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-   -   Understanding ABS (Anti Lock Braking System) (https://www.team-bhp.com/forum/road-safety/74799-understanding-abs-anti-lock-braking-system-4.html)

Exactly, an "ABS" car does not bcom safe on its own. Its only if the driver has the presense of mind to stop worrying about the braking and concetrate on steering the car out of danger, will it help you.

I have seen friends and relatives who have braked in a straightline (expecting the ABS to stop the car) and have smashed into the car ahead.

Having technology like this, without knowing how it works is even more dangerous than not having it at all.

People who dont know about ABS seem to believe its some sort of a miracle system that stops the car, no matter how small the braking distance.

I rem my friend telling me that he is not worried about his Swift having 165mm tyres coz it comes with ABS. Little does he realise that even ABS have a limit to how much braking it can squeeze out of those skinny tyres.

Shan2nu

stupid:
Here is what i understand. Please correct me if i am wrong

ABS actually increases the time required to bring the car to a complete halt
ABS(Present Versions) are not reliable 'cause the number of external variables involved in its working is too high for comfort.

Pray enlighten me; what good is a system like that for a sedate novice driver like me who drives from Thane to BKC in Mumbai mostly in bumper to bumper traffic 5 days a week and at arnd 100-120 on the Mumbai-A'bad stretch of the highway once a month.
For heavens sake i thought; like most other drivers; that ABS would prevent the vehicle from going out of control on hard braking at high speeds and bring the thing to a halt faster.:Shockked:
The only thing it helps me with is that it would not let my wheels lock.
But that does not mean better control on the vehicle does it ??
Wet tarmac or gravel or sand or any other abberration on the runway and the things gonna go all wobbly on me.
And to think that i took a Swift all the way to Spiti and back in high winter trusting my ABS for better control on the vehicle. Shan2nu; i second everything you say.
Scary; i must say.

ABS does not increase the time, it increases the distance to bring the car to a complete halt, that too, if and only if, it senses a wheel lock, not otherwise. ABS is reliable in the sense it does the job it is set out to do - to prevent wheel lock.


ABS is a safety feature. Its not like your car aircon which you need to use regularly. If you are a good driver, you almost never have to get the ABS to kick in, in the first place.

Yes ABS would prevent the vehicle from going out of control by preventing the wheels from locking, but ABS will not bring your vehicle to a halt faster. If your wheels do not lock you can control your vehicle, which is not the case when wheels locks and the car starts to skid.Hence ABS provides better control over your vehicle, if and when the situation arises.

ABS aids in directional stability. Hence it is safer to change direction on wet tarmac, gravel, sand or any other abberration on the runway on a ABS equipped vehicle when compared to a non ABS one.

Thanks Longhorn; i am still not convinced though.
Practically speaking in bumper to bumper city driving; usability of the thing is still an issue with me.
I have been daft in the past for thinking that an ABS enabled car can help me with things it was not intended to solve. Mea Culpa !!
And It aids directional stability at high speeds by preventing wheel locks; i guess that is a plus in my book.
But the fact remains that it is only as good as the driver behind the wheels. So for a novice like me. ABS is a bit of a toss up.

That pretty much sums it up. As I told earlier, ABS is not a one stop solution for all the braking problems. I don't think anybody here has tried to jam the brakes on a slippery surface like a skid pad. In such a situation, once your wheels start slipping, you cannot predict where your vehicle goes. I think there are lots of technology demonstrator videos available in youtube and such, showing the tests of ABS, ESP etc. Finally, it comes to the skill of the person behind the wheel. But then again, there is the saying - Make a system fool proof; somebody will make a better fool.

A very Close friend of mine paid extra money for the ABS in his Fiesta Sxi and he also feels that the ABS definitely does not help in closing the gap. He is infact planning to ask the dealer during the next service if there is a way to disable the ABS or have it completely removed. As said its 'Anti Brake Skid' and it doesnt say anything about closing the stopping distance.

1. ABS increases the time required to bring the car to a halt, as compared to what? Only as compared to threshold braking and this technique is the holy grail novice drivers or even professional racers might not be able to manage on unseen roads and impossible to get right on wet/slippery/uneven/bumpy roads. At high speeds in such conditions, ABS is your best bet. But you have to apply FULL brake pressure.
2. It is for combating the external variables that the present day ABS helps by preventing wheel lock. With the external variables, non-ABS braking would be more disastrous.
3. Spot on. ABS will give you steer control without fearing a wheel lock and without worrying if you've too much or too less brakes to decelerate.
4. It's not the only thing. By preventing wheel lock, ABS gives you steering ability and it would also keep your car straight unlike a car which may spin out of control on wheel lock.
5. It will give you better control over steer, it will let you go on full brakes without worrying about a skid developing.
6. Wet surface and normal braking would result in a faster wheel lock and lesser friction would mean an uncontrollable spin. Nothing helps you better than ABS in this situation. Remember, once you skid on a wet surface (hydroplaning), even if you release brakes, the car may keep hydroplaning till much lesser speeds. An absolute recipe for disaster. This was one of the reasons for the development of ABS because an aircraft simply cannot afford to skid. In sand and gravel, ABS would increase stop distance because locked wheel perform better in this situation. This is why vehicles designed for off-roading would have ABS pulse set lower than road cars.
7. Scary without ABS IMO.

Also, even though wheels locked would give better stop distances over lesser speeds than ABS, the corresponding steer loss is a disastrous combination. Once the speeds go past 100 kmph, ABS would stop faster.

That was one really good response :D

From the Bosch video, it seems that ABS did improve braking distances in conditions we encounter most frequently in India: Dry, wet, and mixed. Broken surfaces are apparently better handled by ABS too, as per the University of Monash report, and Wikipedia. That leaves the omnipresent sand/mud (Bangalore roads). I think as one who drives defensively in most situations, and was done in once where the road surface was broken - ABS makes sense!

I know we'd like to show off our threshold braking skills to friends (Boys will be boys), but that should done by boys in a controlled environment, away from regular everyday traffic. If people start showing off on city roads and highways - well let us not speak ill of the departed.

As rightly said earlier in this thread - ultimately someone has to drive the car. We need drivers' education on maintaining a safe distance from the vehicle in front, anticipation, adhering to speed limits, proper use of signals, and Common Sense (etiquette would be nice too). Without these, even the most technologically equipped car is not going to save anyone. Heck, one can even run too fast around a corner, and slip and fall. Do people need ABS? No, because we have the brains and are supposed to use them.

Slight correction here, DW. At all speeds, a locked wheel takes longer to stop compared to threshold braking. Why? It is because a locked wheel experiences a different coefficient of friction compared to a rolling wheel. It is called coefficient of kinetic friction for the locked wheel and it is the coefficient of static friction for the rolling wheel. And the former has a higher value than the latter which implies better retardation.

Scenario 1: You are descending in a ghat at 70-80 kmph in a shallow blind turn and are on the valley side of the road. It has rained mildly and the road has some wet patches, some small potholes occasionally and the average Indian quality construction. Midway through turn, you see a tree/log fallen on the road blocking it, there's a big wet patch on the road ahead and you know you are too fast. What do you do?

Case 1- Non ABS:
(a) You are an experienced, extremely skilled driver:
You brake so as to stop quickest but can you guarantee, with all your skill, with the vehicle turning, with an unknown road, gravity assisting speed, that wheels won't lock over the wet patch even with light braking? If a wheel locks on the wet patch, you are likely to traverse a tangential line leading off the cliff. If you brake only lightly or release brakes on the wet patch to avoid this, you ram hard into the obstruction (preferred choice in this case).
(b) You are a novice:
You brake hard and steer and either go off the cliff in a skid or ram the obstruction sideways (or front if lucky).

Case 2- ABS:
Regardless of experience, you stomp hard on brakes and steer to stay on road. You stay on road and in case you can't stop the vehicle, you can at least hit the obstruction head on.

Sorry there's a typo there. The coefficient of static friction is higher. So a rolling wheel under threshold braking fares better

Sure. But I was comparing locked wheels with ABS (not threshold braking).
True.
Seems you got confused there. Static friction 'usually' has higher value than kinetic friction. Retardation is best at point of max static friction (threshold braking) just before kinetic friction occurs (wheels beginning to lock with increasing slip).

A clarification is in order.
A car is moving forward and the wheel is rotating freely. The weight of the car is balanced by the road exerting upward normal force and the forward motion is rotating the wheel. The wheel is under static friction since the portion of the tire in contact with the road remains stationary with respect to the road (as is the case when we walk).
Brakes are applied. Forward motion attempts to rotate the wheel while brakeforce attempts to stop the rotation developing some slip. So far as the brakeforce is just the maximum to counter the static friction, there is maximum braking effort (threshold braking occurs here and ABS rapidly varies brake pressure to stay around this point, better the design, closer it will be to ideal). If the brakeforce exceeds this maximum force, the combination of forward motion of the car and retarded rotational velocity due to braking will force the wheel to slide (rub against road surface as when we drag our feet while walking) and there would be kinetic friction instead of static (locked wheels).

You caught it :) the coefficient of static friction may not always be higher.

I don't think so. Locked wheels should still be worse than ABS which, from what data I have so far, gets pretty close to threshold braking.

No, it was a typo. And I'd already corrected it.

Okay, so can we say that the RMS of the braking force applied by ABS should be close to the Ideal braking force, or Threshold? And that the closer it actually is, the better the ABS system?

IMO, the Threshold can only really be identified by a person when all four wheels are on the same kind of surface, and in an emergency if one is on a surface consistently encountered by the said person.

This would also be if all four wheels are on a Good surface (smooth tarmac or concrete), because otherwise the feedback (or lack of it) through the pedals and steering would interfere with the judgement of the driver.

My Logical :P deduction, and 2 cents.

There's conflicting data regarding locked wheels vs ABS. In this link:

http://www.monash.edu.au/muarc/repor...ectiveness.pdf

the study says that at lower speeds, locked wheels may stop earlier than ABS. This was quite a revelation to me.

In this link:

Anti-Lock Brakes (ABS - Wikicars)

It says that ABS performs better in all situations than locked wheels (gives data from a Finnish magazine for stopping from 80-0 kmph for a VW Golf).

If you have any other data, can you post it for our benefit?

------------------
IMO, overall the ABS would be better to have than locked wheels regarding stability in any situation. Consider that only one wheel or two wheels lock which leaves you with some erratic steering ability and this is what might take the vehicle out of control. It's noted that in all situations tested, ABS equipped vehicles always had greater stability and control than non ABS vehicles.

In contrast, it seems to be the unanimous opinion of all studies that though best in outright performance, threshold braking is a near impossible thing to achieve even for skilled drivers in real panic situation and varying external conditions. Also, threshold braking is usually practiced in vehicles with advanced brake systems which would outperform road vehicles any day.

It's known that drivers encountering the pulsing pedal under ABS the first time were likely to instinctively reduce brake pressure thus increasing stop distances. ABS is not a dead-stop miracle as perceived by many, it needs to be worked. This thread is an excellent eye opener for many, I'm sure. :) Modern Emergency Brake Assist systems may help in judging what the driver wants by analysing rate of onset of brakes (slams), vehicle speed etc and modulate brake pressure accordingly.