Do we really need weight reduction on a road unicycle?

Unicycles and Equipment
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Please help me sort this out, I think I may have some confusion here:

Many praise a 36er for its stability, which to my understanding is brought by its great rotational mass. Using a lighter wheel component, e.g. a rim, tube or a tire, while reducing the weight, also effectively reduces its ability to keep the momentum. Is that what we want?

In another aspect, for a smaller wheel, say a 29er that is set up to be ridden on the road (rather than being a muni), is it more desirable to increase its wheel weight rather than decreasing it, to achieve a better stability?

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Rotational inertia is an often underappreciated feature of unicycle wheels.

Whether you want to reduce or increase it, depends on what you are aiming for. Reduce it, if you want to accelerate faster, manoeuvre quicker or climb hills better. Increase it if you want a more stable ride, even out surface irregularities and have more time to react to disturbances.

I’d love to talk and compare values here, but unfortunately it’s not as easily measured as like total unicycle mass or wheel radius.

As for a 29er for road riding, I personally would aim for typical muni weights there, and certainly not typical road bicycle weights.

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I think the stability of a 36" is only partially do to it’s great rotational mass and mostly due to the better “rollover ability” of a large diameter wheel. Large wheels are less sensitive to bumps, because they hit them at a less extreme angle.
Another factor adding stability is moment of inertia (in the turning axis), which is obviously related to rotational mass, but not quite the same, because it’s proportional to r², but only linearly depends on mass. Essentially meaning that increasing diameter is a much better way to increase this moment of inertia than adding mass.
For these reasons, a 29" with the same weight wheel as a 36" will feel the same accelerating (and keeping it’s momentum) forward on a smooth surface, but the 36" will still feel more stable when it comes to turning and handling uneven pavement.

But then there is the likely more interesting question too: “Does one want stability?” Which I think has to be answered on an individual level, but my gut feeling says:

  • Better riders will tend to prefer a less stable/more responsive setup, because they can give exactly the right input. They can use that to do more “agressive” cornering, accelerations and braking.
  • Worse/beginning road riders will prefer a more stable setup, because it forgives a wrong input. They likely aren’t very agressive in their maneuvering anyway, so they don’t get as much benefit from having the unicycle feel more responsive.

(Of course this is to be seen within reasonable limits, it won’t hold for 50 kg wheels…)

I can’t quite spell the reasoning out, but I think there is also an argument to be made that the more rigidly connected the rider is to the unicycle, the less important the mass of the unicycle itself is for stability. The idea would be that if you are well attached to the unicycle, it doesn’t matter how much momentum the wheel has vs. how much momentum you have. But if the connection is floppy, if the wheel gets slowed down the rider shifts around/gets thrown off the unicycle, so having a wheel that is more difficult to stop is a bigger advantage.

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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.

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Yes, I think the r² part was what missing in my formula. Gotta learn some Newton!

Did you mean a 29" with 1.5x wheel weight ((36/29)² = 1.54) has the same feeling of acceleration and keeping of momentum as a 36"?

Good point! I think a safe answer will be, it’s all about finding a balance between stability and responsiveness. As skill improves, responsiveness is becoming more desirable than stability.

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This is a very good and important point. I can max out speed on my 26er at 15 km/h in a commute, but only marginally faster on my 36er, because I’m much less confident on the 36er on an urban road. This may not have something to do with the wheel weight though; the 36er is definitely more stable, but it’s also inherently a more scary machine to ride.

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No, for acceleration forward/backward, it’s really only the mass that matters. For a cylindrical shell, (which I would say is a pretty good approximation for a unicycle wheel) I=mr²
But then: E_kin=(mv^2)/2+(I*w^2)/2 for a rolling wheel we know: w=v/r if we assume no (or negligeable) slip.
So E_kin=(mv^2)/2+(mr^2*(v/r)^2)/2 =mv^2 , the radius cancels out with the assumption of no slip and the weight being centered at the outside of the tire. So the mass remains the only deciding factor for the forward rolling motion. This is where the “wheel weight counts twice” comes from by the way.
If we assume the weight to be at a different place, it’s E_kin=(1+k/r)mv^2/2 with k/r being the ratio between the “radius of giration” and the rolling radius. I would assume k/r to be reasonably constant between wheelsizes, since most of the mass is the rim and tire.
You can also show the radius cancels out for acceleration, but that is slightly more involved and needs some sketches to visualize.

What makes this less intuitive I think is that the ratio between cranklength and wheelsize obviously plays a big role, which is rarely kept constant and you are also rarely finding a 36" wheel that weighs the same as a 29" wheel. And of course this is only in the forward direction, to twist the wheel when turning, the radius has a significant influence.

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The idea that a lighter weight wheel makes sense for faster acceleration and maneuvering but I don’t think that a lighter weight wheel will always improve the hill climbing ability.

When you look at the total weight of the uni and rider, there will be some saving with a lighter wheel but the amount is small. However the flywheel effect of the wheel is needed to keep the wheel rotating between power stokes. At some point a lighter wheel will not have enough mass to store the pedal stroke energy and release that energy past the dead zone to the next pedal stroke. It would be interesting to see some real world testing on how varying the wheel weight helps or hinders the hill climbing ability on different grades.

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Ok. So we can start asking around…
@JimT Do you climb better with your 36" heavy Coker uni or with your 36" less heavy Nimbus? :wink:

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I don’t have a range for grades any steeper then 17% but on my driveway that is up to 17% I’d say my heavier Coker is better. I can get a good speed up before the steepest part and the Coker just seems to power up the hill better.

It would be interesting to find a gradually increasing steeper slope and see what one will go the furthest.

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Touching on the weight part too, I’m intentionally trying to lose 10lbs before Unipacking season here in late April in Canada starts. Doesn’t matter how much money you throw at your uni to shed 5lbs max off a stock uni or so based in my experience

Rider weight will always be more impactful. I threw about $1000 CAD at gear and got my base weight down to 10/11lns or 4.5-5kg. losing the 10lbs/4.5kg is going to be more effective for me losing the body weight and I’m saving money on less marijuana and chips and takeout too.

Given not everyone is in a position to lose weight due to different life circumstances but if it’s and option for you it will be cheaper than wheel weight savings in some cases and let you accelerate faster.

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Wow, thanks for the in-depth explanation!

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It’s always a valid point that reducing the weight of the rider is more effective than of the cycle itself. But I think we are more of talking about the rotational mass here, while the rider does not rotate. I agree it’s nearly pointless to shave off some hundreds of grams from the frame or the saddle (of course, does not apply to hopping), but weight reduction on a rotating wheel will always make some difference.

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This is a very interesting perspective. For a shorter, more gentle slope, when you can have a “approaching run”, your can store more kinetic energy in a heavier wheel, making the climbing effectively easier. But for a long steep slope, like those one in San Francisco, it could be impossible for a 36er to climb, but could be done by a smaller wheel (e.g. 20er). Do you attribute this to its wheel weight or diameter?

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Smaller wheels work better for steep hills because of the the ratio of the crank length to the wheel diameter. The second factor is the rotational mass of the wheel required to get past the dead zone of the wheel.

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This and above points for climbing. It will be one of the factors you put into your builds. My terrain is usually pretty flat and I’ve got multi hole pedals to help with climbing as needed.

There are times I though I’d need the extra holes but a break and just sending hills has been my strategy.

My light weight 29" did climb a lot easier than the 3.0" current build, but I’ve pushed up enough hills that I’ve gotten better climbing techniques and can do most hills with the heavier build.

I value the stability and have adapted to the climbing as needed.

If I lived in the bay area though and hills were everywhere it would change what factors I prioritized for going faster.

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I can add that if you aim for a really light weight uni, than you can achieve again a medium rotational mass and inertia adding a tire insert and running tubeless: you’ll get all the benefits from the tire sealant and all the benefit from the tire insert. Please avoid to comment about the tire insert if you didn’t try it before in a unicycle wheel!!! It is great if could stand the 100-200 gr of rotational mass increase

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I didn’t know the existence of tire inserts, good to learn! I’m not gonna go tubeless tho, as mentioned in the main post, this build will be mainly for road.

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Another factor adding stability is moment of inertia (in the turning axis),
which is obviously related to rotational mass, but not quite the same,
because it’s proportional to r²

Well, sort of. I think that’s only meaningfully true if you hold the angular rotation speed constant, but if we hold speed constant then angular rotation speed is inversely proportional to wheel radius.

I think we’d have to say more precisely what it is we’re trying to compare, and what we are holding constant, in order to get this right, and I haven’t done this type of physics in a while.

I agree with several people who have said that the most important thing is probably how easily a big wheel can roll over obstacles.

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Guys, please help me. I own the lightest 36wheel ever built maybe. I need to know the weight of your inflated wheel (tubed or tubeless+sealant as you prefer) without disk, cranks, spacers. Keep the bearings and the bolt.

I forgot to weight a common wheel for comparison (and I don’t how enought spares to build a second one)

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