# Bike crash physics

Note: This is all hypothetical for a story I’m writing. Please don’t think I’m psychotic.

Say someone is riding a bicycle down a street. Say a car comes up fast behind them, and hits them dead-on, front bumper to rear bike wheel.

What’s the most likely ‘trajectory’ of the person? I’m assuming that they’d be thrown backwards off the bike, into the windshield of the car (since the object they were on suddenly accelerated faster than the person was). However, I’m not certain about this, and I’m not exactly going to experiment.

Additionally - if, say, the driver should choose not to stop once hitting the biker, what happens? Would it be likely for the person to slide off onto the road?

Assume that the car is going a moderate speed - about 30 mph.

The trajectory would depend on the difference of velocities between the biker and the car, and whether or not the biker got entangled in the bike (or the bike in the car).

You can subtract the biker’s speed from both the car and the biker, so that you’re working with the biker stationary and the car at the difference in speeds. For a car going 30 and a biker going 20, the trajectory of the biker relative to the car will be the same as if the biker was stopped and the car going 10.

If nothing gets tangled in anything, then the car hood just has to be under the biker before the biker’s center of gravity is under the level of the car hood. I went out in the cold and took some measurements for this. I figure that my CG while biking is about 4 feet off the ground, and my car hood is about 2.5 feet off the ground. So the fall is 1.5 feet. From seat to back of tire on my bike is 2 feet, and from front of bumper to front of hood is half a foot. So if I was hit on my bike by my car, I’d have to fall 1.5 feet in the same amount of time as the car traveled 2.5 feet for me to end up on the hood.

Doing the math, that means that my car would have to be going about 5.6 MPH relative to the bike. So if the car was going 30, I’d have to be going 24.4 MPH or less to end up on the hood. Otherwise I’d stick to the grill or go under the car.

If the hood is higher or the bicyclist lower, then the car has to be going faster. If the hood is higher than the CG of the biker (like if it’s a kid, or a recumbent bike) then there’s no way for the biker to end up on the hood.

The trajectory from the hood would continue to depend on the relative velocity. If the biker breaks in the windshield, then he might get stuck there. If the velocity is fast enough, then maybe he could get bounced up the windshield and over the car. It also depends on how squarely the biker is hit, the shape of the hood and windshield, prevalinig wind, whether or not the biker has recently eaten beans, and so on.

Thorough enough, or shall I go on?

I saw an accident like this when I was in college. (Yeah, lucky me.) What happened was that the bike acted as a catapult lever-as the car hood pushed the rear wheel forward faster than the front wheel was moving, the front of the bike responded by flying up. The rider was actually tossed entirely over the car as it passed under him. (The collision wasn’t quite a straight rear-end; I think he probably would have landed on the trunk in that case.) This was at about 20 mph, I’d say-the cyclist stopped suddenly & the car right behind him couldn’t … At a higher speed, the bike frame might not be rigid enough to do this-it would probably crumple without giving that kind of boost. But that particular accident was certainly quite a sight.

This happened to me when I was in college. I was stopped to make a left turn when a car hit me smack from behind. He was skidding to a stop, which makes it possible for me to tell this story without drooling.:rolleyes:

I was tossed onto the hood and smacked the windshield quite hard. My bike, of course, was totalled. The seat stays (running from the back of the seat down to the rear wheel) were bent into a concave curve. If the car had been going faster, I figure I would have been tossed up in the air by the lever action of the car’s bumper against the back of my bike. Since the car would still be moving, I’d probably have landed on its roof. It’s not that I would have been flipped backwards as much as the car would move under me as I went straight up in the air.

Kids, don’t try this at home! :smack:

Ouch.

Thanks for the info (though the character in my story who this is happening to isn’t happy…)

This is just a little to close for comfort as I am recovering from a similar type of accident.
I was riding to work when I was struck by the mirror of a passing pick-up followed by the fender of the trailer he was towing. He was doing 65 and my speed was appx. 20. I was hit in the back of my left leg and thrown up and to the side away from the truck. I was airborne about 5 feet up and landed appx 25 feet away. Most of my hamstring was torn away and my femur was broken. NinjaChick,email me if you want further details for your story.

I have a recumbent bike like this that has a seat with a backrest. I can only theorize what might happen, but it might be substantially different than a bike with standard geometry.

I wouldn’t go backward off the bike unless there was enought force to tear the seat loose. It’s possible the front would flip up in some wheelie-like maneuver, but it seems equally possible that my rear wheel would crumple under the impact and I would go under the car. But I think that is also possible on a norma bike – if you destroy the rear wheel’s ability to roll, the rider might be thrown over the handlebars if they stay mounyed during the initial impact.

I once had a rather opposite accident on a normal bike. Coming down a steep hill, the traffic in front of me stopped short and I actually rear ended a car, dead on from behind. I went on to the trunk and my head put a dent in the trunk lid. The wheel did a pretty much perfect job of transferring the force to the frame – the wheel was undamaged, but the down tube (between the handlebar stem and the pedals) got bent about 5 degrees, totalling the frame and making the bike unrideable.