# Braking Distance, Skids & ABS

• B
https://en.wikipedia.org/wiki/Braking_distance

"Braking distance
refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop."

I'd like to know something. When the brakes are fully applied and before the car stops. Are the wheels still rotating or are they simply skidding in the roads?

If they are still rotating.. then that means they are not locked and so ABS (Anti Brake locking System) won't work?

berkeman
Mentor
https://en.wikipedia.org/wiki/Braking_distance

"Braking distance
refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop."

I'd like to know something. When the brakes are fully applied and before the car stops. Are the wheels still rotating or are they simply skidding in the roads?

If they are still rotating.. then that means they are not locked and so ABS (Anti Brake locking System) won't work?
What reading have you done on ABS? There are sensors and actuators involved...

berkeman
Mentor
ABS (Anti Brake locking System)
BTW, that is Antilock Braking System,,,

I have two cars, one with abs.. one doesn't.. for now. I'd like to understand the concept of Braking Distance that doesn't involved ABS but just Braking distance. See http://www.drivingtesttips.biz/stopping-distances.html

If your car is travelling at 30mph and you hit the brake. it would still move 14 meters! Does this mean when pressing the brake, the wheel doesn't stop suddenly but have to use friction to stop the rotations? Or does braking distance means the wheel suddenly stops and the car is skidding 14 meters. Again this is assuming no ABS and just plain concept of car braking distance.

rcgldr
Homework Helper
Normally braking distance is based on static friction, so the tires are rotating and not skidding.

Braking distance is based on static friction, so the tires are rotating and not skidding.

And if it rotating.. it means you can still steer the wheel without ABS??

The purpose of ABS is electronic "threshold braking" (where you press and release and press the brake dozens of times a dozen to avoid lockup.. which ABS did everything for you using sensors and actuators automatically).. I know the concepts of ABS having read numerous articles and even youtube about it). So I guess if the wheels are still rotating.. then ABS is not necessary, right? In slippery roads, what concept in physics wherein the wheels have more tendency to lock up.. because somehow the lessen traction makes the braking force larger than skidding force.. what are the correct terms for this descriptions. In such case. ABS is necessary.

berkeman
Mentor
And if it rotating.. it means you can still steer the wheel without ABS??

The purpose of ABS is electronic "threshold braking" (where you press and release and press the brake dozens of times a dozen to avoid lockup.. which ABS did everything for you using sensors and actuators automatically).. I know the concepts of ABS having read numerous articles and even youtube about it). So I guess if the wheels are still rotating.. then ABS is not necessary, right? In slippery roads, what concept in physics wherein the wheels have more tendency to lock up.. because somehow the lessen traction makes the braking force larger than skidding force.. what are the correct terms for this descriptions. In such case. ABS is necessary.
It takes more skill than the average driver has to perform maximum braking without skidding the tires, so that is why ABS is so popular. Race car drivers do it routinely, but the average driver in a panic stop situation almost always ends up skidding the tires, so they lose steering ability in the skidding stop.

rcgldr
Homework Helper
And if it rotating.. it means you can still steer the wheel without ABS??
If steering, then the braking force has to be reduced to compensate for cornering force.

The purpose of ABS is electronic "threshold braking" (where you press and release and press the brake dozens of times a dozen to avoid lockup.. which ABS did everything for you using sensors and actuators automatically)...
or ABS just looks for a tire that is rotating slower or faster than the other tires, or any tire decelerating much faster than the car would be able to decelerate, and adjusting brake pressure on that tire to compensate without actually locking up the tire even for brief moments. Wiki article:

http://en.wikipedia.org/wiki/Anti-lock_braking_system

SteamKing
Staff Emeritus
Homework Helper
I have two cars, one with abs.. one doesn't.. for now. I'd like to understand the concept of Braking Distance that doesn't involved ABS but just Braking distance. See http://www.drivingtesttips.biz/stopping-distances.html

If your car is travelling at 30mph and you hit the brake. it would still move 14 meters!
What? You thought cars stopped instantaneously when the brake pedal was pressed? Haven't you ever observed how other vehicles come to a stop?
Does this mean when pressing the brake, the wheel doesn't stop suddenly but have to use friction to stop the rotations?
The kinetic energy of a moving vehicle is converted to heat by the friction of the brake pad rubbing on the brake rotor or the brake shoe rubbing on the brake drum in older cars.

This heat is supposed to be dissipated, but if it is not, then you'll experience brake "fade", and it will take longer to stop. Disk brakes are less susceptible to fading, but heavy braking can cause disk brakes to fade.
Or does braking distance means the wheel suddenly stops and the car is skidding 14 meters. Again this is assuming no ABS and just plain concept of car braking distance.
What happens to a tire when it skids on pavement? Doesn't it leave, you know, skid marks? Doesn't it make a loud skidding sound? How long do you think your tires would last if every time you braked you skidded to a stop?

CWatters
Homework Helper
Gold Member
So I guess if the wheels are still rotating.. then ABS is not necessary, right?

Correct.

The stopping distance is determined by friction. The coefficient of static friction is usually higher than the coefficient of kinetic friction. One purpose of ABS is to keep the wheels rotating so that the stopping force is determined by static friction (rather skidding = kinetic friction).

In slippery roads, what concept in physics wherein the wheels have more tendency to lock up.. because somehow the lessen traction makes the braking force larger than skidding force.. what are the correct terms for this descriptions. In such case. ABS is necessary.

The wheels are more likely to skid if the coefficient of static friction has been reduced by rain. So you are more likely to need ABS to keep the wheels rotating.

berkeman
Mentor
Just to clarify, you still get skid marks with most ABS systems, bit they are modulated skids so that the wheels are turning enough of the time to allow limited steering capability.

rcgldr
Homework Helper
Just to clarify, you still get skid marks with most ABS systems, bit they are modulated skids so that the wheels are turning enough of the time to allow limited steering capability.
The image shows what happens when a common controller is used for more than 1 tire. As mentioned in the wiki article, the smarter ABS systems have a separate feedback / controller systems that control each tires individually, not in pairs, so the pulsed skidding effect is eliminated. The high end ABS systems uses in some racing cars allows for optimal slippage, even while cornering (perhaps using onboard telemetry as part of the feedback), but I don't know if any street cars have such systems.

berkeman
Mentor
The image shows what happens when a common controller is used for more than 1 tire. As mentioned in the wiki article, the smarter ABS systems have a separate feedback / controller systems that control each tires individually, not in pairs, so the pulsed skidding effect is eliminated. The high end ABS systems uses in some racing cars allows for optimal slippage, even while cornering (perhaps using onboard telemetry as part of the feedback), but I don't know if any street cars have such systems.
Interesting, I did not know that. Are most ABS systems on cars now the improved type? Or does it depend on how expensive the car is?

rcgldr
Homework Helper
Are most ABS systems on cars now the improved type? Or does it depend on how expensive the car is?
My guess is that newer cars would have the improved ABS systems, but I'm not sure. Price could be a factor, but as an analogy, some cheaper cars like the VW Passat have dual clutch transmissions as an option, while other car makers only offer this on specific and usually higher end models.

berkeman
berkeman
Mentor
The image shows what happens when a common controller is used for more than 1 tire. As mentioned in the wiki article, the smarter ABS systems have a separate feedback / controller systems that control each tires individually, not in pairs, so the pulsed skidding effect is eliminated. The high end ABS systems uses in some racing cars allows for optimal slippage, even while cornering (perhaps using onboard telemetry as part of the feedback), but I don't know if any street cars have such systems.
Who says you can't teach an old dog new tricks?!

http://www.toyota-4runner.org/attachments/5th-gen-t4rs/74262d1351041072-abs-explained-image.jpg

davenn
ABS systems come in 2, 3, and 4 channel varieties.

2 channel systems control the wheels in pairs. These systems are the norm in small front wheel drive economy cars. Each front wheel is paired with the rear wheel on the opposite side.

3 channel systems are the norm on mid range cars and pickup trucks. Each front wheel is controlled individually, and the rear wheels are controlled as a pair.

4 channel systems control each wheel individually.

A lot of newer systems also have an 3 axis accelerometers and a yaw rate sensor to measure the actual movement of the vehicle for comparison to the wheel speed sensors. These systems can be integrated with 2, 3, or 4 channel systems, though I don't know of any 2 channel systems currently using it, as those are typically economy priced systems anyway. Different manufacturers have different names for these systems like "dynamic stability control" or "stabilitrak" This was the racing technology rcdlgr was talking about in post 12, and it's now very common in production cars. I'm sure there are newer refinements being developed in the racing circuits though.

berkeman