Car crash physics

August 21, 2006 2:52 PM

You're parked at the lights with your foot on the brake pedal. Someone rear-ends you. Does the ABS trigger?

And does it depends on the type of ABS?

posted by twine42 to science & nature (9 comments total)

According to this article, the ABS sensors are "looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration." Wikipedia has a similar explanation.

If you're already stopped with the brakes firmly applied, there won't be any sudden change in wheel speed to trigger the ABS. (Your wheels will stay locked, and the wheel speed will remain zero.)
posted by mbrubeck at 3:05 PM on August 21, 2006

Typical ABS systems are based on reading the spinning of the wheel. If you're holding down your brake pedal hard enough, those wheels should be locked and wouldn't actually move - your car would either stop the car that hit you or would just flat out skid.

So I would say no.
posted by jedrek at 3:06 PM on August 21, 2006

On follow up, I should use preview.
posted by jedrek at 3:07 PM on August 21, 2006

The airbags did not deploy when exactly this scenario happened to my son three years ago. He was at a light and a cell-phone blabbing yuppie in an SUV slammed into the back of his Neon.
posted by LarryC at 3:23 PM on August 21, 2006

So this video is fake?


posted by MonkeySaltedNuts at 3:44 PM on August 21, 2006

That video is fake, but the question was about ABS, not airbags.
posted by MarkAnd at 4:01 PM on August 21, 2006

I was rear-ended while I was stopped in traffic not too long ago, and my foot was knocked off (off of the brake pedal, not off of my leg) upon impact. So, in order to keep your foot firmly on the brake pedal in that situation, I would guess that you'd have to be hit pretty softly -- probably softly enough that ABS wouldn't come into play anyway.
posted by jknecht at 7:51 PM on August 21, 2006

Airbag triggers are totally different. They measure acceleration, not wheel speed, which means they could be triggered if you're rammed while stopped with the brakes applied. But they're designed to trigger only in a frontal collision, so being rear-ended won't do it.
posted by mbrubeck at 10:30 AM on August 22, 2006

jknecht - I was rear-ended while holding the brake pedal down, and my foot remained squarely on the pedal. Of course, I heard the brakes screech behind me and I adrenalated and stomped on the brake pedal, so I may have been better braced than you.

On the other hand, I once t-boned a car that ran a red light in front of me. I had time to stomp the brake once again, and this time - probably via impact - I bent the brake pedal right to the floor. I was pretty amazed.
posted by Mr. Blint at 12:50 PM on August 22, 2006