Big Wheel Frame Building
We have been moving slowly forward with the frame.
Mitered the down tube and brazed it in place onto the head tube. Then mitered the other end of the down tube to the seat tube/bottom bracket joint, clamped it into the frame-bulder's jig, then tacked it into place. Starting to look a bit like a bike frame.
Up to this point, the joints and lengths were somewhat "open", meaning that they didn't have to fit anything more that slightly accurately, as the other end would be trimmed or adjusted for. But for the top tube, the miter joints had to be very accurate, as the tube needed to drop into place between the head tube and the seat tube.
I measured everything and set the angle and mitered the end that goes against the head tube. Went very smoothly. Then it was time for the critical seat tube mitre on the top tube. I measured everything twelve times, checked angles, rechecked the length, marked it out carefully, measured again, set the level so the miter angles would match. Then tightened it all down in the mitering clamp and checked the measurements and level again, then held the rest of the frame assembly up to it to make sure I was cutting the right angle.
Rodney, my mentor, had joined me by this time, and he carefully suggested that I back off the mark a half-inch or so, and then hand-grind the joint into place. But the other end of the tube had gone so very smoothly, that I thought I could just cut it into place in one fell swoop. Which, considering out equipment, was a bit foolish.
Our tooling consisted of a nice drill press, a hardware-store hole-saw, and a clamp that holds the tubing at a measurable angle -- probably accurate to within two or three degrees. Vertical alignment would have to be eyeballed with the help of a level on a surface that wasn't really level. Not exactly "machine shop" quality. So backing off a half-inch would have been an excellent and safe choice.
I did relent to the point of backing it off about 3/32 of an inch. Then pulled the spinning cutter down onto the tube. Hmm. When the tool dug in, it seemed like it was exactly on my mark, and wasn't backed off at all. Good thing I'd backed it off.
The cut started smoothly, with the vibration trying to loosen the tube clamp, and the cut trying to spin the tube, with me pulling the handle with one hand and trying to hold everything straight with the other. Rodney was squirting oil on the cut to keep it smoking instead of overheating. All business as usual.
Then about halfway through, the cutter caught on something. Clonk! The drill press stopped suddenly (loose belts) and the force of catch spun everything that could spin. The tube was out of place, the angles were off, the clamp had moved on the drill press bed. Rats.
We eyeballed everything back into place and expected to be ordering a new top tube the next day for a fresh start. We finished the cut and pulled it out of the machine. Didn't look too bad. Then I set it into place on the frame. And it slid right into place and stopped. The miter looked spot on. What? How did we manage that?
So despite the messy approach, we got a good cut and accurate miter, and just a couple swipes with the file with get it to fit just right. Nice. And purty durn lucky.
Next step will be to tack it into place with the torch.
A Brief History:
Most mountain bikes today evolved from modified balloon-tired "cruiser" or "klunker" bikes. Thus, they have 26 inch diameter wheels. Or, actually, it's the wheels plus tires that measure about 26 inches.
But almost from the start of this modern age of mountain bikes, some people have wondered if that size wheel was really the best size for the job. Over the past few years, there have have been growing numbers who have experimented with, or wholeheartedly embraced a "new" wheel size: the 29 inch wheel. Which, for starters, isn't particularly new. The rim is the same diameter as the standard road bike rim. But with a big, fat tire, the diameter makes it to about 29 inches.
Why does this even matter? The theory is this: a larger diameter wheel will roll more smoothly over uneven terrain. Think of your shopping cart, with it's little tiny wheel. That wheel will fall into very small cracks and be jarred by small pebbles. Things that a regular mountain bike wheel would roll over smoothly.
The 29 inch wheel takes it a step further than the 26 inch wheel. But is it a good step? The larger diameter wheels also have a longer patch of tire on the ground at any one time -- a good thing for traction. The spokes are longer and thus theoretically weaker -- a bad thing. Larger diameter wheels have more inertia -- a bad thing when you're trying to get them rolling, but a good thing when they're already moving.
I've been curious. But I've also been satisfied with my 26 inch wheel bikes. I've test ridden a couple Big Wheel bikes. Which was interesting. The bikes were also single speed bikes. My reviews were mixed. Not so fond of the single speed. But I liked the big wheels. Would I like a big wheel bike of my own?
My landlord builds his own steel bikes. He's a road bike rider, but he's built mountain and cyclocross bikes, too. He had an extra tubeset for a 29 inch wheel bike, and asked if I would be interested in making one. Heck yeah, I would!
So he lent me a book on drawing up a frame. I took some measurements from other big wheel bikes. I did a preliminary drawing. And then we drew it up full size, with all the angles and lengths.
Then I did some brazing practice, heating up a tubing joint with a torch and melting brass rod into a fillet. First try was a bit blobby and ugly, but the next wasn't too bad. We got out the actual tubing and mitered the end of the seat tube to snugly cup onto the bottom bracket shell. Then I brazed in on. Bang! The frame was started!
Since then we mitered the down tube to the head tube and brazed it. Now I'm mitering the down tube to the bottom bracket shell and notching it to fit against the seat tube.
All this is taking place in small chunks of time on evenings and weekends. Won't be done quickly. Fun, educational, and interesting. Looking forward to riding it, too.
Check back for updates!