Build a tandem frame from scrap

My 1979 project was inspired by tandems which my friend Sheldon Brown had built.  Please also read his Web page which includes much useful information I won't repeat here. The new information in this article concerns:

• Using MTB and hybrid bike frames;

• An good, easy way to make a bottom tube assembly (which connects the front and rear bottom brackets);

• A simple and accurate technique for aligning the front and rear frames;

• Using spliced sections of tubing to make longer tubes;

• Choosing frames and adjusting frame alignment to achieve the optimum trail in the front fork;

• Cobbling together an idler pulley for the synchronizing chain..

Building a tandem frame from scrap frames is not a project for everyone. But it is many times easier than building a frame from scratch. Most of the work of framebuilding has already been done for you. You have to join parts at only a few places, and you do not need specialized measuring equipment.

You need three frames

You will need three steel frames. Preferably, at least two of the frames -- which will become the main parts of the front and rear frames -- should have 1 1/4 inch diameter down tubes and top tubes. Preferably, the frames should be of good-quality tubing, but not butted (thinner at mid-length). Plain-gauge tubing is stiffer, of importance in a tandem. Besides, you will be joining tubes at locations which would be the thin parts of butted tubes.

The frame to be used at the front should be of a suitable size for the tandem's captain. This frame may be damaged at the rear (broken dropout, bent seatstay). Its front triangle must be intact and in good alignment. A head tube designed for a 1 1/8 inch diameter fork steerer is advantageous, though not necessary.

The rear frame should fit the stoker. In case there may be more than one stoker, a short frame with a long seat tube can accommodate a tall stoker perfectly well. A too-tall frame is not as good but still workable. As long as the saddle can be lowered enough, a short stoker can clamber up onto it while the captain holds the tandem upright.

The rear frame can be damaged in the head tube area. It is acceptable for the down tube and seat tube to be crumpled or bent just behind the head tube, but otherwise, the frame should be in good alignment. 

You will cut up the third frame for additional parts It could be a road frame or MTB frame, but it should be of sturdy construction, especially in the chainstay area.

Sway-backed or hump-backed?

Your choice of frames affects the geometry of the tandem frame, and we need to discuss this now.

A tandem's bottom brackets ought to be somewhat higher than a road bike's, to allow the pedals to clear the road surface in spite of the longer wheelbase. Because of the rough terrain in which they are used, MTBs and hybrids also have higher bottom brackets than road bikes. The usual bottom bracket height of an MTB is 11 to 12 inches (280 to 305 mm).

A good bottom bracket height for a road tandem is 11  to 11 1/2 inches, so you may want to adjust the bottom bracket height a bit.

Seat tube angles are of little importance, as the saddles may be slid forward or backward in the seat posts. The front head tube angle is important, as it affects a bike's steering. Lowering  the bottom brackets will make the frame swaybacked --  rear head tube and seat tube angles steeper, front head tube and seat tube angles shallower. Vice-versa for a humpbacked frame.

Hybrid bikes are made for use with tires with a radius of approximately 350 mm (13.75 inches), and MTBs, approximately 324 mm (12.75 inches). Using a hybrid bike frame at the front of the tandem, but with an MTB front fork and wheel, will require the frame to be somewhat "humpbacked" to achieve the desired front bottom bracket height. This odd geometry is desirable because it will reduce the trail at the front fork.

(Trail, or caster, is the distance between the extended centerline of the steering axis and the middle of the tire-road contact patch. Just as with a furniture caster, the trail causes the wheel to track straight ahead. Even though the bicycle's front fork is bent forward at the bottom, the angle of the head tube places the tire contact patch behind the steering axis.)

Too little trail makes a bike's steering feel unsure, and too much trail makes the steering feel heavy. Due to the greater weight of the cyclists and different load distribution, a tandem handles best with about 28 mm (1.2 inches) of trail, about half as much as with a solo bike.

For the head tube angles shown in the table below and a 12.75 inch (324 mm) outside radius of the tire, the center of the contact patch will be located as indicated below. The couple of degrees' increase in head angle achieved by using a hybrid bike frame and MTB wheel will bring the trail into the desirable range, all other things being equal.

Trail may also be adjusted by choice of a front fork, or by rebending the fork blades. Rebending the fork blades near the bottom, the usual approach, will change the spacing between fork dropouts and brazed-on brake bosses, an undesirable complication.

Some cantilever brakes and V-brakes do allow limited adjustment of the brake pad position, but still, rebending the fork blades is a less viable option with a hybrid bike or MTB front fork than with a typical road fork which is made for use with a caliper brake. Rebending the blades near the top, above the brake bosses, is difficult and runs a serious risk of weakening the fork -- so it is better to establish the trail by choice and alignment of frames.

Getting started building

Are you ready to get started? OK, let's go.

Your tandem will have a dual bottom tube, made of the chainstays of the front frame and a set of chainstays cut out of the "additional parts" frame. The additional chainstays will be brazed onto the front of the rear bottom bracket. The resulting double bottom tube will be extremely stiff against compression and bending..

Brazing is the preferred technique to assemble the parts of the tandem frame. Unlike welding, brazing does not heat the steel of the frames to the melting point. Brazing is quicker to learn, and requires less specialized equipment. Brazing also is more forgiving than welding -- for example, you can braze to a bottom bracket shell without a risk of damaging the bottom bracket's internal threading.

Now we start cutting up frames.

Cut through the chainstays of the front frame just ahead of the rear dropouts. Then remove the seatstays by cutting them off at the seat tube.

Next, bend the front frame's chainstays down and inwards until they are approximately horizontal and parallel to one another. At this time, there is no need to align them precisely.

Cut the chainstays out of the "additional parts" frame, along with a sector of the bottom bracket shell, making a "pair of trousers" assembly. You will braze the "pair of trousers" assembly onto the front of the bottom bracket of the rear frame, facing forward. Grind the sides of the bottom bracket sector so it is narrower than the bottom bracket to which will be brazed, and cut a semicircular recess into the bottom bracket sector to fit under the rear frame's down tube. This way, you can avoid melting brass onto the ends of the bottom bracket shell where you would have to remove it later.

Make sure that all the surfaces onto which you will flow brass are clean and shiny. Clean away enough paint that you will not burn paint as you braze.. A strong chemical paint remover, followed up by a once-over with sandpaper or a file, will do the job, though sandblasting is faster. If the frame is painted internally, you must remove the paint there too, or it will burn. Brazing outdoors is preferable if you can not be sure that you have removed all paint in the area that will be heated.

Now braze the "pair of trousers" assembly onto the rear bottom bracket. Precise alignment is not necessary at this time. Have a pair of scrap bottom bracket cups installed in the bottom bracket shell while you braze, to keep it from warping due to the heat of brazing.

Decide how much distance you want between the tandem's bottom brackets. Modern tandems generally have a spacing of 26 to 30 inches (66 to 76 cm). A taller stoker requires a longer frame. Shorter stokers can be accommodated with handlebar adjustments, so it is safe to make the bottom bracket spacing longer rather than shorter.

You will splice the small ends the "pair of trousers" assembly and the front chainstays, halfway between the bottom brackets.   The neatest way to connect them is to cut them so they have the same diameter at their ends, and splice them using an internal or external sleeve to align them and strengthen the joint. See the page on splicing tubes for instructions.

On my tandem, I cut one set of chainstays longer than the other, and slipped its thin ends inside the thicker ends of the other set. This approach allows easier adjustment during building, but the appearance will not be as neat as with the splicing method.

Aligning and brazing the bottom tube assembly

At the next step. you will align the frames and braze the bottom tubes together.

I made a frame alignment jig using an L-section aluminum beam for a "backbone", clamped to the seat tube and down tube of both frames. This jig positively aligns the frames with each other. You could also use a steel beam, or even a wooden one if it has been jointed and planed so it is straight. Aligning the frames using this jig is the only critical measuring operation in building your tandem!

As the diameter of the seat tubes of my frames was 1/8 inch less than that of the down tubes, I placed 1/16 inch thick scrap metal shims between the seat tubes and the aluminum beam. Wooden blocks on the near side of the tubes prevent the clamps from denting them (see photo below).

The vertical distance (perpendicular to the string and parallel to the centerline of the frame) between the string or rope and the bottom bracket axis is the difference between the tire outside radius and the bottom bracket height. How to calculate this? Here's an example.

My tandem was to use 26 x 1.75 inch (ISO 47-559) tires, with an outside radius of approximately 12.75 inches (324 mm). The desired bottom bracket height was 11 inches (280 mm) rear and 11 1/4 inches (286 mm) front. The vertical distance from the rope to the front bottom bracket axis was therefore 12.75 - 11.25 inches, or 1.5 inches (37 mm); and to the rear bottom bracket axis, 12.75 - 11 inches, or 1.75 inches (44 mm). I placed the front bottom bracket slightly higher because it is far from any wheel, making a pedal more likely to catch if the road surface is uneven.

Aligning and brazing the rear top tube

Remove  the alignment jig to test that the bottom tube assembly is correctly aligned when unsupported. At this stage, you can still bend the bottom tubes to adjust the bottom bracket height.

The bottom tubes alone are not rigid enough to hold the frame's geometry steady for installation of the rear top tube, so reinstall the alignment jig after you have performed the measurement and adjustment.

Angle the rear top tube upwards as necessary so it approaches the front seat tube just below the seat tube clamp. The frame will be weaker if the rear top tube attaches farther down the front seat tube. To angle the rear top tube upward, you may need to saw it off entirely and splice it back into place, or make a saw cut at its top just ahead of the rear seat cluster, then bend it upward and braze over the cut. You will almost certainly have to lengthen the rear top tube to make it reach the front seat tube. Braze the rear top tube to the back of the front seat tube, just below the front seat cluster.

Adding a lateral tube

The frame will be stronger and stiffer if you add a "lateral tube" extending from the head tube to the rear bottom bracket. This tube will be in two pieces, one brazed between the head tube and front seat tube, the other, between the seat tube and rear bottom bracket. The "lateral tube" should preferably meet the head tube just above the front down tube, to strengthen the frame against head-on impacts.

You can use the rear down tube as part of the "lateral tube" if you cut a notch in its underside just ahead of the rear seat tube, so you can bend it down. Then braze over the gap which has been closed up by bending the tube.

Another possibility is just to extend the rear down tube to the top of the front seat tube, but this approach does nothing to stiffen or strengthen the front half of the frame.

The idler pulley and idler pulley bracket

Like the chain of a one-speed, derailleurless bicycle, the synchronizing chain of a tandem (the chain which connects the two bottom brackets) is adjusted so it has the smallest possible amount of slack while still running freely. On a derailleurless bicycle, the adjustment is made by moving the rear wheel forward or backward slightly before tightening its axle nuts. But the synchronizing chain of a tandem does not connect to the rear wheel, so wheel position can not be used to adjust it.

The synchronizing chain of high-quality commercially-made tandems is adjusted using an eccentric bottom bracket for the captain's crankset. Ordinary solo-bike frames  will not be equipped with an eccentric bottom bracket. The large-diameter bottom bracket shell of an Ashtabula bottom bracket, for one-piece cranks, can be adapted into an eccentric bottom bracket for three-piece cranks, but Ashtabula bottom brackets are not generally used on high-quality bicycles in adult sizes.

Sheldon Brown describes several ways to adjust the synchronizing chain. I prefer an adjustable idler pulley. A slotted fitting on the frame allows the pulley to be moved up and down to adjust the chain. .

Place the idler in the untensioned, return run of the synchronizing chain, where it causes very little power loss. The return run is the lower run with single-side drive or rear crossover drive, but it is the upper run with front crossover drive, as shown in the photo. (See my article on tandem gearing for descriptions of these drive systems.) Front drive is advantageous when using an idler, because it places the idler up inside the frame where its bracket can be secured to a frame tube at both ends and is better protected from damage.You could use the same idler for either front or rear crossover drive by using large enough synchronizing chainwheels and placing the idler between the upper and lower limits of the chainwheels where it doesn't interfere with the chain either way.

An idler is quicker to adjust than an eccentric bottom bracket if you must convert the tandem back and forth between a kidback and the rear bottom bracket. With front drive, there is no need to remove the rear pedals from the rear cranks. Just strap the cranks horizontal. Chain slack with the kidback can be adjusted by moving the kidback slightly up or down the rear seat tube.

The return run of the synchronizing chain comes under tension and the driving run goes slack when one rider backpedals against the other. Therefore, the idler must be securely attached to the tandem's frame. Using a rear derailleur's two-pulley sprung cage would risk the chain's coming off.

I'm not comfortable with the wear life of the tiny idler pulleys seen on some commercial tandems. You can make a better idler pulley out of a freewheel, by removing the pawls so it turns freely in both directions. Most one-speed freewheels have a sprocket with 1/8 inch wide teeth, but the wider chain for this type of cog runs perfectly well on chainwheels and sprockets made for derailleur chain. A multi-speed freewheel may be slightly heavier, but offers you a choice of sprocket positions to adapt to different chain alignments, and a lighter chain. The idler shown in the photo is bolted to its bracket with the small-cog side facing in, so the cover plate bears against the bracket.

The slotted bracket for the idler pulley could be made from a length of thick sheet steel, or one of the rear dropouts from your third, "spare parts" frame as long as this is a good-quality, thick, horizontal  dropout. The idler is attached to the slotted bracket with a bolt and washers. If you install the slotted bracket so the idler is in its slackest position with a new chain, you can get away with relatively little adjustment range -- when you can't adjust any more, it's time for a new chain.

Unless you can install the chainwheels first and find the correct position of the idler through experimentation, you will need at least 1/2 inch of adjustment in chain length if you have a choice of even or odd numbers of teeth for the synchronizing chainwheels -- otherwise 1 inch -- but, as Sheldon has pointed out, it is advantageous to use synchronizing chainwheels with an even number of teeth.

The idler has to move more than an inch to adjust the chain by an inch. To get enough adjustment range, you may have to make a bracket out of something other than a dropout, or keep a selection of idlers with different numbers of teeth. The greater the angle the chain makes around the idler, and the closer the idler is to one or another of the chainwheels, the shorter the slot in the bracket can be.

If you use front drive, it is a good idea to place another idler in the return run of the long drive chain, just ahead of the rear bottom bracket. This idler keeps the long, slack return run from flopping around. A two-pulley sprung idler from a rear derailleur is preferable for this use, as it takes up chain, improving shifting with wide-range gearing. Any idler in this position must have a chain cage to keep the chain in place, as this chain is tensioned only by the spring in the rear derailleur.