So there’s 2 pieces to this, 36" will roll over stuff better due to angle of deflection from the biggers wheel, and more weight in a wheel will make you more stable/bigger tire will absorbs the bumps better
From personal experience, I find I ride faster on heavier wheels. Part of this is that yes a lighter uni will be easier to accelerate/decelerate and you’ll need less energy per spin but I found that you also are more likely to be bumped around on the uni.
I ride road and love doing 40-80km rides and have gone up to 100km on my uni on light gravel, chunky gravel, and road.
I have built or owned the following unis so my experience is based on this:
- 36" with standard rim and heavier tire
- 27.5" fat
- light weight 29" - 9lbs with a 21int diameter rim, and everything was made to be lowest weight without carbon
- 29 x 3.0" tire
As I’ve be Unicycling I find my speed was faster in short sections on the light 29" but I had to slow down due to lower weight and stability compared to my heavier unis as you have said. You can’t ride fast if you don’t have the control/skill/confidence for the level of stability your terrain requires.
I’ve been riding 100mm cranks on my 29" and found that I was riding my 9lbs uni just as fast as a build I did recently this year with the 29" rim on the hatchet and a 3" tire. Though it was a heavier wheel it has a flywheel effect for me and will keep going at its high speed whereas my light 29" would slow down much easier. It had a 2.25" tire though. I find my bigger tire (2.6 or a 3.0") was more stable and something in the middle like a 2.4 or 2.6 would probably be a good middle for my riding. The light uni did climb an accelerate very well though.
On a long ride for me, if you’re able to keep your average speed up by slowing down less for sketchy terrain you’ll be faster in the long run.
If you’re only riding road, light weight and skinny tire will probably b the fastest like a road bike, but people don’t take those on gravel for similar reasons as above.
Also an underrated aspect of speed is also fitness. 2021 my average speed on my 29" was 14kmh, after a winter of riding my elliptical for an hour each day I was able to get up to 16kmh average this season from better fitness and more time in the saddle. You are the engine of your uni so the more powerful you are the better you’ll ride. If you ride a heavier uni you’ll also adapt to the extra weight with time.
Some things king good pedals/shoes/handle will also help with control but there’s plenty of other threads talking about that.
My thinking is that for each rider there’s going to be a nexus between:
- wheel size
- wheel weight
- tire size and pressure
- terrain ridden
- skill and experience
- contact points (pedal/shoe/handle or bumper)
For me a 29" tends to be fastest because most of my riding is urban and I feel safer on it than I did on a 36" because I ride next to cars in painted bike lanes. I also ride in the winter so I need a uni that is prepared for sketchy terrain without constantly changing the tire.
In the future I might go back to my 9lbs build in the future but I don’t think I have the skill to control it right now. But it’s going to be a build for next year I think.