Yes, motorcycles commonly have a smaller rear brake disc compared to the front brake disc. There are a few reasons for this design choice:
Weight Distribution: Motorcycles are typically designed with a forward weight bias, meaning there is more weight on the front wheel. When braking, weight shifts forward, increasing the load on the front wheel and reducing the load on the rear wheel. Since the front brake handles a larger portion of the braking force, it requires a larger braking surface to effectively dissipate heat and provide stopping power.
Braking Dynamics: During braking, the weight transfer to the front of the motorcycle results in increased traction on the front wheel and reduced traction on the rear wheel. As a result, the front brake is more effective in slowing down the motorcycle due to the higher grip available at the front. The smaller rear brake disc helps prevent excessive rear wheel lock-up or skidding by reducing the braking force applied to the rear wheel, which can cause instability or loss of control.
Cost and Complexity: From a manufacturing standpoint, using a smaller rear brake disc can help reduce costs and complexity. Rear brakes generally experience less stress and heat compared to the front brakes, so a smaller disc size is sufficient for the rear braking requirements. This allows manufacturers to optimize costs and weight distribution without compromising safety and performance.
It's important to note that while the rear brake is still essential for overall braking and stability, the front brake provides the majority of the stopping power. Proper braking technique involves utilizing both brakes together, with more emphasis on the front brake. The distribution of braking force between the front and rear brakes is typically controlled through the motorcycle's braking system, such as the master cylinder and brake calipers, to provide balanced braking performance.