91 Chrysler Le Baron Rear Bumper

[msquared48]

Question is, are the two bumper springs meant to absorb 100 percent of the horizontal force of a rear end collision at any point up to their failure at full compression? To put in a numerical perspective, if the impact loading is 2000 pounds, and say the capacity of the two springs is 3000 pounds asorbed at 100% compression, will the occupants feel any force? If not, why not? If so, why so?

Mike McCann
MMC Engineering

 

[IRstuff]

You hit a car, it's going to move, so the occupants will feel the jerk, that's why they get whiplash. isn't it?

The usage of the term "absorb" is misleading, I think. "Absorb" implies that the springs magically suck up the energies, but, the springs compress, and then they want to uncompress. Not that much winds up as heat.

TTFN

FAQ731-376

 

[Bribyk]

The springs reduce the peak force felt by spreading the impact pulse over a longer time period.

 

[msquared48]

Bribyk:

Do you have any study/test references with any energy dissipation curves with respect to the time of the impact for these or similar bumper springs - really a damper? Anything relating to accelerations seen with respect to the percentage of the permanent compression of the spring?

As I understand it, the "springs" as I referred to them, permanently compress on impact to a length that depends on the force seen in the impact, having to be replaced after each rear end collision. So these are really energy absorbing mechanisms, returning no energy back to the overall system.



Mike McCann
MMC Engineering

 

[SomptingGuy]

The impact loading (expressed as a force) will be dependent on the springs themselves. The job of the bumpers (or crumple zone) is to dissipate the kinetic energy of the impacting vehicle. The softer the springs (or more compliant the crumple zone), the lower the peak force will be. If the springs are ideal, all of the momentum of the impacting vehicle will be transferred to your LeBaron.

- Steve

 

[GregLocock]

I'm guessing that these things are concertina'd or trombone type stocky steel hollow columns? The work done is in plastically deforming the hinges, thus to a first approximation the energy absorbed is proportional to crush distance. In the olden days the entire crash system of cars was developed in the lab by crushing things like this. Now we get little node-pushers to do it instead.

I'd be slightly surprised if there were no published papers on this, albeit not your model in particular. Oh, and glass fibre tubes were in vogue for a while.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dgallup]

I saw a student competition once where the winning entry was rows of beer cans sandwiched between two wooden boards. That was before they started crushing them on their foreheads.