If you only use one brake while driving or riding a bike equipped with two brakes attached to both wheels equally, several things can happen depending on which brake you choose to engage:
Front Brake Only: If you only use the front brake, the weight of the bike will shift forward, increasing the braking force on the front wheel. This can lead to rapid deceleration, potentially causing the rear wheel to lift off the ground and destabilizing the bike. It may result in a loss of control and potentially cause the bike to flip over (commonly known as an "endo" or end-over-end accident). It is important to note that modern motorcycles and bicycles are designed to handle the braking force applied to the front wheel, but sudden or excessive front brake application can still be dangerous.
Rear Brake Only: If you only use the rear brake, the bike's weight distribution remains relatively unchanged. The braking force will be applied to the rear wheel, causing it to decelerate. While this approach may slow down the bike, it can result in longer stopping distances and potentially lead to rear-wheel skidding or locking. Skidding can cause a loss of control, particularly in wet or slippery conditions. Additionally, relying solely on the rear brake may not provide sufficient braking power for quick stops or emergency situations.
In summary, it is generally recommended to utilize both brakes in a coordinated manner to ensure optimal braking performance, stability, and control. This allows for efficient weight transfer and balanced deceleration. By using both brakes, you can maximize the braking power, reduce stopping distances, and maintain stability during braking maneuvers.