Bumpers aren’t for bumping–now they are for energy dispersal
I had a little fender bender last week. Nothing major and no one hurt (very low-speed thing), but I had to get an estimate done for the repair. While I was chatting with the guy doing the estimate I mentioned how my mom used to say “bumpers are for bumping” … sorta in reference to parallel parking…now a days though, not so much. In fact your car bumper is more for esthetics than collision protection. The protection parts are behind the bumper. The bumper, in fact is designed to shatter and therefore disperse the energy of the collision instead of absorb it.
Pretty amazing that cars are safer now, but the parts tend to just explode instead of twist, buckle and bend.
Tags: car safety
May 17th, 2007 at 12:43 pm
Part of the issue is that the old style bumpers were heavy. You were basically talking about a steel bar six feet long, and sometimes close to a foot high. That’s quite a bit of steel. And, that extra mass that was being hauled around tended to lower fuel economy numbers.
The other issue is that those old, solid bumpers didn’t protect too well, even given their weight. If you figure the kinetic energy (E=.5*m*v**2) of a moving automobile, you know there’s quite a bit of energy there, even if the speed is quite modest. Then, use Newton’s F=ma equation to find out the forces involved in a collision, with the vehicles stopping in an incredibly short distance(s=v*t+.5*a*t**2), and you find that the forces involved are astronomical, usually high enough to bend even those old heavy steel bumpers (e.g., push the deformation from the elastic region to the inelastic region:
http://en.wikipedia.org/wiki/Deformation
http://en.wikipedia.org/wiki/Young%27s_modulus
).
The modern bumpers are designed to give to absorb part of the energy. By giving, they lengthen the distance over which the vehicles stop (Well, not by much, but definitely better than .01 inches.). So, the acceleration is decreased, and the force applied to the bumper is lessened. As a result, the newer, “energy absorbing” bumpers may well be able to cushion the collision without as much damage to the entire vehicle as the older bumpers.
Of course, a side effect of this is that since the newer bumpers are designed to collapse, you don’t really want to push things with them. Thus, bumpers are no longer for bumping.
Dave
P.S. Good luck with ICBC. ;-)