3 ms·

Pro teams are going 28 mm or even 30 mm at the front and 32 mm at the back. Some measurements from Tour de France 2023 at [1] One of the reasons why larger tir

Pro teams are going 28 mm or even 30 mm at the front and 32 mm at the back. Some measurements from Tour de France 2023 at [1] One of the reasons why larger tires are better is that the size of the contact patch to the road depends only on the internal pressure of the tire. A 20 mm tire and a 40 mm tire have the same contact patch but their shape is different. The one of narrower tire is aligned back to front so the wheel is less round. The contact patch of the wider tire is aligned right to left, so the wheel is more round and it rolls more easily without dissipating energy in elastic deformations. You'd have to inflate the narrower tire to double pressure to compensate, but then the ride is less comfortable and you can double the pressure only up to a point. That was about rolling resistance, then there is aero. Wind tunnel tests discovered that tires should connect smoothly to the profile of the rim without making the classic shape of everyday bike's wheel: a wide tire in narrow rim. They must have the same width instead and a fatter rim is more aerodynamic than a skinny one. Hence the drive to wider tires. However there is catch. 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. You add mass to the frame and all the static parts of the bicycle if you really have to. And finally a very large tire (100 mm or 4 inches) is obviously too wide to have a reasonable front surface at high speeds. The sweet spot nowadays seems to be around 30 mm. Note that UCI mandates cyclocross tires to be 33 mm max, and they must work in muddy conditions. Road bikes are at almost those widths. [1] https://escapecollective.com/what-we-learned-by-measuring-tyre-widths-at-the-tour-de-france/ https://escapecollective.com/what-we-learned-by-measuring-ty...

scott_w 3y ago
> 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.

scott_w 3y ago

> 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.