2 speed folder redux

New frame for the 2 speed folder

After riding my 2 speed folder around Mt Hood last year (168 miles, 10,500 ft climbing, 15 hours) it struck me that this versatile bike has a lot of potential, and worthy of a specific build of its own. The frame was left over from a previous frame upgrade project. I needed to bend the rear end of it from 135mm to the 110mm width (for the BMX type single speed hub) and I altered the fork to fit the narrower 74mm front hub. That got it on the road, but making a frame from the ground up would make a better, lighter, and smaller (when folded) bike.

The basic design for folding is the same as my previous portable bikes. This 2 speed version, discussed on a previous post  folds down to 28 x 19 x 11″ in one minute. I can put it in my lap on a bus. Makes a solid package that is easy to carry. The new version is one inch shorter when folded, and a pound or so lighter. The current one is about 24lbs and a bit overbuilt. Some tubes on the new one are of a smaller or of a thinner gauge.

This 2 speed format is simple, light, and is really all I need for day rides or commuting. There are 2 chainrings, and one 15 tooth freewheel cog, tensioned by a derailleur that is set to stay positioned below the one cog. The main gear (66″, 52×15) gets me pretty much everywhere. I use the lower gear (48″ 39×15) on longer, steeper hills. I shift it manually by deflecting the chain onto the smaller chainring with my foot, or guiding it into the bigger chainring manually with my finger (while riding usually). The upshift is not as easy; I can do it while riding by reaching down and guiding the chain. But it gets my fingers dirty so I tend to avoid shifting unless I really need to. But having the low gear makes climbing mountain passes so much easier. And there is always one more gear- walking. Sometimes thats a nice change of pace!

The old bike
The old 2 speed folder at Buzzard Point, on my 168 mile day ride this past fall. Mt Hood in the background. Black tote bag hangs from brake levers and straps to head tube.
folded up.jpg
The old one, at a bus stop, with tote bag. It’s way too easy to bail out of a  ride…
The New Bike
Front bag hangs off brake levers, with a lower velcro strap as well. Smaller wheels make a bag like this possible. National forest service colors.
At Council Crest in Portland. The fenders negate most of the weight saving I made.
The most obvious change with this new bike is the triangulated seat post extender.
When folded up, this bike is a steel fortress. I can stand on it without bending/breaking anything. The exterior is mostly just blunt and durable.
Folded up, it’s eleven inches wide. It can sit upright like this, or on its side as seen below. It’s 27 inches tall. I could throw it down a grassy hillside,watching it tumble down to the bottom, and it would probably still look like this. All the components are clamped together.
19.5 inches tall on its side.
Traditional drop bars are what I wanted, and are a necessary part of the design. These are 42mm wide, and a perfect fit around the stuff above above the wheels. One side offers up a flat stable space to rest the folded bike on the ground.

A little video of how I fold it up. Takes about a minute. I made fenders for this one, because I live in Portland, OR, and it rains a lot here. That negated most of the weight savings I managed to accomplish, but it had to be done.

Framebuilders notes:
Here are the numbers and details of the new finished bike: 71 degrees head and seat tube, 42″ wheelbase, 17.5″ chain stay, 13mm fork offset, 10 3/8″ bottom bracket height with 40mm tires. The trail calculates out to 59mm, which is in the same range as big wheel bikes.  Stability with a capitol S.
Fork steerer is a 1″ threadless. Sellwood cycles machined the fork crown and head tube, thanks guys! Brakes are 500 reach side pulls, and frame is designed to have them contact rim at fullest pad extension. This will allow up to a 406/42 tire. A studded Schwalbe winter tire (406-42) fits with about 4mm clearance below the brake. Fenders are set at about 8mm clearance. The rear brake is on the chainstay, so the fender does not have to fit beneath it. For the front brake I made a part brazed to the fork which allows fender attachment on either side of it, so the fender does not need to fit under the brake. Tubing is mostly 4130 cromoly steel from Aircraft spruce, brass fillet brazed. Sleeved joints and most small stuff is silver-soldered. Almost all the tubing is .035″/0.9mm wall thickness. Exceptions are clamping tubes, .058; I try to keep them short. Seat tube is 1 1/4, 7″ long, a beefy .065 wall thing reamed to fit a 1 1/8 seat post extender. Fork blades, dropouts, and head tube are from Nova. Steerer is 1″ .058 cromo tube. The 1″ steerer commonly avaliable for bicycles typically has a thicker lower section, but with the reduced leverage of a shorter fork and my 160lbs I think I can get away with this .058 straight gauge tube (and have been doing so for years). One inch .035″ tubing is used for the truss-like frame part that connects the front and rear of the frame. The whole thing was built using straight edges, squares, and a drawing on a board. I don’t have any frame-building jigs like the pros use. Fork was jigged up on a piece of flat plywood, using squares, shims, and pencil lines.

I took an inch off it’s height by making the seat tube shorter. The rear triangle has a shallower triangulation, and the seat post extender is longer now, but I think it’s plenty strong. I could take another inch off the height because there is room to do that. Maybe for the next one… if that ever happens.
Steel as a frame building material
Why use steel? One reason is that steel is the only material I have the skill to work with. But it may be the ideal material for a bike that will see rough handling. Aluminum and carbon fiber for framing material are lighter, but have compromises I am not willing to accept. Carbon fiber abrades easily;  aluminum needs heat treatment after welding, and the tubes need to be bigger for similar strength. Neither material is a good choice for clamped slip-joints, which are all of my frame connections. Steel just makes sense for a project like this.
Weight considerations
I could drop the weight a little (probably less than a pound) by using the hardened thin wall steel bicycle frame tubes wherever possible, but it doesn’t seem worth the effort. The tubes would be more vulnerable to dents, and working with that stuff is harder for a hobbyist like me. Harder to cut, file, and cold-set, no thanks; straight up aircraft 4130 cromoly works just fine.
Titanium could save some weight on a bike like this, but there are many barriers to using it. It is costly, and many of the tube sizes I would need are unlikely to be available. My design uses a dozen different diameters and gauges of tubing. Also, I don’t have the machine shop to cut the stuff; let alone the equipment or skill to weld it. Using titanium may save a pound or so, but the cost benefit ratio is just not there for me.
Very expensive parts could pare the weight down maybe a couple pounds (carbon crank, titanium seatpost, spokes and bottom bracket, Chris King hubs, carbon rear derailleur). Going tubeless may lighten the wheels a bit. But I am okay with the final weight of 23 lbs (before fenders). If it ever got stolen, that would be a lot of $$$ down the drain. A lighter bike is nice while I am carrying it, but I spend very little time doing that. Most of my bike time is spent riding the thing, and I would hardly notice the difference then.
The old 2 speed was over-built, stronger in areas than necessary. I used some smaller and thinner gauge tubes with the new version, where possible. Ended up saving about 1.5 lbs over the older bike. But the new one still seems durable enough for endless miles on terrain so rough I can’t see because of my shaking eyeballs.
 I remember in the 1970s the general advice for bike buying was to tilt a bike to the side and push against the cranks with your foot to see how springy it was (it was better to do it when the salesperson wasn’t looking). A springy bike would have lightweight tubing and be more fun to ride, where a stiff feeling indicates a heavy bike with no flex. The Schwinn Paramount was springy, while the Varsity felt as unyielding as a concrete wall. My older folder, while not unyielding, was closer to the Varsity, while the new one seems springy enough. The new frame/fork/seat post extender with all its nuts and bolts on it weigh about 7 1/2 pounds, while the old one is 9 lbs. To compare, a good quality butted steel frame (with the steel fork) of a road bike may weigh 6 lbs. The extra weight of this folder is not that much of a compromise, considering this bikes impressive multi-modal capabilities.
Some construction pictures:
A new bike always starts with a fork, and there isn’t anything quite like this one out there.

The side pull brake bolts on to a collar that extends down from the tube which clamps onto the steerer.

backside of the fork crown.

The steerer has a cutout that nestles on top of the brake mount tube. This indexes the steerer-fork connection. Makes putting it together quicker, and if the clamp bolt ever failed, the fork would not rotate.

front of fork, with steerer inside it.
Some bike tubing, getting ready to come together.

Bike frame in progress. Folding chainstay done. The drawing for the bike is on the plywood in the background. The main tubes connecting the front and rear are 1″ diameter .035 wall.

This picture shows the pivoting connection of the chainstays. It allows the rear wheel to fold under; the tire resting against the bottom bracket shell. I had to take this joint apart and re-do it, because it was almost one degree off, making the rear wheel crooked compared to the seat tube line when folded under. I suppose at the time I was not that focused on getting that important joint right. Fixing that was was possible to do with brass fillet brazing. I heated the brass, brushing it away and removed the tube. Then I mitered it correctly and brazed it in place again. If I had welded that joint it would not be possible to fix it.
Now that’s a busy fork, finally finished except for paint. Lots of attachments going on. Brazed on pieces for securing bike parts and fenders are seen. A fender reach-around is visible, providing attachment points for front and rear fender halves while avoiding the caliper brake and the steerer clamping functions. 2 items to the right secure the fork to the side of the folded unit. A short fat tube on the left holds the handlebar stem. 2 small tubes behind the right (your right) fork blade connect the main boom to the inside of the folded bike. Near the dropouts are small curved tubes that accept the front and rear fender struts. Small pinch bolts will hold the struts in place.
frame components painted with one coat of rustoleum.
Principal frame components painted with one coat of rustoleum, against the drawing of the bike on a piece of plywood.
Some details
Non-standard components
It’s built with all standard  off the shelf stuff with the exception of the following: A Phil Wood 74mm 32 hole front hub is used. These may still be available from the company even though it is not seen in the catalogue. Rear hub is Shimano DX, with single 15 tooth freewheel cog. Old grand comp 500 reach brakes. Sun CR18 406 rims. 26.8 seatpost fits perfectly in the 1 1/8 .035 seatpost extender tube. MKS quick-release pedals.
The fenders
My first thought was to just do without them to save weight (and the considerable effort involved in making and fitting them). But that limits how much riding this bike will see by too much. Summer is the only season I can count on dry roads and small chance of rain. I don’t like getting sprayed by dirty road water, so I have to use fenders. I doubt I will ever take them off unless I move to a drier place.
These fenders are flat strips; 1/16″ thick cheap hardware store aluminum strips. I bent them around a paint can to give them a curve. While a curved and rounded fender will deflect the water better, there are advantages to a flat strip. Any objects stuck on the tire are more likely to be ejected out the side, rather than jamming somewhere in the fender space. Cleaning mud out of the fender is much easier; all it takes is a stick.
On the negative side, although I find them as effective as rounded fenders most of the time in keeping me dry, speeding thru standing water can cause spray to escape off the sides. I go slower thru standing water to keep from getting wet. Rounded fenders do a good job of keeping the bike clean; flat fenders less so. Another consideration is that flat fenders may need an extra strut because they are more flexible.
The front fender struts are attached to the fork with pinch bolts. If a stick gets trapped in the wheel, as it rotates it will push against the strut, bending it, and causing the fender to jam against the tire. This could stop the wheel if the strut had a no-slip connection (like an eyelet). But with a pinch bolt connection, the strut will pull away, and not jam the fender against the front wheel. Some over the counter bike shop fenders like the ESGE come with an emergency release built into the struts, in case this occurs.
Seat post extender
seat post extender, with seat post and saddle in it.
Seat post extender, with seat post in it at the upper end, and a wood dowel below.
The new triangulated seat post extender uses only .035 tubing throughout, and is much lighter. The main tube extends into the seat tube only about 2 inches. The rearward part of it is braced against the tube extending from the rear of the seat tube. There is a wooden dowel inside the bottom of it that contacts the ground when the bike is folded , keeping it upright.
The upper part of the seat post extender triangle is silver soldered (vs brass brazed) against the main 1/1/8 .035 tube, to reduce distortion, and allow good seat post fit. It provides a lot of strength where the end of the seat post is.  So far it seems quite stiff and is probably a better design than the previous, heavier, straight post.
Some folding attachment details
I’ll end this post with some photos of the little metal gizmos I made that allow me to fold the bike up quickly and securely. Nothing fancy here, just simple bits that work every time and don’t get in the way. I’ve make a lot of them over the years, trying to improve them with each version. They take a long time to make.
The tube sticking out of the fork crown fits into the rear pivot opening. The fender wrap-around attachment is easily seen.
Rear wheel pivot; wheel folded under. Chain held in place by small rail tab. Rear brake clears chainring by a few millimeters.
A tube with an opening fits into this thing. The screw is turned once, it enters the opening, and the tube along with everything it’s attached to stays there. This secures the truss tubeset. If you understand all that, you are really paying attention.
Close up of the (green) seatpost extender in place. The main tube extends into the seat tube only a couple inches. The rear part braces itself against the rearward tube and aligns the assembly at the same time.
The (green) end of the handlebar stem indexes against the collar that sets the headset adjustment. When I put the handlebar stem in, it’s aligned with the wheel automatically.
That’s a nail with a coin brazed at the end that keeps the lower tubes together. When I put the main truss tubeset into its place, I tighten the clamps. But the lower tube can still pull out a couple millimeters under heavy pedaling because it is under tension (the upper tubes are in compression). So I drop this nail in it to lock it in place. This simple solution has worked great over many of my bikes for tens of thousands of miles.
Pin seen in place here. If I forget to drop the pin in, I can hear it jingle against the tube while riding.
Rear brake, and rear wheel hinge seen.
The 2 items clamped to the seat post hold the seat post assembly against the frame are seen here. Upper post fits into main rear wheel pivot. Lower device pivots up and is captured in its mating tube; then secured by one turn of a bolt.
Front fender. Struts held in place by set screws.
Rear fender. Mudflaps are from a plastic peanut butter container. I added another strut after this picture was taken.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s