650B conversion

From time to time I’m asked about off the shelf options for 650B bikes.  A lot of people have heard about the advantages that larger section tires can offer, but aren’t ready to take the plunge and invest in a custom bike.  A few good options are on the market now from Velo-Orange, Soma, Boulder, and La Confrérie des 650 for example.  I’ve assembled a few and their owners are very pleased.

There’s also another option: conversions.  Many bikes that were built for 700C or 27inch wheels can be made to work with 650B.  Often this means a bike that was no longer being ridden, perhaps even on the way to a landfill, will see many more miles of use.  Of course not every bike is a good candidate for conversion, a poorly done conversion looks cobbled together and may not work very well.  Sometimes though, it’s just a question of making a few frame modifications so that the end result is a bike that has as few compromises as possible.

A couple years ago a fellow cycling club member brought me an old Trek he had recently purchased.  I don’t recall how much he paid, certainly less than two hundred dollars.  He also brought a box full of components that had been removed from an early 90s vintage mountain bike.  The Trek frame was in good shape, and its dimensions were correct in terms of fitting the rider.  I installed a pair of 650B wheels with 42mm tires.  The distance from the top of the tire to the underside of the fork crown looked good.  The tire cleared the fork side to side as well, not by as much as it would on one of my custom forks, but adequate.  The bottom bracket height was a few millimeters lower than stock, but again, acceptable. Things weren’t so easy when it came to the rear wheel.  Even with the wheel pulled all the way back in the dropouts, the tire still lightly touched both chainstays yet it was much too far from the chainstay bridge for correct fender placement.  A poor conversion could have been done simply by using smaller tires, but we wanted this one to be right.

After I completely disassembled the bike, I removed both the chainstay and seatstay bridge, and spread the rear dropouts to 135mm to fit the hub provided by the customer.  Then I made some deep indentations in the chainstays to increase tire clearance, and finally installed new bridges with fender mounts the correct distance from the tire.


Here’s what the increased chainstay clearance looks like from below.

I also added brake bosses for cantilever brakes (more repurposed mountain bike parts), a rear brake cable stop, some shifter bosses, and a pump peg.


Brake bosses, cable stop, and seatstay bridge.

After an inexpensive black powdercoat, the bike was ready to reassemble.  I built new wheels for it using the old Shimano mountain bike hub, and a Schmidt front.  I think the wheels were the most expensive part of the whole project.  Wheels and tires are no place to save money, but that’s a subject for another post!  Fenders, a chain, handlebar tape, and a stem were the only other new parts purchased.  I never got a picture of the finished bike, but the customer loved it and rode it later that summer on his first 1000km brevet.

A couple weeks ago the bike came back for some upgrades and some overdue routine maintenance.  Long harsh winters had taken their toll on the braking surface of the lightweight rims, and the old seven speed drivetrain wasn’t doing well either so it was time for some upgrades: a ten speed freehub body and cassette, new shifters, derailleurs, crankset, and rims.  While I was at it, I washed and waxed the frame.

Maybe you have a similar old bike lurking in a dark corner of the basement or languishing in the attic.  Could it be a conversion candidate?


Finished product: a solid and dependable 650B randonneuring bike for a fraction of the cost of a custom.


3 responses

  1. David Beckstead

    Beautiful and cant tell the difference:)
    Great job recycling. Creativity on wheels.
    Dave beckstead

    June 28, 2015 at 12:58 am

  2. Hugo Hoppe

    incredible, this should be an option at every LBS, save on a frame spend it on wheelset and extras and ride out with a proper steed. I must say good on ya for doing the work and making it look right in the end. And any idea how much a chainstay and seatstay bridge renovation would be e$timated at? I see too many bikes gathering dust that could use more friendly sized tires! cheers and keep up the good works

    December 24, 2020 at 4:22 pm

    • Hi Hugo, I tend to agree- it would be nice if more shops spent more energy on keeping old stuff going. Unfortunately, it’s pretty expensive to do this kind of a retrofit. It made more sense a few years ago when there weren’t many off the shelf options for this sort of bike, although it can still be a good option if you already own the donor frame or something like that.

      December 25, 2020 at 7:37 pm

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