Does ABS allow a vehicle to skid?
25 November 2005, 12:04 PM
#1
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Does ABS allow a vehicle to skid?
Right, need some help to settle an argument.
I thought that ABS systems prevented cars from skidding (so you can retain steering control) by releasing the brakes on the point of the wheels locking up/skidding, then reapplying the brakes, and so on, obviously very very quickly. Result being, the vehicle does not skid.
Now a friend has just qualified as a driving intructor for a big national group and has been taught and told to teach her pupils that ABS systems DO allow vehicles to skid, just a lot less than if a vehicle did not have ABS fitted and you slammed the anchors on. She agrees about the function of ABS being to allow you to retain steering control while emergency braking, but is adament that ABS systems will still let a vehicle skid to some degree.
Anyone got a definitive technical answer they can give in simple terms?
Thanks
I thought that ABS systems prevented cars from skidding (so you can retain steering control) by releasing the brakes on the point of the wheels locking up/skidding, then reapplying the brakes, and so on, obviously very very quickly. Result being, the vehicle does not skid.
Now a friend has just qualified as a driving intructor for a big national group and has been taught and told to teach her pupils that ABS systems DO allow vehicles to skid, just a lot less than if a vehicle did not have ABS fitted and you slammed the anchors on. She agrees about the function of ABS being to allow you to retain steering control while emergency braking, but is adament that ABS systems will still let a vehicle skid to some degree.
Anyone got a definitive technical answer they can give in simple terms?
Thanks
25 November 2005, 12:15 PM
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Tyres lock up
ABS releases pressure to wheels turn then applys brake force again (depending on where your foot is on the brake..
In between the above there will be a minimal skid (basically lots of little ones, this is why sometimes abs extends braking distance - try braking in snow, abs is often worse).
Think of it like this (| = braking but wheels turning, _ wheels locked)
Driver would brake like this when skidding
||________________||________________||____________ ____
Or like this
||________________________________________________ ____
ABS is like this
||_||_||_||_||_||_||_||_||_||_||_||_||_||_||_||_|| _||_||_||_
Some manufactuers/models are better/worse than others and behave differently. Basically the better ones can minimise the "_"s more. Problem with this method is that if the abs isn't good enough, the car would never stop!!! At the end of the day, the wheels have to stop turning else you'd keep going forever
I am curious to know, however, what happens if you are going sideways (literally) as even if the wheels are turning, they are right angles to the road therefore are effectively skidding.
ABS releases pressure to wheels turn then applys brake force again (depending on where your foot is on the brake..
In between the above there will be a minimal skid (basically lots of little ones, this is why sometimes abs extends braking distance - try braking in snow, abs is often worse).
Think of it like this (| = braking but wheels turning, _ wheels locked)
Driver would brake like this when skidding
||________________||________________||____________ ____
Or like this
||________________________________________________ ____
ABS is like this
||_||_||_||_||_||_||_||_||_||_||_||_||_||_||_||_|| _||_||_||_
Some manufactuers/models are better/worse than others and behave differently. Basically the better ones can minimise the "_"s more. Problem with this method is that if the abs isn't good enough, the car would never stop!!! At the end of the day, the wheels have to stop turning else you'd keep going forever
I am curious to know, however, what happens if you are going sideways (literally) as even if the wheels are turning, they are right angles to the road therefore are effectively skidding.
Last edited by Dracoro; 25 November 2005 at 12:18 PM.
25 November 2005, 12:15 PM
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I'm sure the car will still skid to some extent as the brakes are applied again. it's the time when the brakes are not applied that allows you to have some steering control.
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25 November 2005, 12:17 PM
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It is true that although ABS will allow you to brake in a straight line with less chance of the wheels locking up except possibly on snow or gravel, there is no guarantee that with a side load on the tyres that the wheels will not slide due to the additional forces on the tyres causing loss of sideways adhesion. It is all a function of the speed, sideways forces involved, and the slipperiness of the road.
So you cannot depend totally on being able to retain steering control while braking.
Incidentally, the definition of a skid is when the wheel locks while going in a forward direction, and a slide is when the tyre loses sideways adhesion.
Les
So you cannot depend totally on being able to retain steering control while braking.
Incidentally, the definition of a skid is when the wheel locks while going in a forward direction, and a slide is when the tyre loses sideways adhesion.
Les
25 November 2005, 12:19 PM
#11
Well, ABS does allow the wheel to lock up and and skid. But only for a very short space of time, you are talking fractions of a second.
It needs to do this as a wheel needs to lock or skid before the system can actually detect a skid scienario and activate the abs (wheel speed sensors - a locked or skidding wheel rotates slower than the ones with grip).
It will rapidly remove braking force from the affacted wheel(s) and then re-apply the system braking pressure causing it to lock/skid again (assuming grip is still poor or pedal pressure is still high). It will do this many times per second.
If the wheel didn't lock or skid, then the ABS system would deactivate and you'll be back to normal brakes.
So she is right in the sense that a car does skid. Even if it it only for fractions of a second
Oh, and on some older ABS cars, the system would not trigger at all below 5mph.
It needs to do this as a wheel needs to lock or skid before the system can actually detect a skid scienario and activate the abs (wheel speed sensors - a locked or skidding wheel rotates slower than the ones with grip).
It will rapidly remove braking force from the affacted wheel(s) and then re-apply the system braking pressure causing it to lock/skid again (assuming grip is still poor or pedal pressure is still high). It will do this many times per second.
If the wheel didn't lock or skid, then the ABS system would deactivate and you'll be back to normal brakes.
So she is right in the sense that a car does skid. Even if it it only for fractions of a second
Oh, and on some older ABS cars, the system would not trigger at all below 5mph.
Last edited by ALi-B; 25 November 2005 at 12:30 PM. Reason: forgot my glasses
25 November 2005, 12:22 PM
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I was given to understand that the ABS releases the brake pressure when it detects that the wheel has slowed abnormally for its original speed but just before it actually locks.
Les
Les
25 November 2005, 12:24 PM
#13
Yes it does, but for a wheel to be travelling slower then the others it has to be skidding
There is also a decelaration factor added into ABS which uses a basic mathematical equation that dictates that a wheel cannot decelerate faster than a given amount. This does indeed make it respond faster.
If it decelerates faster than the given amount then it can be said that it has lost grip. And is therefore skidding.
There is also a decelaration factor added into ABS which uses a basic mathematical equation that dictates that a wheel cannot decelerate faster than a given amount. This does indeed make it respond faster.
If it decelerates faster than the given amount then it can be said that it has lost grip. And is therefore skidding.
Last edited by ALi-B; 25 November 2005 at 12:28 PM.
25 November 2005, 12:29 PM
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The ABS module doesn't need to see a locked wheel, a significantly lower wheel speed will trigger the ABS, I find it highly unlikely that a wheel would lock, especially on modern systems.
(not all wheels rotate at the same speeds all the time, hence significantly different).
(not all wheels rotate at the same speeds all the time, hence significantly different).
25 November 2005, 12:33 PM
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Originally Posted by ALi-B
Well, ABS does allow the wheel to lock up and and skid. But only for a very short space of time, you are talking fractions of a second.
It needs to do this as a wheel needs to lock or skiding before the system can actually detect a skid scienario and activate the abs (wheel speed sensors - a locked or skdding wheel rotates slower than the ones with grip).
It will rapidly remove braking force from the affacted whee(s) and then re-apply the system braking pressure causing it to lock again (assuming grip is still poor or pedal pressure is still high). It will do this many times per second.
If the wheel didn't lock or skid, then the ABS system would deactivate and you'll be back to normal brakes.
So she is right in the sense that a car does skid. Eeven if it it only for fractions of a second
Oh, and on some older ABS cars, the system would not trigger at all below 5mph.
It needs to do this as a wheel needs to lock or skiding before the system can actually detect a skid scienario and activate the abs (wheel speed sensors - a locked or skdding wheel rotates slower than the ones with grip).
It will rapidly remove braking force from the affacted whee(s) and then re-apply the system braking pressure causing it to lock again (assuming grip is still poor or pedal pressure is still high). It will do this many times per second.
If the wheel didn't lock or skid, then the ABS system would deactivate and you'll be back to normal brakes.
So she is right in the sense that a car does skid. Eeven if it it only for fractions of a second
Oh, and on some older ABS cars, the system would not trigger at all below 5mph.
i decided to have some fun last time in germany, so i was giving it some large in the snow, and went to do a slide. the abs kicked in, and the olf pedal graunching was heard and felt, the car did slide to halt albeit in a very short space, but it was sliding.
all the abs does as most and yourself have stated is stop the locked wheel from staying locked. it will noit overcome the laws of physics and prevent or stop the car from sliding. its because youve overcome the drag coeficient for the road surface you have started to skid, in brute force terms, all the abs does is give control back to the wheels, which by way of physics / drag stop the slide
M
25 November 2005, 12:33 PM
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It is probably down to fractions of a second Ali-B because the wheel will show an instant deceleration at the moment that it is about to lock. That ties in with the timing factor that you mentioned. It would be interesting if a design engineer was to explain it to us completely. I was going by what we were told about aircraft ABS systems. Luckily you are not expected to do much fast cornering in one of those.
Les
Les
25 November 2005, 12:37 PM
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25 November 2005, 12:38 PM
#20
Originally Posted by Leslie
It is probably down to fractions of a second Ali-B because the wheel will show an instant deceleration at the moment that it is about to lock. That ties in with the timing factor that you mentioned. It would be interesting if a design engineer was to explain it to us completely. I was going by what we were told about aircraft ABS systems. Luckily you are not expected to do much fast cornering in one of those.
Les
Les
) ...I dig it up and post it
25 November 2005, 12:41 PM
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I don't have a definitive answer but I'd suggest looking at the STOPTECH website as they have an article on ABS systems.... Aimed at people who replace parts of their braking systems but the theory I guess is the same.
IMO ABS systems are tuned so the best results are achieved under most driving conditions. The mechanical components can only work from feedback of various sensors so the electronics either detect the skid or try to guess when a skid is likely. At some point there is likely to be a skid or lots of skids separated by releases of the brakes.
J.
IMO ABS systems are tuned so the best results are achieved under most driving conditions. The mechanical components can only work from feedback of various sensors so the electronics either detect the skid or try to guess when a skid is likely. At some point there is likely to be a skid or lots of skids separated by releases of the brakes.
J.
25 November 2005, 12:55 PM
#23
Here we go, not the most informative unfortunately. But The other lot I have is about 150 pages and is 30meg, so I'm not posting that
http://putfile.com/pic.php?pic=11/32806461036.jpg&s=x12
http://putfile.com/pic.php?pic=11/32807002540.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806503378.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806515744.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806531287.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806542228.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806552863.jpg&s=x12
Note on technical English: Substitute skid for slip
I forgot to mention the g-force sensor that many cars have which works in conjunction of the wheel speed sensors in calculation of decelaration.
Edit: fixed page 2 now.
http://putfile.com/pic.php?pic=11/32806461036.jpg&s=x12
http://putfile.com/pic.php?pic=11/32807002540.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806503378.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806515744.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806531287.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806542228.jpg&s=x12
http://putfile.com/pic.php?pic=11/32806552863.jpg&s=x12
Note on technical English: Substitute skid for slip
I forgot to mention the g-force sensor that many cars have which works in conjunction of the wheel speed sensors in calculation of decelaration.
Edit: fixed page 2 now.
Last edited by ALi-B; 25 November 2005 at 01:00 PM.
25 November 2005, 12:56 PM
#24
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Wow, thanks for all the answers/info!
So, we're still not sure/100% if the systems allow the wheels to lock momentarily for a fraction of a second then release/reapply etc....or if the system releases/reapply's the brakes on the point of the wheel about to lock as the system realises that a skid/locked wheel is immanent.
However, the fact that a wheel can still bhe rotating but also skidding adds to the slight confusion
i guess it's the older systems that allowed the wheels to lock/skid for a brief moment while the newer ones are more advanced and able to predict the onset of a lock/skid a lot better through improved electronics/processing of sensor information meaning that the wheels aren't allowed to lock.
So, we're still not sure/100% if the systems allow the wheels to lock momentarily for a fraction of a second then release/reapply etc....or if the system releases/reapply's the brakes on the point of the wheel about to lock as the system realises that a skid/locked wheel is immanent.
However, the fact that a wheel can still bhe rotating but also skidding adds to the slight confusion
i guess it's the older systems that allowed the wheels to lock/skid for a brief moment while the newer ones are more advanced and able to predict the onset of a lock/skid a lot better through improved electronics/processing of sensor information meaning that the wheels aren't allowed to lock.
25 November 2005, 01:00 PM
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Originally Posted by Nat21
Wow, thanks for all the answers/info!
So, we're still not sure/100% if the systems allow the wheels to lock momentarily for a fraction of a second then release/reapply etc....or if the system releases/reapply's the brakes on the point of the wheel about to lock as the system realises that a skid/locked wheel is immanent.
However, the fact that a wheel can still bhe rotating but also skidding adds to the slight confusion
i guess it's the older systems that allowed the wheels to lock/skid for a brief moment while the newer ones are more advanced and able to predict the onset of a lock/skid a lot better through improved electronics/processing of sensor information meaning that the wheels aren't allowed to lock.
So, we're still not sure/100% if the systems allow the wheels to lock momentarily for a fraction of a second then release/reapply etc....or if the system releases/reapply's the brakes on the point of the wheel about to lock as the system realises that a skid/locked wheel is immanent.
However, the fact that a wheel can still bhe rotating but also skidding adds to the slight confusion
i guess it's the older systems that allowed the wheels to lock/skid for a brief moment while the newer ones are more advanced and able to predict the onset of a lock/skid a lot better through improved electronics/processing of sensor information meaning that the wheels aren't allowed to lock.
25 November 2005, 03:20 PM
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Thanks for all that Ali-B, that does tie in pretty well with what I was taught about the aircraft systems, or as far as I can remember anyway. I found it pretty interesting and it just goes to show that it is a pretty complicated bit of kit.
Les
Les
26 November 2005, 01:25 AM
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Originally Posted by Nat21
So, we're still not sure/100% if the systems allow the wheels to lock momentarily for a fraction of a second then release/reapply etc....or if the system releases/reapply's the brakes on the point of the wheel about to lock as the system realises that a skid/locked wheel is imminent.
Originally Posted by Nat21
However, the fact that a wheel can still be rotating but also skidding adds to the slight confusion .
.Most systems don't work below a certain road speed because you would never come to a complete standstill, and you don't really need ABS at walking pace. Manual for MY02 WRX states "The ABS system will not operate when the vehicle speed is below approximately 10km/h (6mph)."
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