How to build up your own tandem crankset

John S. Allen

This article has been translated into Portuguese.

Most commercial tandem cranksets are very similar, and rather expensive. Building up your own crankset gives you a fine opportunity to try new ideas and/or save money. If you know how to assemble and rebuild bicycle components, a tandem crankset isn't hard to build. In fact, you can assemble a fine tandem crankset from solo-bike parts. You can even build a tandem crankset from the leavings in bike shop spare parts bins. Parts for the most common triple cranksets are compatible across many brands and models.

On a tandem, cranksets with relatively large chainwheels are preferable to the newer mountain bike cranksets with tiny chainwheels. The smaller the number of chainwheel teeth and the slower the chain moves, the higher its tension and the more rapid the wear to chain, chainwheels, sprockets and bearings. To the degree possible, use large rear sprockets, not a tiny chainwheel, to get low gears on a tandem.

The simple way to build your own tandem crankset: single-side drive.

Let's look first at the simplest way to assemble a tandem crankset: single-side drive. This is illustrated below, looking down from the top of the tandem. The front crankset is to the right, and the freewheel is to the left. Notice that both chains are on the right side of the tandem: the drive chain from the rear crankset to the rear wheel, and the synchronizing chain which connects the two cranksets.

Single-side drive
^ | Drive chain	^ | Synchronizing chain

Single-side drive requires no special tandem parts. You use two ordinary solo bike cranksets. Because the chain tension from the front crankset is transferred along the right side of the tandem through both chains, stresses on the bottom brackets and cranks are no higher than in solo bike use. You can use almost any cranksets, except the cheapest ones with the spider riveted onto the right crank. A single-side system is light, too: only two (instead of three) of the tandem's four cranks have spiders, and the bottom bracket spindles can be shorter on the left side.

The main disadvantage of single-side drive in the past was that you couldn't easily use triple drive chainwheels. This problem has gone away. Modern wide-range front derailleurs require 10 mm or more of clearance between the largest drive chainwheel and the right crank. And the smallest chainwheel has its own, separate set of attachment bolts. For these reasons, most modern triple cranksets let you install a fourth chainwheel at the outside. Aha! -- your synchronizing chainwheel!

Common cranksets, like the one shown in the photograph below, use 110 mm and 74 mm bolt circle diameters. Use a racing triple chainwheel bolt set (a special-order item from most bike shops) to secure the synchronizing chainwheel to the spider along with the outer and middle drive chainwheels. The long sleeve nuts of the bolt set should mount from the inside, to secure the heavily-stressed drive chainwheels.

Right crank with four chainwheels for single-side drive The three outer chainwheels are attached using a racing triple bolt set, with a thin washer in addition to the spacer between the outermost (synchronizing) and second (outer drive) chainwheels

John S. Allen photo View this photo against a snappy full-screen black background

To clear the front derailleur, your outboard synchronizing chainwheel should be a few teeth smaller than the largest drive chainwheel. Don't use a synchronizing chainwheel of less than 40 teeth, though, or it will wear quickly and is more likely to interfere with the crank.

As the photo shows, the crank spider does not directly support the synchronizing chainwheel. Use a chainwheel with recessed bolt holes, as shown, so the bolts will position the chainwheel accurately and hold it securely. For chain clearance, add a set of thin washers to space the synchronizing chainwheel a bit extra far from the drive chainwheel. The washers which pad the chainwheel spacers of recent Shimano cranksets with thin, dished chainwheels are ideal for this purpose. To improve clearance, use narrow chains. You may have to remove chain-catching pegs from chainwheels and/or the right crank, but you won't miss them: the synchronizing chain takes their place.

So the synchronizing chain runs straight, use a long front bottom bracket spindle, or else chainwheel spacers and washers, to line up the captain's single chainwheel with the synchronizing chainwheel on the rear crankset.

Building your own crossover drive system

In a crossover drive system, the synchronizing chain is on the left, as shown here:

Crossover drive
Synchronizing chain | v
^ | Drive chain

 A crossover drive system lets you use triple drive chainwheels without adding a fourth chainwheel to your drive crankset -- but a crossover drive system is more difficult to build than a single-side system.

Your major challenge: left bicycle pedals are left-threaded so that pedaling forces tighten rather than unscrew them. For this reason, three of the four cranks in a tandem crossover crankset are special. There are two left-threaded cranks with chainwheels, and there is one right-threaded crank without a chainwheel.

Left-threaded pedals and cranks are an invention of the Wright brothers, bicycle builders from Dayton, Ohio. (They also built airplanes). Using parts made for one side of the bike on the other side makes your tandem team into Wrong brothers (and/or sisters), but you can get away with it, thanks to thread-locking compound. Here's how:

• You will need three right cranks and one left crank; three right pedals and two left pedals. If you can find left and right pedals identical except for the threading (usually, double-sided pedals), you will only need one left pedal. Pedals made for toeclips and straps are common in bike shop spare-parts bins, since many people have traded these in for locking shoe-pedal systems. Find a shop that sells a lot of one model of mid to high-end bike, and you stand a good chance of acquiring all the pedals you need at a good price.

• If the left and right pedal shells are different, you will need to rebuild two left pedal shells with right-threaded pedal spindles and one right pedal shell with a left-threaded pedal spindle.

• Aligning the chainwheels is easiest if you use cranks and bottom-bracket axles of the same make and model. If you don't, then be prepared to experiment with bottom bracket axles and chainwheel spacers. Take extra care that the two left cranks are aligned so the synchronizing chain will run straight. Remember, you will need at least one bottom bracket axle with ample length on both ends for the drive crankset, which has chainwheels on both sides. Many mountain bike axles are long enough.

• A crossover system stresses the cranks heavily. Most good mountain bike cranks, and many others, will do the job. Check and recheck the crankbolts and bottom brackets in your first few days of riding with the new crankset. Don't use cottered cranks with crossover drive, at least not on the main drive crankset.

• You must secure the reverse-threaded pedals to the cranks with removable thread-locking compound such as blue Loctite«. This is available at auto parts stores and hardware stores. If you use steel cotterless cranks (sometimes found on Fuji bikes...) and a long pedal wrench such as the Park pedal wrench, you can get away without using thread-locking compound, as long as you tighten the reverse-threaded pedals extra-well. Some people, including a high-end tandem builder which will remain nameless here, have converted crank threading using Helicoil« threaded inserts -- but the necessary enlarging of the pedal hole does weaken the crank somewhat, and if you are going to all that trouble, you might as well just buy a ready-made tandem crankset.

• Use high-grade (racing quality) bottom bracket parts, especially in the main drive crankset. Its bottom bracket must withstand twice the usual stress. If using conventional bottom brackets, use 11 loose bearing balls in each side rather than bearing retainers, most of which have only 7 or 8 balls.

Advantages of crossover drive

Why go to all the trouble to build a crossover system, when single-side drive is so much simpler? There are several reasons.

• A crossover system is more conventional, if that's what appeals to you. As long as all the cranks match, hardly anyone will notice that your crankset is a do-it-yourself job. Very rarely, someone may stare at your bike and exclaim "That's a new one for me, when did Shimagnolo ever made a tandem crankset in the 335SXL MTB gruppo?" You could shrug your shoulders and smile, or else relieve this person's curiosity with a detailed explanation. Who knows, it might be the beginning of a beautiful friendship, and we tandemists do need friends to ride our tandems with us. (But be warned: a person who asks this type of question is either the local Shimagnolo sales rep, an obsessive bike parts junkie, or both.)

• A crossover system lets you use a kidback adapter with its chainwheel on the left side. (You can, however, run a kidback's chain diagonally down to an additional chainwheel on the right front crank of a single-side system. Turn the kidback upside down to place its chainwheel on the right, use symmetrical pedals or exchange pedal spindles, and Loctite« the pedal threads of the kidback.)

Front crossover drive

A major, but little-appreciated advantage of crossover drive, is that you can use front drive. Front drive makes a lot of sense on a road tandem. In front drive, the drive chainwheels are on the front crankset, as shown here:

Front crossover drive

Synchronizing chain | v

^ | Drive chain

Front drive has some important advantages:

• You can use any chainwheel-sprocket combination without chain angle problems, since the long front chain reduces the angles.

• The drive chain lasts longer, because the wear is spread among more links, and so the sprockets also last longer.

• Both riders can see and hear the front derailleur.

• Stresses on the bottom brackets and the frame are lower than with rear crossover drive.

• You can switch easily and quickly from the conventional rear crankset to a kidback by using a chain that runs diagonally from the kidback down to the front crankset, and lashing the conventional rear crankset to the frame so it does not turn. You do not have to remove the rear pedals when switching to the kidback.

There are disadvantages too:

• Both sides of the bike have chains, making it more difficult to lift the bike without getting chain dirt on yourself. The long chain may soil the rear seat tube;

• There's more chain, and so the bike weighs a little more (but probably, this is less important than the increase in drive efficiency due to smaller chain angles)

• You have to make sure that the drive chain clears the right rear crank. Generally, it will if the rear bottom bracket axle is as long as the front one.

• If you ride over big bumps, the long drive chain can flop around. Front drive doesn't make much sense for off-road riding. Even on a road tandem, it's a good idea to put a chain idler just ahead of the rear bottom bracket to control the long, slack lower run of the drive chain. But the need for an idler can become an advantage: a sprung, two-pulley idler made from a rear derailleur chain cage takes up chain, increasing derailleur capacity, and improves shifting.

I'll finish with another advantage: because all the gears are usable, front drive is ideal for half-step gearing -- but that is a subject for another article.

[contact John S. Allen by e-mail]

Last revised 5 June 2004