Tech FAQ: 650B or not to be?
I saw an article a while ago that said back in the day the MB builders would have been wiser to use 650 rims instead of 26″. Probably true, but it impressed me as being kind of irrelevant at this point. Anyhow, I saw an advert recently for a new bike with 650 rims – something along the lines of “rolls like a 29er without the extra weight”. As they say, you can’t get something for nothing, and if there ain’t no extra weight, to me that meant there ain’t no extra rim. So I checked on the Sheldon Brown website for wheel sizing and here’s what I found in terms of ISO sizing.
29er (700C) – 622 mm
650C – 571 mm
26-inch MTB – 559 mm
So the 650 is only 1/2-inch bigger than a 26-inch MTB rim, and it’s a full two inches smaller than a 29er. About the only thing it has going for it in my mind is that if you cared, you could put a slightly larger wheel in most any new 26-inch disc brake frame. But it’s only a ¼-inch difference in radius – a silly 2 percent difference. Is this really as silly as it seems on the surface? Am I missing something?
You are indeed missing something. You’ve made one of the classic mistakes about tire sizing, namely not realizing that many sizes that have the same, or almost the same, designation are actually completely different sizes.
The “650” size that was a popular mountain bike tire size in the early 1980s (remember the Finnish Hakkapeliitta tires?) and that Kirk Pacenti recently worked so tirelessly (pun intended) to resurrect is actually 650B. The 650C size you got off of sheldonbrown.com is a road size and is the “26-inch-wheel” size that is (or was) so popular in triathlon. The ISO (or ETRTO) bead seat diameter (BSD) for 650B is 584mm, not 571mm. While this is only a 6.5mm increase in radius, it is closer to in-between in size, as the BSD is 1.5 inches smaller than 700C (or 29er) rims and one inch bigger than the 26-inch mountain bike size.
One application I see for 650B tires is for smaller riders who would otherwise have trouble with toe overlap of the front tire with a 29er and for whom the extra weight of the 29er would be a much bigger percentage of their body weight and hence take a bigger percentage of their power output to overcome. These riders can get some of the rolling advantages of a 29er without the weight or toe overlap problems of a 29er. And, as you say, you can jam a 650B disc-brake wheel into some bikes meant for 26-inch tires (be careful of doing this on the front; you must ensure, either with internal spacers or by some other method, that the front tire won’t hit the suspension fork’s crown on compression. Deflate the fork to check for this before you ride it).
As for the history, 650B was a common road tandem wheel size in Europe, but its original off-road popularity was dependent upon the Hakkapeliitta tires, which Gary Fisher imported from Finland at the urging of Tom Ritchey and the late John Finley Scott (arguably the inventor of the mountain bike, apparently murdered by his handyman in 2006). The reason wasn’t necessarily the larger tire diameter. “Everyone was looking for more knobs back then, and these looked great in the catalogue,” recalls Fisher, so he ordered a large shipment.
Indeed, the tread was deep and the knob shapes were more refined than the plain, rectangular blocks on 26-inch knobbies of the day.
Fisher only placed the one order, as Hakkapeliittas proved susceptible to sidewall cuts.
Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.
Follow Lennard on Twitter.