Story of the hacky tandem

Origins

This summer, my main bike gave me a lot of ... adventure. I replaced the drivetrain, the rear wheel, and the cleats, and got it into tip-top shape before the Seattle to Portland. The night before the STP, I tuned the last bit of slop out of the front derailleur. For the first 50 miles, the bike was wonderful. My riding partner Chris frequently heard me exclaim "it's perfect" and "shifts like butter!" Then, two miles from the lunch break in Spanaway, suddenly the bike felt shifty. I pulled out of the mob. Back tire? No, not flat. Front tire? Nope, it's firm too. Egads! The frame broke!

Chris finished the ride. I made my way home by Pierce County bus and bumming a ride from a friend. I spent the next few weeks shopping for a new bike. I checked out folders, recumbents, electrics. I eyed a tandem... how cool would that be? Wow, they start at $2000.

Finally, I decided that I liked my Softride so much that I'd just buy a new frame from the factory. I had Speedy Reedy move my headset, bottom bracket, and beam to the new frame, and I put everything else on. After a nearly complete dis- and re-assembly, I felt quite empowered -- bike repair isn't even a little scary.

Thoughts drifted back to that tandem. I started reading a bit about tandems on the web, and came across Sheldon Brown's Homebuilding a Tandem page. Hey, why stop at putting components on a frame? Why not stitch together my own frame?

I was sitting in a boring meeting one afternoon in August, 2006, and I ended up doodling around with the build-your-own-tandem idea. I drew these sketches:


I note that I remarked "build it? (can amateurs weld well enough?)" ... and on the same page proposed that it should also have a very strange feature: rear seating for the captain! Mostly, that's a matter of cable routing, but the main trick is arranging for the remote steering.

Parts is Parts

Obviously, I'd need a frame to assemble. I planned for this bike to carry big riders, so that meant building on mountain wheels and brakes. I scanned craigslist and called the LBSs. Everyone pointed me to the same place: the famed Free Rack out front of Recycled Cycles, next to UW. The Free Rack isn't just a metal structure, it's an institution.

There wasn't much there on my first visit, but even if there would be, I needed to know that I could finish the job with parts. I went inside Recycled Cycles, and found the bins: stems, $5. headsets, $5. Handlebars, $5. Cranks: $10 Bottom brackets (behind the counter): $10. Shifters, $5. Brakes, $5. Seatposts, $5. I got the idea: If I couldn't find what I needed on the free rack, I could finish with parts at $5 apiece.

A future trip to the free rack turned up a Nishiki frame with a servicable set of cranks on a nearly-siezed bottom bracket. It was a big 22" frame, perfect for the bigger rider (me). I also hauled home Magna, Greatland, and Huffy frames.

I decided that I should probably try to buy a batch of parts in one shot, just to make it easier to get matching ones. I bought a 20" Trek 820 from Craigslist for $80. I should have checked it more carefully: the Doofus owner had replaced a roller in the chain with a nut (!), which had destroyed the entire drivetrain. I did use the frame, fork, stem, and wheels (after replacing a broken spoke), brakes, derailleurs, and levers.

Frame design

On Sheldon's tandem page, I learned that the main consideration with tandem frame design is to fiddle with the original frame angles to give the bottom brackets higher clearance. Secondarily, the long wheelbase makes the tandem steering very stable, so the fork rake can be reduced.

I laid my bikes out on the shop floor in about the right place and took a photo, over which I sketched the additional frame members I would add. The front frame was the Trek 820 (Frame #1), and the rear frame the Nishiki (Frame #2).

In retrospect, another crucial design consideration for my proposed rear steering was that the head tubes should be close to parallel. Luckily, they happened to end up about right, and the multi-axis rod-end bearings made up for any difference.

Assembled frame. Top to bottom, yellow ovals identify top joint, diagonal tube, bottom joint.

top joint

Sheldon's design made an easy joint between the rear frame and the front by bisecting the rear head tube, giving an easy surface to weld to the front seat tube. That wouldn't do for my bike: I needed the rear head tube intact so that the captain's bars could turn. Instead, I included a 9" tube (that's Frame #3: Magna) between the rear head tube and the front seat tube.

I expect that tube to carry a compression load. (Notice here that I'm trying to sound like a mechanical engineer. I'm not. I'd be delighted to know whether my wild guesses were right, ideally before the bike breaks in use.)

bottom joint

Sheldon's design called for bending the front chain stays up to join the rear downtube. On my longer frame, they wouldn't reach. Instead, I used a second set of seat stays from a fourth frame (Frame #4: Greatland). The bottom bracket tube of the extra stays is bisected for an easy weld onto the outside of the bottom bracket of the rear frame. The dropout ends are cropped and ground until they mate over a 1.5-inch surface with the existing dropouts of the front frame.

I expect the bottom member to carry a tension load: The wheels push up on the ends of the bike, and the riders push down in the middle, so the wheels want to move away from each other.

diagonal tube

This arrangement leaves the center of the frame a four-sided figure, renowned for its nonrigidity. I resolved this by adding a diagonal tube, in two segments, from the bottom of the front seat tube, "through" the rear downtube, to the top of the rear seat tube. I lucked out: that line makes approximately a 70-degree angle with the rear downtube, which is an angle that appeared in two places on Frame #4, so I could apply the same trick of reusing an existing joint, and bisecting the intersecting tube to arrange an easy weld.

I have no idea whether this member is compressed or tensioned. Maybe it depends on the relative weight of the riders.

Note that in four places (both ends of the top connector, and the far ends of the diagonal member) I really did have to fashion my own round-to-round joints. I still have no idea how to do this correctly. What I did in fact was grind away the ends of the joining tube until it mostly fit in the right place, grinding and fitting repeatedly. With time, this got easier, as my grinding wheel took on the radius of a bike tube.

This meant that it was difficult to precisely control the length of the final result. I did the top connector first, so its length didn't much matter. For the diagonals, each half is actually two telescoping tubes. (Happily, Frame #4 included two sizes of tube that nested well together.) I ground the difficult half to fit, then nested the tubes, placed the whole assembly, and extended the tubes to fit snugly. In retrospect, this worked out pretty well: it let me arrange the tubes to snuggle right into the corners of the main frames.

Frame assembly

The next step was to actually weld the parts into one frame.

I read that someone had succeeded in performing some bike welding using the Mapp-Oxy kits from Home Depot ($50 kit); their only complaint was that you go through oxygen bottles ($8) quickly. I tried using one of these kits to braze some scrap, and in the 20-minute life of the oxygen bottle, I couldn't get a consistent-enough flame long enough to make the work piece hot enough to braze. It was infuriating. I took the whole mess back to Home Depot.

The next weekend, I priced out 115V stick welders at Harbor Freight and Home Depot ($100-300). Then I went down to Central Welding Supply in Fremont to return unused goggles and flux. For yuks, I asked what the cheapest welder they had was, and they pointed out the Lincoln 135ST MIG welder, mine for mere a $489. I hesitated quite obviously. The Central guy said, "well, you could go get a cheapie from Home Depot or Harbor Freight, but it's just going to drive you nuts..." Okay, I'm inclined to believe him: the last one did! But no way I can part with $500 for a goofy project like this.

It turns out, though, that Central rents the same welder: $40/day (I finagled the weekend as a day), plus $20 for a tank of Argon/Nitrogen, and another $20 in consumables (welding wire and tips). You should understand that I have a whole few hours of welding experience behind me: I managed to get by my four-week gas and arc welding class in high school, and more recently, my father-in-law taught me to braze. The Central guy assured me that I'd really love the MIG welder.

I came home, plugged that baby in, and set to work on my practice piece. After about three minutes, I was producing almost pretty beads along straight joints. (As it turns out welding over braze metal doesn't come out so well.) In just a couple hours, I had the frame together. I really did love that MIG welder! It was like a magic marker for steel. The most challenging joints were between 1/8" thick and 1/16" thin tube (see below), and even those I worked out very quickly.

Wow! The big question (can I build a frame?) is answered! I tossed it into the trunk, and joyfully headed down to Recycled Cycles to pick out parts to install. I got there an hour before closing. Shido and at least two other shop guys helped me pick out a seat post, a bottom bracket, a headset for the rear bike ("say, how do these threaddy things work?"), a set of cranks, and some handlebars. I bought new three chains and a set of cables.

One helpful staffer started clipping cable housing to fit, and got a couple cut too short before I realized he was measuring to the front handlebars. I explained my wacky plan to his surprise. He even taped all ten pieces to the frame so I could remember which was which. The bike got a lot of appreciative (and other) looks. Shido hefted it and said "boy, that's heavy -- I mean, solid!"

I went home and test-fit stuff all night. The angle grinder got pretty toasty as I flattened off the weld near the rear BB opening and cleaned off other messes. I still had the welder for another day, so I spent Sunday preparing the parts for the remote steering.

Remote steering

The remote steering consists of a pair of levers on each end, coupled with a pair of 1/4" steel rods. The rods are redundant; I rode the bike around the block with only the first rod installed, and the steering felt solid. In the original boring-meeting sketch, I had envisioned a pull-pull cable arrangement. I found this pushrod arrangement on the web used in remote steering for some recumbent contraption (pot, kettle, black).

The rods attach to the levers via Wicks MM-4 rod end bearings. The inner race of these clever bearings are spherical, so besides the required rotational freedom, the bearings allow the rod's vertical angle to vary by a few degrees. This has the benefit of covering over remarkable amounts of slop in construction, and even accomodating slight nonparallelism between the axes of the two headsets.


Front lever plate.

Front lever plate, front view.

Connecting rods.
Rear lever plate, at the bottom of the rear head tube.

The rods are bent out to go around the front seat stays, back in to stay as close as possible to the front top tube (since the front rider's legs have to move around there), and at angles at each end to position the rod end bearings parallel to the lever plates. I wasn't excited about weakening the rods by bending them, but in practice, I don't feel any mush in the steering, even bouncing over curbs. (A nice thing about the simple coupling is that it would be straightforward to replace the rod connection with an alternate design. A boating friend suggested push/pull cable.)

At the ends of the rods are two 3/4" 1/4-28 threaded couplers. Each is drilled out halfway to 1/4", and brazed onto the end of a rod.

The lever plates are rhombuses cut from 1/8" soft steel bar from Home Depot. Yeah, waaaay too thick. 1/16" with a stiffener wrapped around the outside would probably have been just fine. Each has a big hole drilled in the center (7/8" in the front, to mate with the stem; 1" in the back, to accomodate the steer tube), and two 1/4" holes drilled to provide a 2" lever arm.

The front lever plate is welded onto the front stem, high enough that there's plenty of stem left to insert into the front steer tube. The rear lever plate is welded to the bottom of another threaded steer tube from which the fork was removed. Not quite that simple, really: I couldn't find a long enough steer tube for the rear head tube. So I had to assemble a longer one by taking two pieces from two salvaged forks and coupling them with another tube inside. I got the welded plates pretty darn close to perpendicular to the tubes using a right-angle welding magnet ($3.99 at Harbor Freight).

Assembly and fitting

I added as many parts as I could over the next couple weeks, until I had a list of metal parts that needed welding. Our family went to visit my in-laws. You have to understand my father-in-law, Dick. He used to race cars as a hobby. On weekends, he's in the shop, Speed Channel blaring over the sound of the air-powered cutter. He's hacking up the aluminum bumper frame on his new Mini to install an oil pressurizer to improve performance in turns. Other people's houses have a Mother-in-law apartment; Dick has a son-in-law garage.

Projects for the weekend:

  • braze the couplers onto the steering rods.
  • Cut the rear stem diagonally and re-braze it to be nearly vertical: this let me get the rear handlebars far enough back to turn freely without bothering the stoker's backside, and far enough forward to clear the captain's knees.

    Modified rear stem.
  • Braze a segment of steer tube to the front top tube to provide a place to mount a set of non-steering handlebars. This turned out to be a bad idea.
  • Braze together a cable stop to replace the one I cut off to make room for the aforementoned steer tube.
  • Braze on various appendages to support the synchronizing chain idler pulley.

    Synchronizing chain

    The reader will no doubt express shock and amazement to learn that the synchronizing chain is a little weird on this bike, too. First, the cranks are of unequal size. I have a higher cadence than almost any other rider I see, so I thought it would make sense to put a smaller crank on the rear end of the sync chain. I was concerned that somehow the equal-sized cranks were crucial to rider balance, but (a) that doesn't appear to be so, and (b) if it turned out so, it's an easy fix.
    Synchronizing chain.
    That front chainring is not just bigger due to perspective, it's really a bigger chainring.

    One trick for the homebuilt tandem is providing chain tension. Sheldon's cheapest suggestion is the idler pulley, so you can guess what I chose. I cobbled way too much steel into place to hold a salvaged derailleur spring and pulley assembly to serve as the idler pulley. This was a bad idea for several reasons. The first one I noticed was that with the pieces brazed in place, I couldn't tension the spring in the derailleur as I installed it.

    The second problem made the first one moot: a spring-loaded idler doesn't work! Unlike the coupling to a freewheel, the two riders can load the chain in either direction. When the front rider stopped pedaling and I continued, the bungee that replaced the un-tensioned spring loaded up, and the chain slackened and jammed or fell off. I added still more steel to install an adjusting screw, and now I can set the chain tension rigidly.

    Idler pulley assembly, view from left. Idler pulley assembly, view from right.

    It's still not ideal, because it makes an awful lot of rattle as the chain rides through. I'm not sure why. The pulley is probably too cheesy; I should probably use a real sprocket on a bearing.

    If I were to do this again, I think I might consider some wacky arrangement in which the front rider's bottom bracket is mounted independently from the frame, enabling its position to be adjusted up to half an inch front to back; sort of a poor-man's eccentric bottom bracket.

    First rides

    After I had all the parts on, I rode it up and down the street at Dick's place. The steering was much more fun now that I could pedal and brake, too. Dick saw the bike and said, "hey, that's really trick!" The perfect opportunity to offer him a ride: no race car driver is going to chicken out of a ride on an innocent little bicycle, is he?

    Dick and I lumbered down the street for a few houses, made a crazy turn through the neighbor's loop driveway (and their lawn), and back to the house. It worked!

    Rigid handlebars considered harmful

    Starting and balancing were a challenge. I found that I had to verbally communicate every turn. Otherwise, I'd lean left and start to steer left, and Dick would lean right to balance us before he knew what I was planning.

    My brother-in-law Rick joined me for a couple slightly longer test rides, and suggested that both the starting and balancing problems were due to the front rigid handlebars. We realized that the stoker in the front had neither visual nor tactile balance cues about what the captain was up to. Rick suggested putting the handlebars into the steering tube, where he could feel my control inputs. About thirty seconds with a 6mm wrench got the bars out of the rigid stem and into the front steering stem (the one with the lever plate welded on, mounted in the front fork). The next ride went much better. That left the exposed rigid steewith a tennis ball to protect the front rider's equipment. More than once as we were learning to stop smoothly, Chase said "Thank you, Mr. Wilson!"

    Riding with Rick also exposed the spring-loaded idler pulley problem described above.

    A longer trip

    The next day, back at home, I strapped my four-year-old Eliot's trail-a-bike onto the rear seatpost, and the two of us rode the now three-seated contraption down to Recycled Cycles, using only the two big cogs, to look for a cassette to replace the one chewed up by Dr. Doofus' Nutty Nut-Chain.

    We got a lot of great looks. A neighbor pointed out that we appeared to be missing a rider, and I replied "we're looking for a volunteer!" No takers.

    No cassette at Recycled Cycles, either. But a block after we left the store, the sync chain fell off. It turns out that the front bottom bracket had disintegrated, spilling ball bearings into the street (thanks again, Dr. Doofus!). The rear steering paid off: we simply looped the chain over Mr. Wilson, and rode away, the captain still coupled directly to the drive wheel.

    A later trip to Recycled Cycles yielded a healthy bottom bracket; with a new store-brand cassette, the bike was back in business. I also salvaged a chrome rack with an elegant "rust" patina from the free rack, and added toe clips to the captain's pedals.

    The maiden voyage

    The next weekend, my friend Chase bravely volunteered not only to help me move a freezer, but also to test-ride the tandem. We made it about three miles, down to the Burke Gilman and back. We tried lots of techniques for starting and stopping. His main complaint was that his hands hurt on the low handlebars; I'll have to find some cruiser bars to bring the grip position up to a more comfortable spot.

    Stops were a little abrupt at first -- if the passenger steps off before the bike stops, the brakes suddenly have twice the effect, and the bike stops more suddenly. ("Thanks, Mr. Wilson!")

    Starts are still a bit of a challenge. The most successful approach we found was to have the stoker step on and ride normally while the captain runs on the pavement. Once we have enough speed to establish balance, the captain leaps onto his seat, feet floating untethered. Finally, the captain watches his pedals long enough to join the rhythm. It's clumsy, but it was the best we came up with.

    A less-successful approach was to have the stoker provide an initial burst, then stop pedaling so the captain can step onto the bike on a pedal. This didn't work as well for two reasons: First, if the bike is pointed up hill, that first burst of speed doesn't last long enough to maintain balance, and suddenly you're stopping again. Mr. Wilson suggested that we avoid approaches that produce unexpected stops. Second, the captain still has to look at the pedals to figure out where to stand, because of the non-1:1 ratio of the sync chain.

    Chase found turns a little disconcerting, because of the need to lean in when not in control. I still think that the turning stoker handlebars made this more tolerable than it was with rigid bars.

    We found that having our feet coupled was unexpectedly tricky. We are used to the freedom to coast or backpedal whenever we like, but it's quite alarming to suddenly work against the other rider. We practiced verbally coordinating coasting.

    In summary, though, apparently Chase liked the bike enough to want to try riding it from the Captain's seat. That's a good sign.

    Toe clips considered harmful

    The toe clips turned out to be an awful idea, too. When I ride my road bike with my Frogs, I enter them gracefully mid-stroke, usually only a stroke or two after starting. Toe clips, however, require that I coast and stop the stroke long enough to flip the clip to the top of the pedal and stuff my foot in. Stopping the stroke twice is pretty unpleasant for the other rider. Perhaps I'll try to pick up some shoes and some used clipless pedals, but no matter what, the clips have got to go.

    Cost

    Somehow, I derive way too much pleasure from extracting a bike from mostly salvaged junk. Here are the parts that went into this beast:
    pricepartswhere
    $80 Abused Trek 820: frame, fork, headset, derailleurs, brakes craigslist
    $20Two new chainsRecycled Cycles
    $20New cablesRecycled Cycles
    $70Used BBs, stem, crankset, headsetRecycled Cycles
    $12New 7-speed cassettePerformance
    $28MM-4 rod-end bearingsWicks Aircraft Supply
    $24Steel rod, bar & hardware for remote steeringTacoma Screw, Home Depot
    $0Salvaged cranksetWright Bros. Cyclery
    $0Salvaged chainRecycled Cycles free rack
    $0Salvaged stemfree rack
    $0Salvaged handlebarsfree rack
    $0Salvaged seatfree rack
    $0Salvaged frames (4)free rack
    $80Welder rental and consumablesCentral Welding

    I spent a little on tools that I should probably blame on this project, too.

    pricepartswhere
    $20Hole saws. The steel discs are still wedged into both.home depot
    $10Chain toolRecycled Cycles
    $15Crank puller & splined BB toolWright Bros. Cyclery
    $20Welding gloves, cleaning brushesHarbor freight

    All told, this bike set me back about $400. Fifty came from a "commuter reward card" (for biking to work), and another $180 came from selling a broken computer on eBay. More patience and trips to the free rack could have saved as much $100. Access to a welder (at a local high school or community college) could have trimmed another $40. I've been sorely tempted to try my luck with the $110 Harbor Freight welder.

    Later developments

    I've since replaced the front straight bars with a set of high-rise bars from a Murray kids' BMX bike. Chase reports that they're much more comfortable. And more ridiculous-looking, which is a nice bonus.

    I rode the bike solo to work on the Burke Gilman (23 miles), and my friend Jeremy rode the bike home with me that night. Another friend, Chris, rode with me from work to Martha Lake. After that ride, I had accumulated over 100 miles on this crazy machine.

    I decided that it had earned its keep: it deserved a paint job. It's not much to look at, with areas of burned paint and flux slag. As of this writing, I've stripped it back down to individual parts, and stripped and brushed almost all the paint off. When spring rolls around, I'll take it outside with a few spray cans.

    Update. Well, I finally stripped and painted and stripped and painted it. (Turns out, when the spray can says not to apply another coat after an hour but before 24 hours, they're not kidding.) After the second stripping, while the frame was naked metal, my friend Jeremy called. He said he was planning a crazy get-away for his wedding, and he and his fiance thought that departing by tandem bike would be just the thing. (Wow! A real use for the bike!) And, by the way, could it be red?

    Well, there you have it. Liesl hopped over the front bar with ease and grace, and they rode off into the night. The bike appears in the Seattle Metropolitan Bride & Groom article. (end update)

    I'm kind of hoping I can get it in shape to ride the STP next year, or perhaps a trip over the Cascades to Yakima. Victims welcome.

    It makes me want to devise other crazy contraptions. How about a tandem tricycle that doesn't require athletic confidence to start and stop? Or a unicycle? Or a three-seater? The hard part there is getting all those volunteers. And a welder.

    Kudos

    First, a huge thanks to Sheldon Brown and his shop for putting online what is hands-down the most comprehensive guide to everything anyone needs to know about bikes ever. It took me from nervous bike mechanic to building an entire bike. The "homebrew tandem" page was a crucial starting point, but just as importantly, I could learn about every the little component on some-or-another specialized page of the site. The resource is literally inspiring. Thanks.

    Next, Recycled Cycles deserves credit for being just exactly the sort of place one needs to actually pull something like this off in a month: bins of parts and a staff who thinks this sort of behavior is reasonable.

    Charles Hadrann of Wright Bros. Cycle Works in Fremont teaches classes and offers a coop shop space (for a modest lifetime membership fee). He taught me about bearings, wheelbuilding (not used on this project), and cold-setting frames. A salvaged crankset and some tools came from Wright Bros.

    And, of course, I have to give credit to those who actually rode this thing with me: Dick, Rick, Chase, Eliot, Jeremy, Chris, and Christina. I list them in order, because those first on the list had the most guts.


    LBS: Local Bike Shop.
    Updated 2008.08.15.