Why is it so much harder to keep up speed on your bike when you’re in the grass than when you’re on dirt or hard soil?

Question

It feels like the tiny little speck of grass shouldnt bring that much friction. Is there something I’m missing?

In: 2

Answer 1

It’s not so much the grass itself, its the fact that grass grows out of soil, which is soft.

Compare the effort of walking on paved road vs walking on a sand beach. Your feet sink in a little on every step, and that sinking-in energy is subtracted from your momentum and added to energy you need to put in to keep going forwards.

Biking on a grass field is the same way. As you go along, the soft soil is deforming some under the tire as you go. Where did the energy to compress the soil come from? Well it has to be from you and the bike, so ultimately that means it’s coming from your legs. It saps your momentum and makes you work harder than biking on a hard surface. Now you’re providing the energy to move forwards, energy to flatten grass, AND energy to compress soil. On a road there’s only moving-forwards to do.