Team-BHP - Understanding ABS (Anti Lock Braking System)

The data is from the same link posted earlier. Says that ABS still manages to be slightly better than locked wheels on dry pavement irrespective of speed. I suppose the discrepancy could be down to the calibration differences between different systems?

I've only two or three other sources. One is the same as your link. The second is 5 Environment, roadway, and vehicle. And last, there are quite a few demos done by various auto shows/ manufacturers who use the same car and disable ABS for one test and enable it for a second, identical test. The demos are particularly convincing proof that ABS reduces stopping distance compared to a locked wheel.

My original question was whether ABS beat threshold braking so there is the assumption that ABS should easily beat locked wheels in that question. Now, the revelation that locked wheels might beat ABS is quite new to me. I didn't think it was actually possible. Could you quote the exact sentences in that link that said locked wheels were better? I skimmed through it but its pretty big so I think I missed that bit.

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Originally Posted by Delta Wing (Post 1724291)

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).

Even for a skilled driver in a non ABS situation, if the wet patch is only under 1 or 2 tyres, when braking, the wheels on the wet patch will lock, while the others wont, leading to loss of control. I agree with you that ABS is certainly the safer option. agree:

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Originally Posted by Delta Wing (Post 1724291)

Quote:

Originally Posted by Delta Wing (Post 1724291)

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.

My take on this for the sake of debate is this.

If you are an extremely skilled driver, I would say you are better off without ABS in this case. If you know how to drift, (and I'm not talking about rookies here, you need to be a real pro) you could use the wheel lock to your advantage. Here you could induce a drift and come to a stop quicker than what you could wih ABS. When the wheels lock, the friction will be higher, so it should be easier to stop when compared to the ABS equipped vehicle, where the wheels will not lock and therfore the friction will be lesser. The loss of steering control in case of a normal driver, does not affect the pro. He knows how to correct oversteer, control the vehicle, and bring it to a halt with minimum damage. A normal driver will be better off with an ABS equipped vehicle.

Quote:

My original question was whether ABS beat threshold braking so there is the assumption that ABS should easily beat locked wheels in that question. Now, the revelation that locked wheels might beat ABS is quite new to me. I didn't think it was actually possible. Could you quote the exact sentences in that link that said locked wheels were better? I skimmed through it but its pretty big so I think I missed that bit.

If you watch videos of ABS tests, you will notice that some ABS systems lock the wheels as the car is about to come to a halt.

Hers an example - http://www.youtube.com/watch?v=VdoBgowjbR4

Moreover on loose gravel/soil, locked wheels tend to dig in, making the car stop faster.

Budleee Rants and Raves: ABS

Shan2nu

Longhorn, I'm not sure about the claim that locked wheels have more friction. As I mentioned eariler, and DeltaWing elaborated, locked wheels face less friction which is why they take longer to stop. This is because of the difference in the friction coefficients for the two cases.

Shantanu, yes, in gravel, snow etc. wheel lock helps. But on tarmac, it doesn't help at all. As for ABS locking up as the car comes to a halt, that's because it deactivates below 5kph as mentioned by your source. I suppose that's because it is impossible to distinguish between locked wheels and a stopped car at this stage.

Lemme summarize

Scenario:-
I am on the Highway doing a steady 80kph; a rainy day in mild traffic.
Suddenly a dog decides to test my driving skills and waltz across the highway a couple of meters from me.
Me being me and loving the dog more than my car; stand-up on the break lever using all my 92 kgs to good effect and steer the car to the right.

Case 1 :- Car with ABS

I skid but i go roughly where i intend to go 'cause my ABS prevents the wheels from locking up. That means i can control the drift to some extent 'cause i retain control over the wheels. (Provided i also retain my presence of mind)

Case 2 :- Car without ABS

I do a drift of my own 'cause i have no idea what my wheels are gonna do. I either end up on my head or hit the divider side on. I total my car and break a few bones (if i am lucky)
Period. Finito.

@Mclaren Rulez: The document is edit protected so can't paste text from it. The info is on page 19 in ABS Track Test section.

@Longhorn: In the scenario I described, what if one induced a drift and then hit the wet patch? The unpredictable car behaviour might be disastrous. And would we prefer to hit an obstruction sideways?

@nairanupg: You WON'T skid with ABS, that's the whole point of ABS. You just might avoid the dog with correct input of steering. Without ABS, it's favourable to sit on the brakes just till they threaten to lock and only steer a little.

The study of RACV mentions that in panic, drivers tend to focus more on steer than brakes and also apply excessive steer input.

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Originally Posted by McLaren Rulez (Post 1725335)

@ McLaren Thanks for the clarification.

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Originally Posted by Delta Wing (Post 1725545)

@Longhorn: In the scenario I described, what if one induced a drift and then hit the wet patch? The unpredictable car behaviour might be disastrous.

That's why I said the driver should be a pro.

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Originally Posted by Delta Wing (Post 1725545)

And would we prefer to hit an obstruction sideways?

Well it depends on the type of obstruction and the speed at impact. On a vehicle that is not equipped with airbags, you may have better chance of survival if you hit an obstruction sideways due to two reasons.

1. The speed of your vehicle will be considerbly lesser if it is drifting sideways than when you collide head on.
2. When the impact is spread over a larger area, the absorption by the body shell should be better than when is is spread over a smaller area.

It all depends on the situation. Sometimes frontal impact is better, at other times side impact will be better.

Quote:

Originally Posted by Delta Wing (Post 1725545)

The study of RACV mentions that in panic, drivers tend to focus more on steer than brakes and also apply excessive steer input.

This could be the reason why ABS equipped cars topple in an accident. We can't blame ABS for the driver's mistake. Excessive steering inputs at high speed is a recipe for disaster, ABS equipped or not.

Quote:

Originally Posted by longhorn (Post 1725737)

Side-Impact accidents have a very significant likelihood of injury/fatality, as cars don't have a significant crumple zone to absorb the impact forces before an occupant is injured.
The speed of the drifting vehicle may or may not be lesser with locked wheels than in a vehicle with ABS on full brakes.
Even without airbags, with seatbelts, you have a better chance of being safe in a frontal impact given the same speed.
On a wet surface, the vehicle may start hydroplaning and thereafter skill is irrelevant. You leave brakes and wait for wheels to unlock before reapplying brakes. There may not be enough time. So in this kind of a situation, even for a pro the ABS option would be best IMO.

A significant factor in the studies quoted hitherto has to be taken into account. These studies examined reported crashes as their data. There's no evidence to indicate how many accidents were actually saved by the ABS because an averted accident would simply not be reported.

So firstly Im no expect in braking systems this must be said hoever here is some of my expereince.

It is quite diffcult to learn to threshold brake i know because I have been on various courses and instruction from tutors on how to do this and its difficult but amazing effective once you have mastered it however there a few assumptions that threshold braking is better than ABS that usually dont hold on the road but do on the track.
1: The road surface (max friction coef) has to be very similar for both front wheels.
2: Car must be in very good repair with both brakes creating the same brake force and neither disc being warped.
3: The tyres must be the same compound and pattern and running at the same temperature at the time of incident

On a track as an example (from memory) I could get ABS braking to stop me in about from 30-35 meters from 60mph and about 20 meters without.

[quote=McLaren Rulez;1725335]Longhorn, I'm not sure about the claim that locked wheels have more friction. As I mentioned eariler, and DeltaWing elaborated, locked wheels face less friction which is why they take longer to stop. This is because of the difference in the friction coefficients for the two cases.
QUOTE]

I belive this to be correct, on a well designed car there is more friction between the brake pads and the disc vs the rubber contact patch and the road. So if you keep the wheel turning you can use the braking system friction rather than the road fiction.

Quote:

Originally Posted by McLaren Rulez (Post 1725335)

Shantanu, yes, in gravel, snow etc. wheel lock helps. But on tarmac, it doesn't help at all. As for ABS locking up as the car comes to a halt, that's because it deactivates below 5kph as mentioned by your source. I suppose that's because it is impossible to distinguish between locked wheels and a stopped car at this stage.

Yes this is true, the car does not know that you havent in fact stopped and it doesnt really matter anyway.
If you look at the abs/dsc wheel sensors at the 4 hubs (assuming its 4 channel ABS) they have a large number of raised bars or holes or lines or whatever method the car manufacturer has employed if you move too slowly you cant see any pulses generated in the sensor so you dont know you are still moving. Its the same effect if you have an old mechanical speedo and move very slowly your speedo cant detect the movement and reads zero or flickers till you hit at least 10kmph
This is the same

Quote:

Originally Posted by aaggoswami (Post 1714675)

1) An example is G2HC i.e. NHC. The initial version had an issue of wheel lock, but the issue was solved with ABS. Here actually ABS helped.

Sorry to pick up an old thread but wanted to clarify did the NHC ever come with ABS in India, I am asking this because I recently met with a shunt (estimated repairs at 1Lakh, mostly cosmetic) which I could have easily avoided did I have ABS, the NHC kept skidding on locked wheels as if on rails without cutting down on speed resulting in a crash, I have since then been digging into threads on threshold braking and came across this post.
Also is the NHC known for these wheel locks under hard braking and is there some thing that can be done. I think its also because of the narrow 175 tires. Thanks.

ABS is more useful on slippery roads with frozen snow known as "black ice". Before ABS became the norm, you had to pump the brakes in the older cars and then brake. If you didn't do that and just hit the brakes hard your car would not be able to grip the road, the brakes would lock and your car could spiral into a 360. While rain drenched roads are bad, they are nowhere as slippery as iced roads. So except for a few mountain regions in India where it snows - the necessity of ABS will be largely negated.

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Originally Posted by invidious (Post 4091051)

A So except for a few mountain regions in India where it snows - the necessity of ABS will be largely negated.

I don't where you got your info but evidently you can do with some more reading around these. ABS simply prevents wheel lockup that happens in panic braking situations. It is superfluous to state that at high speeds if the wheels suddenly lock up it can lead to catastrophic and unpredictable results like slide, skid, uncontrolled spin or even toppling of the vehicle.