A few years ago in Germany (where else...) some major bike mags decided to get to the bottom of rolling resistance once and for all. They kitted out several bikes with more wires than a spiderweb and went for it. I'm not going to translate all those articles (it's not my mother tongue for starters) and computer read-outs, but here's the gist:
- it's not because it's got big, fat knobs that it rolls slower. Actually, some pretty agressive treads outperformed all semislicks on the market. Construction of the carcass is crucial to rolling resistance."
Not quite convinced on this point, shaving all the knobs except the outer row on the old Nokian Gazzaloddi 3.0 tires made them noticeably faster. I agree that supple sidewalls make tires faster on soft and uneven surfaces.
"- adding a tube increases rolling resistance x-fold. Friction between the tube and tire is responsible for that. Tubeless is the way to go."
So far all the attempts I've heard about to make a winter tubeless setup for the big tires and rims at low air pressures in the cold have failed. If somebody could figure that one out it would be a big breakthrough.
"- the lower the tire pressure/wider the tire, the less rolling resistance on uneven terrain. Two reasons for that.
First: a skinny, hard inflated tire presents a long, narrow footprint. A fat, underinflated tire presents a short, wide footprint. The latter has the least rolling resistance (don't ask me why, it's physics)."
I think it has to do with how far the tire is sinking into the snow. Short and wide could be compared to a rolling pin on dough, long and skinny is more like a pizza cutter.
"Second: every pebble wants to stop a tire. A hard tire bounces over it, resulting in an upward movement and deceleration. A soft tire deforms and rolls over it without vertical movement, which results in less rolling resistance".
"- a heavier wheel takes more watt to accelerate, but once going pays dividends when the going gets rough. I forgot and am too lame to go look for the actual figures but I think it took like 5 extra watts to accelerate a heavy wheel vs a light one to 15 mph.
But it took 20 watts extra per 200 yards to maintain that light wheel at that speed over rough stuff vs the heavier one."
"That pretty much sums up what we've all felt: a soft, fat tire is a drag on pavement but seems to accelerate when the going gets rough.
Vs a skinny tire at mega psi: great when smooth, but hits a wall when hitting rough spots.
And explains why our very fat tired bikes are faster on rough terrain than they appear at first sight.
Another reason why fat bikes are so good on rough stuff is the rotational force. If you want to experience it, grab a spinning wheel by both hub ends and try to turn it. It's difficult and it's a main reason why we stay upright on a two-wheeled contraption.
A heavy wheel generates more force than a light one and very much wants to keep in the same plane when spinning. Slippery roots or rock gardens don't impress heavy wheels as much as lighter ones.
So don't follow skinny tired folk on their terrain... try to lure them onto yours