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Discussion Starter #1
I was talking with a guy at work who said that cars with anti-lock brakes actually have a longer stopping distance in snow than cars without. He claimed that when the wheels lock up they actually cause the car to start pushing the snow, which slows the car down. This doesn't sound right to me. Also, I would think that once the wheels lock up, you could no longer steer.
 

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Correct to both of you.

I believe the car with ABS does take longer time to stop in the snow. But when you stop hard, your car doesn't slide around. It only stop in the straight line direction, unless you turn your steering wheel. :cool:
 

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The whole point of ABS is for the car/truck to stop straight. You do not necessary stop faster with ABS, but with ABS the pulsating of the brakes helps the car stay in control. A car without ABS will just slide until the snow buildup in front of the car stops it or the momentum fades. With ABS, the brakes will be applied and reapplied continuously to allow you to still have control of the car. Try stoping a car without ABS and turning and you really won't be doing any turning and a car with ABS will allow you to turn albeit a little slower.
 

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agree with JTM and kishino...

It's true that ABS brakes can increase stopping distances in snow. This can be very noticeable at speeds under 5 mph when you are driving toward a wall or some other barrier. So much so, that I would suggest vehicle manufacturers only make this feature (ABS brakes) active above 5 mph.
 

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Case in point....

I used to own a BMW 735 iL (1989 model) that was equipped with ABS brakes, whereas my Toyota 4-runner (1988 model) is not. I owned both vehicles at the same, hence comparing braking performance was easy. At high to moderate speeds, especially on dry surfaces, the BMW's braking performance was vastly superior to that of the 4-runner. The BMW would brake straight as an arrow at all times. However, when I drove into my garage when the floor was slippery (snow on the tires or wet cement floor) the BMW would take about twice the distance to stop as the 4-runner. I had a scare several times with the BMW because I thought I was going to drive through the back wall of the garage into my living room! The ABS system would continually release the brakes to prevent any skid even though I was only going 2 - 4 mph! If the ABS system would have automatically deactivated at very low speeds, braking would actually have been enhanced. :4:
 

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oceanMDX,

interesting point. I never experience any ABS activities while I am driving in 2~5 mph on the drive way. I did experience ABS kicks in while I am backing out of driveway at approximately 10 mph (due to the downhill) while the snow is cover the driveway.
 

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Non ABS brakes can stop faster in snow, unfortunately you usually end up with a tree or another car stuck to your front bumper. You can anticipate longer braking distances, loss of control is a whole different animal...

I do wish there was a way to defeat the ABS when going down a steep grade on sand or gravel, there you need to plow a furrow to stop.
 

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kishino said:
The whole point of ABS is for the car/truck to stop straight. .
I thought the whole point to ABS is to be able to have steering control while doing a emergency breaking. So you can swerve.
 

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Warzau said:


I thought the whole point to ABS is to be able to have steering control while doing a emergency breaking. So you can swerve.
What I meant was that you can stop straight with the ABS as you can control the steering. Without ABS you can stop but have no control over the car,no steering.
 

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The newer ABS valves...

...are ORDERS OF MAGNITUDE faster at cycling than even those from a year or two ago.

The result is that the "pulsing" takes MUCH less pressure off the tires, and stopping distances remain impressively short in virtually all conditions that there is some traction (on glare ice and similar conditions your only chance is with studded tires)

BTW In regards to the "snow wedge" effect it is important to remember that this only applies in a circumstances where the characteristics of the snow will actually pack against itself, NOT when it is already compressed into a slickened surface. Same thing for gravel, in fact, if you've even tried to slow a mountain bike on gravel you may have seen the irregular 'grabbiness' of the tread against the stones -- and the unfortunate effect is that pressure to slow you & bike on smooth ground will lock-up your wheel and FACE PLANT you into the gravel...
 

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In theory, the whole point of ABS is to retain control while braking, regardless of car velocity (velocity being speed AND direction, or changes in direction), as you guys pointed out.

Just this past weekend, I was travelling maybe 35mph in my girlfriend's BMW 528i. It has all-season tires (that err, need to be CHANGED cuz they are worn out rather heavily) and I approached a traffic light. About 25 ft before the traffic light (or pedestrian X-ing) there was ICE. I braked, and the car slid forward with barely a slow down, the ABS engaged, but it REALLY started to stop when I pressed VERY HARD down on the brake pedal and the ABS did its thing. Then I said to her "ok, w/o these Anti-lock brakes we woulda had a very bad situation on our hands with that car that just went by us perpindicularly". :eek:

So for me, ABS rules, no matter the surface.
 

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Stomp and pray vs pulse braking

RTK said:
I was talking with a guy at work who said that cars with anti-lock brakes actually have a longer stopping distance in snow than cars without. He claimed that when the wheels lock up they actually cause the car to start pushing the snow, which slows the car down. This doesn't sound right to me. Also, I would think that once the wheels lock up, you could no longer steer.
Both of you are correct in what I call the "Stomp and Pray" method of braking. Some varieties of snow will pack under the locked wheels and decrease stopping distance, as will a large enough quantity of gravel.

If all you do when braking is stomp and pray for it to be enough, ABS will always win. Hopefully you can do better than that!!!!

If you can, he's still right but for a different reason - even the average joe can outbrake ABS on low-traction surfaces (ice, some varieties of snow) with a little practice on one of several "intelligent" braking schemes. I was taught "pulse braking" where one applies the brakes moderately hard until a wheel locks up, then releases them, then re-applies, etc... much the way ABS does, but much slower and with a much more pronounced "square wave" look.


When I got my first ABS-equipped car some 10yrs ago I did reasonably controlled testing and found that on ice I could easily out-stop the ABS by pulse braking. In fact, I believe ABS actually helps the average joe pulse-brake because there's immediate audible and (usually) tactile feedback when the wheels lock (the ABS starts banging away) - that's your key to back off and take another bite. After quite some time, I finally convinced myself that this works because of physics ;)

ABS tends to keep the car's attitude level, while pulse braking upsets the car into a nose-down attitude which actually increases the force on the tires, increasing their braking effectiveness.


The key is of course to get a feel for your vehicle/tire combo and how it behaves. I generally drive comfortably within my traction limits on ice and rarely get so far into it that I have to pulse brake. (Playing around is entirely another story!)

Richard

P.S. If you try this crap with summer tires on ice at right around 32F, no combo of braking technique or technology will keep the shiny side up! Decent all seasons or snow tires will make a world of difference, and then allow you to get into subtleties as above.
 

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Warzau said:


I thought the whole point to ABS is to be able to have steering control while doing a emergency breaking. So you can swerve.
I haven't had to use my ABS for any snow conditions yet..(I live in Georgia now) I will say I had ABS on my Integra and it indeed allows you to steer away from trouble. I agree totally that the distance is longer but you can steer yourself out of an accident instead of "spinning circles" down the road.(uh..done that too) All in all, I am pleased that ABS is standard on all vehicles now. I had to pay extra for ABS with the Integra but it saved me more than a few times.
 

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Discussion Starter #14
I actually thought ABS would be able to stop more quickly on dry or wet pavement because the frictional force between the tires and the road actually decrease once the tires lock up. From what I remember from a physics class I took, the frictional force between 2 materials is different depending on whether or not the materials are sliding across each other (in the case of a lock up). The dynamic coefficient of friction is always less that the static coefficient. ABS attempts to maximize the friction between the road and the tires by preventing the sliding.
 

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Hey RTK...

I took that physics class too, but I don't remember you. ;)

You're right on with the physics, but the ABS system - at least what I had on my 89 BMW - would not operate at maximum static friction all of the time. That is to say, once the system detected impending lockup, it would release more than enough braking force and not reapply it quickly enough for optimal performance. According to what renov8r has said, this problem apparently has been ameliorated in the last year or two with better ABS systems. Since I'm here in Mexico, I haven't been able to check how the ABS system in my MDX works in the snow. I should also mention that what you said regarding the physics ignored the effect of snow "damning up" in front of the tire when the wheel locks - when that occurs, resistance to forward motion increases, which reduces stopping distances. This would be particuarly true when the snow is warm enough to stick together (packing snow).
 

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ABS and Snow

If we only drove on snow covered roads, there would be no ABS. There is no doubt that ABS is a disadvantage on snow. However, for those living in the snow-belt, how much do we actually drive on snow covered roads? Well plowed roads is what I experienced living in Green Bay for two years. Heck, the highway department guys are out clearing the roads before the flurries begin, so it seemed. On well plowed, but still slippery roads, ABS has my vote. Packed snow, typically encountered within the subdivisions, becomes ice within a few days of a storm and the accumulation effect of conventional brakes is lost anyway. If we all lived in rural areas that commonly received snow measured by the foot, manufacturers would equip vehicles with an ABS defeat feature. But we don't. Therefore, ABS makes sense for the masses. Manufacturers build vehilces for the masses.
 

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RTK said:
I actually thought ABS would be able to stop more quickly on dry or wet pavement because the frictional force between the tires and the road actually decrease once the tires lock up.
Sure, that's true and all but it's only one dimension of the problem. Think of the braking force as dynamic - duration of application * force of application. ABS yields duration approaching 100% but at a much lower force than pulse braking. Pulse braking might be 75% duration but at twice the braking force of ABS.

If downforce were the same, ABS would always win.
ABS doesn't always win.
My theory is that downforce is the missing factor.

Richard
 

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Discussion Starter #18
Solomon,

This may be inaccurate, but I thought that the ABS system applied as much braking as possible to the point where the wheels lock up. It then reduces the braking to get the wheel spinning again. It repeats this cycle to keep the wheels on the verge of locking up.

I'm not sure how pulse braking can apply any more braking pressure (than being at the verge of lock up). The nose dive effect you described might cause more weight on the front wheels but would actually be doing that at the expense of the rear wheels (since all that is happening is that the weight is being redistributed).
 

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RTK said:
I'm not sure how pulse braking can apply any more braking pressure (then being at the verge of lock up). The nose dive effect you described might cause more weight on the front wheels but would actually be doing that at the expense of the rear wheels (since all that is happening is that the weight is being redistributed).
Actually the weight is being focused on two smaller contact patches vs distributed over 4. Thus the braking effectiveness is actually higher with more weight on the front than if it's spread out evenly.

I don't claim that this is proven, but it's the best explanation I've come up with to explain what actually happens. If all the counter arguments were true, a person in the car would not be able to out-brake the ABS system in that car. I have done it personally under reasonably controlled and repeatable conditions, and I've had other people reproduce the same results in their cars.

Note that on dry pavement I personally cannot beat the ABS, so the low friction of ice or slush may be in some other way compromising the system.

Richard
 

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How about the vibration when the anti-lock kick in, it feels like a massage chair, or an earthquake depending on how you look at it.
I personally would rather pump the pedal myself, I feel I had more control that way.
 
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