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Old 11-25-2011, 07:51 PM
 
8,233 posts, read 10,892,213 times
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Default Pumping Brakes for Faster Stopping in Snow

I just watched a video supposedly from Tire Rack on AOL Channel (which I am not linking because it has an advertisement embedded) which basically says to pump brakes for stopping in snow for cars without traction control. Now, I have never understood why that would be of any benefit at all unless your vehicle was veering off course as a consequence of the braking. In fact, I assume it would increase stopping distances. Now, I never have any problem controlling the direction of my car on slippery conditions. If it is too slippery to stop and the vehicle is not going a good direction, I know how to correct for that and do. Is there any other reason for this "brake pumping" that is touted by so many other than for the fools who do not notice that their vehicle is no longer going down the road in a straight line but is headed for the ditch? It is quite possible to have all wheels "locked up" but stay perfectly on course. In such case is pumping a help or a hindrance?
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Old 11-25-2011, 08:29 PM
 
Location: The Triad (nc)
16,344 posts, read 18,921,876 times
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Quote:
Originally Posted by Wilson513 View Post
...which basically says to pump brakes for stopping in snow for cars without traction control.
And rain or any other time you need to "trim" your speed.

Quote:
Now, I have never understood why that would be of any benefit at all unless your vehicle was veering off course as a consequence of the braking.
Then it would be too late.
The idea is to keep you on course so that veering doesn't begin in the first place.

Quote:
In fact, I assume it would increase stopping distances.
Ironically not... or at least not significantly.

These are all the reasons that carmakers have shifted to fulltime ABS and other traction control.

hth
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Old 11-26-2011, 12:36 AM
 
Location: Not far from Fairbanks, AK
12,619 posts, read 12,785,117 times
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Quote:
Originally Posted by Wilson513 View Post
I just watched a video supposedly from Tire Rack on AOL Channel (which I am not linking because it has an advertisement embedded) which basically says to pump brakes for stopping in snow for cars without traction control. Now, I have never understood why that would be of any benefit at all unless your vehicle was veering off course as a consequence of the braking. In fact, I assume it would increase stopping distances. Now, I never have any problem controlling the direction of my car on slippery conditions. If it is too slippery to stop and the vehicle is not going a good direction, I know how to correct for that and do. Is there any other reason for this "brake pumping" that is touted by so many other than for the fools who do not notice that their vehicle is no longer going down the road in a straight line but is headed for the ditch? It is quite possible to have all wheels "locked up" but stay perfectly on course. In such case is pumping a help or a hindrance?
On a car with ABS you should not pump the brake pedal. Just step on and hold the pedal. But on one without ABS, you pump the brake pedal to avoid locking the brakes. Do you know what ABS does for you? It pumps the brakes in rapid succession (you can feel the pulsations on the pedal). So if you pump the brake pedal of a car that does not have ABS, you basically are being the ABS And yes, ABS reduces stopping distances.
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Old 11-26-2011, 12:38 AM
 
Location: North Pole Alaska
842 posts, read 2,125,979 times
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Pumping your brakes is to keep the tires from locking up. Same thing ABS does in modern cars.

The tires achieve the highest coefficient of friction just before locking up. After they lock up there is less friction and your stopping distances will actually increase.

Now with that being said DO NOT pump the brakes if your car is equipped with ABS unless you can pump them 60 times a second.
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Old 11-26-2011, 08:17 AM
 
8,417 posts, read 7,420,482 times
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Quote:
Originally Posted by RayinAK View Post
On a car with ABS you should not pump the brake pedal. Just step on and hold the pedal. But on one without ABS, you pump the brake pedal to avoid locking the brakes. Do you know what ABS does for you? It pumps the brakes in rapid succession (you can feel the pulsations on the pedal). So if you pump the brake pedal of a car that does not have ABS, you basically are being the ABS And yes, ABS reduces stopping distances.
Quote:
Originally Posted by usafracer View Post
Pumping your brakes is to keep the tires from locking up. Same thing ABS does in modern cars.

The tires achieve the highest coefficient of friction just before locking up. After they lock up there is less friction and your stopping distances will actually increase.

Now with that being said DO NOT pump the brakes if your car is equipped with ABS unless you can pump them 60 times a second.
What they ^^^ said.

Traction control only works for acceleration, not braking.
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Old 11-26-2011, 08:22 AM
 
8,233 posts, read 10,892,213 times
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Quote:
Originally Posted by RayinAK View Post
And yes, ABS reduces stopping distances.
But why? The physics escape me. I see reasons why ABS would increase stopping distance if vehicle directional control was not an issue. But why would ABS reduce them. What physics principle could be at work?

Don't get me wrong, I think I avoided a very serious accident only thanks to ABS, I just can't see why it would help in straight line stopping if directional control was not an issue.
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Old 11-26-2011, 10:12 AM
 
8,417 posts, read 7,420,482 times
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Quote:
Originally Posted by Wilson513 View Post
But why? The physics escape me. I see reasons why ABS would increase stopping distance if vehicle directional control was not an issue. But why would ABS reduce them. What physics principle could be at work?

Don't get me wrong, I think I avoided a very serious accident only thanks to ABS, I just can't see why it would help in straight line stopping if directional control was not an issue.
Once you start sliding because of wheel lockup, you aren't stopping as quickly as if you aren't sliding. ABS keep you from sliding, generally. If you're sliding you have little directional control, even for minor adjustments.
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Old 11-26-2011, 10:53 AM
 
Location: Northern MN
3,870 posts, read 4,374,514 times
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The pumping the brakes came from the push to get folks from locking up the brakes and sliding.
You loose your stering if the wheels are not turing thus you can't change the direction you are going and avoid getting into trouble.

When you lock them up snow builds up in front and under your tires, if you release the pedel the tire can turn and get rid of this and gain some traction for when you replay the brakes or so you can steer away from trouble..

but I'm sure you have found a lite pressure on the peddle is even better than the skid release, skid release, ie pumping the brakes.
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Old 11-26-2011, 11:03 AM
 
629 posts, read 380,651 times
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ABS essentially pumps the brakes, it just does it faster then you can and independently on each wheel. You don't need to pump as much as keep it right at the point of where it still spins vs locks up, pumping back / forth over this zone is what they are talking about.

If your sliding / locked up - you're not slowing down really, and you can't steer the car.
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Old 11-26-2011, 12:01 PM
 
Location: North Pole Alaska
842 posts, read 2,125,979 times
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Quote:
Originally Posted by Wilson513 View Post
But why? The physics escape me. I see reasons why ABS would increase stopping distance if vehicle directional control was not an issue. But why would ABS reduce them. What physics principle could be at work?

Don't get me wrong, I think I avoided a very serious accident only thanks to ABS, I just can't see why it would help in straight line stopping if directional control was not an issue.
Surface area and friction.

When the tires are turning there is friction against the brake pads/shoes and rotors/drums and tire contact patch.

When the tire is locked up the only friction that you have is the tire contact patch.
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