It seems to me like mountain bikers are always looking for more tire while to roadies 34-38 seems like a lot of tire. Perspective is everything.
Anyone racing on a 650b setup? Do you notice a difference in speed at all?
I would imagine this is similar to MTB when they debated 27.5 vs 29 wheels. The 27.5 will have slightly quicker acceleration (better for a lot of cornering) but a 29 will have more āspeedā and offer better stability.
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29ā tires are also better for rolling over/getting over obstacles.
The only difference between mountain bikes and gravel bikes is that many gravel bikes are designed to also work with 700c/28ā/ā29āā road tires, and going for a smaller wheel size for your wide tires means that the bike will handle very similarly to the bike with road wheels.
Not for gravel, but have played with the idea of 27.5+ vs. 29" for MTB. I donāt have a lot of quantified data, but my perception was that on a course where the obstacles were largely ālots of rootsā, riding on the plus tires (2.8-3.0") felt faster than the narrower MTB tires. Certainly smoother ā and I equate smooth with fast. This was just perception, though, I havenāt timed segments in any scientific way. (I know Iāve read results form experiments when 27.5+ was first coming out that showed 27.5+ tires were faster.) OTOH, Iāve since switched to 29" tires for more technical and (big) rocky courses and do feel that the rollover is a bigger factor there. I seem to get hung up less, which may be in my head, but certainly thatās the conventional wisdom of the larger diameter tire+wheel.
Iād extrapolate that to say that if your 650B is a significantly bigger volume and your riding conditions/surfaces will make use of that volume in smoothing out your ride and preventing mitigating fatigue that this is probably the faster tire. This assumes apples-to-apples tire compound comparison. And it ignores differences in aerodynamics and weight.
Itās worth reading this: Myths Debunked: 700C Wheels Are NOT Faster ā Rene Herse Cycles
(But like everything from RH/BQ, IMHO, also worth being very skeptical of their methods and conslusions.)
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I agree with many of your points, especially concerning volume on rough surfaces. Where I found a big issue with 27+ v. 29 was in a couple of places: cornering at speed on firm surfaces, and in tire mass/durability. The wider tires, at lower pressures, would deform in turns to the point of requiring lower speeds to hold lines. The second is the unfortunate trade-off- rubber is heavy! in order to keep tire weights reasonable, thinner casings are used.
In the context of gravel, these are kinda moot. 700c tires have gotten close to (or equal to) wider 650b tires, neither of which are balloony enough to have a big effect on cornering under most gravel conditions.
As far as that Rene Herse article, the methodology is utterly ridiculous. Rumble strip grooves are rarely deeper than 20mm, and are narrow enough that the angle of attack delta between 650b and 700c is negligible. Big, chunky grave/trail would be more a valid, although less consistent, test surface.
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My gravel riding is in Northern Virginia primarily (where you are also based, I believe?). I use one bike for road and gravel, and only had 35 mm tires for a few years and then switched to 40 mm last summer. Before that, I was on a road bike running 32 mm tires for a few years. Iāve not run into many situations where I felt that too-skinny tires were really holding me back.
Friends with wider tires sing their praises, but my slowpoke descending is much more of a liability than running 40 mm tires.
That is particularly true on the hundreds of miles of gravel roads in Loudon County, many of which honestly would be fine with tubeless road tires that have beefy sidewalls. Same with the C&O Canal towpath (at least the portion close to DC).
If my bike could accomodate wider tires, I would try them and might love them. Iām just saying, as long as you arenāt bikepacking (or maybe doing very long rides, where the cushioning would be particularly appreciated), I think 45 mm is plenty wide to allow you to ride most of the roads in NoVa.
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@losifer , great points. Yeah, absolutely re: cornering. I also struggled with durability of the plus tires: either they weigh north of 1kg or they are frequently flatting. In the end, I think Iāve reconciled myself that a 2.4" 29" tire is wide enough to give me the float I need (especially running tire inserts now, so more confident in lowering that pressure). I also felt like the 2.8" tires were this weird compromise where they generally could be run very low-pressure and made the trail feel great (with caveats re: cornering) but taking them off any larger drops would bottom out unless they were pumped up close to what Iād normally run for a 29" tire. Cracking my second carbon rim also pushed me toward a more conventional 29" setup [and alloy rims].
And 100% on the RH article. I think itās always good to question these ideas like rollover practically speaking, but I agree that their testing is always super sus. I use their articles as a way to prompt some reflection and not as any sort of scientific study 
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These are all great replies and points, thanks all!
The reason I asked is because I can easily find a carbon 650b wheelset with an internal width of 25-29 for a solid price. Finding a 700c wheelset 25-29 internal has been tough. Though I keep my eye out on the Hunt renew website. The reason for it is I have a Racemax which having a wider internal wheel is going to help with a 40mm tire not balloon and get too tall and rub the frame. Most of my gravel racing will be on mostly smooth Colorado/Arizona/Utah dirt where I donāt see myself needing much more than a 40 measured.
Though I do have a spinery gx wheelset that is 24 I think for 700 just in case but would like to go wider. I have some 50mm deep hunt wheels but they are narrow (19)
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I live in Prince William County but we have surprisingly little gravel. I definitely need to make it up your way and onto the C&O Canal Towpath.
With you 100% on all points!
Take a look at Scribe Wheels for some well-reviewed and reasonably priced wheels. Iām considering a pair myself.
I have had a similar experience to several on this thread. I have a 2018 Crux Elite which is listed as a max 38 mm wide tire (for the rear due to tolerance due to the dropouts). I echo what other ppl have said, it matters what surface you ride, what your bike handling skills are, and what your weight is, as all of that relates to the pressures you can run. Iām a āClydesdaleā at 200 lbs and 6ā4ā and consider myself more of a roadie. So a 38 mm tire for me is going to be a different experience compared to a 150 lb rider with off road handling skills. That said, in hindsight I wish I could run a 42-45mm tire on the rear as I think it would help me a lot in races where it gets chunky, which for races Iāve done is where the selection occurs for the podium. Life being life, I would prefer to spend my riding time on my road bike for pure time efficiency, knowing the time it would take to get to trails and practice skills just isnāt conducive to where Iām at in life. So in short, yeah for me having a bike that is a bit of a ācheat codeā for the sections where coefficient of friction is really most against me would have been better. But hey, live and learn.
Depending on what rim you are running, you may be able to squeeze a 42 in thereā¦my buddy has the 2016 Crux (basically the same as you) and he has used 42ās, but it was tight and he has relatively narrow internal width rims.
That said, I got 40ās in my 2017 Crux on 23mm internal width rims with no issue.
The real answer is of course 42. I guess if itās not spinning nice and freely between forks, then itās big enough, no?!
But just so subjective and case sensitive, because what I feel plays nice might not feel nice for you. Or for you. And even if your wider setup is faster than my narrower one, or vice versa, we both still might be an hour slower than Swenson or Schurter on that course, no matter did they choose wider or narrower ones.
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My response isnāt Crux-specific, but rather a general cautionary tale. I have a 2018 Scott Addict Gravel. The stock tires were 35s, but I mounted 40s last year and raced them in conditions that I expected to be dry, but that became muddy unexpectedly due to a morning downpour. I ended up wearing down the inside of one chainstay, and the repair was costly and left me without use of the bike for about a month. Now I run 40 in the front and 35 in the back. This is 100% YMMV, just noting that itās a high-consequence problem if a tire that looks like it will clear the chainstay all of a sudden doesnāt because it loses pressure or picks up mud.
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Yupā¦there is always a risk of clearance issues in mud when you push the limits of tire width.