I'm considering converting one of my 25" framed bikes originally meant for either 27 or 700c (can't remember which) wheels to 650b so I can run a 32-38ish tire. I have a few requests/questions:

I'm afraid it will look stupid. Pretentious? Maybe. I'm afraid of what such a large frame with such a smaller wheel will look like. Does anyone have any pics of similiar conversions on a larger frame?

More importantly, I know the way to figure this out is to measure, but does anyone know if Mafac Center pulls (the gold ones - competitions) will clear a 650b in a frame meant for a 27? How about a frame meant for a 700?

What about fenders. Will a fender marked as meant for 700/27 wheels work ok on a 650b conversion?

Anything else I need to worry about?

Please bring on the pics/opinion/comments. I'll post more details about the rest of the build later.

maybe something like this..

Thats Bill Walton of the Blazers. And those wheels are 700C. Dunno how tall he was though..

sorry, I meant 25 inch frame, not 25 foot.

I was wondering the same thing recently. If you run a 38-40mm tire on a 650B rim, I think you want a fender designed for that, which would be a 48-50mm 650B fender such as those that VO sells.

I think many 650B conversions cut it pretty close on brake reach so one intended for 27" would be pushing the envelope. It's nearly impossible to say, though without putting a wheel in there and mounting a brake.

For the reach you have to have, I'd rather have the additional efficiency (strength) of dual pivot, extra long reach sidepulls (like Tektro R559s) as opposed to a centerpul. The lower the brake pad sits in the slot, the less effective it is.

I do not think 650B wheels with 38-40mm tires will look bad so long as there are fenders. Quite the contrary, I think it'll look really cool.

There are lots of pics of taller frames converted to 650B in the 650B threads that are within this forum. Nlerner has done a few really nice looking jobs.

The wheels might look a little smaller than normal if the bike was meant for 27", but if it was a 700c bike then the wheel/tire combo of a 650bx38 should be pretty close to a 700x23. I think it would look just fine.

As for brakes, like you said, you'll just have to measure to see. If it was built for 27's and could accommodate big tires then they might not work, but if it was 700c and built with tighter clearances then they probably will (given, I don't know reach numbers on those brakes).

I'm not sure about the fenders, but going back to a 650b tire/wheel combo being close to the same size of a 700x23, then I'd think that fenders for 700c wheels would be what you want.

25" 72' PX10 converted to 650B. This is just after the initial build, and some parts have changed since (tires, crank, bar, stem, and brake levers):



It originally came with tubular wheels of course so basically 700c. To reach 27" to 650b is a real stretch and it can be done to reach, but not many long reach brakes will work. I am using Tektro R556 brakes which are 55-73 reach. The front and rear pads are down toward the bottom of the adjustment (measured around 68-70mm if I remember correctly). For 27">650b you might need around 75-80mm or even more. Fenders shown in the picture are 700c Planet bike Cascadia plastic fenders.

Thanks, Colonel! Here's the tallest frame I've converted: a 24" PX-10. The friend I sold it to is still running it as a 650B w/ Pacenti PariMoto tires and V-O 45mm fenders:



Neal

ok well I'll spill the beans. The frame I've been considering is a '68 Carlton Flyer that I've been wanting to set up with a IGH, drop bars, kinda like an old raleigh lention or club racer type thing. I'm considering 650b because I also want to run a wider tire to accommodate my new commute which will be a few miles on a hard packed gravel trail. I was also hoping to use my gold Mafac brakeset, but I suppose I'm ok with saving that for my px-10 build.

So basically, I have no idea if my Carlton was meant originally for 27's or 700s. I got it as a frame-set. I'll have to measure and see what's what.


BTW - 2,000th post!

Congrats on 2,000.

A 1968 Carlton may well have been built for 27". You really need to know. If you've got even a 700c wheel, stick it in there and see how your available breaks reach. Then measure.

I've recently gone thru a very similar process. I acquired a great '63 frame, built for 27" and fenders. I spent a lot of time exploring a 650B conversion. In the end, however, I've been convinced to go 700c with 35mm tires.

First, I found that even 700c wheels required long reach brakes. I've gone with the R559. But, skipping many other details, the final straw was Chris K, the owner of V-O, who looked at me and said, "Why would you do that? The handling will be much better with the 700c and you can run as wide a tire as you want."

Heck, I went to him for validation. I got common sense.

That said, I still look longingly at those Grand Bois 42mm tires... sigh.

OP: please read this article. Gives some helpful measurements:

https://www.bikeman.com/content/view/1161/33/

Some factors you might want to pay extra-careful consideration to are the BB drop, tire clearance at the chainstay about 320mm from the axle, and brake reac. Leicalad gives good advice above: throw some 700c wheels in there to see what kind of reach you'll need from the brakes. It'd be great to have some 23c tires on them, so you get an idea of what standover and BB drop would be like with some 650x40b tires on it.

If you have a ton of clearance with 700c wheels, i'd do as leicalad did and just run fat 700c tires/wheels. If you have your heart set on 650b, and you feel comfortable that you'll have enough space between the BB shell and Mother Earth, but you're worried about brake reach, consider some drums on the IGH and front hub. They make it a moot point, and they are great for commuters. Will save you $$ and effort in terms of replacing 650b rims down the line, too.

hth
-rob

Tall frames look goofy no matter what. Just do it.

Just a bit of rear triangle math:

Mafac Competitions come in two reaches: shorter is 44-54 mm, longer is 49-59 -- and the "adjustable" one with slot in the arch is 51-66. I doubt your gold calipers are the latter.

650B rims have bead seat diameter of 584 mm, 38 mm less than 700C, meaning that to convert from one to the other you need brakes with 19 mm more reach. Add 4 mm to that for converting from 27" (630 BSD) to 650B.

My guess is that your Carlton frame probably has an axle-to-brake mounting hole dimension of about 368 mm (plus 7, minus 2). So to use 650B rims (radius from brake track to axle about 292 mm), you need brakes with reach about 76 mm. You might make do with Weinmann 750 centerpulls, but I'm pretty sure that Mafac Competitions are out of the question.

This one isn't a conversion, but it's a tall hardtail MTB from the early 80s that should give you an idea of what the wheels will look like in the frame.

so I think sticking with 700's might be the move, that is if I'm OK with 32mm or thinner tires. Here's some info:

- with 700 rims the distance from the center of the hole for the brake in the rear to the middle of the rim is 63.5mm.
- with 700 rims the distance between the chain stays where the tire would be is about 38mm. How many mm's do they say you need on each side of the tire for enough clearance?
- the mafac's just barely are long enough for 700's, though its so close it's hard to tell until i had them completely set up and adjusted.

The mafac's arent getting used either way because now that I look at them again, they just don't look right with chrome lugs and a green frame anyway. So basically I'm sure I could get away with 700's with a 32c tire and weinmann or some other type of longish reach center pull. Anything with a 63.5mm reach.

I still don't know what the frame was designed for with those measurements. I'm guessing maybe 27 's since 63.5 seems like a longish reach.

Mafac Racers would probably do you, and they're not so expensive.

I converted a Schwinn World a few years ago for my daughter. It was, of course, a 27" wheel bike. I think I may have had to file the pad holder slots a bit to get proper reach. I also recently converted a Raleigh Competition in the 65cm size. I used old Raleigh 3 speed fenders. I don't think it looks too odd, but I like the look of tall frames.

holiday76, I know you kept your Carlton Flyer 700c, but I thought this would be apropos the discussion of tall bikes and 650B conversion: this is my 25" Shogun frame converted to 650B. I do think that really large frames look funny when converted, but not if large fenders are used and the tire-fender clearance is kept fairly large:



As for using 27"/700c fenders with 650B wheels, if they're aluminum, I suppose the arc can be reshaped smaller by creative manipulation, but on my latest 650B conversion (new Soma mixte frame designed for 700c), I decided to stick with true 650B fenders rather than try to reshape an existing 700c set.

thanks for bringing this back up. Your Shogun is inspirational!
I think about doing this to some bike I own every once in a while, but it hasnt happened yet.

I do think that yours looks great though, large frame or not.

Maybe the PX-10 will get converted..

But didn't you tell me that you did not like lager tires because of the increased rolling resistance?

Yu. And I hate the ride of mountain bikes too, but I still own one