406 · custom design · experimental · folding bicycle · hand built · portable · small wheel · trail

Portable bike # 14

Still rideable with your standard touring 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. So I figured I would make a new one with bigger tire clearance; having signed up for the Oregon outback ride (although I cannot make the group ride this year).  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- 20×2.4 (or 406-62). Outfitted with narrower 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 20×2.35 BMX tires installed the folded bike gets about a half inch wider and taller, but the same width. It still meets the 62″ length/height/width requirements for airline baggage and Greyhound bus, with 2″ to spare.

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 inch Demolition Machete 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 3rd and 4th hole, and using a variety of spoke lengths. I built such a wheel and the blog post is here.

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 thought it was a bit 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 a 20 mm 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.
Clamps are silver brazed on. I am holding a mitered tube the will mate with the lower clamp and be brazed on to the seat tube.  It’s a beefy 1 1/8 .049 piece., like the one installed in the upper clamp. Mitering took 20 minutes using a grinding wheel and a file. It is spot on; the angle when held up against the seat tube is perfect, and there are no gaps or crescents of light when viewed against a light background to assess the miter.


The boom tube was a bit off when all was brazed. The head tube was about 1 degree off  compared to the seat tube. They should be exactly parallel.  Here  I am cold setting it in a makeshift way. It helps to have a lot of spare steel tubes around to use a leverage. The cinder block the boom tube is inside is part of my very heavy work bench. Although I had a lot of mechanical advantage here, it took a surprising amount of force to straighten it out. This part is probably overbuilt. I may use 1″ x .035 tubes for the next one.


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.


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: 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. It can use tire widths from 40 to 63mm. The trail measurement with 25mm fork offset looks like 50mm.  My main criteria for good handling is when a slow uphill with weight on the front is easy to manage. 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.

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.

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 2×2″x8′ and of the thickness I wanted. 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 of the front wheel 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 dérailleur needs to be more rearward because of the smaller wheel.
setting up to braze on small connector pieces


The front bag is an experimental thing made out of a discarded Weber barbecue cover. It’s been on a few short tours already and about a year of commuting. It 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 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 that I will take on board with me. The tent, toolbag, and a few other items are in the bike bag as well.


From arrival at the bus station to all bagged up like this takes an easy 10 minutes. If rushed I could probably bag everything up in 5 minutes.


Bag unfurled and ready to drop the folded/clamped together bike into. It takes a just a couple of minutes to fold it up.
I remove the left pedal and put it in the crank arm backwards, before I put it in the bag


bike is inside bag


Move along, nothing to see here. Just another boring piece of luggage. Telling a transit authority that a bicycle is in that bag is  just asking for trouble. If they ask I say it’s “bike parts”.

Bikepacking with a folder. The bigger tires make a huge difference, rocky and gravelled roads are much more rideable and pleasant. I lower the PSI to 20 and it’s full speed ahead. Huge front bag holds food, stove, and misc items. One gallon of water in 2 bottles straddles the top-tube. Sleeping bag and clothes in dry sack on top of rack. Tent and inflatable pad in burgundy bag underneath. Tools in blue bag above crankset.


Camping spot I like near Larch Mt, in the Mt Hood natl forest.
New rack is 5″ wide and very long. Inner tubes work really well to tie stuff down. 4 posts pointing down near each corner provide a place to lash the tube to. Rear light is in a protected spot.



A new rack was needed. It offers more space and stability for the camping stuff. The rack is not removed during the fold or bagging. Still 60 linear inches, total. Still meets max size requirements of 62″.
Fort Rock was the end of the line for us due to time constraints and commitments.


On the OC&E. When herding cattle, always wear a helmet
I had fenders and Tim did not. When following the cattle, abundant soupy poop would come out of the agitated bovines. Never was I more thankful for my fenders. Poor Tim got splattered.


ready to head out from Klamath Falls
Tim riding the Pugsley on the OC&E with 2.4 Maxxis tires. They rode over almost everything without issue.


Our first campsite along Five Mile Creek during the oregon outback ride.

A few mixed photos from our trip above. We left the route at fort rock because of prior commitments. I will return to finish the route soon, probably from north to south, and will use the folder again. While there were some short sections that made me pine for a fatbike, overall the folder did perform admirably with the 20×2.35 tires. Its a good size tire for all around mixed riding on road or trail and it was a huge improvement over the 2.0 tires I used last year.

Returning to Portland the quick way, we took McKenzie pass over the cascades. It was closed to motor vehicles but was  opened for bikes/peds.  It was an awesome ride.


Addendum: Crooked river to the finish

I took the cobreeze bus from Portland out to Bend to ride another section of the outback ride. While the drivers are nice, the business office is not and I do not recommend them. Being able to bag the bike and get on the bus made the trip very easy. The driver had no idea there was a bike in the bag; it was just another piece of luggage.

From Bend, I rode 30 miles to the Crooked river; here descending towards Prineville reservoir.


My first camp at trout creek, in the Ochoco mts.


a bough shelter somebody made, near my camp


following the dirt roads and double-tracks towards Ashville


Gravel roads after Shankio; riding by wheat fields as far as the eye can see. Imagine a lot of headwinds as well.


Another camp in a cemetery near Grass Valley. There was a lot of wind so I didn’t bother using a tent.


Along Old Moody road, after finishing the ride at the Deschutes river. A U shaped valley overlooking the Columbia river.


I rode all the way back home to Portland thru the columbia gorge.


My last campground near Wyeth. Wild camping near a tiny hidden cemetery.  Cemeteries can make good spots for camping if you do not fear the spirits.

I am looking forward to finishing the last 60 miles of the route I have not ridden thru the Deschutes forest sometime this year.


bicycle · bicycle framebuilding brazing fillet silver brass bike frame · folder · folding · portable

The portable bike

Here is a group of photos of my most recent 20″ wheel portable bike. I made about a dozen of these (similar to this one) over more than a decade. This one has all the R&D from lots of miles and lots of experimentation from the previous ones. There is a big bone pile of what did not work well, and what I improved upon. This particular bike is about 3 years old and has at least 12,000 miles on it. A touring journal using this bike is here. I did a full randonneur series on this bike a few years ago.

Disclaimer: This is only a photo essay of my experience in an alternative bike design! I am not suggesting you build your own bike or copy this example! Save yourself the hazard and the hassle and go buy a folder at your local bike shop! Or you can ask your local framebuilder to look at this webpage and make one for you. But after looking at this they will probably say “no way”, or “are you out of your mind?”

There is perhaps enough information here for an experienced framebuilder to make this (email me if any questions), but it would be quite a challenge for anyone else. A lot of work goes into a project like this. I could probably build a conventional not-fancy bike frame in 3 or 4 of long days, but this one would take 2 or more weeks. So many small parts that need to be brazed, and there is a lot of creative jigging to set things up. The result is a very reliable all purpose bike I would take anywhere, commuting, touring, dirt road or light trail riding. And it gets very small in about a minute and a half.

What does it have? Built in front rack, bolt on rear rack, drop handlebars, fenders, front and rear generator lights.
Triple chainring, 135mm rear triangle with a 9 speed cassette hub and a rear disc brake. It weighs 31 pounds as you see it here.
It gets small in about a minute and a half. 27x21x11″. Without the rear rack and fenders, it would be 19″ wide instead of 21″. Props itself up like this, or on its side. I can pick it up from anywhere and it all holds together.

Design considerations:
It’s like a cross between the elements of a Brompton and BikeFriday. It gets smaller than a BikeFriday, and in less time, and holds together without needing a bag.. But it still has all the full bike features of a BikeFriday (drop handlebars, full range of gears, etc). It’s not that much bigger than a Brompton when “folded”, and does not take that much longer to fold it, but it is much more bike than a Brompton is.

The key here is that there is only one folding function on this bike: pushing the rear wheel under. The rest of the process involves separation of frame elements and clamping them together to form the “folded” package. That allows a frame design with less limitations dictated by folding requirements, and allows the use of standard bike componentry. Tubing is almost all cromoly aircraft tubes fillet brazed together. There is silver brazing where there are “lugs” (seat post extender, connecting tubes at rear of main boom tube). Geometry is pretty standard; 72-73 angles, 40″ wheelbase, 25mm fork offset (short fork offset because of small wheels), 10.25″ BB height. It duplicates the position I have on my regular big wheel road bike. Lots of low gears of course, and the 53×11 top gear is an acceptable 90″. I don’t spin out that often.

A 20″ (406) wheel size is the best option for a bike like this. This tire size is available everywhere, from bike shops to variety stores, and there are lots of choices in this size from many makers. The Schwalbe marathon supreme 406-42 is a wonderful light, durable, and supple tire, and my current favorite. The front hub is a 74mm wide type found only on folding bikes. Formerly, I used the standard 100 mm wide hub, but now that high quality 74mm front hubs are available, there is no reason not to use them. It saves an inch on the width of the folded size, and that is huge. The front hub on this bike is a Schmidt SON XS 28 hole generator. The rear hub is a Shimano 135mm with a disc brake. 135mm is the standard mountain bike/internally geared hub size, and I don’t want to be limited by a narrow rear triangle. The rear disc brake allows me to brake without wearing the small rims down. With rim brakes, I would wear out 3 rims a year, since I live in rainy Portland, OR. Sometimes the worn rim would just collapse, causing the tire to explode off the side. The front brake is a linear pull rim brake which I use only occasionally, for hard stops. 

Front rack is brazed in place. Edelux light has a special protected spot where it won’t get knocked out of place. There is a linear pull brake with a travel agent (for road levers). Just below that is a fat tube sticking up. When folded, the handlebar stem clamps there. SON-XS 74mm front generator hub is visible. The fenders are 3 thin layers of cedar, glued up together with gorrilla glue, using the wheel with the tire on it as a form while the glue was setting. Duct tape held things together while the glue set.

Looking down at the handlebars. A clamp device brazed on the stem is seen here. It holds the bar end shifters. The bar ends are threaded together so they stay parallel, and the clamp secures it all in place. This is a nice way to shift gears, and works well with the folding. I have never seen it on any other bike. Does that mean I invented it?

The boom tube. Two 1 1/8 tubes (.035). Connects steering to seat tube area. On the left, handlebar stem clamps on top, and forks clamp below. There are indexed to hold them in place should a bolt break, or if I forget to tighten them in a moment of cognitive decline. On the right, an upper and lower lug (1 1/4 .058) to accept the assembly on the seat tube. Tightened by levers brazed to 5mm bolts. A pin goes thru the lower lug and tube to resist pulling forces. A cable keeps it from being lost. All the unpainted tube connections are lubed with Boeshield. Once a month or so I clean and lube all the places where one tube inserts into another tube.
Connection at seat tube. The short robust seat tube is 1 1/4 .065, reamed with an adjustable reamer to accept the 1 1/8 .058 tube seat post extender. I can’t have any play in the seat post extender; it must fit like a normal seatpost in the seat tube. All the others are slip fit tubes, using .058 walls, so that the next 1/8″ size down has a little wiggle room. It’s also the clearance I need for brazing, if they are to be connected.  That curled thing you see with a brass bolt thru it fixes the boom tube in place when the bike is folded. In front of that is a St Christopher medal. Patron saint of travelers. They used to sell them at bike shops. This one came off my 1950s Ideor.
Rear rack has posts on each corner to lash stuff down with. They allow me to put a sack of groceries on top of the rack and secure it in place with a bicycle inner-tube (narrow 700c tubes work best). I wonder why custom racks (or stock/aftermarket racks) don’t come with these.  Someday I will paint it.

The hinge to drop the rear wheel down. A 1 1/8 .049 tube extends from just above the bottom bracket. It meets a wide clamp; you can see the bolt just above it. That grips a hinge pin, a 5/8″ cromoly tube.. The hinge pin has the chainstays clamped tight on the left and right sides. The hinge pin is greased, and the central clamp bolt controls how easily the wheel drops down. The front derailleur is clamped to a tube that puts the derailleur at the proper angle. Since the rear wheel axle is lower, the derailleur needs to be rotated back a bit.

Wheel in place. “Seat-stays” connect to upper frame assembly; it just drops into place. Nothing is needed to retain it. There is a bolt I can put thru there that will keep the rear wheel from being pushed down. I use it when I am touring and have a lot of weight over the rear wheel. That way, if I lift the rear of the bike, the wheel doesn’t drop. Lower seat stays are 1/2″ .049, upper 1/2″ .035. They look thin, but seem to do the job just fine.

Folding it up
First I drop the rear wheel down. The chain is captured on the hinge; a small collar of metal keeps it there. The tire butts up against the bottom bracket which keeps it from moving. Whenever I park the bike, I drop the wheel like this. It keeps the bike from rolling or falling. The fork is 90 degrees vertical now and the front wheel is resistant to turning, so I can lean it securely against anything. And it looks too weird to steal. The saddle is chest high; “the person riding that bike must be 8 feet tall” is what I hear people say.

next I take the seat extender out and fix the seat assembly against the side of the frame with 2 connecters I made.

Now I split the bike; loosen the clamps, pull the pin, separate halves of bike, and fix the fork against the frame with 2 other connecters I made. Also, I pull the wire connection for the tail light. Or if I forget it just pops out.

now I remove the boom tube and the handlebar stem. I fix the boom tube in place between the wheels.You can see the handlebars laying to the right.

Next I clamp the handlebar stem in its place, just in front of the fork, and I am done. A cutout in the front rack allows the stem to sit flush. About 75 seconds total to fold up.

Eleven inches wide; and all the fragile and greasy stuff is inside. The exterior of the package is pretty durable.

Looking at the seatpost extender from the bottom up… First there is a peg of wood that helps hold the bike upright. Then there is the 1 1/8 .058 tube that fits into the seat tube. A 1 1/4 .058 tube goes over it, and it is silver brazed. The .1 1/8 .058 tube only goes up inside the other tube a couple inches. A 1 1/8  .035 tube takes it from there to the top. The .058 tube has big cutouts in it, less material as it goes up. Kind of like a lug, putting the reinforcement where it is needed, mostly at the bottom. Then you see the seatpost (26.8). There is a clamp thing with an arm coming out that pegs into a nut brazed to the left chainstay. An upper clamp on thing pegs into a spot behind the top of the seatpost as you look at the photo. I made a lot of seat post extenders that failed (bent) or were too heavy before I came up with this one. It is not heavy and it will not bend under my 165lbs.

Small parts and connections to hold the bike together 

Upper seatpost attachment. The short tube on the seatpost slides into the tube assembly that is clamped on the lower chainstay/pivot-pin tube (the one behind the bottom bracket). Almost all these little pieces in the following pictures are adjustable where they are clamped to. If I use a bigger tire (which would put the wheel in a slightly different place when kicked under) or different seat, I can adjust the pieces so they will still fit together.

This thing clamped on the seatpost has an arm that engages into the nut brazed onto the chainstay (vertical tube to the right. The bolt on the left provides friction so when I push it up and the tab enters the nut, it stays there.
seatpost assembly secured now.
Boom tube to seat tube connection. A nail drops thru the lower connection to resist pulling forces. If it was not there, the tube would slip out a couple of millimeters or so after some heavy pedaling. This pin (nail) idea has worked great over tens of thousands of miles. The holes have not elongated and the pin has not developed notches. There is probably a more elegant solution to this issue but why fix what is not broken.
Handlebar stem clamped on right side of front rack. Built in recess on the rack to accommodate the stem is seen here.

Left side of fork crown closeup
Upper left fork crown, with short tube (about 4cm long) projecting to the left. It is about a quarter inch inside the pivot pin already.
Fork crown alignment tube is now pushed all the way inside the pivot pin. The fork crown and rear pivot area of the bike are held together now.
The metal tab is pushed down behind the chain rail. This keeps the fork from coming out of the pivot pin.
The fork would still pivot forward or backward if not for this. Look at the 2 short horizontal tubes brazed to the fork leg on the right. They will engage with the chainstay when the fork is pushed into the pivot pin (fork is not pushed all the way in place yet, in this picture). That keeps everything solid. The quick release nut for the front wheel you see here is cut a bit shorter. It rests against the quick release nut for the rear wheel, also cut short. Brings the wheels closer together, making the folded package smaller.

The boom tube about to be hung onto the left leg of the fork crown. The brass bolt drops into the small hole you see there, and it stays in place just fine. Gravity and the confined space between the wheels keep it there. To remove, I just lift it out.
Stem to fork connection. The same clamp tightens it on here and the on the steerer.

Tucked under a table.

fits almost anywhere.

To carry, I lean the leather saddle against my hip and hold the tube where the handlebar stem connects. Or I just grab the horizontal tube at the top of the folded bike.

Here is a airline legal size collapsible box I made. About 60 inches length/width/height added together. Limit is 62″. Plywood sheets drop into internal pockets to assemble from flat.

When I remove the left pedal, the bike just drops in.

To carry stuff, a bag I made loops over both brake levers, and is supported by the rack below. A velcro strap fixes it against the headtube, if I remember to secure it (it isn’t essential). The bag is about the size of a grocery sack.