The ABS discussion thread

[Santoshbhat]

Originally Posted by Parthasarathig

now i feel kinda scared. Then what is the safest option for driving on rainy roads? I frequently travel in buses too on highways. So how safe are buses with regards to this? Especially ksrtc buses.
The only safe option im guessing now is to drive very slowly in watery roads.
And regarding gravel, tcs does help. As ive seen many landrover videos having a tcs option labelled as gravel.

TCS will help in preventing wheel spin when accelerating on slippery surfaces. I don't think TCS plays any role in stopping the vehicle.

@Mods : We are drifting OT here. Request move this discussion to the appropriate thread.

[raj_5004]

Originally Posted by Santoshbhat

You are dead right. Precisely for this reason, sometimes I wish car makers gave us an 'ABS off' switch. At times on roads with lot of loose gravel and sand, the ABS simply unlocks the brakes and car just does not stop. A good driver one can easily modulate the brakes based on feel of grip avaialble/ loss or trasction etc... With ABS there is nothing the driver can do. At times I have have had to jump over some pretty nasty speed breakers coz the ABS just would not allow the brakes to grab the wheels.

Well, it is not necessary that a car without ABS will stop in time on slippery surfaces. A car without ABS locks the tyres which causes a lot of screeching which makes the driver think that the braking distance has reduces, which is not entirely true.

Atleast, with ABS, the car remains in control of the driver.

Yes the idea of having ABS is to retain steering control which is not available when the wheels lock up. But what do you do when there is no room to steer the vehicle out of harm's way? You just want to minimise impact by reducing the speed of the vehicle. If in such a situation, the road is slippery, ABS can be counterproductive.

I have seen a lot of drivers pumping the brakes of a car which has ABS. This is wrong and negates the efficiency of the ABS system. In cars equipped with ABS, just slam the brakes and stand on it. And if there is room, just steer away your car. I am sure, even on a narrow road, you will definitely get atleast a few inches on either side to squeeze your car and prevent a major shunt.

I am not against ABS per se. My point is that it has its downsides too. People have many misconceptions about ABS, which can be pretty dangerous.

Of course, every system will have it's downsides. But I feel the ABS has more pros than cons. If driven by someone who is well versed with the system, I feel it greatly reduces the chances of an accident.

Originally Posted by vikram_d

The post with regards to which I wrote mentioned fine stone particles on the road. In such a case ABS would not help in anyway at all. It would lengthen the braking distance and would not help in steering the car.

It is not necessary that a car without ABS would have been better in that situation. Who knows, the wheels might have locked and it may have had a nastier crash...

Originally Posted by swarnava.m

So, in layman terms, in a single statement, how right or wrong is it to say that ABS actually increases our stopping distance? In other words, it reduces the braking capacity as compared to a car without ABS?

Well, it depends.

A car with ABS may or may not increase the braking distance. There is no hard and fast rule. On slippery surfaces, if a car without ABS keeps on skidding, would we say that it is safe or the braking distance was less?

In other words, what would you want - an out of control car with lesser braking distance (if we assume that) or a car which is completely in control with marginally greater braking distance? I prefer the latter. The former would freak me out!

Originally Posted by Off Roadie

I agree to your points here but let's say if I were driving during the rains I would like the option of keeping the ABS off throughout.
There are quite a few instances where one may like to keep the ABS off so it is always better to have an option than not have one at all.

The main objective of ABS is to use it on slippery roads. Switching it off when it rains defeats the whole purpose.

I have had an experience where I could save my car from a shunt (T bone) because a moron taxi driver cut in front of me without judging my speed. With or without ABS, my car would not have stopped in time. I simply slammed the brakes and swerved the steering to the right. There was no skidding or any drama whatsoever. The car braked hard, I could feel the pulsations of ABS on my brake pedal, it turned well and stopped in full control. Yes, there was some construction going on and the road was (is) filled with gravel and small pebbles.

In this case, I think even the EBD helped. I have personally felt the ABS + EBD work in my car when I did some hard braking on the corners of some nice ghats.

And for those who feel ABS always increases braking distance, watch this video, especially when its 2:15 minutes -

http://www.youtube.com/watch?v=mKiTAcXK6M4

Even read the comments below, similar to what we are discussing here!

[Santoshbhat]

Originally Posted by raj_5004

And for those who feel ABS always increases braking distance, watch this video, especially when its 2:15 minutes -

Thanks for the informative video. But do note that the video was made by the company that developed the system

Googled a bit on ABS.

Quoting a few lines from Wikipedia on the subject

On high-traction surfaces such as bitumen, or concrete, many (though not all) ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those that would be easily possible without the benefit of ABS

In gravel, sand and deep snow, ABS tends to increase braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring.

Some vehicle manufacturers provide an "off-road" button to turn ABS function off. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid, though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice. On a very slippery surface such as sheet ice or gravel, it is possible to lock multiple wheels at once, and this can defeat ABS (which relies on comparing all four wheels, and detecting individual wheels skidding). Availability of ABS relieves most drivers from learning threshold braking.

Anti-lock brakes are the subject of some experiments centred around risk compensation theory, which asserts that drivers adapt to the safety benefit of ABS by driving more aggressively. In a Munich study, half a fleet of taxicabs was equipped with anti-lock brakes, while the other half had conventional brake systems. The crash rate was substantially the same for both types of cab, and Wilde concludes this was due to drivers of ABS-equipped cabs taking more risks, assuming that ABS would take care of them, while the non-ABS drivers drove more carefully since ABS would not be there to help in case of a dangerous situation.[19] A similar study was carried out in Oslo, with similar results

ABS is definitely a must have safety feature and there's no debating that. But the system also has its limitations which I think many drivers are unaware of and take it for granted they are 'safe' since they are driving ABS equipped cars.

[swarnava.m]

Originally Posted by raj_5004

...And for those who feel ABS always increases braking distance, watch this video, especially when its 2:15 minutes...

I am not saying that ABS necessarily increases braking distance. However, in the video itself, there is a little contradiction. If you observe carefully, when the graphics illustrates the functioning of ABS, following conclusions can be drawn.

1. Without ABS, on slamming the brakes, they work at full power, therefore locking the wheels.

2. When ABS is added to the above situation, a lock up is detected in a wheel by the ABS module using a speed sensor. Then, as the narrator says, the braking power is regulated. Now, it is probably logical to assume that since the ABS is in action when the braking power is maximum, hence "regulating braking power" would only mean reducing braking power, so the wheels would no longer be locked.

3. If the above point is true, then this means that the driver of an ABS equipped vehicle must be fully aware of this aspect. Otherwise, only due to the ABS branding on the boot, he is over confident about his car's braking capacity, which is actually (maybe) marginally lower than that of one without ABS. Often, this can be the cause of an accident in an ABS equipped car.

[Thad E Ginathom]

The greatest danger of ABS is that people use it as an excuse to drive more badly.

Originally Posted by Parthasarathig

IIRC the skidding of tyres in rains happens due to something called aquaplaning.

Aquaplaning is a specific phenomenon. a different thing altogether. Nothing can save you from aquaplaning except having enough clear road to slow down and put the wheels back on the ground. It's frightening!

Originally Posted by Parthasarathig

now i feel kinda scared. Then what is the safest option for driving on rainy roads?

Slow down. Be more willing to change down, and do it sooner.
Read the road further ahead, increasing anticipation. Much bigger distances between you and the vehicle in front.
No sharp steering movements.
Drive smooth; avoid heavy feet on gas or brake.

Of course, many of those piece of advice can be made impossible by other drivers. Wet road driving requires quite a shift in driving style but the majority of drivers make no change at all.

Originally Posted by raj_5004

Atleast, with ABS, the car remains in control of the driver.

I think you mean the driver remains in control of the car

Originally Posted by swarnava.m

1. Without ABS, on slamming the brakes, they work at full power, therefore locking the wheels.

2. When ABS is added to the above situation, a lock up is detected in a wheel by the ABS module using a speed sensor. Then, as the narrator says, the braking power is regulated. Now, it is probably logical to assume that since the ABS is in action when the braking power is maximum, hence "regulating braking power" would only mean reducing braking power, so the wheels would no longer be locked.

3. If the above point is true...

I don't believe that it is true.

What I understand is that the maximum braking efficiency is at the point just before lockup. ABS seeks to keep the brakes at this point.

A skidding car does not have efficient braking. Braking is no longer in the picture as soon as the wheels stop turning

[raj_5004]

Originally Posted by Santoshbhat

Thanks for the informative video. But do note that the video was made by the company that developed the system

............

ABS is definitely a must have safety feature and there's no debating that. But the system also has its limitations which I think many drivers are unaware of and take it for granted they are 'safe' since they are driving ABS equipped cars.

Totally agree. That is the reason I said -

Originally Posted by raj_5004

Of course, every system will have it's downsides. But I feel the ABS has more pros than cons. If driven by someone who is well versed with the system, I feel it greatly reduces the chances of an accident.

A car with ABS may or may not increase the braking distance. There is no hard and fast rule.

Originally Posted by swarnava.m

I am not saying that ABS necessarily increases braking distance. However, in the video itself, there is a little contradiction. If you observe carefully, when the graphics illustrates the functioning of ABS, following conclusions can be drawn.

1. Without ABS, on slamming the brakes, they work at full power, therefore locking the wheels.

2. When ABS is added to the above situation, a lock up is detected in a wheel by the ABS module using a speed sensor. Then, as the narrator says, the braking power is regulated. Now, it is probably logical to assume that since the ABS is in action when the braking power is maximum, hence "regulating braking power" would only mean reducing braking power, so the wheels would no longer be locked.

3. If the above point is true, then this means that the driver of an ABS equipped vehicle must be fully aware of this aspect. Otherwise, only due to the ABS branding on the boot, he is over confident about his car's braking capacity, which is actually (maybe) marginally lower than that of one without ABS. Often, this can be the cause of an accident in an ABS equipped car.

Regulation of braking power does not necessarily mean reducing braking power. It just means modulating it in such a way that the wheel does not lock.

Originally Posted by Thad E Ginathom

I think you mean the driver remains in control of the car

Oh yes, sorry!

[vibbs]

Originally Posted by swarnava.m

2. When ABS is added to the above situation, a lock up is detected in a wheel by the ABS module using a speed sensor. Then, as the narrator says, the braking power is regulated. Now, it is probably logical to assume that since the ABS is in action when the braking power is maximum, hence "regulating braking power" would only mean reducing braking power, so the wheels would no longer be locked.

Braking power and Braking efficiency/braking ability are two different things in my opinion. Braking power is the power being supplied to the brakes.

If the wheels lock, no matter how much power you increase the braking efficiency/ ability goes for a toss.

Reason: Sliding friction is less than rolling friction. When the wheels are running there exists rolling friction between the tyres and the road.

To stop the car, you apply brakes which slow down the tyres and bring it to a halt as soon as possible.

However it is essential that there exists sufficient friction between the tyre and the road. If in the process of braking the wheels lock, there wont be any rolling friction but only sliding friction which will invariably increase your braking distance. So even after having great breaking power, the braking distance would increase. Idea is to supply the right amount of power to the brakes so that they slow down the wheels as soon as possible without locking them.


ABS reduces the braking power supplied to the wheels if it detects locking, so that the wheels dont lock and rolling friction does not vanish.

Basically by reducing the braking power when there is a possiblity of wheel lock, ABS increases the efficiency and invariably gets the braking distance down unless in certain conditions when Locking wheels would favour the reduction of braking distance.

[Thad E Ginathom]

Very nicely put, vibbs

This sliding/rolling thing is the key. I didn't have the terms, so I didn't know how to explain it.

[swarnava.m]

Originally Posted by vibbs

Braking power and Braking efficiency/braking ability are two different things in my opinion. Braking power is the power being supplied to the brakes.

If the wheels lock, no matter how much power you increase the braking efficiency/ ability goes for a toss.

Reason: Sliding friction is less than rolling friction. When the wheels are running there exists rolling friction between the tyres and the road.

To stop the car, you apply brakes which slow down the tyres and bring it to a halt as soon as possible.

However it is essential that there exists sufficient friction between the tyre and the road. If in the process of braking the wheels lock, there wont be any rolling friction but only sliding friction which will invariably increase your braking distance. So even after having great breaking power, the braking distance would increase. Idea is to supply the right amount of power to the brakes so that they slow down the wheels as soon as possible without locking them.


ABS reduces the braking power supplied to the wheels if it detects locking, so that the wheels dont lock and rolling friction does not vanish.

Basically by reducing the braking power when there is a possiblity of wheel lock, ABS increases the efficiency and invariably gets the braking distance down unless in certain conditions when Locking wheels would favour the reduction of braking distance.

Okay thanks.!

That pretty much explains it in a most efficient way..

Now I can go and show off to less fortunate people (read.. non-bhpians)!

[jkdas]

Originally Posted by Off Roadie

I agree to your points here but let's say if I were driving during the rains I would like the option of keeping the ABS off throughout.
There are quite a few instances where one may like to keep the ABS off so it is always better to have an option than not have one at all.

Why?

My experience using it on my bike ( Apache RTR 180 ABS).
Its bloody awesome. I can repeat that line again!

For example, thanks to monsoon and road conditions, you dont know when a new pothole has developed on your daily route to home or office. I bet that only coz I had ABS I could brake hard and slow down the bike in time to avoid/or ride down that pothole safely. Without ABS, I would have damaged the disc or even myself. Also, your confident factor rises a lot.

[Santoshbhat]

Originally Posted by vibbs

Reason: Sliding friction is less than rolling friction. When the wheels are running there exists rolling friction between the tyres and the road.

Isn't sliding friction supposed to be greater than rolling friction? Otherwise cars would be sliding all over the place rather than rolling on their wheels.

[raj_5004]

Originally Posted by Santoshbhat

Isn't sliding friction supposed to be greater than rolling friction? Otherwise cars would be sliding all over the place rather than rolling on their wheels.

Imagine a very very steep slope. A slope in which a car with all wheels locked will easily slide down because of it's weight & gravity.

Now, in such a situation, imagine a car with all wheels locked. It will slide all the way down and hit the ground. Now imagine a car with a very good driver and without locked wheels. What the driver does is that, he carefully and judiciously applies the brakes in such a way that the wheel dont lock. Imagine this and you will realize that the car will come to a stop before reaching the ground.

Alternatively, ask any off roader. If they are stuck on a steep slippery slope, they will not use full braking power. Infact they will use just the right amount of brakes for the vehicle to come down slowly and steadily. Once they slam the brakes and if the wheels lock, then the vehicle will slide down without control and there is no way he will come down safely. That is the reason if the wheels lock, they release the brake pressure till the wheels start rolling again.

Many expensive SUVs have 'Hill Descent Assist'. It does the same job - bringing down the vehicle with little braking so that the wheels dont lock.

In all the above examples, the effort made is to keep the wheel rolling and not allow it to lock. Point made above is, the driver needs to apply just enough pressure for the vehicle to slow down BUT without locking the wheels. This complex job is done by the ABS on the driver's behalf.

[Parthasarathig]

Originally Posted by Santoshbhat

Isn't sliding friction supposed to be greater than rolling friction? Otherwise cars would be sliding all over the place rather than rolling on their wheels.

Yes sliding friction is greater than rolling friction. Which means we're stuck without an explanation once again.
Class 8 physics. We are talking about the wrong quantity. What the influencing part is Coeeficient of friction. This determines the property of friction of a particular substance.

Road or tarmac has a higher coefficient of sliding friction than gravel. Hence here sliding friction is greater than rolling friction and our cars therefore dont skid away to glory.

When however the tyre travels on "loose" gravel, the gravel has a lesser coefficient of sliding friction than rolling friction this is due to the uneven nature of gravel surface also the surface area of the gravel is less as a result of this lesser amount of friction is exerted on the gravel by the tarmac below. Imagine gravel being sandwiched between the tyre and the road.



So in other words gravel reduces the sliding friction between road and tyre. The tyre then continues to skid as long as it is on the gravel surface. Once back on the road the surface area of tyre in contact with the road increases and so does the coefficient of sliding friction. Then the tyre starts to roll as the coefficient of rolling friction is less.

However in case of a patch of road made up only of gravel and stuck together with some adhesive, like tar or pitch, behaves as a large unit and here the friction is completely offered by the gravel as the entire surface area of the gravel in toto is very high thus increasing the coefficient of sliding friction. And when this happens the coefficient of rolling friction either stays same or decreases both of which lead the wheels to roll.

Loose gravel is the culprit here for causing the skid. Its the same as a hubcap of some vehicle being stuck under your tyre as it happened with one BHPian KarthikK. His bikes front wheel locked up and skidded as the hubcap came in between the road and the tyre. Whereas his rear tyre kept rolling and slowed the vehicle down as it was still fully in contact with the road, thereby having a higher coefficient of sliding friction which thus prevented his entire bike from skidding on the road and thus he escaped unhurt.

From the above i am thus led to conclude that as long as your wheels are rolling and slowing down you are much safer than when wheels lockup without ABS or in a skid and you have a very dangerous situation develop in front of you.

Hope this solves the doubts.

[vibbs]

Originally Posted by Santoshbhat

Isn't sliding friction supposed to be greater than rolling friction? Otherwise cars would be sliding all over the place rather than rolling on their wheels.

I m terribly sorry. I mixed up the terms pretty badly. What I meant was static friction and dynamic friction and not rolling and sliding friction. I cant edit the post too now.

Static friction happens between two bodies that dont move relative to each other. Example a wheel and a road when the wheel is turning.
I hope you can visualise it when a wheel is turning the point of contact on the wheel with the road hardly moves wrt to the road, it just rolls over.

Dynamic friction is between two bodies when there exists relative motion between two bodies. Eg when a wheel is locked and is sliding. The point of the tyre in contact with the road moves on the road.

Dynamic friction is always less than static friction.

This link can be checked for a brief insight on static and dynamic/kinetic frictional coeficients.
In general between all types of surfaces normally the dynamic friction is less than the static friction. difference becomes greater in case of wet roads.
http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html

Originally Posted by jkdas

Why?

My experience using it on my bike ( Apache RTR 180 ABS).
Its bloody awesome. I can repeat that line again!

Yes you are right in wet road conditions, the water film acts as a lubricant and once the wheels are locked, this lubrication would reduce the dynamic friction a lot. Under dry circumstances, the difference between the dynamic and static frictional coefficients may not be as pronounced as in case of wet conditions.

[Off Roadie]

Originally Posted by jkdas

Why?

My experience using it on my bike ( Apache RTR 180 ABS).
Its bloody awesome. I can repeat that line again!

For example, thanks to monsoon and road conditions, you dont know when a new pothole has developed on your daily route to home or office. I bet that only coz I had ABS I could brake hard and slow down the bike in time to avoid/or ride down that pothole safely. Without ABS, I would have damaged the disc or even myself. Also, your confident factor rises a lot.

Am using my cellphone hence am using a cut paste from my scorpio review thread.

ABS: SLE owners rest assured that you have the safer Scorpio. I loved the way my CRDe locked her wheels under hard braking and one could modulate the brakes by controlling the pressure on the brake pedal. Simulating conditions and trying out the ABS had given me a good reassurance of their working but when it was crunch time I was pretty disappointed. First case was while I was speeding down from Rothang pass heading to Manali, I had crossed the nightmarish stuff and it was smooth tarmac all the way down, however accelerating out of a hairpin I was amazed to see a stretch of broken (kacha) road right ahead and I slammed the brakes. For while it felt like I was rolling down hill in a car that had empty metal drums for tyres. The ABS is super sensitive and the whole car just continued forward with just a marginal reduction in speed, fortunately the road was not all that bad and we just bounced around a bit but the bottom line is that the brake pulsation was way too much and the reduction in speed was way too less. At that moment I actually thought that my brakes were not working only when I collected my thoughts a while later and pondered over the incident did I realize what had happened. The brakes behaved similarly when I had to brake hard and perform and emergency lane change on the Mumbai - Pune expressway in the fog one rainy night but I was better prepared to handle the situation then. Oh and I was not slamming the brakes and performing the lane change simultaneously so please don't think that the ABS helped me perform the lane change, I needed the brakes to shed my speed but they were not all that effective. That said under normal driving conditions I have found the brakes to be perfectly normal in terms of feel and response it was just disappointing when slammed hard at high speeds. One may have to be supersensitive with the break pedal in case of emergencies which will need a lot of composure on the driver's part as a natural tendency will be to slam the brakes hard. Now please don't start thinking oh the Scorpio is not good maybe the Safari is better. Both these SUVs are under tyred and I don't expect the Safari to behave all that better.

See I base my opinion on my undertyred SUV while you base it on your bike and we are comparing apples to oranges here.
I think we are right in our own perspective and lets leave it at that.

At everyone else, people we are not driving trains on metal tracks here. Each vehicle and it's tyres are different and are being driven on a host of surfaces in a variety of conditions. The dynamics are always different.
There is no doubt that ABS is a crucial safety feature but there are conditions where having the option of turning it off is required and the safer thing to do.