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How to build up your own tandem crankset
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by John "Cranky" Allen
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Why and How

Most commercial tandem cranksets are very similar, and rather expensive. Building up your own crankset gives you a fine opportunity to try different ideas and 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 common 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 chainwheel 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.

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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 in the picture, and the freewheel is to the left. Both chains are on the right side of the tandem: the primary chain from the rear crankset to the rear wheel, and the synchronizing chain which connects the two cranksets.

Single-side drive

Single side drive (1.1 KB GIF)

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Primary chain

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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.

It is usual to place the synchronizing chainwheel to the inside of a single primary chainwheel. That way, the right front crank is closer to the centerline of the bicycle, improving cornering clearance. With two primary chainwheels, the synchronizing chainwheel can be either on the inside or the outside, but it must be smaller than the nearer primary chainwheel. It may have to be very small if on the inside, increasing stress and wear. A triple crankset can allow the synchronizing chainwheel to be on the outside, where it need only be slightly smaller than the outer drive chainwheel -- with somewhat reduced cornering clearance.

But what if you would like to use triple primary chainwheels? It can be done! Modern wide-range front derailers require 10mm or more of clearance between the largest primary 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.

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 primary chainwheels. The long sleeve nuts of the bolt set should mount from the inside, to secure the heavily-stressed primary chainwheels. For chain clearance, add a set of thin washers to space the synchronizing chainwheel a bit extra far from the primary chainwheel.

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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 primary) chainwheels.

Right crank with 4 chainwheels (25 KB JPEG)

To clear the front derailer, your outboard synchronizing chainwheel should be smaller than the largest primary chainwheel. Don't use a synchronizing chainwheel of much 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 if you are using four chainwheels. Use a chainwheel with recessed bolt holes, as shown, so the bolts will position the chainwheel accurately and hold it securely. You may have to remove dropped-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, preferably also place the single front chainwheel as far outboard as possible without the chain's interfering with the crank, so you can use a shorter bottom-bracket spindle. If you are using the same type of crankset at the front and four chainwheels at the rear, both bottom-bracket spindles will be the same. You might use a longer bottom bracket spindle instead, but this will reduce cornering clearance.

Thanks to Stu Giesecke for the photo of his tandem with single-side drive, synchronizing chainwheel on the outside. Says he: "I have ridden a same-side setup like this for thousands of miles. thanks, SB."

Stu Giesecke's tandem with single-side drive

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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
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Crossover drive (1.1 KB GIF)

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Primary chain

 

A crossover drive system lets you use triple primary chainwheels without adding a fourth chainwheel to your drive crankset

A do-it-yourself crossover drive system is more difficult to build than a single-side system. You must use a three-piece crankset for the primary crankset, so it can have chainwheels on both sides. Square-taper or ISIS cotterless cranks and bottom brackets are suitable and widely available.

Wrong-Brothers Cranks and Pedals

To build a crossover drive system with cranks made for solo bikes, you have to put right cranks on the left side of the bicycle, and a left crank on the right.

Left bicycle pedals are left-threaded so pedaling forces tighten rather than unscrew them. For this reason, three of the four cranks in a tandem crossover crankset are special. Two left-threaded cranks have chainwheels, and one right-threaded crank has none.

Left-threaded pedals and cranks are reputedly 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:

A pair of Wrong Bros. pedals -- right spindle in left pedal, left spindle in right pedal.

DSCF0134wrong bros pedals.jpg (17350 bytes)

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 good reasons.

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Front crossover drive

With crossover drive, you can use front drive. Front drive makes a lot of sense on a road tandem. In front drive, the primary chainwheels are on the front crankset, as shown here:

Front crossover drive

Synchronizing chain
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Front crossover drive (1.1 KB GIF)
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Primary chain

Front drive has some important advantages:

Chain idler

There are disadvantages too:

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.


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