So I'm considering converting my Gitane to 650b because I want to use it on Randoneer/Brevet type rides this next year. The reason I'm considering changing it is because the fender clearance is really minimal and I can't fit anything larger than a 23c tires.



I've never ridden a 650b bike so I'm not sure how it's going to change the handling and/or rolling resitance. Ideally I would keep it 700 if I could fit larger tires, but I think that is the only way I'm going to get more clearance.

I also would like to keep the bike looking "vintage". I plan to add a small front rack to run something like the lil baggins from Rivendell, and probably a medium sized seat bag or rack top bag of some sort.

If the 650b conversion doesnt seem right for a rando bike, I'll probably just build up a different bike for my rando bike as I have all winter to think about it.
I've love to get anyone's input with experience on this matter.

I think 650B was utilized (if not conceived) for the very reasons you are contemplating it.

cool, that's what I was hopign but wasnt sure. I didnt know if it would have much effect on handling or speed.

Don't know if you saw this on the Riv website...

https://www.rivbike.com/article/misc/...0c_conversions

Another even smaller effect is the small decrease in geometric trail due to the smaller wheel. But we are talking about a 1-2mm difference (calculator here) so it's not noticeable.

In terms of handling well bigger tires are more "grabby" when turning. In terms of rolling resistance I don't think there is a big difference. They are definitely more comfortable. Note that I haven't tried 650b specifically but 700x35c and balloon 26x2".

I've run 700c x 32. Much nicer ride, not perceptibly slower than 700c x 23.

I don't know what the geometry of your Gitane is; the head tube angle, at least, looks pretty steep, and one of my 650B conversions, a 1978 Raleigh Competition GS, is probably similar. It went from a somewhat twitchy sports bike to a much more sure-handling sports-tourer. The only thing to make sure of is that your BB is high enough. What's the measurement from the center of the BB to the ground currently with 700c wheels?

Neal

Will the 650B rear tire have enough clearance at the BB?

I can check tonight, but do I do that with or without tires installed?

Velo-Orange has plenty of goodies for the 650B wheel size too

Aaron

With wheels and tires. You want to see what the results will be of dropping the bike a couple of centimeters (though that'll vary depending on which 650B tires you go with) and whether pedal strike will be a problem.

Neal

with wheels and tires the height to the bottom of the bb shell is about 9.25 inches or 23.5 centimeters. that's with the 23c wheels on it now. The crank arms are 172.5s if that matters, although I' might be changing them to 175's

edit
so according to my calculations, a 700c wheel has a radius of 311mm and drop is wheel radius minus bb height. that would be 76mm or 311 - 235 . According to Rivendell anything over 70 and you will probably have pedal scrape.

so maybe I should be ditching the 650b conversion idea for this bike...

Yeah, 9.25" with 700c wheels is pretty darn low (assuming your measuring from the ground to the middle of the BB). I believe you need a minimum of 10.5" to avoid consistent pedal strike after conversion, and 175mm crankarms will put you seriously scraping the ground.

Here's a quote from Ed Braley's page on the topic of 650B conversions:
"The bottom bracket will come down a bit with 650B tires, you can expect a lowering of about 8mm-10mm converting from 700x23mm tires to 650x38B tires. For reference, the old racing standard for bottom bracket height is 267mm, and that was determined when people rode toe clips and straps on 170mm cranks. It's still a good guideline. So a good 650B conversion candidate will have a bottom bracket height of about 275mm with 23mm 700C tires. More is better, but anything lower than 270 is risky."

Neal

Seems like you've already figured out some of the issues. The best candidates are already relaxed tourer frames, with relatively high BB.

I have a 650b page here.

I wonder if a cyclocross frame would be a good conversion choice. They have high bottom brackets for clearance.

Dan

oh. I measured from the ground to the bottom of the bb shell, so thats why my number is tad low. It's ok though, I think I've already decided to keep this setup and use this bike as my weekend club riding bike. I'm looking into converting one of my other bikes to my rando bike instead. Maybe my old Grand Prix instead.

STill, thanks for the help. I learned a few things.