|Still rideable with skinny tires, but it gets more challenging down the trail.|
Having ridden portable bike #13 for about 6 years, I came to the point where I needed to make a new one from scratch. The event that was a catalyst for this was my ride on the OC&E trail I did last year. The largest tires the bike fits are 50mm (2.0). It just wasn't a large enough footprint to ride on the surfaces I encountered on the trail. A bigger tire would make the gravely and loose sections easier to negotiate. I signed up for the Oregon outback ride this year and I plan to bring it. It will be easier to take Amtrak with this bike, just fold it up and put it in the luggage area as I board the train.
The new fork would be an inch taller. The rear triangle would need extra space. The new design would increase the folded size slightly, but it would allow for the fattest tires out there for the BMX size- 20x2.4 (or 406-70). Shod with 406-50 schwalbe big apple tires, the folded size would be 28x11x21 (that includes racks and fenders) on the new bike. With the super-fat 20x2.35 BMX tires installed the folded bike gets about a half inch wider and taller, but the same width. Still meets the 62" length/height/width requirements for airline baggage and Greyhound bus.
|Demolition 20.2.35 tire on a CR18 rim with a 74mm Phil Wood 32 hole front hub. The new fork (not yet finished) offers plenty of clearance.|
|2.0 wide Schwalbe big apple tire on left; 2.35 tire on right. A significantly bigger cushion of air. A bigger apple.|
|Side view comparison. 2.0 on left, 2.35 on right.|
A fat tire like that is probably better if installed on a wide rim. On this narrow Sun CR18 (23mm wide) rim it has a rounder profile and at lower pressures it may cause the bike to feel squirrelly in turns if at low pressure (the sidewalls would be better supported on 50mm wide rims). But with my 28 hole Schmidt generator hub (they do not make a 36 hole in the 74mm width) my choice of rims is limited. I may map out a spoking pattern so I can use a wider 36 hole rim, skipping every 4th hole, and using a variety of spoke lengths.
I will also stiffen up the main boom structure connecting the seat tube to the forks/handlebars. Existing tubes were butted (9/6/9) and I found it to be too flexy when riding a heavy front load while speeding downhill. There will be other refinements as well.
|jigging up the fork. I ended up giving it 25mm offset.|
|Bike plan drawn up life size on a piece of drywall.|
After making the fork, I drew up a plan for the frame. 72 degree head and seat tubes. Bottom bracket height 10.5 inches with the smallest tires I intend to run (40mm)
|making some smaller parts. This is the lower clamp. 1 1/4 .058 tube with the tangs; 1 3/8 clamp silver brazed around it, and slotted. There is no way you can over tighten this thing.|
|Above, seat tube/bottom bracket, with extension for swing arm. Below upper boom tube connected to head tube extending below, rust covered. And the clamps, awaiting to be brazed on.|
|Everything lines up...|
|just about to braze the seat tube area.|
Flat tables and fancy machinery is not necessary for building steel bikes, unless you are a pro who does it for a living. To check for parallel, I sight tubes lined up against the daylight coming in the window, just like Norman Taylor did when making Jack Taylor frames in the 80s. There is a great BBC documentary, part of which is devoted to the Taylors and their frame building.
I spent an hour working on this 1/2" tube. Bent it with heat to match the tire curve. It was an eighth of an inch too short though, to my chagrin. So I made another one, with greater concentration on the task at hand.
Rear dropouts about to be brazed on the chainstays. The wheel is used in the jigging. This way I am assured that things will be aligned. The tacking heats the dropouts a lot. When I first used this wheel the bearing grease melted out and rubber seals melted. So I use it just for jigging during frame making.
After tacking the dropouts, I take the wheel out and finish the brazing.
|main hinge area here|
|Lots of clearance with these 2.35" tires. Whether the chain clears the tire when in the biggest cassette cog is to be seen. (postscript note: the chain does clear the tire).|
|angles distorted by phone camera. Or the photographer. Head and seat tube are parallel.|
I am checking frame angles with this Very Straight Board the bike is on. I used an iPhone with a level app. After zeroing the board, I found the headtube and seat tube to be 71.8 each, with the bottom bracket height at 10 5/8 inches with 50mm tires. I can still use 40 mm tires with this bike, but smaller tires than that could cause pedal strike in corners. I have no intention of ever using tires on this smaller than 40mm; the usual tire would be 50mm, and the dirt road tire would be 60 or 65mm wide. The trail measurement with 25mm fork offset looks like 50mm. I don't know if thats what I want but I will soon find out. My main criteria for good handling is when a slow uphill with weight on the front is easy to do. Because that's how I spend most of my time.
|measuring frame angles..|
|seatstays, or the equivalent, are installed now|
|A tube is needed to connect the seat and chainstays. It doesn't need to be very big.|
|new curved connecting tube in place|
|disc brake supporting tubes in place before brazing|
A disc brake mount dropout for a bike like this does not exist. So I put something together. First I clamped the brake onto the rotor of this wheel, by tightening the brake adjustment wheels. The brake is centered and the pads are aligned to the curve of the rotor. The black mounting bar the brake is clamped to has adjustability relative to the brake. I made sure it was centered; and tightened in a neutral position. I had made a couple of T shaped steel tubing pieces that the brake mount bolts go thru. They are in place and ready for brazing. You can see a sheet of aluminum behind them which will protect the rotor and hub from the brazing flame.
|after tacking. Now I remove the wheel to finish brazing|
|Brazing is finished.|
|What a messy workbench. This is how I set up brazing the cantilever bosses on the fork.|
Frame and fork components displayed here. Still need to braze on the front rack, front derailleur, and finish numerous small brazed-on details related to the folding of the bike. I have a feeling I will be painting this one with a brush and rustoleum.
|setting up for brazing a small supporting tube onto the fork crown|
|Making a set-screw attachment for the front fender struts.|
The bike is rideable and I couldn't wait to try it. The first thing I noticed was the larger amount of trail in this geometry compared to my last bike (which had 42mm of trail as I recall). It rides easily hands-off. I notice it dives into corners a bit, and that taking corners at speed is easier with this bike than the last. The self-centering effect of a lot of geometric trail is something I have been missing. I also find it needs less correction when going slowly uphills. I hit the nail on the head this time with the design. 72 degree head angle (actually 71.8) with 25mm fork offset. I measured the trail to be about 50mm. Big wheel bikes tend to have trail figures in the 50 to 60mm range. Should a small wheel bike have less trail to achieve good handling characteristics? I don't know, but I find that this combination works very well. The bigger tires add some "pneumatic" trail perhaps, but there is a major change in how this bike handles compared to the last.
|front fender and rack installed. It took a whole day to do this.|
I made wood fenders in the past, but would break one or 2 a year. They just don't hold up, especially with a folding bike. I needed aluminum strip 2" wide and 1/16" thick for the fenders. Seems this is not available, but I found an angle strip at home depot that was 2x2"x8'. I cut the strips from that with the sawzall. Bent them around a pail, and began the laborious process of fitting them. The fender struts slide into a small tube brazed to each fork blade. A set-screw holds them in place. If a stick becomes lodged in the spokes and pulls the fender strut- causing the fender to be forced against the tire- a crash could result. This design makes it likely that the fender strut will be pulled out so the fender will not jam against the tire.
|still need front derailleur and rear fender/rack setup|
|Front derailleur, fender and rack added|
To keep the folded size as small as possible, the fender is run over the top of the rack. It clears the 2.35 wide tire by about 1 cm all around. A dynamo taillight is tucked underneath and protected by the rack itself. The front derailleur is tilted backwards so that it is positioned properly to work with the lower cassette position.
|setting up to braze on small connector pieces|
An experimental front bag made out of a discarded Weber barbecue cover. It's been on a few tours already and about a year of commuting. The front bag is involuted and folded at the back. It unfurls to become a bag that you can put the folded bike into.
The bag is really more like a soft basket. It is double thickness in the involuted mode and is quite substantial. Weighs about 1.5lb. It can hold an enormous amount of stuff while providing easy access, even when riding.
The back of the bag is folded and strapped. At the bottom it bunches around the headtube. The upper strap goes around the handlebar stem.
|Bike is in the bag here at a greyhound bus station. Next to it are 2 grocery sacks full of camping stuff. The tent , toolbag, and a few other items are in the bike bag as well.|