Bicycle Mechanics - 26" wheels on 700c frame?

I have a new project: building a city bicycle around a Nexus 8 hub. It should have front generator hub, fenders, rack and not be too heavy. I'd prefer a steel frame with rigid fork.
700 road frames are nice, but they haven't enough clearance for 32-35 mm tires and fenders. I know some have used 650b/584 rims, but these (and tires) are very hard to find where I live, so I thought about fitting 26"/559 rims. For the brakes some kind of drop bolt could work.
The difference in diameter is about 35mm, so the BB would be 18 mm lower.
What do you think?

I think it's a poor idea. You aren't going to find drop bolts that will come near that radial difference and lowering the bb 18 mm is a lot and invites many pedal strikes. Find a 26" old-style rigid/hardtail MTB frame to build up. They are a dime a dozen.

Pick a hybrid frame. Keep the 700c wheels.

The only easy way to do that is using drum or roller brakes (Sturmey-Archer make a front dynamo hub with a drum brake). The rear hub, if you want to use a Nexus, would have to have the roller brake with the band fitting, rather than the one that bolts to a disc brake mount. You could just use BMX sidepull brakes without drop bolts, and they might fit, but you'd be mounting the shoes right at the end of the brake arms, so the leverage of the whole system would be rather poor, which would give you very little braking power.

I've got 26" wheels on an old Raleigh built for 27 1/4 inch wheels, using SA drum brakes front and rear, and pedal strikes aren't an issue. You may not be as lucky as me though.

A far better solution would be to use a hybrid frame, or an old rigid MTB frameset, or failing that an old touring frame or something.

Find a 26" old-style rigid/hardtail MTB frame to build up. They are a dime a dozen.

That would be the easiest route!

http://i36.photobucket.com/albums/e26/sstulken80/Diamondback/pre-commute.jpg

That said, 700x23C and 26"x2.1" tires have roughly the same diameter, so if you were making that switch the bottom bracket wouldn't drop at all. Caliper brakes are pretty much out, as a drop bolt won't help road bike calipers fit around a 2" tire. ;)

If you go for an old rigid mtb, then double check that it doesn't come equipped with annoying and uncommon parts, like 1-1/2in threaded headset, U-brakes, proprietary seat post size or obscurely shaped seat tube.

to be really safe, just get an old trek mtb. Those are really common and about as standard as vanilla.

i tried it on an 85' trek elance road bike frame. i ran a coaster brake rear hub.

brakes were no problem with the coaster brake hub. lower BB didn't seem to be much of an issue for the short time i had it set up like that.

the biggest problem i encountered was a horrendous front fork wobble when riding hands free, even for an instant, at anything greater than about eight MPH. that was the only real problem, but it was game over for me.

it may not happen to you...

Thanks all, I think I'll go for an old MTB frame as recommended.

With using an old rigid MTB frame, you could stuff the largest 700c tires in the frame. I was test fitting a pair of 700c wheels with 40mm tires and they fit fine under a earlier 90's Trek 830 Mountain Track frame. And standard road caliper brakes fit fine. I ended up tearing the bike back down though, was just too tall, had a 32.5 inch standover height.

My city bike is a Dahon Cadenza 8 with Alfine , Shimano dynamo hub, disc brakes, rack, fenders and 26" wheels. It is a sound concept and works very well. The frame and fork are aluminium but, because of the folding mech, is not lightweight. Using Big Apple tyres, I dont find the frame uncomfortable or too stiff.
You need to consider chain tensioning. MTB frames with horizontal dropouts are rare. You can get singlespeeds with track ends. An external bolt eccentric bottom bracket works well but other types can get creaky.
The missing part of my build is a split rear triangle for belt drive.
You could use the Versa shifters with drop bar but I dont think you gain anything over a good flat bar for city riding.

When you buy a mtb frame check to see what the bb drop is. On a lot of the older frames, through the mid 80s, the bb drop is almost non-existent, so standover height is greater, center of gravity higher, etc. Also on the older mtbs the tt length is longer than "normal", keep it in mind when planning your build. I have a mid 80s Bianchi grizzly and a Peugeot urban express that both have these characteristics.

Brian

If you go for an old rigid mtb, then double check that it doesn't come equipped with annoying and uncommon parts, like 1-1/2in threaded headset, U-brakes, proprietary seat post size or obscurely shaped seat tube.

to be really safe, just get an old trek mtb. Those are really common and about as standard as vanilla.

I had an old Trek with a 1 1/8 threaded headset and some strange seat tube shape. It seems even they gave in.

While it does make the build harder, none of those are really deal breakers- even the dreaded u-brake works pretty well.

If you wind up doing this anyway, think about BMX calipers instead of a drop bolt. They'll let you run wider tires.

even the dreaded u-brake works pretty well.

I have never understood the hatred people have for U-brakes. We see a lot of BMX bikes at my co-op, all of which have u-brakes if they have anything, and my experience with them is the same as yours. U-brake mounts on a frame certainly wouldn't put me off it.

Caliper brakes are pretty much out, as a drop bolt won't help road bike calipers fit around a 2" tire. ;)

No they're not; on that yellow MTB with the drop bars I'd use the cheap nutted dual-pivots I saw linked yesterday.