This question got into my mind when I am cleaning my motorcycle.
If spring shock-absorbers like the one below absorbs shock from road imperfections, reducing strain in the motorcycle body as your drive, then why does car manufacturers use rigid bumper for cars instead of semi-rigid or flexible bumpers?
Isn't it good for the car that its bumper dissipates some energy in collision so that the time of impact increases, therefore reducing impact force? What are the pro's and con's of this idea?
To answer your later questions first:
Isn't it good for the car that its bumper dissipates some energy in collision so that the time of impact increases, therefore reducing impact force?
Yes, it is good, for the reasons you describe. Your misconception, however, seems to be that a "rigid" bumper would not do these things. The bumper, and indeed much of the car bodywork constitutes a "Crumple Zone", which is designed to exhibit exactly the behaviour you describe, by increasing the impact time, and reducing the consequent peak force.
To answer your first question:
If spring shock-absorbers like the one below absorbs shock from road imperfections, reducing strain in the motorcycle body as your drive, then why does car manufacturers use rigid bumper for cars instead of semi-rigid or flexible bumpers?
The key difference between using a spring to reduce the impact force vs using a crumple zone, is that the spring returns that force. In a big crash, you really don't want to be bouncing backwards. Bumpers are optimised for safety in an emergency, rather than day-to-day driving into things, as it were.
it is a good idea. 70 years ago, bumpers were built out away from the bodywork by about 5" to 6" to take hits without damaging the car, but styling changed and by 60 years ago, bumpers became part of the bodywork. 40 years ago, shock-absorbing bumpers were required by law in the USA, but they made the cars look ugly so people didn't like them. the law was revoked and now the bumpers do not have to sustain shocks without damage.
The difference is the amount of crash energy dissipated and limited play course of a spring loaded dumper. The energy absorbed by a spring is the area under force line: 1/2 k.x^2.m. - m is the bumper mass. which is very small compared to inertia of the car. And at the end of it's course it delivers the huge leftovers of impact energy to the rest of the car while now adding it's own mass to attacking car's inertia.
The idea behind integrated crash structural design is a highly energy absorbing gradual and ductile crumpling of the entire structure surrounding the occupants as a scrifice zone while reducing the negative acceleration to tolerable levels. No loose part is going to be of any help. If you look at the corrogated pressed patterns on the body frame of the new cars you see they have been designed to absorb much energy in a crash while crumpling away from the cab of the car. You will have the entire front of the car's mass and length to help dampen the impact.
