On descents you can also pick up a lot of speed by getting a heavier bike and supersizing your meals at McDonalds.
Speed on descents isn't really that relevant - you're descending, so unless you're racing it's practically effort-less. What matters for people biking normally (i.e., not recreational road bike racing) is assistance in the 10-15mph regime, on flat terrain or climbing.
Large changes in rolling resistance matters here, like going from an almost 40W tire to a 20W tire. But aerodynamics have a very minimal effect here, and going below 20W tires won't really make a meaningful difference in biking effort compared to the effect on your butt and wrists.
Going from 20W tire to 10W tires is going to be massive even more so for recreational not so fit riders. Such a rider may output 120-150W on average. Getting 20W free (there are two tires) results in a difference comparable to installing a small motor on your bike.
No: A 20W tire is a 20W tire at 18 mph. At 10 mph, it's roughly a 10W tire (rolling resistance should be mostly linear at these speeds). Going from 20W to 10W tires therefore only saves you 10W total at those speeds.
At 120W, you are idle pedalling. You do not really pedal lighter, so saving a few watts mean an increase in speed, but as both rolling resistance and aerodynamic drag goes up with speed the gain becomes small.
At normal climbs - the time where you'd wish pedalling was easier - you're either 1. spend in the area of 200-300W maintaining the same speed without increasing rolling resistance, making rolling resistance a smaller part of the load, or 2. drop to a granny gear, going very slow to maintain your 120W and thus making the rolling resistance negligible with any tire. In either case, rolling resistance matters much less in climbs.
Yes if you bike slowly, wider tires are better. However I mentioned that in my post. A lot of people who are into biking are in the fitness zone where they are going into speeds where aero starts to matter way more. This is especially true when you consider wind.
> The German magazine TOUR built a sophisticated setup with a motorized dummy rider and found that a 28 mm-wide tire had the same wind resistance as a 25 mm tire when the wind was coming from straight ahead. With a crosswind, the wider tire was very slightly less aerodynamic. Even then, the wider tires required only 5 watt more – on real roads, the reduced suspension losses make up for that.
> To summarize all this research: Narrow tires (<25 mm) are slow. Above 25 mm, the width of your tires are won't change your speed on smooth pavement (at least up to 54 mm wide tires). On rough surfaces, wider tires are faster. That doesn't mean you can just slap any wide tires on your bike and expect it to go fast.
25 and 28 are basically same width. Also wind tunnel testing is not really representative of real world condition given real world turbulence, but I dunno if that would really affect results.
Im just against the general saying that wider tires are faster. This implies that fat bike smooth tires are the fastest, but we know that this is ridiculous. 25 to 35c probably doesn't matter too much aero wise at regular cycling speeds, but going all the way into 40-50c does.
The issue is that narrow tires have a very, very inefficient suspension behavior, and road surface and rider movement causes it to be constantly exercised. That does not mean that you gain anything from going nuts - there are diminishing returns just like there is for slimming the tire for aerodynamics.
What tire is best depends on the surface, the rider and the speed. On a perfectly paved road at high speed and with a light, stable rider, 25 might be right. On regular roads, 35 might be faster. On a bumpy mess, "smooth fat bike" tires would indeed be faster still - assuming you can stay upright without the tread. There's a reason you don't take a road bike with 25mm tires to do downhill mountain racing.
Speed on descents isn't really that relevant - you're descending, so unless you're racing it's practically effort-less. What matters for people biking normally (i.e., not recreational road bike racing) is assistance in the 10-15mph regime, on flat terrain or climbing.
Large changes in rolling resistance matters here, like going from an almost 40W tire to a 20W tire. But aerodynamics have a very minimal effect here, and going below 20W tires won't really make a meaningful difference in biking effort compared to the effect on your butt and wrists.