25mm vs. 28mm....for speed
 

Hey all. This topic has probably been hammered to near lifelessness, but I'd like some perspective as it relates to my intended application. I'm building a TT bike and will be looking at new wheels. I typically ride 28's on my road bikes. But for going as fast as I can in a straight line, will 25's be superior? If so, why?

Also, can I get recommendations for a fast tubeless tire?

Also, can I borrow $25 for a McRib sandwich to sustain my training? Thanks!

It’s probably a wash, in terms of what you’d actually see in the numbers, although theoretically the narrower tire should save a bit in terms of aerodynamic drag. Probably that theoretical difference is easily eaten up between factors like tread pattern difference, rim size/shape, and frame design elements.

If maxing out potential gains is what you’re after, go 25mm width rubber in this case.

As for which tire specifically, have a gander at BicycleRollingResistance and grab any of the top tires:

https://www.bicyclerollingresistance...d-bike-reviews

I’m a fan of the Michelin Power Cup R TLR tires, but more for overall road performance than just straightline speed. The Vittoria Corsa Speed TLR is the champ (by the numbers), though.

Thanks, Chaad. The Corsa is a 28 only, correct? Great chart. I see several 26mm on there. That seems like an odd size. A few 23's. Do you recall those super narrow 18mm tires? It seems like they be best, being so narrow.

There is a bunch of info on the web about tire width, aero, rolling resistance, and on... We all have our opinions. Some colored by 'science', some just our own prejudices or impressions.
If I was doing what you're doing, I would go to the manufacturers for the wheels you are considering purchasing. If they have some strong science and experience in performance wheels (like ZIPP or HED), aero and the use intent; give them info about you and the use intent. They prolly have tire preference/optimum for the wheels you are considering and will suggest and reinforce their suggestion.
If you're gonna buy some wheels from some source which doesn't have that experience, then don't expect much beyond what you already know.
You gotta start somewhere, and tire selection can always change...
Ride On
Yuri

Tires that have a width of 28mm can be faster on real roads with imperfect surfaces than narrower ones.

The rolling resistance isn't significantly different, and the additional cushion and stability reduce fatigue.

Well, you have to consider all elements of performance, not just tire width. Even if the wheelset was optimized for aero with 18mm rubber, any gains there could—and certainly would— be overwhelmed by losses in rolling resistance, rider fatigue, bump compliance and grip.

If the riding surface were perfectly smooth (like an indoor velodrome) then those other elements don’t factor in as much, but on real roads, and at real tire pressures, it’s well established that wider tires greatly reduce rolling resistance by shortening the contact patch. Even on a smooth track, depending on rider weight, there are physical limitations to how high the tire pressures can go to get the smallest, fastest, contact patch, so 19mm and 23mm rule there.

Too many variables to say whether one width is faster than the other. I will say this though, 2 seasons ago I ordered some new tires. My road bike doesn't have a lot of room for wider tires, so I ordered the same 25's. The actual measurement when mounted was 28. The new 28's are faster than the old 25's, but they are a different brand. I do run them at a little lower pressure, not too great a difference tho. Seems like pro racers are running wider tires and getting away from the skinny, higher pressure tires of the past, if that's any indication.

Too many variables, indeed. Lots of videos from GCN about this so you might want to check on YT.

Part of this answer will be defined by your wheels, how wide are they? Flo cycling did some testing on this using the rule of 105 (rim should be 105% of tire), they found that with their wheels 25c was aerodynamically faster (roughly 104% vs 97%) going to 28c had lower rolling resistance. The combined difference had 28c barely edge things out, with a difference of .09w on the least aero rim to .43w on their most aero rim. That would suggest that wider still would have had enough aero disadvantage to not be worth it though I wonder if it also due to the tire not being a less optimal shape when going wider which could effect the amount of reduced rolling resistance. Course their test might be old enough 30c wasn't popular yet. The result is that I'd very much base it on your wheels. If you have an older, narrower rim and you'd drop somewhere below that 95% rim width, I'd be disinclined; if you have newer, wider rims I'd go wider if the frame can handle it.

Why not conduct your own experiment and share your results?

As others have alluded to, the rim and tire width pairing is more important than the absolute width of the tire. First, consider the wheels that you may own, since that would dictate things. If you have flexibility there or have yet to make a purchase in wheels, consider the courses that you will time trial on and select something that will pair appropriately, for example go wide if the courses warrant 28c. Just make sure to pay attention to outer rim width and tire width and as always, make your own measurements since different combinations have different real world measurements.

Lastly, there is a wealth of information in Slowtwitch and some of the British time trialing forums about this stuff, highly recommend. Our own "33' racing subforum used to have a pretty trafficked TT thread, but things have been a bit slow. Good luck

Lastly, as CAT7RDR mentioned, if you have flexibility in equipment, empirical testing works well and you'll find that each athlete has their own unique optimization. The Chung method is fairy easy to execute with the free version of Golden Cheetah and the aero lab extension. The method isn't too hard to learn.

There are (at least) three things to consider:

1. Which tire matches your rims best?
2. How fast do you expect to ride?
3. How rough/smooth do you expect the road surface to be?

I would think you'd want to use a 28 on the rear. I suspect (but don't know) that if the road is smooth, you're fast, and you have the right front rim, a 25 might be marginally faster than a 28. But because having to keep two different sizes of tire is kind of a pain, and sometimes the road isn't smooth, I might run 28 on the front.

^
This sort of matches my experience. My TTs are mostly on smooth courses and the random Flo deep front that I have matches very nicely with a 25c Corsa Speed. I'm a 27-28 MPH ITT kinda guy and 29-30 MPH TTT guy. I have a Zipp Super 9 in the rear with a 28c that bulges a little, but I keep it for comfort and don't mind the aero matching as much since it's not the leading edge (front).

I'm a smaller dude at 65 kg and although my w/kg is high, my absolute watts and early stage optimization put me a notch or two below the supersonic TT guys. Still good enough to get me 3rd overall in the Valley of the Sun Stage Race last year in the 35+! 10 mi TT day one.

Unless you ride on perfectly smooth tarmac I would always go for the larger tire especially on a TT bike where weight is not the main concern but more so aero and rolling resistance. My debate would be to use dedicated TT tires vs regular race ones

I heard somewhere that some folks were using 25 front and 28 rear for time trials, on wheels with rim widths that were optimal for each (see post above about the rule of 105. That gives you narrower where it is cutting through the wind and more compliant in the back where the aerodynamics are less crucial. I can tell you that the one bike I own that can fit 28s is my fastest bike, but it would be ridiculous to claim the wider tires are THE reason why.

Ben O'Connor talks about using 25F/28R on his TT bike at 5:25...

See?

Added that fourth thing. A 25c tubular (that measured out exactly inflated to 110 psi brand new) came to a stop in one revolution; bike on the stand. My Pro Miyata with a very short Cyclone sidepull brake. Tire rubbed the bottom of the calipers (but cleared the brake bridge).

I am a fan of running the same or larger tires in front. 1) road adhesion issues and resulting crashes are nearly always "prettier" when it is the rear wheel that slides out. 2) the racing adage I heard many times in my racing days - you want your fastest tire in back. On that Pro Miyata, I run 25c front, 23c rear. Love it. (And pay more attention to parallel cracks in the pavement than I do on my other bikes.)

Edit: I don't time trial. Yes, that bigger in front is aero slower. And again, those words of those club vets when I started racing - crashes are slow. Both in that race/ride and they too often impact entire seasons. I chose tires with that in mind. (And love that Vittoria has brought back that sweet ribbed tread I took so for granted that I never thought about it until it disappeared around 35 years ago.)

25 or 28 mm McRib sandwich ?

I think you're the only person who got that. You made my day! ;)

Quick side question...anyone know if the Mavic SSC Carbon Cosmics fit 28 mm tires (or "tyres".) The wheelset is a few years old.

Thanks!

Dude, McRib is ribbed; the cross-sectional width varies between 25 mm and 28 mm, like the latest aero wheels.

For sure you can put 28s on them, but the question is whether or not that’s a good choice for aerodynamic benefits, and the answer is probably to almost certainly, no.

I’m not a Mavic expert, but do know they were very late to the wide game. Those wheels were, at some point, 13.5mm internal, 19mm wide external. Dunno if that changed at some point and yours could be wider, so you should probably measure.

But going with what we know, No: 28s would be terrible for aero on those. Even 23s would be lousy from that perspective, creating a step-down at the sidewall/rim interface, something that’s quite the spoiler to laminar airflow.

If those ate what you’ve got, I’d abandon concerns about aero effects, and just put the best and widest tire the frame will accept in hopes of minimizing rolling resistance.

According to this road.cc review the Mavic SSC Cosmic Carbon has an internal width of 19 mm.

Review: Mavic Cosmic Carbon 40 wheels | road.cc

1. While 28 mm tires will fit, the setup would not be aero.
2. My GP5K nominally 25 mm tires actually measure 27 mm wide when mounted on my CF wheels having an internal width of 19 mm and sit flush with the 27 mm external width of those wheels. Better, but still not optimal aero.
3. Optimal aero supposedly has the wheel slightly wider than the tire, i.e., 105% rule. As the review says, good results were obtained with 23 mm tires.

Been awhile but I believe they came up with 32mm being the fastest

My SL8 came with the Roval Rapide CLII wheels and the front one has a 35mm external width (wide!) while the rear one has a 30mm external width. The wheels are said to be optimized for 26mm tires and it's said to be backed up by wind tunnel, software simulation and real life testing. This said, my rims are larger than my tires, and this, even if I swapped the 26mm tires for 28mm tires this year.