The ABS discussion thread

[Sutripta]

Originally Posted by DerAlte

The ABS ECU *doesn't calculate vehicle speed* - it doesn't need it.

Any need for 'Estimated Vehicle Speed'?

On another matter, below a (pretty low) threshold, I believe it's common to switch off ABS.

Regards
Sutripta

[im_srini]

Originally Posted by Viju

Just posting a capture from the Ritz Owner's Manual Page 64 for reference.
PS : This need not be the case for all OEMs / models.

Originally Posted by Viju

All they are saying is that the ABS will not work if the vehicle speed is below 10 km/h.

I'm with Viju on this, remember reading somewhere ( I think the Gen-1 Swift's Service Manual ) that the minimum speed for the ABS is 11 Kmph.

Originally Posted by Sutripta

On another matter, below a (pretty low) threshold, I believe it's common to switch off ABS.

Initially was under the impression that there was a minimum "resolution" for the ABS reluctor ring + sensor combo ( sort of like that with mouse wheels ).
The Swift came with 165/80-R14 wheels, which means that at 10 Kmph, the wheel RPM is ~ 1.4 RPS ( is 1 RPS at 7 Kmph ).

But then came across this IEEE document which I think indicates the real reason.

RotationalSensors.PDF

To quote from the document [ Page 19 ] :

...
Saturation Effect
There is also an important saturation effect here.
If the angular speed is too small compared to the sample interval, then (10) collapses, since no pulse will be available.
...
This gives a lower bound on the sampling interval for a given minimum speed.
As a practical consequence of this, the ABS has to be turned off for low speeds.
A remedy would be to sample the analogue sensor signal, rather than converting it to a square wave, and use the phase information, but this would affect the price of the sensor.
...

Disclaimer : I'm no automotive engineer, hence 'am completely out of depth here & could be barking up the wrong tree...
.

[DerAlte]

Originally Posted by Viju

... I know of multiple models in which the transmission output speed was unreliable ...

:O Difficult to believe it is not reliable - it is a simple reluctance sensor near the output drive shaft.

... they are just giving an approximate vehicle speed below which the WSS pulses may no longer be reliable for the functioning of the ABS. ...[/quote]

Originally Posted by im_srini

I'm with Viju on this, remember reading somewhere ( I think the Gen-1 Swift's Service Manual ) that the minimum speed for the ABS is 11 Kmph. ...

The Swift came with 165/80-R14 wheels, which means that at 10 Kmph, the wheel RPM is ~ 1.4 RPS ( is 1 RPS at 7 Kmph ). ...

I used to drive a rental VW Jetta (with ABS) in Germany, 20 years back. Everyone was headed home early one day as there was a weather announcement of impending bad weather. All of a sudden it started snowing heavily. First time experience in those conditions, and I was crawling (with heart in my mouth) looking for a slot to park the car on the street near my house. The speed was really low - low enough that the speedo needle was just intermittently flicking above 0. Braked hard (pushed pedal suddenly) when I saw in the ORVM someone pulling out, and promptly ABS acted - causing a loud shudder through the brake pedal and making me almost jump out of my skin.

The lower limit is just an arbitrarily declared figure to put the monkey on someone else's shoulder.

Originally Posted by Sutripta

Any need for 'Estimated Vehicle Speed'? ...

Not sure I understand why it would be required, but no, Prof!

Originally Posted by Sutripta

... below a (pretty low) threshold, I believe it's common to switch off ABS. ...

Manually? There is no switch, unless a switch is manually added by vehicle owner in his off-roader. ABS doesn't act on a wheel only if that wheel's sensor is faulty. There is no other way to switch ABS off.

[batterylow]

Originally Posted by DerAlte

Manually? There is no switch, unless a switch is manually added by vehicle owner in his off-roader. ABS doesn't act on a wheel only if that wheel's sensor is faulty. There is no other way to switch ABS off.

I am curious why most bikes equipped with ABS provide a facility to switch it off, while cars only have an ESP off switch.

To enable pulling off stunts maybe!

[Viju]

Originally Posted by DerAlte

:O Difficult to believe it is not reliable - it is a simple reluctance sensor near the output drive shaft.

Unfortunately, I cannot name all the models, but an example in India is the A-Star Automatic. I am talking from first hand experience as I am involved in ABS/ESC development for passenger cars and motorcycles for a living.

Originally Posted by DerAlte

The lower limit is just an arbitrarily declared figure to put the monkey on someone else's shoulder.

Spot on! This is the fact. These are all disclaimers to avoid market claims. I just didn't want to put it out bluntly.

Originally Posted by DerAlte

Manually? There is no switch, unless a switch is manually added by vehicle owner in his off-roader. ABS doesn't act on a wheel only if that wheel's sensor is faulty. There is no other way to switch ABS off.

There is no manual switch for ABS in passenger cars, unlike for ESC/ TCS (many motorcycles do have ABS ON / OFF switches though). But to switch ABS off, all you have to do is open the fuse box cover and remove the correct fuse for the ABS pump motor. The ABS warning lamp will come ON and ABS will be disabled.

What Sutripta was talking about was turning off the ABS through the software logic below a certain speed which again I personally don't think, is the case. Even at <5 kph, if there is a big enough slip (an example being hard braking on wet ice), ABS control will start.

Cheers

[ILTDrive]

Can ABS be fitted into a non ABS vehicle? I have a 2013 Dzire VDI which did not have ABS, however the newer model of Dzire VDI has ABS.

[Thad E Ginathom]

Originally Posted by DerAlte

I used to drive a rental VW Jetta (with ABS) in Germany, 20 years back. Everyone was headed home early one day as there was a weather announcement of impending bad weather. All of a sudden it started snowing heavily. First time experience in those conditions, and I was crawling (with heart in my mouth) looking for a slot to park the car on the street near my house. The speed was really low - low enough that the speedo needle was just intermittently flicking above 0. Braked hard (pushed pedal suddenly) when I saw in the ORVM someone pulling out, and promptly ABS acted - causing a loud shudder through the brake pedal and making me almost jump out of my skin.

I tried the ABS on my Rover 620 (British-built Honda). It was on ice, and less than walking speed. Juddering in the foot told me that it was working.

As I understand it, ABS operates just before wheel lockup, to prevent the wheels becoming locked. It must work then, when the wheel rotation speed is almost zero. I don't see any relevance in the original speed of the car at the moment that the breaks are applied. Am I missing something?

[batterylow]

Originally Posted by ILTDrive

Can ABS be fitted into a non ABS vehicle? I have a 2013 Dzire VDI which did not have ABS, however the newer model of Dzire VDI has ABS.

Its not advisable to retrofit ABS. Factory fit is the best bet!

[DerAlte]

Originally Posted by Viju

... I am involved in ABS/ESC development for passenger cars and motorcycles for a living. ...

Jolly good - we need more people like you pitching in.

Which company, may one ask?

Originally Posted by ILTDrive

Can ABS be fitted into a non ABS vehicle? I have a 2013 Dzire VDI which did not have ABS, however the newer model of Dzire VDI has ABS.

It is possible if the base model is the same - theoretically. The snafu is that Suzuki will charge you for all the components at Spares rates, making it economically unviable!

Originally Posted by Thad E Ginathom

... As I understand it, ABS operates just before wheel lockup, to prevent the wheels becoming locked. It must work then, when the wheel rotation speed is almost zero. I don't see any relevance in the original speed of the car at the moment that the breaks are applied. Am I missing something?

You are missing nothing at all, sir!

The key criteria is the rate at which the (rotating) wheel is slowing down. The ABS ECU just calculates angular speed difference every few milliseconds. If the deceleration rate is higher than a trigger point (which is a pointer to impending wheel lockup), ABS acts. It has nothing to do with the speed of the vehicle.

[Chetan_Rao]

ABS works on the (sensor-measured) likelihood of losing traction and locking wheels, not driving speeds! Someone better acquainted with the technicalities can explain the exact mechanism, but that's the gist of it.

If an ABS-equipped vehicle loses traction at single-digit speeds, ABS will still kick in.

[Viju]

Originally Posted by DerAlte

Jolly good - we need more people like you pitching in. Which company, may one ask?

Sure. I currently work for Continental Automotive Corporation, Japan as a Vehicle Test Engineer for Motorcycle ABS.

[itwasntme]

In my Cruze, ABS definitely works regardless of speed.

Just this morning outside my sons' school, I parked at around 5 km/hr and the ABS immediately kicked in as there was a layer of very slippery mud.

My overall take is that even at very low speeds on icy/muddy roads, ABS will avert the common rear-ending incidents because 99% of Indian drivers don't know how to maintain a safe distance in such conditions.

[commonman]

One of my friend's Ritz ZXi (ABS) suddenly lost the brakes on a downhill as she was unable to stop by pressing the pedal.Some how she stopped the car and she went in to tears as there were school children in front of her car.She couldn't bear the fact that, if something had happened to those kids, how will she live with that. Later on, upon inspection,we found that the car had no fuel and may be because of that, the vehicle had lost the brakes. Even if the vehicle is switched off,there will be some amount of brake.Then why did this happen? Currently, there is no issue with the brakes.The car is braking and running good.Can the experts comment ?

[vibbs]

Originally Posted by commonman

One of my friend's Ritz ZXi (ABS) suddenly lost the brakes on a downhill as she was unable to stop by pressing the pedal.Some how she stopped the car and she went in to tears as there were school children in front of her car.She couldn't bear the fact that, if something had happened to those kids, how will she live with that. Later on, upon inspection,we found that the car had no fuel and may be because of that, the vehicle had lost the brakes. Even if the vehicle is switched off,there will be some amount of brake.Then why did this happen? Currently, there is no issue with the brakes.The car is braking and running good.Can the experts comment ?

It is a little confusing. You say that there was no fuel in the car. Does this mean that the car was switched off and coasting downhill?

All vehicles now a days have power assisted brakes. Which means the brakes will cease to function/ become extremely hard if the engine is switched off. The residual vaccum in the system may help you with some braking action initially but as you keep pumping the brakes you will lose that too in no time. Only hope might be to literally stand on the pedals.

It is highly unsafe to coast downhill on a car with no power.

[saket77]

Originally Posted by commonman

One of my friend's Ritz ZXi (ABS) suddenly lost the brakes on a downhill as she was unable to stop by pressing the pedal.Some how she stopped the car and she went in to tears as there were school children in front of her car.She couldn't bear the fact that, if something had happened to those kids, how will she live with that. Later on, upon inspection,we found that the car had no fuel and may be because of that, the vehicle had lost the brakes. Even if the vehicle is switched off,there will be some amount of brake.Then why did this happen? Currently, there is no issue with the brakes.The car is braking and running good.Can the experts comment ?

Originally Posted by vibbs

It is a little confusing. You say that there was no fuel in the car. Does this mean that the car was switched off and coasting downhill?

All vehicles now a days have power assisted brakes.

Although the advise by Vibbs is spot on, but even if the car is out of fuel, but rolling downhill in a gear (not advisable to do so), the engine will keep generating the required vacuum for the brakes as the engine will be driven through the rotation of wheels through the transmission. However, the brakes would be lost if the transmission is set to neutral.

Regards,
Saket