27.5 wheel in 26" MTB frame
 

The nutshell question is, do you think the cantilever brakes on an old 26" mountain bike have enough upward adjustment to accommodate 27.5/650B rims? My calculation for the difference in radius is almost exactly 1/2" (12.5mm). Is this within the normal adjustment range of typical canti brakes - or is there some way to make this work if it isn't?

This is an old fully rigid MTB. The frame is not pristine, but is in very good shape, no dents or cracks, just patina. I'm thinking of converting it to a more all-around, drop bar, gravel-ish bike to leave in our travel trailer which we keep in another state and use for a couple of months at a time. I have and have had very nice gravel bikes which I have been carrying back and forth. I'd like to come up with a cheap, but adequate bike to leave there and end the schlepping. Mostly fitness and leisurely rides paved and gravel roads and paths. No big mileage stuff, 5-25 miles here and there, no need for a super bike. I think I can do the conversion for $500 +/- since I have a set of 10 speed road shifters, a good enough crank and some stems and handlebars "in stock". Don't need new brakes, seat post, saddle, etc. The only expensive thing I'll need is wheels, and even with them, I'm thinking of a pretty modest price range or maybe even build my own. I'll probably go with a MTB cassette and rear derailleur. The RDs of basic quality that work with Shimano 10 speed road shifters (I.e. Deore 8 and 9 speed) are usually pretty reasonably priced on EBay - I've made that conversion a few times on my wife's road bikes as well as my original "cross/gravel" bike.

As for wheels, I pretty much need to get new wheels anyway since the old ones are in pretty rough shape including the rims and hubs. I think I'd like to go with 650B with ~35-40mm tires.

I've comfortably set up several bikes for myself, and have done some rough measurements. I think I can get the fit OK on this old frameset. It's nothing like modern mountain bike geometry.

So I think the only technical issue will be the rim-brake interface.

Any words of advice?

I expect you'd have the brake pads sliding into the tire on heavier braking since they won't be pushing perpendicular to the brake track.

There's plenty of 559 tires in the widths you want.
Why create a can of worms?

Usually not without a little modification.

Unless you can get your hands on some Avid Tri-Align brakes or the like.

Since you say you want a bike for low mileage gravel work, I'd probably tend more towards just getting some long reach roadie brakes and some 700c wheels with 32-ish grave tires on there. The BB is usually kinda high with that setup, but, for short miles, NBD

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If you're in a big city, it would probably be way cheaper/easier just to get a decent 26er wheelset and tires, though.

Yep, around here 26" wheelsets are a dime a dozen. If it was me I'd just use 26" wheels and be done with it.

-Ken

That is a lot of brake pad adjustment but, beyond that, will the frame and fork clear the larger diameter tires? Is there enough room under the fork crown and seat stay bridge? I agree that a new set of 26" wheels makes more sense.

Years ago I "roadified" a rigid, hardtail Trek 7000 MTB, converting it to drop bars using an old set of STI brifters, etc. but I kept the OEM 26" wheels.

This is the solution to a problem nobody has.

While 27.5 wheels are common, there are few that are rim brake compatible. By doing this conversion you will be creating multiple headaches and gaining approximately nothing. Get a set of 26" wheels.

I am doing a similar build w/700c wheels on a lovely old Trek 990. ( Made in USA! )
Would think the ability to do this varies from MTB frame to frame depending on the frames' structure: where the brake bridge is etc.
Some MTB frames work better than others.
They make some rather long-reach calipers, so you might have to dig in some parts bins until you find one which works.
For the front you can use a 700c cyclo-cross or gravel type fork and have more possibilities.

and yeah keep in mind: Certain tire manufacturers make super fast , narrow and streetable 26 tires...

OP wants to do a gravel bike build, may not want narrow tires

Another vote for staying 26". A 650x38B wheel is the same diameter as a 26"x2" wheel, so you'd only be cheating yourself out of potential tire width within the frame's clearance.

Depends on the frameset and the brakes. You're in a better position to know this than we are since you have the bicycle on hand. If not, Paul Motolites will likely work as a solution, albeit a pricey one, and the brake leverage may or may not end up attractive.

What's the motivation for using 650b over 26er?

You need to first do some simple math. The difference between 26” mtb rim dia (559mm) and a 27.5/650b rim dia (584mm) is 25mm or 1”. Which means you need to raise you brake pads up 12.5mm, or 1/2”.

These are theoretical numbers and the brake surface diameter is not going to be the bead diameter, but you will need about 1/2” of space in the brake pad mounting groove. You might pick up a tiny bit more if you lengthen the groove slightly.

As others have said, the brake pad might contact the rim at too much of an angle if you are stretching the limits to reach the rim brake surface. A narrow rim “might” help to square this up a bit, I’m not sure. But I would think finding a 14mm or 17mm wide 650b/27.5 rim might be a little tough.

Someone with more experience in vintage 650b rims can offer a better perspective. But I doubt modern 27.5 rims are that narrow.

John

The other significant issue is that you lose the proper mechanical advantage. So you'll find that the brakes which previously were easily applied and modulated well with only two fingers on the lever will now require a pretty Herculean degree of squeeze to stop very well.

Your goal for a nice older bike is to make up a casual round town and gravel trail bike.... So just stick on some suitable tires. A great option for that style are Kenda Krad tires. Smooth enough to be fast on roads yet with an open enough face to the block pattern to have a reasonable grip on packed gravel. If the Krad tires are no longer available then something similar. There's still a pretty good array of 26" tires out there.

In the distant past before disc brakes were common I looked at the idea of an MTB with 700C wheels. There was simply no way to make the pads line up without extending the adjustment slots on the V brake arms. And regular cantilever brakes did not even get that close to working out. The amount needed and the lack of feel meant that it was a non starter. I had to wait for disc brakes to become a touch more mainstream.

I ended up converting the fairly high end Stumpjumper shown below to 700c with a disc hub and 700C rim build and used it for sloppy weather commuting for about 6 years until I retired. If you can see what this is you know it wasn't a cheap option either. The VERY next year Kona introduced their disc version of their Dr Dew bike at less than half the price.......



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Just kind of thinking out loud and looking at options. Interestingly, I ran into a guy the other day with the same sort of idea I have (drop handle, rigid MTB conversion). He has been using 26" wheels, and like most of the comments above, agreed it's the far simpler solution and satisfactory for a do-it-all bike. I am reconsidering!

I appreciate all the comments. Maybe this thread will be a short cut to decision making for someone in the future too.

I think 26” has been unfairly vilified and sidelined as a somehow unworkable wheel size. While I do own several bikes with 650b wheels, when I ride my 26” singlespeed, I’m honestly impressed by the quick acceleration, snappy handling, and good climbing ability. I run 2.3” tires, and tackle pretty much any surface I encounter with relative ease. Given the opportunity to convert it to 650, I can’t think of a reason why I would.

Yeah, as long as I can get the nice Black Chili versions of the Conti Race King for the front and Speed King for the rear (or some other tires are able to work as well) I’ll keep riding my old MTB as a 26er. It will be something to consider when those tires aren’t available in the 26” size. Going bigger diameter than 26x2.2” doesn’t add much for me. 26x2.2 can be great on gravel and stone trails. It’s about the same diameter wheel as a standard road wheel and that size was chosen for good, practical reasons.

Well put Rolla. And totally agree.

A big part of why the early MTB's went with 26" is to avoid toe strikes against the front tire when turning on the trails. Some of the road bikes with 700C I've had did in fact cause a toe strike when maneuvering at very low speeds and bigger bar angles. Normally they simply don't ride that slowly so it's sort of a non issue. And I would imagine that on occasion a gravel and casual trail bike might well have the same low speed and tight maneuvering to deal with and might well be better off with the smaller overall tread diameter.

Makes me wonder how well the 29'er MTBs are for this. I can't say I've even tried a test ride on one up to this point.

Thats my thinking.

I understand if it’s for gnarly single track with big rocks, but for how a lot of riding is, cruising around the neighborhood, MUPs, rail trails, etc. I think 559 is great.

The earliest MTB's were long-wheelbase Schwinn Excelsior single-speed cruisers with 26" wheels. All the subsequent moutain bike designs were developed based on those original bikes. As someone who sold Schwinn, Bianchi, Cannondale, Specialized, and Trek mountain bikes beginning in the early 1980s (from the first year that each company hopped on the MTB bandwagon), I can confirm that the issue of potential toe overlap never crossed anyone's mind in those days. Why would it?

Would pop over to the BF Drop bar MTB conversion page first Show Your Vintage MTB Drop Bar Conversions - Page 295 - Bike Forums

For the wheels, a quick look through the above and the big thing that seem to have been missed, is that the requirement is for 27.5/650B rims with braking surfaces (as you say in the first line for Canti brakes) they don't really exist, as the spec for 650B MTB wheels came along after rim brakes had been replaced for MTB by discs (yes there are a very few available, by these are going to be rim only requiring hubs = build etc and then the cost has to be factored and question why not just buy a 27.5 bike).

For disc brakes, can it be done, yes, but your run into geometry problems, tire width problems, as many 650B tires will be wider than an old 26" frame can take, even with the width your looking at, being as they will be in a different position to 26" wheels, it's a hard question to answer with the very limited info given.

For the rest of your plan, what is the base bike? and how long is the head tube/is the steerer threaded or threadless, as this will you really want a bike with a long head tube, as you will be bringing the bars up and closer to you with drops, if you have a threaded steerer/stem this can compensate for a short head tube, but a threadless you really want a long head tube.

Is a 26" MTB an easy conversion? yes on the surface, but be prepared to spend a lot of time to get it to be comfortable vs a purpose built gravel bike.

Perhaps because those early designers were smart enough to use 26" sizes to avoid the issue?

Of the five bikes I've got here currently three of them have 700C wheels. Two of them have geometry that can cause me to toe strike if my foot is in the wrong place and the bars are over quite strongly like they would be during a tight low speed turn. The other is a cyclocross bike which seems to have more room between the bottom bracket and the front axle and thus narrowly avoids the toe strike issue. But not by much.

Most, if not all, of my bikes have had toe overlap since my early racing days in the 1960s. I've never worried about it.

I can see why you came up with your guess that the early designers were concerned about toe overlap, given your experience with your bikes, but that wasn't the case. Again, they used 26" wheels simply because the Schwinn Excelsiors happened to be easily procured adult (i.e., 26" wheel) cruisers. No one was using 700c wheels in the U.S. back then except bike racers, and up until sometime before the early '80s the only 700c wheels available used tubular tires rather than clinchers.

It is true that cyclocross, which required bikes with 700c tubular tires, was a sport in Europe as far back as around the time of World War I, but it was largely unknown in the U.S. until comparatively recently. So cyclocross racing has been around much longer than MTB's, and yet cyclocross bikes have never used 26" wheels, so that also seems to disprove your theory.

The earliest mountain bikes were used mostly for downhill racing on Mt. Tamalpais at first, which quickly revealed that the original coaster brake hubs were hopelessly inadequate for the task of stopping the bikes under those conditions. (The grease would boil out of the hubs - hence the racers dubbing the most popular downhill trail Repack.) So a cottage industry developed to add rim brakes and then derailleurs (for riding to the top of the trails rather than walking the bikes up), followed by custom-built chro-moly frames with cantilever brazeons, etc.

Point being that no designer deliberately chose 26" wheels for use in mountain biking. For racing long, rough downhill runs on a mountain, the choice was adult cruisers with 26" wheels or nothing.

In short, I'm pretty sure that toe overlap was never a factor in the use of 26" tires for the early MTB's, but if you can come up with any citation of evidence from that era that attests otherwise, I'd like to see it.

You're giving them to much credit. It was just economics, practicality, and availability. Here's mountain bike pioneer Gary Fisher on the subject:

Yeah, I don't get this why bikes with 26" wheels are such a rarity. After my 12 year old son outgrew his 1x bike with 24" wheels, he tried a few XS bikes with 27.5" wheels, but they were all too big for him. Thank goodness Trek released a Wahoo with 26" wheels.