The Counterintuitive Physics of Turning a Bike (2015) [video]

An uneven amount of weight is distributed to each tire through a turn. Tire temperature, tread, pressure, etc impacts the surface in contact with the road. Even slicks don’t have a uniform pressure across the surface in contact with the road. etc etc.

The result can be oversteer (when the back slide more) or understeer (back slides less).

And it can change depending on road conditions, tire inflation, velocity, acceleration/deceleration. And of course changes radically from vehicle to vehicle.

Covered in depth in the movie Cars "You need to turn left to turn right". :-)

Of course the same thing applies to two wheeled vehicles also - see trail braking.

https://www.sae.org/news/2018/10/2018-yamaha-niken-chassis-t...

I'm just a software engineer, not a mechanical engineer, so any explanation would fall short. However, I've been surrounded by world-class Mech E's for decades, so I can remember these conversations being out of my depth!

I've also done urban commutes on bicycle and motorcycle for decades, so I can say given my experience, this is much easier to feel than reason about. You can tell by watching a person ride at licensing whether a person has integrated this feeling or is still reasoning about it. Hope this helps.

That is, the amount of "misalignment" between the wheel relative to the direction of motion.

Imagine a wheel on a treadmill, with the wheel angled one degree to the right, but held in place so it's kind of slipping slightly. This would have a slip angle of 1 degree.

The fundamental thing is that for a tyre, the lateral force generated by the tire is directly related to the slip angle (also the vertical load and the camber, but these can be removed for a simple model). Slip angle is the main thing.

If you know the relationship between slip angle and lateral force, you can model the movement of a car given the angles of the 4 wheels.

You can't go around a corner on a bicycle or motorbike at more than about 5mph without counter-steering. You have to tilt the bike into the turn or it will just fall over. You tilt by briefly steering in the opposite direction, which rolls the bike in reaction. This is counter-steering.

Braking while leaned over applies a torque to the steering column towards the side which is closer to the road, i.e. steering into the corner. This torque is due to the width of the front tyre; the contact patch is at an offset to the steering axis. This torque counter-steers the bike upright.

Braking also changes the geometry on most bikes as the forks compress and the bike dives at the front, steepening the steering angle, which makes steering more responsive, so the upright counter-steer is more pronounced.

There's also target fixation. You tend to steer where you look, and if there's an enormous hazard coming at you, you risk steering right into it. Look for the escape routes.

There's also a ground rush panic effect. When something is coming at you really quickly, your instinct is to tense up and brace for impact. When landing a parachute, you're trained to look at the horizon and depend on peripheral vision to feel the ground coming up to meet you, so you flare at the right time. On a bike, it ties in with target fixation; don't look at the fast thing coming at you, look at the escape routes.

You don’t have to steer anywhere to start a turn, in a sense. You’re always turning and only have to stop countersteering at the right moment.