ABS doesnt work at slow speed.

silverdragon
Demi-God

Busters Stunt Double, .. because "Life" is something that happens when your planning something else.

Posts: 19,452

ABS doesnt work at slow speed. Aug 1, 2015 7:05:55 GMT

Quote

Post by silverdragon on Aug 1, 2015 7:05:55 GMT

Its sort of common belief that the ABS on cars does not work under something like 5 to 10 mph.
That Speed is Dependant on who you talk to and how many pints they had at the bar?...

This is a common myth from the common person who isnt a mechanic, so, even if its a "simples" answer for those mechanically minded, I suggest its worthy of testing for those of us who perhaps are not.

I tend to believe this myth anyway, but maybe for different reasons than obvious?... experience, in slow speed sudden stops, I dont "feel" that sort of same feeling as the ABS at higher speeds.
TLW has also suggested a side myth, if you manage to lock up all four wheels, the car thinks it has stopped, and will cancel the ABS system.
I have heard this before....
But how would you lock up all wheels when you are moving (above the speed of ABS being active) anyway?...

Testing, ask the car manufacturers...
Is this dependant on the car and ABS system makers?...
Is it also dependant on what type of ABS you have?...
How does it sense speed anyway?...

OR, and this is a hypothesis I have formulated all on my own, without any other help, is it that it doesnt have time to make any effect anyway?...
Being the vehicle is travelling that slow anyway, you dont get enough rotation of the wheel for the ABS to pulse the brakes for you.
Thinking on at 5mph I could probably haul my car or a truck to a dead stop in one rotation of the car wheels or less, of course thats easier with a truck wheel being larger....
I am suggesting that maybe the ABS just cant function that fast, and in that case, the human is faster than the machine, we can react and stop the vehicle before ABS realises what we are doing?

Any mechanics out there know for sure on this?..

-x-clacks-GNU Terry Pratchett----Release the Clan Mac Feegle, awae hame ye scunners.[p]Most of my Job is avoiding Stupid. ...You cant Fix Stupid. Especially when you turn Left at Hammerhead.

the light works
Demi-God

Posts: 38,322

ABS doesnt work at slow speed. Aug 1, 2015 11:24:26 GMT

Quote

Post by the light works on Aug 1, 2015 11:24:26 GMT

Aug 1, 2015 7:05:55 GMT silverdragon said:

Its sort of common belief that the ABS on cars does not work under something like 5 to 10 mph.
That Speed is Dependant on who you talk to and how many pints they had at the bar?...

This is a common myth from the common person who isnt a mechanic, so, even if its a "simples" answer for those mechanically minded, I suggest its worthy of testing for those of us who perhaps are not.

I tend to believe this myth anyway, but maybe for different reasons than obvious?... experience, in slow speed sudden stops, I dont "feel" that sort of same feeling as the ABS at higher speeds.
TLW has also suggested a side myth, if you manage to lock up all four wheels, the car thinks it has stopped, and will cancel the ABS system.
I have heard this before....
But how would you lock up all wheels when you are moving (above the speed of ABS being active) anyway?...

Testing, ask the car manufacturers...
Is this dependant on the car and ABS system makers?...
Is it also dependant on what type of ABS you have?...
How does it sense speed anyway?...

OR, and this is a hypothesis I have formulated all on my own, without any other help, is it that it doesnt have time to make any effect anyway?...
Being the vehicle is travelling that slow anyway, you dont get enough rotation of the wheel for the ABS to pulse the brakes for you.
Thinking on at 5mph I could probably haul my car or a truck to a dead stop in one rotation of the car wheels or less, of course thats easier with a truck wheel being larger....
I am suggesting that maybe the ABS just cant function that fast, and in that case, the human is faster than the machine, we can react and stop the vehicle before ABS realises what we are doing?

Any mechanics out there know for sure on this?..

to give you clear objective statistics on this: when I am coming into my station in a bit of a hurry - gravel driveway - the ABS is cycling until the vehicle comes to a complete stop. in my old car - I managed the full lockup at around 20 MPH. 4 wheel disc brakes, and maximum braking pressure with the expectation of a stomp-and-steer stop.

the mechanical bits are: the ABS sensors have to be forgiving enough not to panic if you put your foot on the stop pedal while you are driving in a short circle. at low speed, this may result in not being able to detect a skid.

silverdragon
Demi-God

Busters Stunt Double, .. because "Life" is something that happens when your planning something else.

Posts: 19,452

ABS doesnt work at slow speed. Aug 2, 2015 5:03:58 GMT

Quote

Post by silverdragon on Aug 2, 2015 5:03:58 GMT

to give you clear objective statistics on this: when I am coming into my station in a bit of a hurry - gravel driveway - the ABS is cycling until the vehicle comes to a complete stop. in my old car - I managed the full lockup at around 20 MPH. 4 wheel disc brakes, and maximum braking pressure with the expectation of a stomp-and-steer stop.

the mechanical bits are: the ABS sensors have to be forgiving enough not to panic if you put your foot on the stop pedal while you are driving in a short circle. at low speed, this may result in not being able to detect a skid.

What I expect in more modern fly-by-wire systems is an accelerometer somewhere that kind of senses you is still moving forwards?... or even backwards?... and yes, I have reason to believe ABS works backwards.
I have reason to believe, but, hell, I may be wrong on that as well... is it worth asking that question?

-x-clacks-GNU Terry Pratchett----Release the Clan Mac Feegle, awae hame ye scunners.[p]Most of my Job is avoiding Stupid. ...You cant Fix Stupid. Especially when you turn Left at Hammerhead.

the light works
Demi-God

Posts: 38,322

ABS doesnt work at slow speed. Aug 2, 2015 15:33:25 GMT

Quote

Post by the light works on Aug 2, 2015 15:33:25 GMT

Aug 2, 2015 5:03:58 GMT silverdragon said:

to give you clear objective statistics on this: when I am coming into my station in a bit of a hurry - gravel driveway - the ABS is cycling until the vehicle comes to a complete stop. in my old car - I managed the full lockup at around 20 MPH. 4 wheel disc brakes, and maximum braking pressure with the expectation of a stomp-and-steer stop.

the mechanical bits are: the ABS sensors have to be forgiving enough not to panic if you put your foot on the stop pedal while you are driving in a short circle. at low speed, this may result in not being able to detect a skid.

What I expect in more modern fly-by-wire systems is an accelerometer somewhere that kind of senses you is still moving forwards?... or even backwards?... and yes, I have reason to believe ABS works backwards.
I have reason to believe, but, hell, I may be wrong on that as well... is it worth asking that question?

if the system includes electronic stability control, it may add accelerometer input, but I have no experience with it other than the ESP platform on our two newest engines is a right pansy. I can think of only a couple of our drivers who have never had it engage when there was no need for it to.

c64
Evil Senior Member

Posts: 3,440

ABS doesnt work at slow speed. Aug 16, 2015 13:37:07 GMT

Quote

Post by c64 on Aug 16, 2015 13:37:07 GMT

Aug 1, 2015 7:05:55 GMT silverdragon said:

Its sort of common belief that the ABS on cars does not work under something like 5 to 10 mph.
That Speed is Dependant on who you talk to and how many pints they had at the bar?...

This is a common myth from the common person who isnt a mechanic, so, even if its a "simples" answer for those mechanically minded, I suggest its worthy of testing for those of us who perhaps are not.

I tend to believe this myth anyway, but maybe for different reasons than obvious?... experience, in slow speed sudden stops, I dont "feel" that sort of same feeling as the ABS at higher speeds.
TLW has also suggested a side myth, if you manage to lock up all four wheels, the car thinks it has stopped, and will cancel the ABS system.
I have heard this before....
But how would you lock up all wheels when you are moving (above the speed of ABS being active) anyway?...

Testing, ask the car manufacturers...
Is this dependant on the car and ABS system makers?...
Is it also dependant on what type of ABS you have?...
How does it sense speed anyway?...

OR, and this is a hypothesis I have formulated all on my own, without any other help, is it that it doesnt have time to make any effect anyway?...
Being the vehicle is travelling that slow anyway, you dont get enough rotation of the wheel for the ABS to pulse the brakes for you.
Thinking on at 5mph I could probably haul my car or a truck to a dead stop in one rotation of the car wheels or less, of course thats easier with a truck wheel being larger....
I am suggesting that maybe the ABS just cant function that fast, and in that case, the human is faster than the machine, we can react and stop the vehicle before ABS realises what we are doing?

Any mechanics out there know for sure on this?..

This is correct. Classic ABS can't really know when the car is stationary. Also at very low speeds, a locked wheel can brake better than a rolling wheel on loose dirt, snow and wet leaves. Also at this kind of speed, steering isn't important any more since the car is about to stop soon anyway.

ESP whic is much more advanced can calculate the moment where locked wheels are better in the particular situation so in most cases (good road), they wait longer to lock the wheels.

The speed where ABS locks the wheels depends on the car and if you are hitting the brakes at the low speed or you have braked down from a greater speed. Also it depends on the circumstances.
If your car is half on the road or one or more wheels on a road marking, ABS will lock the wheels with the most grip first while keeping the low grip wheels turning longer. Unless (ESP) the car might start to spin caused by asymmetric grip.

c64
Evil Senior Member

Posts: 3,440

ABS doesnt work at slow speed. Aug 26, 2015 11:43:04 GMT

Quote

Post by c64 on Aug 26, 2015 11:43:04 GMT

I have fixed a broken ABS system the other day.
The major symptom was that the ABS kept working at low speeds. This was very uncomfortable and the braking distance to stop was increased dramatically. This was highly dangerous since when you roll to an intersection and the guy in front of you suddenly stops, you could crash since your braking performance is decreased.
Also in order to work, the ABS must sense a sudden deceleration of the wheel in order to know that it is going to lock. At low speeds, this can't work so the ABS is hammering the brake of the wheel very intense and releasing the pressure for long periods. This is not only highly uncomfortable, it also can make you spin out of control on snow since you have single wheels locked at a time. Even the steering wheel was jerking to one side - not good!

There was a sensor grinding on the ABS ring. Changing the sensor and ring had fixed the problem. There was no error logged so the mechanics were unable to find the cause. I found the problem pretty soon by just looking closely at all relevant ABS parts.

the light works
Demi-God

Posts: 38,322

ABS doesnt work at slow speed. Aug 27, 2015 1:02:58 GMT

Quote

Post by the light works on Aug 27, 2015 1:02:58 GMT

Aug 26, 2015 11:43:04 GMT c64 said:

I have fixed a broken ABS system the other day.
The major symptom was that the ABS kept working at low speeds. This was very uncomfortable and the braking distance to stop was increased dramatically. This was highly dangerous since when you roll to an intersection and the guy in front of you suddenly stops, you could crash since your braking performance is decreased.
Also in order to work, the ABS must sense a sudden deceleration of the wheel in order to know that it is going to lock. At low speeds, this can't work so the ABS is hammering the brake of the wheel very intense and releasing the pressure for long periods. This is not only highly uncomfortable, it also can make you spin out of control on snow since you have single wheels locked at a time. Even the steering wheel was jerking to one side - not good!

There was a sensor grinding on the ABS ring. Changing the sensor and ring had fixed the problem. There was no error logged so the mechanics were unable to find the cause. I found the problem pretty soon by just looking closely at all relevant ABS parts.

here, having the ABS trigger at low speed is usually an indication of a bad wheel bearing - as the wheel starts vibrating at low speed, and makes the ABS controller think it is doing the wrong thing. - and the bad wheel bearing often DOES make the brakes work wrong on the affected wheel.