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How do bicycles manage to turn without falling. According to my intuition when the rider tilts the bike it should start falling down. In reality though bicycles manage to continue moving without falling because of their speed. Does speed generate a force that counter the weight?

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Have you ever felt about to tilt when turning shapely while driving fast in a car? The biker feels the same (and this has a bigger effect on bikes and trucks with higher centers of mass), so he makes a "counter" tilt to compensate.

When the wheels on a vehicle turn, the centre of mass (CoM) wants to continue. This feeling is the well-known "centrifugal" effect. The whole vehicle tilts a tiny bit now that only the wheels are being pulled in by friction - car suspensions e.g. compress a bit at the outer wheels and elongate a bit at the inner wheels of the turn.

During this tiny tilting, the CoM is moved. It must stay between the car wheels. If it is further out than the outer wheels, the car has been tilted enough to roll over.

Same for a bike - or a truck for that matter.

  • A truck has larger wheel distance, but also a CoM higher up. So a small tilting moves the CoM further.

  • On a bike the "distance" is the contact to the road of just one wheel width. This is why a bike will never stand by itself. The driver must constantly place his weight correctly to keep the combined CoM directly above the wheel's contact point with the road. (there are some small gyro forces helping him as well that depend on the wheel masses and how fast they spin - those are the reason a bike wheel can't stand on its own, but it can roll on its own).

When the bike turns, the same tilting tendency as for the other vehicles happens, and the biker must compensate by intentionally tilting opposite. He doesn't fall down, because the swing in the turn tries to lift him up to tilt the other way out of the circle.

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  • $\begingroup$ Strange as it may seem when a motorcyclist wants to make a turn he actually steers in the opposite direction. It's called counter steering. The heavier the bike and the faster the speed the more counter steering is noticed and needed. At slow speeds and on a light bike a rider's lean will move the bike over. I had a 800 pound Harley and at higher speed I had to really push the handle bars hard to the left to go right and vice versa. Look it up. It's and interesting topic that will help your understand of the turning dynamics of a bike. It's called counter steering. $\endgroup$ – Lambda Mar 13 '17 at 1:49

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