Stopping distance in snow when ABS is engaged.

[haktae]

First of all, let me emphasize that this is not about the beaten-to-death discussion about whether ABS helps or not. Nor what is the best way to stop in snow.

To make it specific, here is the situation.
- A modern midsize car equipped with ABS with relatively new all season tires.
- Road condition is packed snow. Temperature is around 20.

Let say you are going at around 40 mph and want to come to a complete stop.
You start to depress the brake pedal and at some point, ABS engages. You are feeling this unpleasant vibration. Your brake pedal is not fully depressed yet.

Now, my question is:
ONCE ABS IS ENGAGED, does FURTHER depressing the brake pedal has any effect on the stopping distance?

[Brendan@bwalkauto]

ONCE ABS IS ENGAGED, does FURTHER depressing the brake pedal has any effect on the stopping distance?

No. The ABS is going to give best available braking per the current traction. Pushing it harder doesn't change how much grip you have.

[Zz_Radish_zZ]

Now, my question is:
ONCE ABS IS ENGAGED, does FURTHER depressing the brake pedal has any effect on the stopping distance?

No. The ABS modulator will actuate each brake caliper on its own via the individual channels (essentially pistons) in the modulator. Additional brake input (additional hydraulic pressure from the brake master cylinder) will do nothing.

[adrew]

No. The ABS is going to give best available braking per the current traction. Pushing it harder doesn't change how much grip you have.

Is that also true on cars with electronic brake force distribution? Modern cars don't activate ABS on every wheel all at once.

Say ABS activates on one of the front wheels when, say, 2/3 of braking power has been summoned. Then if you stomp the pedal to the floor in a desire for additional deceleration, you will get ABS modulation (and essentially threshold braking) on all four tires, not just the one that was trying to lock up at first. At that point the EBD will search around for traction and brake more effectively, no?

The reason I ask is because I distinctly remember braking hard when someone pulled out in front of me when I was on some worn, wet, slick asphalt a few years back. I nailed the brakes — but not all the way — and felt the ABS clicking away. I quickly became aware that I might not stop in time, so I stomped the pedal and felt the EBD shifting braking power from side-to-side and front-to-back. Very interesting sensation.

[Brendan@bwalkauto]

Is that also true on cars with electronic brake force distribution? Modern cars don't activate ABS on every wheel all at once.

Say ABS activates on one of the front wheels when, say, 2/3 of braking power has been summoned. Then if you stomp the pedal to the floor in a desire for additional deceleration, you will get ABS modulation (and essentially threshold braking) on all four tires, not just the one that was trying to lock up at first. At that point the EBD will search around for traction and brake more effectively, no?

The reason I ask is because I distinctly remember braking hard when someone pulled out in front of me when I was on some worn, wet, slick asphalt a few years back. I nailed the brakes — but not all the way — and felt the ABS clicking away. I quickly became aware that I might not stop in time, so I stomped the pedal and felt the EBD shifting braking power from side-to-side and front-to-back. Very interesting sensation.

You know, that's an excellent point, and I don't know for sure.

In super simple terms, once you've triggered it on a tire, pushing the pedal harder can't get more braking out of that tire, but I honestly don't know for sure how modern 4 channel systems would divvy up braking to the other wheels.

Edit: I'm going to dig around a little more to see what I can uncover. I'd assume triggering ABS should be interpreted to the ABS Controller as "I need to get with the stopping program ASAP" and trigger a full panic stop. Yet the more I think of it, I think we might find different things for different ABS setups (two channel, four channel, first gen, current systems that react to a quick pedal change from throttle to hard brake application, etc).

Interesting topic.

[mike02467]

Another question: If I'm braking in snow in a 2003 Hyundai Santa Fe with ABS and 4wd and auto tranny... why is it that all 4 wheels can lock up without abs enagaging? Same issue with the old 2001 protege5 we had a few times I'd hit the brakes and they'd just lock up and ABS wouldn't even do anything. Is this a mechanical problem or is it just a ****ty ABS system??


Also sometimes if I'm on the brakes in my 2011 civicSI at all and go over a pothole the abs will chatter but the wheels clearly weren't locking?

[Krazee]

Also sometimes if I'm on the brakes in my 2011 civicSI at all and go over a pothole the abs will chatter but the wheels clearly weren't locking?

Probably the traction control system kicking momentarily when one wheel spun faster than the other. If I got aggressive over bumpy roads, my G8 would do the same.

[adrew]

Another question: If I'm braking in snow in a 2003 Hyundai Santa Fe with ABS and 4wd and auto tranny... why is it that all 4 wheels can lock up without abs enagaging? Same issue with the old 2001 protege5 we had a few times I'd hit the brakes and they'd just lock up and ABS wouldn't even do anything. Is this a mechanical problem or is it just a ****ty ABS system??

Also sometimes if I'm on the brakes in my 2011 civicSI at all and go over a pothole the abs will chatter but the wheels clearly weren't locking?

I know some cars can detect if you are on gravel/dirt/etc (and possibly snow) and will allow the wheels to lock up since a wedge of material building up in front of the tire will usually slow the vehicle better than pulsed ABS in that situation. Or it could be possible that the cars you mentioned probably had older, less sophisticated systems that weren't tuned that well for ice and snow. Does the ABS work on dry pavement? I would expect at least a little grinding from the ABS pump before it gave up.

Re: your second point, I have had the exact same thing happen on my old Civic Si and now my Yaris. I assume the tire rebounds from the bump and locks up for a split second while there's less weight on it, thus setting off the ABS temporarily. I have a small but sharp bump at the bottom of a hill, just before a stop sign, on the road that comes out of my neighborhood that will trigger the ABS with almost 100% regularity.

[EKaru]

My father used to own an Audi V8 Quattro, which I almost totaled back in 1990 (causing $18K worth of damage when I rolled it) but that's another story...

Anyways, it had an ABS disable switch in the center console and the salesman mentioned that ABS was to be disabled when braking in deep snow. I'm guessing at that time ABS systems weren't as advanced as they are today.

[haktae]

The reason I ask is because I distinctly remember braking hard when someone pulled out in front of me when I was on some worn, wet, slick asphalt a few years back. I nailed the brakes — but not all the way — and felt the ABS clicking away. I quickly became aware that I might not stop in time, so I stomped the pedal and felt the EBD shifting braking power from side-to-side and front-to-back. Very interesting sensation.

Thanks adrew for sharing your experience and insight. I am asking this question because I had a similar experience. On a snow covered road, someone pulled out in front of me. I stepped on the brake but not all the way and felt ABS vibration. Seemed like I might not have enough distance, so I stepped harder. I do not remember any braking power shifting but I felt like I am stopping better.

[DeckManDubs]

The modern ABS system still sucks in my opinion when it comes to snow. Many times I have driven cars with studded snows and gone shooting past a driveway because the ABS kept feeling lock up (This is what you want to bite into and thru the packed snow). Now take my A4 that I have disabled the ABS...the stopping distance is almost 1/2 that of a car with the ABS.

ABS was designed to allow inexperienced drivers to still have steering input when threshold braking (locked up wheels and trying to turn does not work, laws of physics apply here) This is where pumping the brake pedal to allow the wheels to spin to allow for turning. Yet able to lock the tires up to bite as hard as possible.

[1Qckcab]

Lets see if I can help with this.

ABS is designed to prevent wheel lock-up during deceleration when braking so you can maintain control and steering of the vehicle (Common knowledge, no?) Thus begins the age old debate on whether ABS helps or hinders control of the vehicle, some prefer no ABS, some cannot control a car without it, and sometimes, there are situations where ABS will work faster and more efficiently in a panic situation than any of us could.

That being said, at the point the ABS is activated, more brake pressure would be irrelevant, because you're already at the braking threshold and the wheels are already beginning to lock up. The vehicle uses wheel speed input from wheel speed sensors to determine a sudden drop in speed on a wheel. When that occurs, the ABS module uses a series of electronically controlled valves inside the ABS module to decrease brake pressure from the wheel that is starting to skid and transfer that pressure to the remaining wheels. It does this in milliseconds and monitors every wheel for lock-up. Therefore, more brake pedal pressure does not change the fact that the wheels are already locking up and will not slow the vehicle down any quicker. (Que debate on why ABS is better or worse for you, yada yada ya....)

Edit: Also a great point right above me. I didnt see it as I was typing.

Also, to touch on the Santa Fe/Civic subject. The ABS may not be functioning properly on the Santa Fe (That ABS system is also very primative) and the lock-up you get when hitting a pot hole is the cars ABS module seeing the sudden change in wheel speed at that wheel. You are basically tricking the car at that point and for a moment, it thinks it is stopping too quick.

[DeckManDubs]

My father used to own an Audi V8 Quattro, which I almost totaled back in 1990 (causing $18K worth of damage when I rolled it) but that's another story...

Anyways, it had an ABS disable switch in the center console and the salesman mentioned that ABS was to be disabled when braking in deep snow. I'm guessing at that time ABS systems weren't as advanced as they are today.

Rolled an Audi, dang!

[Zz_Radish_zZ]

Is that also true on cars with electronic brake force distribution?

Yup. All electronic brake force distribution (or whatever fancy term each individual OEM is calling it) really does is manage brake caliper actuations (I'll call this ABS event from now on) based on how fast you're going and what angle you have the steering wheel oriented.

Items you will find in a vehicle with EBD:
- ABS modulator
- Wheel speed sensor at each wheel
- Steering angle sensor (usually mounted in the steering column)

Standard or old ABS is fine for straight line braking events: The brake calipers will actuate/unactuate independently.

Things get a little dicey when you're braking in a corner: Let's say you're negotiating a blind right corner when all of a sudden a wild Juniper Monkeys appears. You instinctively slam on the brakes (whereas you wouldn't if a wild VladGTI appeared before you) triggering an ABS event. If you are driving a car with old ABS you would being to understeer since each brake caliper is acting independently of each other and has no idea that you are cornering. If you are driving a car with EBD the ABS modulator realizes that you are cornering (feedback sent from steering angle indicator) and modulates the passenger side calipers more than the driver side preventing understeer (driver side v. passenger side rate is determined by feedback from the wheel speed sensors).

It's hard to explain this without diagrams and a whiteboard. Hopefully that covers it.

Oh and I'm a chassis engineer for an OEM, AMA.

[VDub2625]

Braking harder means more pressure is available for the ABS system to work with, but once a wheel's limit is reached, ABS does the rest. If the skidding threshold isn't reached, the normal braking force applies.

Basically, the way I understand it, slamming the pedal is the best way to work with ABS, if you're not attempting to modulate at all, since all the wheels will reach their max. threshold and ABS will do it's best to keep it there.

I used to lock up all 4 wheels in snow for fun in my Taurus. it's because if no wheels are turning, the car doesn't know that it isn't stopped (more modern systems may have g sensors to differentiate this).

[1Qckcab]

Braking harder means more pressure is available for the ABS system to work with, but once a wheel's limit is reached, ABS does the rest. If the skidding threshold isn't reached, the normal braking force applies.

Basically, the way I understand it, slamming the pedal is the best way to work with ABS, if you're not attempting to modulate at all, since all the wheels will reach their max. threshold and ABS will do it's best to keep it there.

I used to lock up all 4 wheels in snow for fun in my Taurus. it's because if no wheels are turning, the car doesn't know that it isn't stopped (more modern systems may have g sensors to differentiate this).

Exactly.

[what]

ABS may (will) increase stopping distances but, hopefully, allow the driver to maintain control by preventing wheel lockup.

[XiaoNio]

ABS may (will) increase stopping distances but, hopefully, allow the driver to maintain control by preventing wheel lockup.

This was a bone of contention with motorcyclists for a while. Honda pretty much dispelled it on their new CBR. On wet surfaces, ABS stopped bikes sooner almost every time. If I remember correctly, pro riders were only able to threshold brake and beat ABS ~15% of the time. If professional racers in controlled settings can only occasionally beat ABS, I'm thinking in the real world there's no way I could beat it.

Lesson being, you're almost always going to be able to stop faster with full on ABS braking.

Addressing the issue of the Santa Fe from earlier. If your tires aren't up to it ABS can't really help. In that situation, I'd bet the tires weren't able to deliver enough traction for ABS to really do any good.

[Live-Wire]

Another question: If I'm braking in snow in a 2003 Hyundai Santa Fe with ABS and 4wd and auto tranny... why is it that all 4 wheels can lock up without abs enagaging? Same issue with the old 2001 protege5 we had a few times I'd hit the brakes and they'd just lock up and ABS wouldn't even do anything. Is this a mechanical problem or is it just a ****ty ABS system??


Also sometimes if I'm on the brakes in my 2011 civicSI at all and go over a pothole the abs will chatter but the wheels clearly weren't locking?

IF all four wheels lock up at the same time, the car thinks it has stopped (due to a 0 reading on all speed sensors) and doesn't need to run the ABS. Used to happen from time to time with my Mk4 VW on ice... However the ABS system on my 6 year older Volvo (1993 vs 1999) never got confused in such a manner. My 2001 Audi wouldn't get confused either, unless it was in reverse - possibly ABS wasn't enabled in reverse tho.

So the answer is a cheap and/or poorly calibrated ABS system used on a less expensive car. I believe the Volvo was equipped with another sensor not dependent on wheel speed to detect forward motion, while the VW's cheaper system only relied on the sensor rings. $20k versus $40k car, there has to be some advantages right? My Mk6 Golf isn't able to lock them all up while moving either - I tried... ABS right till the end.

My Mk4 VW would also pulse the ABS while going over potholes... again never had this issue on the Volvo or Audi, or my Mk6 Golf. It went away on the Mk4 when I put on better shocks and tires tho, so maybe you are losing grip momentarily.

[VDub2625]

I had my Cabrio's ABS pulsate over a bump in a turn a few times... made me think something was wrong, glad to hear it's normal.

[TOMPASS]

Probably the traction control system kicking momentarily when one wheel spun faster than the other. If I got aggressive over bumpy roads, my G8 would do the same.

I agree with this. On my Passat, whenever this happens, the traction control light flashes.

[dakotacoates]

In my 1993 UrS4 there was an ABS on/off pushbutton as well as a differential lock button. When the diffs were locked ABS was automoatically disengaged. In the owners manual it specifically stated that when driving in snow having ABS turned off stopping distances were significantly decreased, as the tires would build up wedges of snow in front of them.

Living in Canada i do my fair share of snow driving, and i absolutely HATE ABS. I noticed huge differences in stopping ability with the ABS deactivated. ABS is only there so when someone goes into panic mode and mashes the brakes, they'll still be able to steer rather than sliding. Last winter i had a 98 legacy wagon with 4 michelin hydro-edges all winter long, and i pulled the ABS fuse because when i needed to stop in a hurry, locking up the wheels was far more effective than ABS modulation. I never had a problem.
On a ski trip In my friend's 2005 XC-70 he drank too much at the hill and i had to drive back to the hotel in a snow storm, i almost crapped myself when ABS kicked in and the car wasn't slowing down (Michelin X-ice on all corners).

The way i see it- ABS is there for dry/wet pavement emergency avoidance. not for stopping in the snow.

[Aw614]

Good discussion and very informative

Question, how about those trucks back in the 90s with only rear ABS? What was the point of that instead of having four wheel ABS? Having it in the rear only doesn't seem really safe if the fronts would lock up especially in snow or wet conditions?

[VDub2625]

Good discussion and very informative

Question, how about those trucks back in the 90s with only rear ABS? What was the point of that instead of having four wheel ABS? Having it in the rear only doesn't seem really safe if the fronts would lock up especially in snow or wet conditions?

It allows bigger brakes in the rear, since they have a lot of weight in the back, without the normal rear lockup you'd have from weight transfer in a panic stop.

[mike02467]

Good discussion and very informative

Question, how about those trucks back in the 90s with only rear ABS? What was the point of that instead of having four wheel ABS? Having it in the rear only doesn't seem really safe if the fronts would lock up especially in snow or wet conditions?

cheaper, besides, the rear wheels are always more likely to lock up than the front, even more so in a pickup truck with no weight over the rear wheels.

[VadGTI]

Let's say you're negotiating a blind right corner when all of a sudden a wild Juniper Monkeys appears. You instinctively slam on the brakes (whereas you wouldn't if a wild VladGTI appeared before you)

This explains why I keep getting hit. It's instinct.

[what]

This explains why I keep getting hit. It's instinct.

He wasn't talking about you, you narcissist. He said VladGTI.



Also, way to bring thread back from the dead.

[VadGTI]

He wasn't talking about you, you narcissist. He said VladGTI.



Also, way to bring thread back from the dead.

Note mike02467's sig. I had to track it down.

[what]

Note mike02467's sig. I had to track it down.

My post still stands, Vlad (sans "s").

[alex_bgnet]

I have only felt real use for ABS once when driving in AZ 80mph, 3am in a minivan. Had driven for 6 hours. Road straight and clear. Deer jumps out and I nail the brakes. I really felt how the van started going to one side then ABS corrected. I felt how it played with the wheels and kept it going straight while the passengers were tossed forward.

[JLJetta]

If the ABS system were 'perfect', it would not have to modulate or throb power. It would maintain the maximum amount of braking at all times.

Unfortunately, from a system controls standpoint, that is very hard to achieve for all the various road/tire/car conditions.

[s-rocc]

I did this experiment with my audi 80 quattro with ABS in the snow one time. the car had one of the "ABS OFF" switches on the dash. car had normal all season tires on it. with the ABS on, the car took much, much longer to come to a complete stop, with it off, the tires dug in and it stopped much quicker.

the trade off was that with the ABS on, i was still able to steer.

granted, the car was a 1991, and it was the first generation of ABS technology, i would imagine newer systems are much better.

[haktae]

Well, since this thread was magically revived, I will re-iterate the question.

The question is NOT about ABS vs NO-ABS. I want to know, once ABS is engaged, if further depressing the brake pedal will have any impact on the stopping distance.

So far, one viable theory came up which was related to EBD. Basically, depending on the pressure on the brake pedal, it is possible that ABS is engaged only for some wheels, not all at once.

Is there any chance that the ABS software is set-up such that the braking ability is not at its maximum when the ABS first engages? I know this will not help with the stopping distance, but still give the drive the feel of progressive braking.

[71DubBugBug]

My father used to own an Audi V8 Quattro, which I almost totaled back in 1990 (causing $18K worth of damage when I rolled it) but that's another story...

Anyways, it had an ABS disable switch in the center console and the salesman mentioned that ABS was to be disabled when braking in deep snow. I'm guessing at that time ABS systems weren't as advanced as they are today.

I think that we all need to hear the story of how u managed to flip a 4k pound awd audi

[lowredcabrio]

In my personal experience (newest car I've driven regularly in snow is from 2003, MK4 Golf) once you hit ABS, the ABS pump will modulate the brakes for you using the pressure in the system. If you're already locking up the wheels, no additional pressure is required because the system has enough to work with (cause lock-up). If you started locking up on snow under light brake pressure then came onto pavement, ABS would kick out and you would be braking as you normally would for that pedal pressure. If you stay with the same coefficient of friction (same road surface) the same brake pressure will be used to keep you just alternating between locking up and not.

To answer your question OP: If you're pressing the pedal hard enough for ABS to engage, pushing it any harder will not stop you any faster. That said, if ABS stops while you perfectly maintain brake pressure, you must now have more traction and therefore could give more force to the brakes. This is something ABS could take care of if you had simply floored the brake pedal at the first response. In an emergency, if you have ABS, put your brake to the floor and let the car do the work. If you try to threshold brake in a car with ABS, you will confuse the computer and lose braking power.

If you're a racecar driver, pull your ABS fuse. If not just mash the pedal. Threshold braking requires being at the traction limit which inevitably involves crossing it. At that point the ABS would kick in and totally mess with your braking.