|[Top: John S.
Allen's Home Page]
[Up: Bicycling articles]
[Previous: Raleigh Twenty]
[ Next: Spoke Tension]
[contact John S. Allen by e-mail]
|Bikexprt.com Web Site|
How to build up your own tandem crankset
John S. Allen
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 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.
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:
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:
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.
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 drive has some important advantages:
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.
|[Top: John S.
Allen's Home Page]
[Up: Bicycling articles]
|Contents + 1997 John S. Allen
Last revised 5 June 2004