The first is knowledge of mechanics.
Our bicycle tyres, handlebars and pedals all have some patterns of varying depths, which increase the roughness of the contact surface and friction. So that we can grasp more firmly, grasp more ground and drive more steadily.
Some lubricating oil is often needed on the upper axle to reduce the roughness of the contact surface, friction and the consumption in the transformation of forces so as to achieve labor-saving results.
When the bicycle is pushed forward tightly, the friction force on the rear wheel is forward, which is the driving force of the bicycle. The friction force on the front wheel is backward, which is the resistance of the bicycle. When the bicycle is moving forward by inertia, the friction force on the front and rear wheels is backward, which is the resistance of the bicycle.