Does ABS work when sliding backwards?

Lol, I got you. I think everyone but OP gets it

You are correct the it is the same hardware.

Specifically regarding ABS. If you are braking and one wheel stops turning the computer recognises this as a slide and modulates the braking. If all four wheels stop at the same time the computer thinks you are stopped and does nothing.

Traction control applies when you are accelerating and one wheel begins to turn faster than the others - braking is then applied to that wheel. If all four wheels are spinning but all four are spinning at the same rate, traction control will not kick in.

Same monkeys, different circus.


Edit to address your last comment - it judges the vehicle speed by the average rotational rate of all four wheels. If they all lock at once there is no speed differential at any one wheel therefore no ABS.

 

This discussion did bring up one idea in my head. Normal emergency braking happens when the vehicle is moving, but in this case the vehicle starts motionless.

I'm guessing that ABS is triggered off sudden rotational deceleration of the wheel as opposed to its rotational speed. This section of the wikipedia article leads to believe this.

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There are four main components of ABS: wheel speed sensors, valves, a pump, and a controller.
ABS speed sensors
Speed sensors(Encoders)
A speed sensor is used to determine the acceleration or deceleration of the wheel. These sensors use a magnet and a Hall effect sensor, or a toothed wheel and an electromagnetic coil to generate a signal. The rotation of the wheel or differential induces a magnetic field around the sensor. The fluctuations of this magnetic field generate a voltage in the sensor. Since the voltage induced in the sensor is a result of the rotating wheel, this sensor can become inaccurate at slow speeds. The slower rotation of the wheel can cause inaccurate fluctuations in the magnetic field and thus cause inaccurate readings to the controller.

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In an emergency braking situation of the wheels go from spinning very fast to not spinning at all. In the sliding backwards scenario, the rotational speed doesn't change, so ABS is never triggered. I'm guessing the same is true if you start sliding down a hill forwards from a standstill.

So I think you might be right about the ABS not activating because it "thinks the truck is not moving", but I don't think you are right about ABS being triggered by uneven wheel rotation. I think ABS doesn't care whether the truck is moving or stopped.

Make sense?

 

Are you claiming, that if I smash the brakes while driving forward, ABS will not activate because it thinks I'm sitting still? That has never been my experience. That would completely defeat its purpose. I think we must be misunderstanding each other.

 

I live in WI and can guarantee you that on snow/ice, if you jam the brakes on, you will lock the tires. ABS is a good thing, but it can't solve all situations. That is why you are in control.

As for your original question, I would think ABS would work even if you are going backwards. But, I read your question as saying your wheels were already locked up, at which point, see above.

 

Yep, you nailed it



It is Sunday right? I didn't just dream the whole weekend and this is actually a Friday post?

 

I do not share your experience. I slam the brakes on all the time in the snow and ABS activates. Every snow storm I head to the nearest parking lot to whip shitties and what not. I'm not saying ABS is perfect, but it certainly activates when you slam the brakes on.

 

Ok, but you were saying ABS only activates when there is uneven wheel rotation. I'm talking about something different. I'm talking about the rate of change of wheel speed regardless of differences between the rotational speed of the 4 wheels.

 

When you smash the brakes when driving forward, typically the front wheels lock up before the rear wheels, and that's how the ABS system detects lockup. If all four wheels come to a stop at the same time, ABS activation is not assured. Some models may activate it based on other inertial sensors in the vehicle (those sensors being part of the federally mandated VSC system).

 

Maybe, but I'm thinking it's more likely triggered off the rate of change of wheel rotational speed as that seems more reliable.

 

This section the wikipedia article explains it well.

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There are many different variations and control algorithms for use in ABS. One of the simpler systems works as follows:[26]

1. The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car two to four seconds to stop from 60 mph (96.6 km/h) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.
2. The ABS controller knows that such a rapid deceleration of the car is impossible (and in actuality the rapid deceleration means the wheel is about to slip), so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly before the wheel can actually significantly change speed. The result is that the wheel slows down at the same rate as the car, with the brakes keeping the wheels very near the point at which they will start to lock up. This gives the system maximum braking power.
3. This replaces the need to manually pump the brakes while driving on a slippery or a low traction surface, allowing to steer even in most emergency braking conditions.
4. When the ABS is in operation the driver will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. This pulsing also tells the driver that the ABS has been triggered.

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I learned something today.

I would still like to experiment with sliding backwards down hills. For example, I'm curious if ABS will activate while pumping the brakes with the locker off. Pumping the brakes would allow the wheels to start spinning and then decelerate them. I could try this going forward as well.

Thanks, everyone.

 

My question is not about the ABS being disabled when the locker is engaged. I mentioned ABS being disabled when the rear is locked to explain why my ABS didn't work, but it doesn't explain why it didn't work for the other rigs.

That's a little bit misleading I think; at least in my head. After working though this topic in this thread, I think they key is not the speed of vehicle nor the rotational speeds of the wheels, but rather the rate of change of the rotational speed of the wheels.

 

Fug it! The answer is….

Lawsuit!

Lol!

 

The actual control logic in most modern vehicles is significantly more nuanced than just preventing lockup. A small degree of lockup actually results in shorter stopping distances. The engineers call this "slip ratio". The ideal slip ratio varies according to the specific tire, weight on the tire, and the terrain. A certain slip ratio might work best on dry pavement, another for wet pavement, another with slush and ice, etc. You know how the ABS valve pulses (solenoids opening and closing) when ABS activates? The frequency and duration of those pulses vary according to conditions. In some terrain, full lockup results in shorter stopping distance. See 100 pages of 2nd gen folks complaining ABS being dangerous: https://www.tacomaworld.com/threads/abs-on-this-truck-is-dangerous.68167/ Toyota implemented different ABS calibrations for 4WD in the FJ starting in 2010. The Tacoma got it with the 3rd gen starting in 2016.

 

Ya I don't doubt modern systems are quite a bit more complicated than the simple model we've discussed here. In fact, I would not be surprised if they take into account differences between the 4 wheels rotational speed and rate of change of rotational speed. I was just getting hung up on my perception that people were saying ABS was only triggered by differences in the rotational speed of the wheels, mostly because it sounded like confusion with traction control.

 

Maybe our disconnect is that under ideal circumstances ABS does not allow the brakes to lock up when you slam the brakes on, but when it fails to do that, ABS might not be able to recover because it now thinks "the truck is standing still". It's a bit confusing, because as the driver it can be hard to tell the difference between ABS and the wheels being locked up. The sound of the brake pump on my off-road varies depending on the circumstances and I rarely look at the dash indicators, so I don't always know when it's intervening unless it's in loud mode. It's possible that I have mistaken ABS for locked brakes on some occasions. I'm going to pay more attention and also relearn the brake pumping habit which has gone stale since the purchase of my first car with ABS.

 

That looks like fun. In 30 years of driving in snow and I've never slid backwards down a hill more than a few feet until yesterday. I was happy to keep it on the trail and out of the trees.

 

I assure you ABS does take into account differences in wheel speed when applying the brakes. It likely will engage by slamming on your brakes while going 50 MPH, but at 2MPH I don't believe it would care.

Traction control only takes affect during acceleration, at least from what I could find in the repair manual documentation.

 

Modern systems might take differences between wheels into account, but I do think it will is necessary for preventing lock up during emergency braking. Referencing the Wikipedia article, there is exist or existed 1 channel ABS they had only one sensor and one valve.

I haven’t looked at the speedometer when ABS activates, but it activates at walking speeds. I’m sure there is a threadhold at which it does not activate.

 

My 2003 tundra looked at all. Like I previously mentioned, I had an issue with a rear sensor, causing it to always read 0 MPH. When I applied the brakes, no matter how gently, it would activate ABS.

This might prove useful to you. Not sure if it's out of date or not but at least it's Toyota specific.

 

Exactly...

Let's say best deceleration is at 5% slip. We can assume that on dry pavement, both front wheels will be within a percent or 2 of each other on slip and both rear wheels will be in the same range under threshold braking.

The relationship between front and rear slip levels is variable based on loads, so that has to be taken into account as well.

If the ABS module senses wheel speed differences of say 4% between the two front wheels, 4% between the 2 rear wheels, and 10% between a front and rear wheel, it will begin dumping fluid on the circuit who's wheel is turning the slowest, and doing so progressively until it senses that that wheel has joined the others or returned to within the accepted "normal" relationship range with the others.

These numbers are made up and this is an overgeneralization, but that's the gist of it. Lots of fine tuning and special circumstances go into the algorithm.