Advantage over 700c?

You can fit huge tires and therefore run at lower psi. The rivendell beardsley type touring cyclists seem to like them for this reason.

They're also popular for smaller size bikes that cannot
comfortably fit 700c wheels. And with "hipsters" who
want to do bar-spins.

Anyone know why these are so popular in triathlon now?
It seems like even average size riders are using them.

650 for barspins.

i'm pretty sure the wheels you're talking about are 650B, not C. 650C wheels are either for really small frames, or barspins.

Smaller wheels accelerate faster.
Smaller wheels might make more room for crazy tri frame designs/geometry?

^ My mistake Jesse M! wheel sizes confuse me

Edit: just got schooled here: https://www.sheldonbrown.com/650b.html

Tri riders have the saddle almost directly above the BB, this allows makes it more comfortable when they ride aero, and puts less strain on the muscles they use when running. Having the BB further back means less space for a rear wheel. I think it is mostly habit and ease of parts that means people use 700C wheels. Check out the research by Alex Moulton on wheel size, he made a 20" bike that I think still holds the speed record for riding in an upright position.

Smaller wheels are lighter, but you have to spin them faster so I don't think there is any real difference in acceleration/speed, the conclusions from alex moulton is that tyre construction and pressure makes a much bigger difference to speed and there are just more tyre options for 700C bikes.

The latter seems plausible; who knows? The former is less likely. In time trialling, the name of the game is to maintain the maximum average speed, and it seems larger wheels (well, technically, wheels with a greater moment of inertia) would help this since it would take more force to slow them down. This is similar to one reason many downhill bikes weigh so much -- more momentum means they slow down less when they hit things.

you can fit your foot between front wheel and fork for endo tail whips!

https://www.flickr.com/photos/redridi...7600300276374/

barspin wheelies

i run one in the front for the look of it.

sue me.

i have two bikes,both 650c front wheels w/650 fork thats the way i roll!

in addition to the geometry issues, they are more areo-dynamic since they are smaller. triathlons arent governed by the UCI which mandates 700c front and back on all bikes, otherwise TT bikes would have them too.

could you run a 650 up front, with a fork made for 650 to lower the front end for more aggressive geometry?

you could, but it would lower the bb. depending on the geometry, anywhere from 2–4cm sounds right by estimation (~50mm difference in wheel sizes, bb is roughly halfway in the wheelbase)

They were popular, five years ago or so. Now everything is 700C. My tri bike is 650C; it's a '98 model. 56cm. Now that company only makes 650C bikes in the smallest sizes. A large part of the reason for 650C in the first place was that it was difficult to get the head tube short enough with conventional headsets and quill stems, in order to get the low aero position. My bike is rather absurdly low (I think about an 10" drop, haven't measured it), but the new bikes can do the same thing with 700C and integrated headsets. 650C is marginally more aero too as was mentioned, and that helped its popularity; however, they also have a bit higher rolling resistance than 700C and people have really come to understand how important that is in the last few years. And now that everyone rides deep-section wheels (except me, too broke for that stuff) the aero difference is minimal. Still, a lot of the current small sizes of tri bikes really ought to be 650C for fit reasons (which is the only good reason IMO) and aren't.

+1. This is why 650 tri-spokes were so cheap up until track bike fashion blew them up. All the triatheletes were ditching them for 700 stuff.

not quite. UCI does not mandate a specific wheel size. They actually give a range of allowable wheel diameters (55-70cm incluing tires, I think)

650c fits into this range.
The only requirement would be that both wheels be the same size.

^^^ oops. thanks for the correction.

I think I learned something!

It's been like, 8 months! Thank you!