Without antilock system the fastest way to come to a stop (other then hitting a brick wall) would be apply enough pressure on the brake to just before it locks up. THis is impractable so it’s brake till lockup then release then re apply.
With ABS is the fastest way to just press as hard as you can or is it still better to find that ‘magic’ spot right before lockup (and engagement of ABS). Practically speaking is it better to fully stomp on the brake or just enough pressure to start the ABS? is there any real significant difference between the 2?
Everything I’ve seen says that the best way to stop with ABS is to press hard on the brake pedal, without letting up at all.
Based on personal experience, the only time that isn’t true is when you’re on glare ice, and all four wheels lock up at the same time. In that case, the ABS thinks that you’ve come to a full stop, because there’s no difference in the rotational rate of the various wheels, so it doesn’t do anything to release any of the brakes. In that case, you’re back to pumping and praying.
I tried stopping on wet smooth ice in my '96 Ford Explorer and it does not lock up the wheels or spin out–when you hit them hard the brakes buzz, the brake pedal vibrates and it just comes to a stop really slowly.
~
My experience is based on a '92 Integra, so I’m sure there have been advances in the technology since then. Hell, a new Mercedes S-Class probably determines that it’s going to need to stop several days in advance, does its own forecast of the weather and road conditions, and starts adjusting the driver’s seat to make the panic stop as non-upsetting as possible.
Best advice: hit the brakes hard, and stay on them, unless you’re damn sure they’re not doing what they’re supposed to be doing.
There’s no possible way to find that “magic” spot better than ABS. ABS can detect the slowing of the wheel before traction is totally lost. Even if you could somehow detect when the wheel is about to loose traction, a human being could not respond quickly enough with even close to as much control as ABS does.
All the manufacturer’s that I am familiar with recommend applying the brakes until the vehicle comes to a complete stop, allowing the system to do its thing. Anything else will probably lead to a less controlled and longer stop.
That said, non-ABS braking does better in sand and deep snow. In most other conditions, ABS braking will deliver equivalent or shorter stopping distances but, more importantly, you are much less likely to skid using ABS as directed.
The most primitive ABS systems pump the brakes up to ten times per second which is beyond human capability with non-ABS brakes. Beyond that, ABS can independently control one, any two or any three brakes at a time, which is not yet technologically possible in non-ABS brakes.
Actually, I don’t think ABS reduces stopping distances at all. The fastest way to stop a car is to jam on the brakes and stay on them. However, such a strategy also means you’re much more likely to lose control of the direction of travel. ABS is designed to allow you to stop without losing control of the car.
Without ABS this is not true. THe friction between the tires and the road decreases once the tires go from rolling to skidding - less friction = longer stopping distances.
Don’t mean to be confrontational but that is simply wrong. A skidding car will nearly always (see caveat below re snow and gravel) take further to stop than one that is braking but not skidding. If you don’t believe all of the cites given above then try this research.
If you don’t believe that I can offer a first hand account: About three years ago I had the opportunity to go to the Bosch (manufacturer of ABS equipment) test track in Stuttgart. I test drove a car with disconnectable ABS on the track. In all conditions -wet, dry, slippery- the car stopped significantly quicker in an emergency stop with the ABS turned on. In the case of the slippery conditions (ceramic tiles with water running over them) it stopped in less than half the distance. I had expressed some scepticism about ABS before this but was completely converted afterwards.
However, the greatest benefit of ABS is that you can still control the steering of the car in an emergency stop - something that’s virtually impossible with a skidding car.
Now for the caveats: there is some evidence that inexperienced drivers or people who are inexperienced with the way ABS handles in an emergency stop can increase their risk of some types of accidents but note that this evidence is contentious. Also there is some evidence that some ABS systems take longer to stop a car in snow or gravel emergency stops.
Personal experience driving a 95 Jetta GLX is that on dry pavement, I can stop the car faster if I avoid the ABS coming on like you are asking.
On a rain-slicked road, I perfer the ABS because it allows for more control.
So the answer I’m sure is that it all depends on the car driven, the road conditions, and the driver driving. There will be no standard YES or NO answer…like most questions in life.
As others have said, ABS is not about stopping the car the quickest. In some (possibly many) situations you will get the shortest stopping distance by stomping on the ABS brakes, but their main value is that they allow you to control the car while braking.
It is true that threshhold braking (just before locking up) will stop the car quickest. On dry pavement, a skilled driver can probably stop quicker than many/most ABS systems. However, in the real world you rarely get perfectly dry, even pavement, and are aware and in control enough of the situation to perform perfectly.
Another common complaint about ABS is their handling in deep snow or sand. When you lock up the brakes, piles of material (snow or sand) build up in front the wheels which slows the car quicker. With ABS, the wheels keep spinning and the pile of material never gets the chance to build up.
One final thought, not all ABS systems are equal. Some are 3 channel (one each for the front wheels, one for both back), others are 4 channel (one for each wheel, better) and have differing electronics.
Quoting (from memory) from the ABS training manual that I use in technical training class (I work for a car maker)
ABS has three design criteria[list=1]
[li]Must be automatic. No special levers or buttons to push[/li][li]Must work at all speeds. (as long as you are above the threshold speed, about 3mph)[/li][li]as long as the speed of the car is below the maximum cornering speed of the car, the car must be able to corner.[/li][/list=1]
You will note that shorter stopping distance is not one of the design critera. In fact most of time ABS will stop faster than non-ABS. The exception being as has been mentioned snow, gravel and alike.
Over the years I have noted some thing about people using ABS. Ususally people getting used to ABS don’t hit the pedal nearly hard enough for the system to operate at maximum effect. Just this weekend I was showing a friend how the ABS in their new car worked. I had her do a panic stop from 45 MPH on wet pavement. My instructions to her were “Do your best to break the brake pedal off.”
The first stop did not even get into ABS.
The second got a little ABS pump action torward the end.
The third was about the same as the second.
We then swapped seats and I showed her what the system would do. her comment was “Now can we go back so I can change my panties?”
Also not all ABS systems are created equal. some work much better than other.
The thing is that if surprised, the brake usually comes on so hard and fast that lock-up occurs and braking is less than if ABS was in use.
when prepared on dry roads it is possible to hold the brake at a constant rate just before lock up. ABS is pumping and not at the perfect rate for all speeds - the ABS will pump the same at all speeds as the car slows, more pressure may be applied
You will also note that this is a list under which ABS needs to work- not the effects it must have. Anecdotes aside, the skidding normally increases stopping distances and, in situtations in which ABS comes on, ABS will stop the car sooner/shorter than if it had not. Remember, ABS is only activated if you start to skid.
Ah, no this is not correct.
I don’t have a lot of time right now, but I will try to put this into a nutshell.
The limiting factor in stopping distance is the amount of traction between the tire and the road. More traction, shorter stoping distance. Less = longer.
Now when you step on the brakes the tire starts to slip on the road. This is actually good. The more slip, the greater the traction **UP TO A POINT. ** When the slip goes past about 12% the traction drops off. Picture a graph where the horzontal axis is slip going from zero at the left corner to 100% at the far right. The vertical axis is traction force. As slip increases, traction increases until maximum traction is reached (about 12%) then traction falls off as slip continues to increase (skid). The graph looks almost like a classic bell curve. At the top of the curve is where the greatest braking power is available. what ABS does is if you are in danger of going past sbout 12% slip, the system steps in with enough control to bring you back into the 10-12% slip range. It does not work at a fixed frequency as Blown and injected has posted, it works as needed at speeds of up to 10-15 times per second to keep you in the range of maximum brake efficiency. (BTW a race driver can modulate a brake pedal about 5-6 times per second)
Now to the arugement that a properly trained driver can outstop ABS.
On a low end car with a craptacular ABS system and a very well trained driver, the answer might be maybe.
On a decent car, I would expect that there would be no contest. One of the elctronic scan tools that we use can dispaly the G-force + or- as the car is being driven. When the scan tool is being used ABS is in standby and does not work. Braking to what most people would consider to be on the limit will produce a .5G to sometimes a .6G stop. I think one time I was alble to show .64G. These are severe stops. A 200 lb person would be thrown against the seat belt with over 130 lbs of force. Without a belt you would be on the windshield.
However with ABS on the car will brake with the full traction force of the tires, about .83G. A full .2G grater than I have ever been able to duplicate under controlled conditions. Add into the equation the fact that accidents are, well, accidents. When you are not prepared for them and therfore are more likely to either push the brake too hard and lock up the tires or not hard enough to get max efficiency. Add into this ABS can adapt the braking of each whell to changing conditions. I give the nod to ABS hands down.
Pictue if you will, you have to stop with the right two wheels in some sand scattered on the right shoulder, left wheels on dry pavement. ABS can cope, no ABS either right wheels will lock, or left will not be at max efficiency. So after you start to brake you swerve left and now the right tires are on clean pavement, but the lft have gotten into a puddle of water. Again ABS handles the mission, no ABS good luck. (can you say spin?)
I know a number of pro and semi pro race drivers, they all have and want ABS on their personal cars, because the computer can work faster than they can.
Pretty good Rick for not having much time but something you said goes agaist what I learrned. which is:
There are 2 basic types of friction/traction dynamic and static. Static is the non skidding type where the tire and road are not moving relative to eachother when in contact. Dynamic is when the tire is skidding or moving agaist each other when in contact. Static friction/traction will always be greater then dynamic.
A little trick we use to get cars unstuck in the snow belt states is an application of the dynamic friction/static friction conumdrum.
From time to time the stuck vehicle is found stuck on a slope. It may happen that if the vehicle could be turned sideways, the car could be driven out more easily. How to turn the car sideways? Spin the tires and push SIDEWAYS on the car toward the downhill direction. With just the right conditions, (slope, ice/snow) one person can do this with one hand, whereas, with the tires not spinning, several men could not push it sideways with all their combined might!
Motog’s link did not work for me, was this in the link?
You are correct.
Good ole Mr. Newton, an object at rest wants to stay at rest. This would be the tire/pavement when you are coasting down the road. They are at rest with each other. No matter what speed you are traveling when coasting the tire and the pavement are at zero velocity when compared to each other.
Now when you step on the gas, turn the wheel, step on the brake the tire speeds up, moves, or slows down when compared to the pavement. This causes slip between the tire and the road. Now if the slip between the pavement and the tire becomes too much, then you are in dynamic friction not static. Acceleration, steering and braking all occur in that transition between static friction (zero slip) and full dynamic friction (skid)
The transition static/dynamic is not instantaneous. Remember Mr. Newton the object at rest wants to stay at rest. Therefore if you give them half a chance the tire won’t skid.