Agree, that is exactly what I tought.
I currently have LightBicycle AR45 rims, that are 30mm wide and 21mm internally. I installed a Michelin Pro 5 TLR (32mm nominal) expecting it to be around 32mm since 21mm inner width is kind of narrow by modern standards and that BicycleRollingResistance stated that the tires were true to size. They ended up measuring 34 mm.
I have now a QuickPro Er One and that tire measuring 34 mm in my opinion is very much on the limit. I measured 4 mm of clearance with the chainstays, so I put that in the front (plenty of room for a 35mm tire) and I put a 31 mm (WAM) tire in the back (Hutchinson Blackbird 30mm). At the rear, on the Er One, I would not go past 32 mm anyway. I prefer some extra clearance for any debris that might be picked up from the road.
Also, given the 30.6 mm hook-to-hook distance for the Vaporlite II rims, running a 30 to 32mm WAM tire is the only way to avoid creating a double bulb shape: one for the tire with respect to the hook-to-hook distance, the other one with the hook-to-hook distance and the widest point of the rim. That, I suppose, is not ideal in terms of aerodynamics.
Yes, I agree. I also have a Quick Pro ER One, so I know exactly what you mean regarding rear clearance.
I’ve actually tried a 34 mm tyre on a wheelset with a 26 mm internal width, and it measured around 36 mm WAM. It does fit the ER One, even with my 2x setup, but it’s definitely right on the limit. I put tape on the inside of the chainstays and did a ride to check for any contact, and there was no evidence of rubbing. Even so, personally I wouldn’t choose to run that size regularly. I prefer having some additional clearance for debris, wheel flex, etc.
For me, around 32–34 mm WAM is probably the sensible maximum at the rear of the ER One. 36 mm is getting extreme anyway — it’s still a road bike at the end of the day! 😆
Regarding the VaporLite II, I agree that a 30–32 mm WAM tyre makes the most sense. I have a set myself, although I haven’t ridden them yet because my new SL9 build is taking forever to get finished. From the tyres I’ve fitted, 30 and 32 mm nominal sizes seem to work very well with the rim profile.
I do look at tyre-to-rim transition, particularly on the front wheel, but personally I think people sometimes worry far too much about tiny aerodynamic differences. On the rear wheel especially, I doubt a very small difference in tyre/rim transition is going to make any meaningful difference for the vast majority of us. At WorldTour level, where they’re chasing every fraction of a watt, absolutely. For normal riders, even those racing, I think tyre choice, pressure, rolling resistance and having the clearance you’re comfortable with are far more important.
Depending on the wheelset, I generally run combinations such as 30/30, 30/32 or 32/32. I’m quite happy anywhere around that 30–34 mm actual-width range.
