mathematics wrote:
Mudge wrote:
mathematics wrote:
Mudge wrote:
mathematics wrote:
DrAlexHarrison wrote:
What convinced you that 28mm was faster than 23 or 25mm when considering aero & rr? (and weight, but let's not get crazy down in the weeds just yet).
I'm curious because I was still of the mind that 25-27mm measured width might be faster than 28-30mm widths, in most cases. (Due to aero)
I might be behind the times.
This doesn't always hold true because there's a hundred different tyresand wheels, but generally having a tire that is narrower (when inflated, actual width) than the rim will be more aero than the other way around. To my understanding this is mainly due to the trailing edge, and how horribly inefficient a round body is at the back of the airflow.
You could run a 21-23mm on a 25mm rim, but the rolling resistance in most cases would outweigh the aero benefits. On a 27mm rim you could run a 25mm tyre and get lower rr for marginally worse aero. In all likelihood 21mm rims are faster at higher speeds and 27mm rims are faster at lower speeds, but again, very dependent on rim shape and tire selection.
I realize this isn't helpful at all.
It's not a given that a wider tire is faster when you make appropriate adjustments for pressures. There a number of useful charts in this test report, but so you don't have to do the mental gymnastics, there's a chart that already shows what happens when you make said appropriate adjustments to pressure as the tires get wider. Spoiler: the 23mm is fastest.
https://www.bicyclerollingresistance.com/...prix-5000-comparisonIn these charts the minimum RR is the 32mm with 7.8w where the 23 was the slowest at 8.5w. They do a test with a 15% tire drop, but as they even state, that's not a great test as 15% of a 32mm is more than 15% of 23mm. One of the reasons for running wider tires is to reduce this drop and theoretically reduce rolling resistance. Right below that is a test with a 4mm drop that shows a straight line among tires. This is a better test, but keep in mind that a 4mm drop with static weight says nothing about the tire's ability to absorb road imperfections larger (or smaller) than the diamond plate roller. Stands to reason the 23 would be fastest on a wooden track at high psi and the 32 would be fastest on gravel at low psi, but we intuitively knew that already.
Finally, and most importantly, all of these tests were done on a 700x17mm rim. That setup is going to heavily favor a narrower tire with the hysteresis losses on the ballooned out wide tire being comparatively greater than if they were on a wider rim. The cross sectional degrees that the tire encompasses are greater for a wider tire or narrower rim.
The 7.8w minimum RR figure for the 32mm wide tire came at a pressure that no one would/could ever ride IRL, so that figure is meaningless. IOW, an untenable course of action is not a course of action. You can and should disregard that figure. Whether you want to use the simplistic 15% drop, or extrapolate the resistance by taking reasonable real life pressures from the earlier charts in the report, what you'll find is that for all intents and purposes, there's nothing in it. For a given tire model, when adjusting the pressures relative to the width, the rolling resistance differences are minimal. With that in mind, the question then becomes one of which tire/rim combo is aero optimized.
Given that a 28mm nominal width tire will balloon out to 30-32mm or more on a wider internal width rim, to get the magical 1.05 ratio, the outer width of the rim has to be pretty substantial. It's almost a given that such a rim won't work in a rim-brake bike. Seems to me that for most rims, even on disc brake set ups, the optimal nominal tire width would be a 25, as it will balloon out to 27-28mm or so when inflated. You'll get reasonable comfort/traction benefits, essentially identical rolling resistance, and far better aero drag figures than you would with an even wider tire.
Before anyone pipes in with "but the pros are going to 32s", keep in mind that they're only doing that for road racing, not time trials. The considerations change when drafting is introduced and pure aero optimization isn't so important.
But to the original point, it's likely that the entire system efficiency would be better with a wider aero rim than with a skinny aero rim (disc brakes, as stipulated by OP) no matter the tire choice. Unless it's something ridiculous like a 19mm tire on a 30mm rim. Running a skinny tire on a wide rim may not be super comfortable but is still likely to have better aerodynamics than a wider tire on the same rim. It all comes down to the speed dependent relationship, RR is king at low speed, aero is king at high speed. At the 25ish mph most people tend to race at it's a bit of a toss up for which is more important. That makes sense, if there was a clear way to optimize with wider/narrower tires brands and racers would have shifted towards that.
I'm not discounting the data showing wider tires being faster, I'm factoring in the fact that the only way to get them to be faster is to inflate them to a pressure that is unrideable in the real world. IOW, an untenable course of action is not a course of action. In decision making terms, you stop even considering an untenable course of action.
The conclusion of the test report spells it out:
"A bigger road bike tire (at least the Continental GP 4000 and GP 5000) provides a lower rolling resistance at the same air pressure. You do have to realize that at the same air pressure, a bigger tire will provide a less comfortable ride.
At the 15% tire drop air pressures, which are very close to the recommended air pressures for a given size, the tables are turned and a bigger road bike tire will have a higher rolling resistance."
And of course, there's this:
Rolling Resistance at the Same Comfort Level (4.5 mm tire drop)
Rolling Resistance at the Same Comfort Level (4.5 mm tire drop)TireGP 5000GP 5000GP 5000GP 5000Tire size23-62225-62228-62232-622Inner tubeButyl 100 grButyl 100 grButyl 130 grButyl 130 grTire drop4.5 mm4.5 mm4.5 mm4.5 mmAir pressure92 psi
6.3 bar87 psi
6.0 bar81 psi
5.6 bar75 psi
5.2 bar
Rolling resistance11.3 Watts11.5 Watts11.4 Watts11.4 Watts Tell me again who's ignoring data?
When normalized for reasonable real world pressures, the RR differences are quite minimal (well within the margin of error of the test). Worrying about the 0.1 W difference between tires when inflated to optimum pressure is akin to picking the fly shit out of the pepper. It certainly seems reasonable that once you've selected the right tire model, you select the size that optimizes drag reduction on your race wheels while inflated to the optimum pressure.