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Technology

ELI5: Why does pumping the brakes work better than holding the brakes when your car is sliding on ice?

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There's two types of contact friction: static friction and kinetic friction. Static friction is the force that it takes to push a stationary object and start it sliding/skidding across the ground. Kinetic friction is the force it takes to keep pushing something that is already sliding. Static friction is always greater than kinetic friction.

Cars work by static friction. Even though the tire moves forward, the tire rubber doesn't actually slide along the ground (unless you're skidding or burning out). The point of contact between the ground and the tire is stationary, so the friction between car and ground is static friction. This friction is how the car moves forward - it spins its wheels to push backward on the ground, and so it pushes itself forward.

When your car is skidding/sliding and you slam on the brakes, your tires lock up and start sliding across the ground. Static friction becomes kinetic friction. Since kinetic friction is lower than static friction, this means that the force your car can exert on the ground is lower. This means that your traction is lower, and your ability to control the motion of the car is lower.

By letting off the brakes for a short time (either by pumping with your foot or by having an AntiLock Brake system do it for you), you allow the wheels to roll freely for a bit. This can give them the opportunity they need to start rolling along the road again instead of skidding across it, regaining the static friction and thus your ability to control the car.

Indeed this, a second reason if you're breaking with your front wheels are that you literally cannot steer unless your tyres are rolling, it doesn't matter in which direction they are turning if they aren't also rolling in that direction.

Many times, being able to steer the car is even more important than being able to stop it!

You sound like you now what you're talking about. But more than that, this is the only comment that explained why it works, instead of simply saying you should, which was the

Excellent explanation!

If you have ABS brakes you can keep pressure on the pedal. The crunching sound is the brakes momentarily releasing so your car does not go into an uncontrolled skid, meaning you keep your ability to steer and may avoid a collision. Without ABS continuous pressure may result in your inability to steer while you slide on the ice.

0 points·29 days ago·edited 29 days ago

Threshold braking / pumping the brakes will still be an improvement over ABS.

Edit: my info comes from https://www.youtube.com/watch?v=7_99dN4dVkc

going to go ahead and say for 99.9999% of people, this is the worst kind of information - if your car has ABS, LET IT DO ITS JOB. ABS will pump @ 15x a second, no human can do that. itll allow you both steer and stop.

This is wrong information, and in this situation wrong info is dangerous.

That was only true with 1st Gen ABS systems, and even then only with a trained/proficient driver who knew what they were doing.

Modern ABS typically outperforms even professionals in all conditions at this point.

Even in the 1st Gen stuff, if you weren't a profession driver, you were better off just standing on the brakes.

That mostly, but not completely proved my point.

A professional driver was, surprisingly, able to outperform the ABS system (I've seen conflicting data on this, and it likely depends on the car. Apparently Fords recent ABS stuff isn't quite as good as some others) in perfect, non-emergency conditions.

He still barely managed to outperform it, and this is in conditions where he knows exactly what is going to happen and when. In panic situations, given the muscle memory of driving professionally, its possible he would have continued to outperform the ABS, but its also just as likely it would be a wash, or even a win for ABS.

In terms of regular, everyday drivers? Stick with the ABS, because most of them are barely competent at driving in perfect conditions, let alone advanced driving techniques during split second decision making.

The average driver will be using ABS anyway because their reflex will be to mash the brakes. I simply posted to further some people's understanding (this was an ELI5, not a PSA).

It doesn't on a car with ABS. If your car has ABS do not pump the brakes that defeats the whole purpose. If your car does not have ABS, pumping them may help you retain some control of the car. But again if you have ABS and need to emergency brake put your foot down as hard as you can and keep it there. Your car will shudder or make noise; that is normal.

what if my car has ABS but only because it’s skinny? Does it count?

All cars are beautiful

I chuckled

when driving (slidding) on ice, the WORST thing that you want to happen is for your wheels to stop spinning. Once they do this, you can NOT steer out of the way....pumping the breaks reduces the chances of your wheels locking up while increasing your chances to control the direction of your vehicle...ABS pumps your breaks 15x per second without locking up the rotation.

Both pumping the brakes and ABS help regain traction on the ice. If your brakes are continuously applied, then your tires turn into skis. When you depress the brakes, the tires will continue spinning so that next time the brakes are applied (either from pumping or ABS), it will keep enough traction to reduce your speed some more.

With enough gradual presses/depresses, you'll shave off enough speed to come to a stop. As opposed to sliding dangerously on tire skis.

2 points·1 month ago

The wheels don't just stop suddenly and cause the car to skid. I'm no mechanic but I think abs works in the same way.

I'll just assume you don't have breakes that prevent locking up.
When breakes lock up they can't get any traction. I think of it like the entire force of the car's speed is in every groove in the tread. The tread will smash through anything that I was trying to get a grip on. When you pump the breaks, it'll let the tire get a grip then slow down, get a grip then slow down.
I hope that helps

When you hold the brakes while sliding on ice, your wheels are going to block, and that means that your car is not going to brake at all.

If you pump them, the friction that is generated from the wheels continually stoping and moving, will eventually slow down the car.

ABS system automatically pumps your brakes even if you are slamming them

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