The ABS discussion thread

[vikram_d]

Originally Posted by Taha Mir

Hello everyone,
I have a 2013 Ford Ecosport (ABS equipped). I tried checking to see if the ABS works a couple of days back. While driving ( in an empty lot) at 20-30 kmph I slammed the brakes while turning the steering wheel, both the front wheels locked up and the car skidded straight instead of turning . That's not supposed to happen right? The wheels shouldn't lock up and the car should turn where I want it to. I complained to the service manager during the service and he told me that the ABS only works when the car is doing speeds above 50 km/h. Is that true?

I remember the 1st gen Swift's workshop manual mentioning that the ABS in the Swift doesn't work below 40kmph. So the 50kmph limit on the Ecosport may just be right. But you never know.

[sandeep108]

More knowledgeable people may correct me but I think that there has to be one more safety system that is there in more advanced cars - i.e. EBD/ESP alongwith ABS to allow ABS to work when the steering is turned. Probably ABS will only work in some cars when in a straight line.

In both my Swift and Jazz, I have experienced ABS even at low speeds in traffic when slamming on the brakes, so I don't think speed is of any issue here.

[DerAlte]

Originally Posted by vikram_d

... So the 50kmph limit on the Ecosport may just be right. ...

Originally Posted by sandeep108

... In both my Swift and Jazz, I have experienced ABS even at low speeds in traffic when slamming on the brakes, so I don't think speed is of any issue here.

Originally Posted by Taha Mir

... I complained to the service manager during the service and he told me that the ABS only works when the car is doing speeds above 50 km/h. Is that true?

No, that's incorrect - there is no minimum speed. Or rather, the wheels should have been rotating previously for the ABS to sense a lock condition - it is continuously monitored. (In a 4 channel ABS system) The average rotational speed of the 4 wheels (not vehicle speed) is compared with rotational speed of each wheel to infer locking. Whichever wheel locks (or all the wheels that indicate locking), the ABS system modulates the valves associated with the brake line of that / those wheels.

Originally Posted by sandeep108

More knowledgeable people may correct me but I think that there has to be one more safety system that is there in more advanced cars - i.e. EBD/ESP alongwith ABS to allow ABS to work when the steering is turned. Probably ABS will only work in some cars when in a straight line. ...

No, beyond sharing the valves and piping and running on the same ECU, these systems are independent. EBD is brake force distribution. ESP is a stability program. ABS will work without them.

Originally Posted by Taha Mir

... While driving ( in an empty lot) at 20-30 kmph I slammed the brakes while turning the steering wheel, both the front wheels locked up and the car skidded straight instead of turning ...

Did you try that only once, or multiple times? How far did the car skid?

That is not really the right way to test, nor is dry pavement the right surface. Look for a surface with some sand / gravel / wet clay / water. Brake in a straight line (no, there is no link with steering angle, but even then), and when ABS activates, see if you are able to steer as you want.

[Parth46]

I think there is absolutely no minimum/maximum speed limit for ABS to work.

I've braked hard in my car at a variety of speeds ranging from 30 to 100+ and ABS has worked every time it should have, and I've been able to maintain perfect control over the steering at all times. It's a very important and basic safety feature and there no limitations/requirements in terms of speed. It measures the rate of spin of each tyre and if any tyre starts slowing down rapidly in comparison with the others, ABS kicks in to modulate the braking force and keep the tyre from locking, that's all there is to it.

You can easily test it by driving partially over water, sand, gravel etc by keeping 1 or 2 tyres on tarmac and rest on a slightly slippery surface.

[Jeroen]

Originally Posted by Parth46

I think there is absolutely no minimum/maximum speed limit for ABS to work.

ce.

It depends on the car and brake system. In many cases you might find a minimum speed mentioned even in the owners manual. Honda, or at least some Honda models spring to mind.

The problem is a bit that it is really difficult to make ABS work well at low speeds and its hardly effective anyway, because of eh, low speed to start with. Check your manual, or give it a try. Drive slowly and you might still be able to lock up the wheels. For a very short duration.

Jeroen

[Taha Mir]

I'll update you folks after testing it in an enclosed sandy area.
Thanks for the info @DerAlte, I tried it a couple of times and it only skidded maybe 2-3 feet (Ik its less but it's still not supposed to happen).
@vikram_d, the ford manual says nothing about a minimum distance for the ABS to activate.

[Viju]

It is generally difficult for ABS to estimate the accurate wheel speeds below 10 kph, and many manufacturers do in fact put in a line of caution in the Owners' / Workshop manuals.

ABS not working below 50 kph is non-sense. If any service manager says so, please ask him to furnish company literature from Ford stating the same.

[latentpotential]

At low speeds, with 80% worn out tyres (tread depth of 3 mm left), I notice the ABS kicks in more often than before. At low city speeds, but on wet/ slush/ recently repaired pothole-gravel-strewn-around Mumbai roads. I thus doubt there is some predefined minimum speed below which ABS will not activate, although earlier I would have been of a different opinion - current self-experience states otherwise.

Vehicle is a Ertiga Zdi.

[gthang]

In Motorcycles, there is usually a minimum speed before the ABS system comes online, 5-10 kph. depending on make.

Assume there will be a similar minimum speed before the individual wheel sensor pick up data is processed in cars.

Once online, only relative speed between wheels should matter.

Cheers

[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.

[sagarpadaki]

Originally Posted by latentpotential

At low speeds, with 80% worn out tyres (tread depth of 3 mm left), I notice the ABS kicks in more often than before. At low city speeds, but on wet/ slush/ recently repaired pothole-gravel-strewn-around Mumbai roads. I thus doubt there is some predefined minimum speed below which ABS will not activate, although earlier I would have been of a different opinion - current self-experience states otherwise.

Vehicle is a Ertiga Zdi.

That is more to do with your tyres becomes hard and worn than the predefinied speed. Hard tyres cause ABS to kick early

[DerAlte]

Originally Posted by Viju

Just posting a capture from the Ritz Owner's Manual Page 64 for reference. ...

Strange! Wonder why they would write such a thing. The logic in the ABS ECU, any and all, inferring wheel lock is unrelated to the vehicle speed. Vehicle speed is not even an input to the ECU, nor does it calculate it from the wheel pulses.

Originally Posted by sagarpadaki

... Hard tyres cause ABS to kick early

Hard tyres produce vapors at the contact patch earlier than soft tyres, relieving adhesion. It actually depends on how hard the driver brakes (d/dt?) that causes that condition. As soon as adhesion is relieved, the force countering the momentum of the car is drastically reduced, and that causes the wheel to lock instead of rolling. [Hope someone expresses it better / simpler than my gobbledygook]

Hard tyres also squeal loudly on high-mu surfaces (good roads without sand / mud / water) when wheel lock happens. On low-mu surfaces, one only hears shshshshshsh without ABS.

[Viju]

Originally Posted by DerAlte

Strange! Wonder why they would write such a thing. The logic in the ABS ECU, any and all, inferring wheel lock is unrelated to the vehicle speed. Vehicle speed is not even an input to the ECU, nor does it calculate it from the wheel pulses.

I am sorry, but could you please explain a bit more? Especially this part: "Vehicle speed is not even an input to the ECU, nor does it calculate it from the wheel pulses". How is vehicle speed calculated, if not from the wheel speed pulses?

[DerAlte]

Originally Posted by Viju

I am sorry, but could you please explain a bit more? Especially this part: "Vehicle speed is not even an input to the ECU, nor does it calculate it from the wheel pulses". How is vehicle speed calculated, if not from the wheel speed pulses?

Let me paraphrase from your expression: yes, to calculate vehicle speed, pulses are required from a sensor, whose timing has a mathematical relationship with wheel speed. BUT ... it doesn't have to be directly mounted on the wheels to do it - there are plenty of other places with known mathematical relationship.

The ABS ECU *doesn't calculate vehicle speed* - it doesn't need it. It uses wheel rotational speed for it's logic.

Engine and transmission ECUs, and others (like Speedometer), that need VSS (Vehicle Speed Signal), don't take it from the ABS wheel speed sensors in most makes of vehicles. Many older systems, especially where the vehicle manufacturer made all the electronics themselves - like GM and Ford, shared the ABS wheel speed sensors. In makes with ABS optional depending on model variant, one can't do that (sensors on wheels without ABS is redundant cost). VSS comes from another sensor (IIRC it is in the transmission somewhere - differential most likely). Where else would you get VSS from if ABS is not present?

Another reason for avoiding using wheel speed sensor input for VSS is the variation of wheel speed signals from the 4 wheel speed sensors, and the need for averaging - needing more logic and software. Everything translates to money, na?

[Viju]

Originally Posted by DerAlte

...to calculate vehicle speed, pulses are required from a sensor, whose timing has a mathematical relationship with wheel speed. BUT ... it doesn't have to be directly mounted on the wheels to do it - there are plenty of other places with known mathematical relationship.

Yes, agreed. It can be taken from other places with known mathematical relationship, such as the transmission output shaft.

Originally Posted by DerAlte

The ABS ECU *doesn't calculate vehicle speed* - it doesn't need it. It uses wheel rotational speed for it's logic.

Totally agree. All that the ABS uses is individual wheel slip in relation to a reference velocity.

Originally Posted by DerAlte

Engine and transmission ECUs, and others (like Speedometer), that need VSS (Vehicle Speed Signal), don't take it from the ABS wheel speed sensors in most makes of vehicles. Many older systems, especially where the vehicle manufacturer made all the electronics themselves - like GM and Ford, shared the ABS wheel speed sensors. In makes with ABS optional depending on model variant, one can't do that (sensors on wheels without ABS is redundant cost). VSS comes from another sensor (IIRC it is in the transmission somewhere - differential most likely). Where else would you get VSS from if ABS is not present?

Slightly disagree. In fact, in older vehicles (equipped with ABS), VSS was taken from the transmission output shaft speed sensor (especially for the instrument cluster), and the WSS signals were used for ABS control logic. But these days, many makers have done away with providing transmission speed sensors and just use the vehicle speed signal from ABS (which it calculates, and is available on CANbus anyway).
[/quote]

Originally Posted by DerAlte

Another reason for avoiding using wheel speed sensor input for VSS is the variation of wheel speed signals from the 4 wheel speed sensors, and the need for averaging - needing more logic and software. Everything translates to money, na?

In fact, in today's situation, I know of multiple models in which the transmission output speed was unreliable (A/Ts, DCTs, CVTs etc bring a lot of complication with them compared to the good old M/T), and hence the speed sensor at the transmission was eliminated and ABS output signal is reliably being used by ECM and instrument cluster.

In fact, the saving of money is by eliminating the transmission speed sensor and using the ABS WSS for VSS inputs to other ECUs.

Back to the extract I posted from the Ritz' manual -

Originally Posted by DerAlte

Strange! Wonder why they would write such a thing. The logic in the ABS ECU, any and all, inferring wheel lock is unrelated to the vehicle speed. Vehicle speed is not even an input to the ECU, nor does it calculate it from the wheel pulses.

All they are saying is that the ABS will not work if the vehicle speed is below 10km/h. That does not imply that VSS is an input to ABS. As a reference value to the layman user, they are just giving an approximate vehicle speed below which the WSS pulses may no longer be reliable for the functioning of the ABS. I think this is a fair enough value for the end user.