It feels like the tiny little speck of grass shouldnt bring that much friction. Is there something I’m missing?
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Pavement, and hard surfaces are much smoother than grass.
Grass is also softer than hard surfaces so you sink a bit. When sinking you have to fight against gravity to roll up out of the ever moving depression under the tires. This expends more effort.
Each blade of grass needs to be pushed down and moved, which takes a little bit of energy. A bunch takes more energy, so you lose more momentum the taller the grass. Its like breaking a twig is easy, but breaking a bunch of twigs at the same time is harder.
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.
Simplest answer?
FRICTION.
Pavement is smooth, it has very little ‘drag’ on your bike tires, allowing them to rotate easily.
When you try to pedal through grass, the grass itself, and the soil it’s rooted in create ‘drag’ on your tires, impeding your ability to pedal easily.
Think of it as little ‘bike brakes’. You know how when you engage the brakes on your bike, those little rubber pads ‘grab the tire rims’ and slow/stop you? Well, picture the grass doing the same thing, hundreds of times, but very, very weakly, as the grass flexes when you roll thru it. The soil does a similar thing. But, bottom line, is FRICTION.
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