Why is it difficult to cycle on sand

When we ride a bicycle on a hard road, only a very small amount of the energy is absorbed due to friction between the tyres and the road along the narrow line of contact. The road being hard, does not deform and so does not absorb the energy of the moving cycle.

But, a mass of sand consists of large amount of sand grains, which easily get displaced on application of even a small amount of force.

This displacement causes local distortion or deformation of the sand mass and absorption of energy. We observe that when the tyre is not fully inflated, the contact area with the road is larger and more energy is lost due to friction.

In this case also it is relatively more difficult to ride the bicycle as compared to the case of fully inflated tyres.

So, when we ride a bicycle on sand, the mass of sand gets immediately deformed. As the tyres move, the deformation continues absorbing most of the energy we supply by pedalling.

Further, the sand causes more friction than what a hard road does. Thus, we need to supply much more energy for moving the cycle on sand and so it is difficult.