Few weeks ago a neighbor manage to hit my bike with his car while the bike was parked next to a wall, check the picture.

bike with broken handlebar

One side of the handlebar was at the wall and the door of his truck hit the other end. He hit the bike while he was turning at the corner of the garage.

I'm wondering how fast he was when he hit the bike.

I guess I can use the "bending stiffness" and the centripetal force to check his speed. But I cannot find proper numbers for the aluminium strength.

May someone help me with this calculation?


I know it is a difficult problem. I'm wondering if it is possible to build up a simple model for this.

Assume that one end of the handlebar is fixed and one applies the force as an "end load". I've found this website, now I'm trying to find reasonable numbers to put in:


Next I have to estimate the load with a model for the car hitting the bike. Let's say I know the car mass and speed.

Almost all collision problems require some strong considerations about the collision time. Here I guess one could use the impulse theorem to estimate the average force.

The velocity would be the the length of the circular arc of the bent bar over the collision time, and the collision time should be about the reaction time of a normal person, about 0.2 s maybe?

I can check the mass of the handlebar. This give us the momentum, and using again the same collision time I would get the force to use as the load.

I know it is oversimplified, but does it makes sense? I'm wondering if the numbers will be consistent after I put all numbers in.

If it all works out, I would like to use it as a class exercise in the future.


closed as off-topic by hft, ja72, ACuriousMind, Gert, user36790 Dec 28 '15 at 2:32

This question appears to be off-topic. The users who voted to close gave this specific reason:

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  • $\begingroup$ Try engineering.stackexchange.com $\endgroup$ – ja72 Dec 27 '15 at 23:06
  • $\begingroup$ I agree. This is quite a practical problem and engineers have more experience with this. But I'll try to finish what I wrote above. The only thing missing seems to be a connection between the average force and the car speed. So far I can only think about centripetal force for that, since the car was turning. $\endgroup$ – Gerson J Ferreira Dec 27 '15 at 23:47

I hope this isn't a joke, because it's a real waste of people's time if it is. This one has me scratching my head whether mods should close this or not. There are so many variables to this problem. You're going to need to know how much the truck weighs. But then it depends on the duration of force applied on the bike, and the angle, and where exactly it was applied. You're going to have to make all those measurements before you get anywhere. And then there are issues of friction. The amount of variables and things to consider makes this problem much more difficult than it's worth to solve, I believe. Because even if you did manage to solve it scientifically, the uncertainty is going to be huge unless you have a huge amount of data and measurement. To be honest I think using your own common sense of law and physics as to how fast he was going is probably going to give you a more sensible answer than to try to calculate it yourself (what was the speed limit, were they trying to park or driving normally).

  • $\begingroup$ It is not a joke. I've edited the question to add an oversimplified model that I'm trying to check if makes sense. The speed limit were 10 km/h. He was turning at the corner of the garage, going to his parking spot. But I don't care that much if his speed was legal or not. The issue is that a bunch of people is asking me if I can calculate the car speed, and it seems to be a surprisingly difficult problem. $\endgroup$ – Gerson J Ferreira Dec 27 '15 at 22:03

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