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.
> 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.
> 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.
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.
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.
So, I believe that the only difference between the wheels is the tires, but I'd say that there's probably more difference than the weight, and that would be rolling resistance.
I've got 3 bikes I ride -- a tandem w. 26"xwide supple road tires, a 12kg gravel bike running supple 650x48 (now, but have done 650x42 and 700x35, and the stock tires), and a 8kg carbon road bike running fair 700cx25.
I'm sold on big tires, because narrow/high pressure is too tiring.
Some anecdotal evidence:
The gravel bike shipped with the most horrible tires I've ever used, 38mm, 800g (each), felt like riding through deep sand. I used them for maybe 10 miles total till the good tires were delivered. It wasn't the weight, it was the super stiff sidewalls that just _sucked_ the energy out. I haven't noticed a major difference between the other three different tire sizes I've run on it, but they've all been running the same casing, Rene Herse ExtraLights (but the point of the light is that it's a super supple casing, so low rolling resistance. The 650x48mms are about a 400g tire).
Last summer, the gravel bike was out of commission for a week, so I did my hill workout (6x5 minute reps, + there and back) on it. Went back to the gravel bike the next week. ET difference between the two rides -- 10 seconds over 1.5 hours. Much less than the usual week to week variation on the same bike.
The tandem used to be on Schwable city 40mm tires, which are tough and durable. Went to RH 44mm in front and 53 in back, and average speeds over the next 6 rides went up by 1.5 mph (from ~16 to 17.5).
In short, I don't think weight is a predictor, but tire quality and rolling resistance is. High pressure leads to suspension losses as your body bounces and absorbs energy, stiff tires lead to hysterisis losses when rolling, low pressure, supple is both comfortable and low resistance.
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."
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.
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.
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-ty...