So I’ve got several sets of 700c wheels already, and was curious about optimizing my new Aspero for racing. I’ve got no objections to running 700c, but it’s all I have ever experienced, so a little bit of this is perhaps grounded in curiosity related to the unknown.
Conventional wisdom seems to be that 650 wheels really shine in category 3+ gravel, like Unbound. Do they have benefits on courses that are more category 1 and 2?
I don’t think there’s much point until you get to 2.1+ tire size, which makes it questionable on 1/2 gravel since the benefit of larger tires can be outweighed by the aero losses and weight increase.
I’ve been racing with 650b wheels off and on for a long time and with smaller sizes 38/42 I find them worse on every surface, 48-50 they’re ok, 53+ they’re good and have some benefits in feel and handling over a 700c wheel in the same tire size. For smoother gravel I wouldn’t (and don’t) bother, I don’t think the benefits are there.
I have an Aspero, though admittedly I hardly race. I have some aftermarket 650b wheels with 50 mm actual width tires. I have fun exploring with that setup (sometimes take bandit trails and deer paths I find). It’s good for under biking.
I have some carbon aero 700c with road tires as it’s also my road bike.
But this year I’m going to buy some 40-44 mm 700c tires to put on the stock 700c gravel wheels. Why? Tire selection. There’s vastly more choices in 700c. This year I’m doing more racing and most courses won’t benefit from a 50mm tire and I’ll have more selection for low rolling resistance and sufficient traction tires in 700c. 700c at 44mm is my ideal width for comfort and aero and is about max for the Aspero.
I’ve come across 650 vs 700 discussions that tout some of the benefits of 650 being better acceleration in general and a relatively easier effort going up climbs, plus additional comfort. Curious if you’ve seen that with your Aspero.
Am I understanding you correctly that you go as large as 700x44mm on that frame?
I haven’t tried 650B on gravel, but I did try 27.5x2.8 on my mountain bike and suspect some of the same pros/cons will apply.
To get the same diameter (total) you need to go quite a bit larger width on the 650B tire. Somewhere north of 2.3" to match a 700x45, if my math is correct. This generally adds quite a bit of weight (more than you save from the smaller rim and shorter spokes). And has an aero hit as well.
If you’re ok with a smaller total diameter, you mitigate the weight and aero a bit, but not completely.
I suspect for racing, a 700C or 29x2.1 or 2.3 is going to be faster most of the time. And the 650B would only be better for bikepacking or under-biking on technical terrain.
But, that’s just a guess based on my mountain bike experience.
I’ve used 650B extensively in the past, but have switched over to 700C completely. I think a lot of the motivation early on for 650B was a workaround for the lack of tire clearance on gravel bikes from even a few years ago, combined with a lack of selection for good gravel tires.
Neither of these is really a problem anymore, and I find 700x50C gravel tires do just fine on pavement while (for me, at least) feeling better rolling over very chunky Vermont Class IV roads and such. For the kind of riding I do (which may or may not relate to what you ride) because I don’t spend a lot of time on pavement, I’m settling on something between 700x48C for less technical stuff and 700x50 to 700x53C for the chunky, technical stuff.
Argh. This may be where the Aspero design bumps up against some limits. Cervelo claims max tire width of 40mm for 700c (does anyone know differently?). So unless you’re gaining meaningful width with a 650, it sounds like it’s just not worth it.
Does anyone happen to know offhand - what’s the difference in total tire width between 650x47mm and 700x40mm?
Seems to indicate 45mm is tight on the Aspero.
Total width is dependent on rim width and tire design. But roughly 7mm. 
FYI, the Aspero that OP was asking about has flip chips to allow you to make geometry changes to account for different diameters. On mine I just set it to the more forward position and keep it there despite swapping wheel sizes. Honestly I don’t think it’s a big deal for non-suspension bikes but YMMV.
Inner width for both would be 24mm. I’m staying within the DT Swiss GR1600 family, for now.
I have the original Aspero (not the 5) and per my notes:
Frame clearance for 650b is 59 mm, which means can fit 650b x 51 mm tire with 4 mm clearance on each side. Frame clearance for 700c is 53 mm, which means it can fit 700c x 45 mm tire with 4 mm clearance on each side. Note these are actual widths. Don’t go by the labeled tire width.
Tire width is tire width, regardless of diameter. What do change are diameter and volume. A 700x28 tire is a radius of 343mm and volume 1.25L, 700x44 is 359mm and 3.2L, 650x47 is 359mm and 3.4L, 650x50 is 346mm and 3.9L.
There’s no difference in a 47mm (actual measured width) in either diameter. However, a tire labeled as a certain width is based on an assumed rim width. If you use wider rooms than whatever they assumed, then the tire will actually be wider. Also, tires stretch a bit over time which varies by construction. Then there’s also the fact that the tires aren’t always the width the manufacturer intended them to be. Long story short, check your actual tires on your actual rims and check again after a few hundred miles. None of those factors are dictated by rim diameter.
The other thing is how much clearance do you need around the tire? ISO spec says there should be 6mm on all sides (left, right, top) to the frame to account for rocks and mud. A rock can jam in there and give you a bad day. Mud can build on your tires and act like a belt sander, cutting through your frame (even aluminum!).
I’m comfortable with 4mm on each side. Some people go lower. ISO declares 6 to be safe. Take your pick.
It is, but there is definitely clearance. I used 45’s in '22 on my Aspero for Big Sugar.
Thinking about this question specifically. I think that if you went with ideal 650b vs. ideal 700c on the same bike - 650b can have some benefits/marginal gains IME: