4 ms·

> A wider tire adds more mass to wheels and they are the worst place to add mass to because it works against you every time you have to make that wheel spin fas

> A wider tire adds more mass to wheels and they are the worst place to add mass to because it works against you every time you have to make that wheel spin faster. If I recall correctly, this isn't a big deal because accelerating the rotating mass is assisted by the (equal) mass on the other side of the wheel, so adding weight doesn't really make any difference vs adding that same weight on the frame. Where it can hurt is cornering. GCN did a video that suggested increasing width increases grip and cornering... to a point. Once you exceed that point, you spend more time moving the bike "over the tyre" and have a hard time cornering.

pmontra 3y ago
That's probably the gyroscopic effect. The more massive the wheel is, the more it wants to keep going straight on. So the rim deforms the tire and the rim plus the tire and even the hub and the spokes resist cornering. About the issue of mass on frame vs mass on wheels (most often on the rims) I found this article [1] where an engineer from Lotus states that "In the case of a wheel, because you have a lot of mass distributed around the rim, some distance from the centre of the wheel, that gives it inertia. [...] We can see that reducing the wheel inertia has a beneficial effect over reducing non-rotating mass, but it is very small. In reality, mass saved from the rims of wheels is likely to be less than 10% more beneficial than the same mass saved from the rest of the bike." [1] https://www.cyclist.co.uk/in-depth/do-light-wheels-beat-a-light-frame https://www.cyclist.co.uk/in-depth/do-light-wheels-beat-a-li...
scheme271 3y ago
It is a big deal. Rotational inertia depends on the distribution of weight around the point of rotation (i.e. the wheel hub). The further the weight is from the hub, the more force is needed to accelerate the wheel.
AtlasBarfed 3y ago
The weight-at-the-wheels is apparently the most apparent in the mountain climbs. Lots of pros had specific clauses in their contracts to use certain ultralight rims regardless of the official team kit of the proteam. Anecdotally, I think this is related to the inevitable far lower rpms climbing a hill and that highlights the uneven power output as a rider (even heavily trained pros don't output perfectly distributed power as they turn a crank). At the low RPMs and variant power highlighted combined with the unending pull of gravity, I guess there's lots of micro-accelerations of the wheel rims.
wiredfool 3y ago
For the idealized mass entirely at the rim of a hoop, the effect is exactly 2x. Kinetic energy is 1/2 mv^2, rotational is 1/2 Iw^2, where I = mr^2 and w=v/r. Make the substitutions and it reduces to mv^2 However, while it takes more energy to spin up the wheel, you get that back when slowing down (unless you hit the brakes). It's also a pretty small effect, since the rim masses are only ~10% of a bike's mass, and 1% of the bike+rider mass.
- pmontra 3y ago
  > However, while it takes more energy to spin up the wheel, you get that back when slowing down (unless you hit the brakes). That energy doesn't get back into the legs of the cyclist. So the bicycle with more mass in the rims might coast for a longer time but the cyclist gets tired sooner than the one with lighter rims.
  - wiredfool 3y ago
    You do get that energy back though, because you don’t slow down as much between pedal strokes with the same input power. Or you get a bit more up the next rise, or it burns off against the wind. But either way, it’s energy that is stored and returned, not lost.
    - pmontra 3y ago
      You're right about the smaller slow down between strokes, I didn't think about it. Anecdotally, I have a gravel bike with two different sets of wheels, 28 mm and 42 mm. I use the 28 mm for asphalt only rides, especially uphills, and the 42 mm for mixed terrain flat rides. The 28 mm feels immediately faster, it's like 3 or 4 km/h for free. On the same training circuit the average speed is higher. The only differences between the two sets are: 1) the weight of the tire and the inner tube because the rim and the rotors of the brakes are the same and 2) the 28 mm tire is smoother but the 42 mm is relatively smooth too. BTW, I know that should use a wider rim for the 42 mm but the previous one broke right before one of the announced covid lockdowns and I got what I could find. We could cycle alone during the last lockdown and I didn't want to go asphalt only for a couple of months or so. I'll buy a wider rim next time.

pmontra 3y ago

That's probably the gyroscopic effect. The more massive the wheel is, the more it wants to keep going straight on. So the rim deforms the tire and the rim plus the tire and even the hub and the spokes resist cornering. About the issue of mass on frame vs mass on wheels (most often on the rims) I found this article [1] where an engineer from Lotus states that "In the case of a wheel, because you have a lot of mass distributed around the rim, some distance from the centre of the wheel, that gives it inertia. [...] We can see that reducing the wheel inertia has a beneficial effect over reducing non-rotating mass, but it is very small. In reality, mass saved from the rims of wheels is likely to be less than 10% more beneficial than the same mass saved from the rest of the bike." [1] https://www.cyclist.co.uk/in-depth/do-light-wheels-beat-a-light-frame https://www.cyclist.co.uk/in-depth/do-light-wheels-beat-a-li...

scheme271 3y ago

It is a big deal. Rotational inertia depends on the distribution of weight around the point of rotation (i.e. the wheel hub). The further the weight is from the hub, the more force is needed to accelerate the wheel.

AtlasBarfed 3y ago

The weight-at-the-wheels is apparently the most apparent in the mountain climbs. Lots of pros had specific clauses in their contracts to use certain ultralight rims regardless of the official team kit of the proteam. Anecdotally, I think this is related to the inevitable far lower rpms climbing a hill and that highlights the uneven power output as a rider (even heavily trained pros don't output perfectly distributed power as they turn a crank). At the low RPMs and variant power highlighted combined with the unending pull of gravity, I guess there's lots of micro-accelerations of the wheel rims.

wiredfool 3y ago

For the idealized mass entirely at the rim of a hoop, the effect is exactly 2x. Kinetic energy is 1/2 mv^2, rotational is 1/2 Iw^2, where I = mr^2 and w=v/r. Make the substitutions and it reduces to mv^2 However, while it takes more energy to spin up the wheel, you get that back when slowing down (unless you hit the brakes). It's also a pretty small effect, since the rim masses are only ~10% of a bike's mass, and 1% of the bike+rider mass.

pmontra 3y ago
> However, while it takes more energy to spin up the wheel, you get that back when slowing down (unless you hit the brakes). That energy doesn't get back into the legs of the cyclist. So the bicycle with more mass in the rims might coast for a longer time but the cyclist gets tired sooner than the one with lighter rims.
- wiredfool 3y ago
  You do get that energy back though, because you don’t slow down as much between pedal strokes with the same input power. Or you get a bit more up the next rise, or it burns off against the wind. But either way, it’s energy that is stored and returned, not lost.
  - pmontra 3y ago
    You're right about the smaller slow down between strokes, I didn't think about it. Anecdotally, I have a gravel bike with two different sets of wheels, 28 mm and 42 mm. I use the 28 mm for asphalt only rides, especially uphills, and the 42 mm for mixed terrain flat rides. The 28 mm feels immediately faster, it's like 3 or 4 km/h for free. On the same training circuit the average speed is higher. The only differences between the two sets are: 1) the weight of the tire and the inner tube because the rim and the rotors of the brakes are the same and 2) the 28 mm tire is smoother but the 42 mm is relatively smooth too. BTW, I know that should use a wider rim for the 42 mm but the previous one broke right before one of the announced covid lockdowns and I got what I could find. We could cycle alone during the last lockdown and I didn't want to go asphalt only for a couple of months or so. I'll buy a wider rim next time.