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I realize this seems like a pretty simple issue of drawing a free body diagram, but I just can't seem to figure it out.

If a bike leans, then it must have had a torque that made it lean. I considered the centripetal force, in this case friction, as a possible source for this torque, but that would create torque in the opposite direction. The only remaining torque causing force in my FBD is weight. But weight was there before the leaning, and would only cause a torque after leaning.

If anything, because of centripetal acceleration, observing from an inertial frame of reference, the bike should lean in the opposite direction because of the introduction of the centripetal force. So the question is, what force causes the bike to start leaning?

Qmechanic
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Clovis Nyu
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2 Answers2

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Two things that need to be considered. Often it is not the cyclists weight that causes the lean in. Subconsciously, as people learn to ride a bike, they do something called counter-steering.

So for example coming in to turn right, a cyclist will quickly turn the handlebars left to lean the bike to the right due to centripetal acceleration. Once the lean has been achieved, the handlebars are then turned to the right to turn and balance the bike in a turn. Then to upright the bike, the handlebars are turned slightly harder in until it levels (due to increasing the centripetal force outwards relative to the lean), then the bike is straightened, and the handlebars follow.

People don't believe that they do this, it's quite funny.

AER
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The bike would lean in the other direction, due to centripetal force, without a cyclist. A cyclist leans the bike on purpose into the turn, to counteract said centripetal force.

The torque involved is the cyclist's weight as they lean into the turn.

Chris
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