eli5: Leaning in to physics

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I recently started riding a motorcycle. Why does “leaning in” while turning allow sharper turns at higher speed?

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Your center of gravity is lower so the sideways force is more on the tires. The faster the turn, the more lean needed to keep from tipping over.

It’s not so much that leaning in allows sharper turns as much as it’s necessary to turn at all.

First, think about a regular car. When you turn, there’s a centripetal (center-seeking) force exerted on the car by the angle of the front tires. The reaction is centrifugal– you and the car want to keep going straight, so you are forced to the outside of the turn. A car in a sharp turn has a visible body lean making it tilt outward.

The same thing happens on a motorcycle. But without the second set of wheels for stability, the bike would just immediately fall over. There’s no mechanism for handling side-to-side forces with only two wheels except for leaning.

To get a little technical, the combined force vectors of gravity and inertia make an angle to the ground between 0 and 90 degrees. When turning, you have to lean to match that angle (or steer to match the lean) so the bike doesn’t straighten back out or fall all the way over. Thankfully it’s not quite as hard as it sounds and can be done pretty intuitively with a little practice. If you’ve ridden a regular bicycle you’ve done it there too.

Tires are wide. When you lean in, your contact to the road moves to the side of the tire.

You have to put a lot of weight into the turn to prevent the bike to simply spring back vertical.

Imagine you are driving straight and your wheels are mounted 8 cm to the left. Then in a turn you also increase the g force.

This means, for a 120kg bike, turning about 1,4g, you have to compensate point of contact and mass and g force, .08m x 120 x 1,4 = 13.4 kgm. Let’s say you weigh 67 kg, you need your body to go 20 cm in the turn to compensate that momentum that tends to raise the bike.

It doesn’t really work like this, as when you ride you can shift forces with the handlebar, you steer out to keep the bike in, but having the weigh shifted in makes everything more natural and balanced. You feel a lot more in control.

The way bikes work is something that you can easily do from muscle memory but doesn’t work the way that you think it does. When you are turning on a bike, you point the front wheel in the opposite direction then you want to turn in which tips the bike moving the center of mass. Once it is tipped, you then correct the wheel resulting in you turning and the center of mass returning to the center line of the bike. Bigger turns means bigger offsets which means bigger leans.