Going around corners
Mon, July 15, 2019 
Roll a wheel or for that matter any round flat object on a flat surface and it will roll in a circle. Even something as small as a coin. It will continue rolling in ever decreasing circles until it finally falls and settles in one spot. This is a demonstration of gyroscopic action, and the way it works.
That is, a spinning wheel will remain upright as long as it keeps spinning. When it loses momentum and starts to fall it will turn in the direction it is falling, which is why it rolls in a circle.
This law of physics gives a bicycle a simple built-in self-steering capability. You can demonstrate this to yourself by holding a wheel in both hands by the spindle and spinning it. The first thing you will notice is that the wheel wants to stay upright in the same plane, demonstrating the first law mentioned in the paragraph above.
If you forcibly move the top of the wheel to the left or right as it is spinning it will also turn in the direction you are leaning it. Just as a rolling coin will turn in the direction it is falling. As you lean a bicycle into a corner it will steer itself around the corner.
Let’s not forget the rear wheel. Although it is in a fixed position and cannot turn within the frame, it is still spinning and leaning therefore assisting in steering the bike as a whole around the corner.
Because the steering tube on a road bike is angled forward, usually at an angle of 73 degrees, when the steering is turned, the fork blade that is on the inside of the turn drops and the other side raises. Therefore, the front and rear hubs are not in the same plane. (See top picture.)
If the head angle of a bicycle was vertical (90 degrees.) when you turned the handlebars to round a corner, the front and rear hubs would remain in the same plane.
Going through a turn the front wheel is leaning slightly more than the rear wheel. This adds to the stability of the bike because the front wheel is outside the centerline of the frame.
Because the front wheel is leaning slightly more than the rear wheel, it is turning at a slightly tighter turning radius, creating over steer. This is a good thing, centrifugal forces are pushing the bike wide on the corner, over steer is counteracting this.
Again, the law of physics states that a moving object will travel in a straight line until an opposing force causes it to change direction. These centrifugal we speak of are nothing more than momentum causing the bike and rider to continue straight while attempting to turn left or right.
We lean into the corner; the wheels steer us in the direction we need to go, and gravity counterbalances the forces that want us to keep us going straight.
At slow speeds this is an instinctive move, higher speeds require more skill. Lean too little and you will go wide and off the road on the outside. Lean too far and the bike will slide out from under you, and you will slide across the road in the direction momentum wanted to take you in the first place.
The design of the bike, in particular the frame will give the bike these desired steering qualities. Head angles, fork rake and wheelbase, even the weight distribution of the rider, all play a role. After that it is the skill of the rider. Done right it is a joy to execute, and a joy to watch others do properly.
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Reader Comments (5)
Very good & concise explanation. Thank you!
What do you think of the sudden fascination with scooters, which used to be considered a child's toy, now a legit, and preferred over bikes, transportation mode?
(Compare the irony of bicycles being seen as toys to scooters not viewed that way.)
Is a scooter's steering as safe as a bicycle?
Does leaning help or hinder steering and the stability of a scooter, with its geometry?
Do the companies that unloaded these onto the public know the physics of riding scooters? I know the people don't, they just ride them, which is the same with bicycles.
You don't need to know the physics behind riding a bike, because science only helps explain the act of riding. And riding well has nothing to do with knowing the science of riding a bike.
Hi Dave!
As far as i know, the front wheel is having a greater distance to the center of the circle that the bike is doing while in a turn, thus a bigger radius than the rear wheel.
A good example would be to do a very very tight turn while the tires can leave a mark on the ground. The rear tire will mark a smaller circle, while the front tire will mark a bigger circle.
Thank you for your posts, always a pleasure to read.
Best regards,
Mircea (from Bucharest, Romania)
In a word, counter-steer.
Hi Dave!
If you turn the bars (steering), keep it there, and pedal the bike, then the bike will make a full 360 degrees circle. This whether you lean the bike or not.
There will be two marks on the ground, two circles, one made by the front wheel, and one made by the rear wheel.
The front wheel will mark a bigger circle, which means the front wheel turns at a larger radius (nor tighter).
On another note:
- the virtual extension of the wheel axis that intersect the ground plane (point-A);
- the contact point of the wheel with the ground plane (point-B);
- the distance between point-A and point-B is smaller, tighter, for the front wheel, since the front wheel has more leaning during a turn than the rear wheel.
Best regards,
Mircea