Are Wider Tyres a total WIN? The Aerodynamics. - YouTube
Take a look at this video. Takeaway is: run the fastest tyre you can get your hands on. GP5k was 19 watts faster than Vittoria Corsa Next. 32 vs 28 for the GP5k was within a couple of watts on a 32.6mm wide wheelset. So wheter you run 25 or 28s does not matter as much as running the fastest tyres.
25mm is Cannondaleās spec for the Knot 64 rim pictured. They think that is the fastest combo. You can argue any other details with them!
My advice to the OP is to uses those 28mm tires and not worry about the extra 2 watts unless heās sprinting at the elite level or riding a TT championship.
Ah, sorry, had missed that context and agree there is no way I would be running 25mm on a rim that wide! I race on 25mm but thatās on rims with 28mm external width. Seems a weird spec decision by Cannondale, maybe they got a great price bulk buying 25mm tyres since nobody else seems to want them these daysā¦
even weirder is 35mm wide front wheel on the Roval Rapide CLX, and its aero optimized at 25c / 26c! I get a lot of comments about that front wheel when running aero optimized tires.
this.
You do wonder when a major manufacturer specs that tire/rim combo on their Superbike and says that is absolutely the fastest setup. Specialized specs a 26mm tire on their 31mm wide Roval. (Edit - wow, the front is 34mm!?!)
I would characterize the difference in rolling resistance between a 25mm & a 28mm tire as āsmallā.
The difference in rolling resistance between a tire with an insert and the same tire without an insert is āmaterialā. Weāre starting to talk about integer numbers of watts. So there is that to consider as well.
35mm on the Roval Rapide CLX
aero optimized for 26c tires.
Yes I get comments when running a 26c tire on the front wheel.
At the risk of going down another rabbit hole, Josh from Silca has mentioned that whether the insert touches the side wall of the tire can make a difference in terms of hysteresis, so an insert like the Airliner that is supposed to compress and not touch the sidewall should have less (or no) impact on rolling resistance versus a large insert that puts pressure on the side wall.
No rabbit hole required, testing data here:
Conclusion:
āOur main conclusion is that using the Vittoria Air-Liner Road does not harm rolling resistance. Rolling resistance is exactly the same at all air pressures, even down to 40 psi / 2.8 bars which is a pressure thatās already way too low for a 25 mm road bike tire.ā
I watched that vid, thanks for the link! I also went back and watched the review of the two wheelsets used in that video. There are three nuances worth mentioningā¦basically because they re-enforce what Iāve already mentioned in this thread. Ha! So let me abandon humility briefly here (errā¦maybe I just never adopted it in the first placeā¦anyhow)
1.) The 35mm wheelset has a much narrower rim profile than the 42mm wheelset. So the measured difference between MOUNTED tire width and the widest part of the rim profile is going to me more pronounced on the 35mm wheel than the 42mm wheel for a given tire width. Maybe he reported measured width of the tires after they were mounted? If so, I missed it.
2.) Iāve often speculated that a deeper rim profile is less susceptible to violations of the Rule of 105. But Iāve never had a series of similar rims at differing depths to test the notion. If anybody wants to collab, do get in touch. But it could be part of what we are seeing hereā¦the deeper wheelset outperformed.
3.) The conditions on that ride were very typical for a cross-windy ride at reasonable pace. Yaw angle was in the 7% to 10% range pretty much all the time. So you can see that when the Rule of 105 is clearly violated in a cross windy situation at reasonable speed there is a 6W differenceā¦so thatās the ICAN 28/32 compare at ~22mph. In the same conditions at the same speed if the Rule of 105 is not quite met the performance hit is only a little over a watt (thatās the P42 28/32 comparison).
I guess the other clear conclusion you can take from that data is that the ICAN 35mm wheelset is aero AF compared to the P42. If the Rule of 105 is close on the ICAN it outperforms a much deeper wheelset that has a distinct rolling resistance advantage. By, like, 4 or 5 wattsā¦so, yikes, back to the drawing board Ascent engineers.
Thatās a good point and absolutely true. The Vittoria insert, as far as I can tell/measure, incurs no rolling resistance penalty. But, also, if you want to protect from rim strikes, provides no benefit OR SO I THINK. Iām just going to be honest and admit Iām not passionate enough about the issue to perform (what could potentially be) destructive testing to prove this point! 
The Air-Liner is NOT designed for this situation. These arenāt gravel inserts, they are road inserts that help if there is rapid air loss, or a slow leak.
They did help ~18 months ago in this situation:
No scuff marks or road rash on the rim and I weigh 90+kg.
Last week on a group ride I hit a deep pothole at 28mph and no damage on my tubeless Rapide CLX II rims. Thats the most destructive ātestā Iāve done to date.
Except nobody is really comparing N.EXTs to 5ks. Theyāre cheap, robust winter training tyres. The 19 watts is just extra training resistance!
Yep. Thatās why I didnāt mention them! We were talking about rim-strike protection, not Vittoria inserts. But somebody brought up that the Vittoria road inserts donāt have a RR penaltyā¦which is true.
Agree 100% for the road Air-Liners, but to make things a little confusing they do have a gravel and XC āAir-Linerā, and now also have an āAir-Liner Lightā for XC. The shapes are all a bit different. I bought a set of the Air-Liner Lights to see how they compare to the Tubolights. I suspect they will feel pretty similar.
it started with āHaving tires narrower than your rims means potentially riding on carbon if you get a flat.ā not rim strikes per se. Iāve had tires wider than rims and had rim road rash on both aluminum and carbon. With carbon you want a good warranty. In either case (wide or narrow tire/rim), for road, the Air-Liner will give some protection in this situation. And the more dangerous sudden loss of pressure while traveling at high speeds. Without impacting rolling resistance.
To bring it back to the Specialized CLX 50 wind tunnel video, the results over 40km / 25 miles:
No wind, time saved on CLX50 vs SLX24:
Crosswind:
So that was a mid aero wheel vs a shallow box wheel. Some level of marginal gains but hey I benchmark myself against myself on some local 1 hour segments. So I got aero wheels with great handling in crosswinds because its usually very windy and gusty when I ride.
On to tires and @NoodleNelson question. Practically speaking, even on a 25mph fast weeknight group ride I would not expect much aero difference between 25c and 28c GP5000 S TR tires, or 26c and 28c/32c Vittoria N.EXT tires.
Example. Our Wed ~25mph loop is about 22km / 14 miles, and plenty of crosswind, so the aero difference between running 26c vs 32c might amount to, best case, 6-8 seconds over 34 minutes? Hard to take those wind tunnel results and apply them to a group.
Given that, where I focus is on reducing downtime from a flat. Because with a flat nearly everyone goes bye bye and you are standing on the side of the road. Which is the point of tubeless, because around here, nearly every Wed group ride has somebody getting a flat. And if youāve ever had close calls on tubes/tubeless at high speeds, like I have, the Air-Liner is an extra measure of safety.
shocking news: narrow rims are more aero. The exact opposite of what the industry leaders are trying to sell. I donāt know about the ascent but isnāt it more of an allroad wheelset? With a depth of 42 and 33 width this would be my choice for a fast allroad wheelset.
I think the broader point is to worry less about whether you are closer to 105 or 100 but to ride the fastest tyre. I think he put it in there to give a bit of perspective.
"Typically what happens with the larger tire is the aerodynamics of the whole wheel remains about the same in the low yaw angle situations (where the wind is coming at -7.5 to +7.5 degrees). This yaw angle spread makes up about 85%+ of the time we are riding. The larger tire will have more frontal area, so the aero drag will be a little higher (although the rolling resistance gets a little lower).
As the yaw angle increases (windy days where the wind is coming from the side), that is where the separation can happen as the larger tire creates flow separation over the rim. So in the -7.5 to +7.5 there may only be 5 to 6 grams of drag, in the high yaw angle (windy) conditions there will be 15 to 20 grams of drag difference.
It will also affect the crosswind handling as well. Itās very noticeable on a windy day to go from a 25mm tire to a 30mm tire in terms of the crosswind stability, however that is mainly with the deeper rims (60mm and deeper). With the 44mm, you are fine to ride even 32mm tires and it wonāt affect the stability that much.
Boyd Johnson"
(this was in response to me asking Boyd about 25c versus 28c tires on one of their 27mm external wheels)
Nobody is disputing itās a relatively marginal gain compared to running mountain bike tyres.
Equally, nobody is forcing you to run 28cās on a narrow rim. You could run 25ās, and not incur a penalty. Except if youāve bought hookless rims and youāre carrying a few extra kilos, then itās dangerous so you have to run 28s.
RR at equivalent comfort level for the same tyre is equivalent. Cornering grip is slightly improved on the larger tyre. Aerodynamics takes a hit.
Just put a 25 on the front.
Or donāt, if you donāt race.
Hereās a related link:
HOW WIDE WHEELS AND WIDE TIRES CAN MAKE YOU FASTER - In The Know Cycling
