in a plethora of varying real world conditions, ABS will outperform almost everyone. There is simply no way to manually vary brake line pressure between the 4 wheels.

ABS on motorcycles isn't as safe as people assume it to be, since it's easier and safer to turn the handle when you're not braking hard. In a car one can slide for a fraction of a second while turning without breaking a sweat, but on a leaned bike you are already pushing the limits of traction. Even moderate braking in that situation can lead to a low side. So if any of your friends ride like nothing can happen to them because "they have ABS," show them this video:

https://www.youtube.com/watch?v=p1WSAESCf2Y

Originally Posted by sudev

Though I think here it was more about aqua planning due to highly worn out tyres cabies are notorious in using.

I don't think wet patches on the roads can cause aquaplaning, it would require standing water. If you observe you'll notice the brake lights are lit and hear the cab's tyres screeching due to a lock up. He merely panicked, locked up and drifted to the left side which was drier. Aquaplaning doesn't require one to brake or accelerate hard. If you're going fast enough and hit a patch of standing water, you will aquaplane.

See this video; you can use the amount of water sprayed form the wheels to calculate the water depth:

https://www.youtube.com/watch?v=mERAaeCrj0E

Originally Posted by im_srini

My understanding was that with the whole 4-channel, 3-channel ABS thing, you've got individual wheel control.

A car with ABS will slide in a situation in which the loss of traction is caused primarily by anything other than braking. For e.g. black ice, sand, sudden change in direction (think Scandinavian flick)


So, ABS does not mean you have the shortest possible stopping distance.

It does not mean you will never ever slide.

It does not mean you can drive fast in the rain.

It merely mean you can steer and brake hard at the same time.

On motorcycles, it just means you can save your soft compounds from sliding too much on bad roads, and that your friend riding your bike for the first time will not lock up the wheels and crash.


On a lighter note: The Veyron has ABS... on its handbrake! Still any doubt whether you need ABS?

[Jeroen]

Originally Posted by bravo6

Engine braking on a road vehicle is completely different from thrust reversers on a jet plane. The key thing to note is that the throttle is closed and RPM drops in a car whereas in planes after touching down, the thrust direction is reversed and the turbine RPM increased for more "braking". The speed would continue to reduce till the plane stops, and then would move in reverse. Something that would never happen with regular brakes. Small parachutes coming out of the rear a fighter plane to slow it down after landing would have been a more comparable analogy.

Ships need to use their "reverse gear" to slow down as well, since they don't have brakes.

What I was trying to point out is the many planes have an (ABS) autobraking system that, once engaged, maintain a preprogrammed rate of deceleration. Irrespective of reverse thrust. Very different from a car.

Contrary to popular belief, ships are not stopped quickly by going from full ahead to full astern. All to do about how to get maximum efficiency from a propellor turning through water. Depends a bit whether it is a fixed or variable propellor (in which case there is no reverse gear to begin with) but the principle remains the same. When you are going full ahead, you need to reduce the RPM and or pitch. I can't remember the exact ratio, but I seem to recall that reducing to roughly 30-40% maximum gives you the best "braking". When the speed starts coming down you start reducing RPM and or pitch further. By the time the vessels speed has come down to about 30-40% of its max speed you need to get start engaging reverse gear or the pitch needs to go in reverse. As the speed depletes further you increase the RPM (now in reverse gear or reverse pitch)

For many people, including many mariners and ship captains, the above is very counter intuitive, but it does work. Some modern ships have the above pre-programmed in the engine control system. So even if you slam the telegraph from full ahead to full astern, they will follow the most optimum braking-cruve.

Jeroen

[phynix123]

Originally Posted by im_srini

Hello Phynix,
So the ABS would, on detecting impending wheel lock, reduce braking pressure on all wheels ?
My understanding was that with the whole 4-channel, 3-channel ABS thing, you've got individual wheel control.

ABS works so as to avoid wheel lock-up. So, the brake force on the side having lower value of mu (co-efficient of static friction) will be applied lesser brake force by the system, enabling the driver to steer away.
You are right, for 4-channel ABS system each wheel will get individual brake force control. Which effectively translates into an ABS-EBD system.

[Jeroen]

Originally Posted by phynix123

You are right, for 4-channel ABS system each wheel will get individual brake force control. Which effectively translates into an ABS-EBD system.

No exactly. In all cars ABS works on all four wheels (well, there migth be the odd exception). Each wheel gets monitored individually. The four channel version is the most modern and provides the best charateristics but it doesnt provide full directional control, such as we need when there are two very different surfaces. It does provide some directional control under those circumstances but depending on the variations, not sufficient.

In order to maintain the directional control on the two different surfaces you need to add something beyond ABS.

From Wikipedia:

Typically 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. This process is repeated continuously and can be detected by the driver via brake pedal pulsation. Some anti-lock systems can apply or release braking pressure 15 times per second.[17] Because of this, the wheels of cars equipped with ABS are practically impossible to lock even during panic braking in extreme conditions.

The ECU is programmed to disregard differences in wheel rotative speed below a critical threshold, because when the car is turning, the two wheels towards the center of the curve turn slower than the outer two. For this same reason, a differential is used in virtually all roadgoing vehicles.

If a fault develops in any part of the ABS, a warning light will usually be illuminated on the vehicle instrument panel, and the ABS will be disabled until the fault is rectified.

Modern ABS applies individual brake pressure to all four wheels through a control system of hub-mounted sensors and a dedicated micro-controller. ABS is offered or comes standard on most road vehicles produced today and is the foundation for electronic stability control systems, which are rapidly increasing in popularity due to the vast reduction in price of vehicle electronics over the years.[18]

Modern electronic stability control systems are an evolution of the ABS concept. Here, a minimum of two additional sensors are added to help the system work: these are a steering wheel angle sensor, and a gyroscopic sensor. The theory of operation is simple: when the gyroscopic sensor detects that the direction taken by the car does not coincide with what the steering wheel sensor reports, the ESC software will brake the necessary individual wheel(s) (up to three with the most sophisticated systems), so that the vehicle goes the way the driver intends. The steering wheel sensor also helps in the operation of Cornering Brake Control (CBC), since this will tell the ABS that wheels on the inside of the curve should brake more than wheels on the outside, and by how much.

[phynix123]

Originally Posted by Jeroen

In order to maintain the directional control on the two different surfaces you need to add something beyond ABS.

I had a doubt in the back of my head regarding this. The article is informative. And yes, the solution with two additional sensors for steering and gyroscopic directional sensors is indeed logical. In conditions with widely different surface types on the two sides of the vehicle, ABS alone may not be sufficient, as you've rightly pointed out. Because, simple wheel lock sensors are not enough in such conditions. The ECU may get fooled by continuous contradictory feedbacks when one tries to steer away. Which will effectively increase the braking distance.
Please correct me if my understanding is wrong.

[Jeroen]

Originally Posted by phynix123

I had a doubt in the back of my head regarding this. The article is informative. And yes, the solution with two additional sensors for steering and gyroscopic directional sensors is indeed logical. In conditions with widely different surface types on the two sides of the vehicle, ABS alone may not be sufficient, as you've rightly pointed out. Because, simple wheel lock sensors are not enough in such conditions. The ECU may get fooled by continuous contradictory feedbacks when one tries to steer away. Which will effectively increase the braking distance.
Please correct me if my understanding is wrong.

What happens if the, say left side of your car is on ice and the right side of the car on tarmac and you stomp on the brakes The ABS system will ensure none of the four wheels lock up. But the braking traction on the right side (tarmac) is still higher then on the left side (ice). Meaning your car will start pulling to the right. You need a different set of sensors to detect that and effectively reduce the braking on the right side (tarmac).

Essentially under such conditions the braking on each wheel needs to be ajdusted for the braking action of the wheel on the most slippery surface in order to maintain direction.

Actually whilst I'm typing this I realize the example with ice is actually maybe not that good, because all of these systems will have limitations when the differences in grip on the different surfaces become to big. But the principle is I hope clear.

Jeroen

[sudev]

Originally Posted by Jeroen

What happens if the, say left side of your car is on ice and the right side of the car on tarmac and you stomp on the brakes The ABS system will ensure none of the four wheels lock up. But the braking traction on the right side (tarmac) is still higher then on the left side (ice). Meaning your car will start pulling to the right. You need a different set of sensors to detect that and effectively reduce the braking on the right side (tarmac).

Nope. With ABS what happens can be seen at ~2:17 on wards on the video posted earlier at http://www.team-bhp.com/forum/techni...ml#post3441948

Even with differing surfaces the car can maintain composure.

[bravo6]

Going by the topic title, all this discussion isn't really helping us learn or experience the pros of ABS. What say we meet up in an isolated/traffic-free road and actually practise steering while braking hard? Only then can we instinctively do the same in an emergency.

Originally Posted by Jeroen

What I was trying to point out is the many planes have an (ABS) autobraking system that, once engaged, maintain a preprogrammed rate of deceleration. Irrespective of reverse thrust. Very different from a car.

Mate, I never commented on or disputed that point, since I have no idea about a plane's braking system. I merely pointed out that slowing down by changing the direction of propulsion is not the same as engine braking on cars. Refer below:

Originally Posted by bravo6

Engine braking on a road vehicle is completely different from thrust reversers on a jet plane

---------------------------------------------------------

Originally Posted by bravo6

Ships need to use their "reverse gear" to slow down as well, since they don't have brakes.

Originally Posted by Jeroen

Depends a bit whether it is a fixed or variable propellor (in which case there is no reverse gear to begin with)

Hence the air quotes!

---------------------------------------------------------

By the time the vessels speed has come down to about 30-40% of its max speed you need to get start engaging reverse gear or the pitch needs to go in reverse. As the speed depletes further you increase the RPM (now in reverse gear or reverse pitch)

Like I pointed out earlier, this cannot be termed braking but not engine braking.

[Jeroen]

Originally Posted by sudev

Nope. With ABS what happens can be seen at ~2:17 on wards on the video posted earlier at http://www.team-bhp.com/forum/techni...ml#post3441948



Even with differing surfaces the car can maintain composure.

So how does a 'regular' ABS system do that? How does it compensate for the different friction on the left / right side of the road?

[SS-Traveller]

Learning too much about ABS / EBD / ESC = risk compensation? Worth it?

[Jeroen]

Originally Posted by SS-Traveller

Learning too much about ABS / EBD / ESC = risk compensation? Worth it?

Learning about ABS and all the other abbreviations is just understanding technology. Taking risks because of them is all down to human behavior. A whole range of different disciplines and sciences of which I have very limited understanding or affinity towards or appetite for. But I can ask my wife to join the forum if you like. She specialises in all that sort of stuff.

But seriously I would like to understand how ABS can distinguish between slippery surfaces and adjust the brakes accordingly only based on wheel rotation, or lack of.

Jeroen

[bravo6]

Originally Posted by Jeroen

So how does a 'regular' ABS system do that? How does it compensate for the different friction on the left / right side of the road?

ABS is all about differences in wheel speeds. It will compare the speed of each wheel to the others:left to right as well as front to rear. Locking up is locking up, whether it happens on tarmac, dirt or ice. The wheel speed will be zero in that instant, and is vastly different from any other wheel with traction and that is all the system cares about. The system will then relieve pressure on not only the locked wheel, but the wheel on the other side (left or right) as well till the two rotate at a similar speed.

If it didn't, the car will veer off course towards the side with greater traction.

Though it keeps the car going straight while braking, this is the Achilles' heel of ABS and the reason ABS vehicles take a longer distance to stop on extremely low traction surfaces, where locked wheels would dig in and stop way sooner.

The system would also allow the wheel speeds to vary within a certain preset range of one another before kicking in, without which it would be impossible to steer while braking hard at the same time.

By keeping the left/right wheels speed differences in check, unintended turns are avoided. Pretty much any 4 channel ABS should be able to accomplish this.

NB: Turning sharply when tyres on one side are on, say ice, will still cause you to spin out. This is when VSC, ESP, etc. come into the picture: to maintain direction when you're not on the brakes.

On motorcycles, the front/rear difference can be used to mitigate the rear wheel from lifting (stoppie). However the shortest stopping distance is achieved when there is maximum weight and hence maximum traction on the front wheel, which happens when the rear wheel is just off the ground.