Bicycle Technology: The Tandem

Tandem is the Latin term for “at length”, and it is used to describe bicycles for two or more riders sitting in line, each contributing to the propulsion. Although just about every tandem you see these days is designed for two, machines for three or more riders can be built and were in fact used extensively around the turn of the century. The longest tandem on record accommodates 36 riders — who have so far not managed to cover the distance of the machine’s own length. The modern two- man tandem, by contrast, is a highly efficient bicycle.

16.1. Excellent lightweight European touring tandem by Mittendorf.

16.2 and 16.3.American challengers: Burley’s crude but well-engineered Bossa Nova (top) and Samba Mixte. Rear seat stays and chain stays are made of one continuous tube, wrapped around on both sides.

Interest in the tandem has increased significantly in re cent years, so that it is now relatively easy to buy quality machines of this kind in a wide range of sizes. And just as important, tandem parts and accessories are becoming more readily available. That is one essential factor for keeping a tandem on the road, since many of the parts used differ from those installed on other bicycles.

Although tandems are efficient, enabling two riders to cover greater distances at higher speeds than they could individually, they are a bit of a nuisance in other respects. Shear size makes it hard to live with them. Transporting one is at least as difficult as finding a convenient place to store it, and when it does break down, the second rider is stranded as much as the first, making it hard to get help.

All these are mere sober, practical drawbacks, that don’t weigh up against the pure delight of riding such a machine in unison at high speeds and over long distances in perfect cadence. A properly designed and constructed tandem is indeed so reliable that it does not break down more frequently than any other bike — practically never if properly maintained.

The reason why tandems do present a special challenge to their designers and manufacturers lies in the fact that a higher load has to be carried on a longer structure, at the same time propelled by a greater output applied in two different locations. Combining two bottom brackets, two saddles and two handlebars together so that the whole can be balanced and steered safely and predictably is no mean feat, but it can be done when it is designed properly.

Unlike the riders of single bikes, those on a tandem are usually unable to relieve the load when riding over bumps in the road. This often gives the tandem the feel of having lots of dead weight. Actually, the machine it self is not all that heavy: typically 1.6 times the weight of a single bike of comparable quality — or 0.8 times the weight of the two single machines it replaces. What gives the feeling of dead weight is the fact that the wheels are so far apart that the riders’ weight can’t be shifted. Consequently, any jolts really give the bike a beating, requiring an even more sturdy construction of bike and components.

Tandem Frames

The tandem frame is some 60cm (24”) longer than that of a single bike. Since longer members are basically more flexible than shorter ones, it should be obvious that the tandem frame, which also has to carry double the load, must be specially constructed if it is not to flex excessively, which would lead to unpredictable steering and balancing characteristics. Fig. 16.5 shows how the various static and dynamic forces act upon the tandem frame.

16.4. A number of different tandem frame variants. The ones in the UI column are those with high torsional rigidity — the thing that matters most on a tandem.

16.5. Loading sketch of tandem frame during riding, showing reaction forces at wheels and the torque axis.

The dynamic, or variable, forces generate a torsional effect along the axis that runs from the rear wheel axle to the bottom of the headset, as shown in the same illustration. In addition to being strong enough to carry the vertically applied static load, the frame has to be so rigidly constructed that the resulting torsional deformation is minimized. Ideally, this is achieved with a very large diameter tube that runs along the torsion axis. In practice, several different methods have been tried, as shown in Fig. 16.4.

Of the various details represented in Fig. 16.4, the ones on the LH side tend to be the better solutions, since on these, a very rigid member braces the frame close to the torsional axis. The details shown on the right, on the other hand, are obviously designed with at best the static loads in mind (including the recently popular design per detail D). In fact, some of these designs are not even adequate from the static standpoint: none of the designs that provides a low instep, be it in the front, in the rear or for both riders, is statically or dynamically adequate unless extremely heavy walled and large diameter tubing is used. A high top tube has to be used on tandems — even for those women who would prefer a lower instep (fortunately a rapidly receding demand in the US, though still very common elsewhere).

A relatively popular solution is the one with two small diameter diagonal bracing tubes, referred to as twin laterals or mixte tubes. This design is inherently inadequate, especially for a lightweight machine used for touring. Some of the problems of tandem design are lessened by using special tandem tubing, made by at least three of the major tubing manufacturers (Reynolds, Columbus and Ishiwata) and by most of the others on special order in some minimum quantity. Obviously, tandem tubing must be of greater diameter and greater wall thickness. Besides, some special tubes are not found on other bikes, such as the tube connecting the two bottom brackets (referred to as bottom tube or ‘drainpipe’) and the diagonal reinforcing tube. Those two tubes are particularly critical, as are the down tube and the head tube. All these tubes, being both long and heavily loaded, should have greater diameter than what is found even on mountain bikes.

16.6. Dutch recreational tandem. Insufficiently rigid, despite relatively high weight.

Tandem Frame Geometry

To be at all comfortable in the long run, the distance between the two seats is quite critical. Racing tandems are kept rigid by minimizing this distance, and they are a royal pain in the neck for the rider in the rear, referred to as the stoker in tandem parlance, while the one in front is known as the captain. For non-competitive purposes, the length of the rear top tube should be at least 65 cm (26”) — preferably up to 75cm (30”).

16.7. French recreational tandem. Little rigidity due to low rear top tube.

Because tandem frames have some rather complicated joints that differ from those that are standard else where, it is quite common to build tandem frames with out lugs. Often fillet brazed joints are used instead, al though the large tube wall thicknesses justify the use of TIG welded joints, leading to considerable cost and weight savings. In fact, Cannondale builds its unique aluminum tandem frame this way, and that frame is extremely light despite its very large diameter tubing.

Tandem Steering

It took a long time for tandem builders to give up on trying to do the impossible: the earliest tandems and other multi-rider bicycles were invariably designed in such a way that both riders could influence the steering. Although that may seem socially responsible, it did make tandem riding extremely dangerous until this idea was given up. For the last hundred years now, tandem riders seem to accept that only the captain can steer the bike, while the stoker just has to trust him (Although I don’t understand why that should be so, it is an almost universal custom today that the man of a ‘mixed’ couple rides in the front, while the woman takes the back seat.)

To achieve a reliable tandem steering system, the fork should be made of thicker tubing than on a regular bike. The oversize headsets with matching steerer tubes recently introduced for mountain bikes are the answer to a maiden’s prayer for tandem builders and riders as well. These are so much more rigid that they provide an entirely new sensation of control over the bike’s steer ing, especially on less than perfect road surfaces.

16.8. Tandem rear handlebar stem for installation on front seat post.

The front handlebars can be normal, like on any single bike of the same kind, although a slightly wider and perhaps heavier version may be justified to overcome the greater steering forces. For the rear handlebars, I recommend choosing moderately curved bars installed with the ends pointing forward. This shape allows the rider in the rear to reach forward for a low position or hold the bars near the center to relax. An additional advantage is that these forward pointing bar ends pro vide excellent locations to mount the gear shifters. The rear handlebars can either be installed with a special stem clamped around the front seat tube, or with a special clamp that is attached directly to the frame.

16.9. Special tandem rear handlebars with stem.

Tandem Seats

Since most tandems are used as touring bikes, typical touring or mountain bike seats are usually installed. Certainly for the stoker, who usually cannot anticipate road shocks, finishing up fully exposed to them, a sprung leather saddle is recommended. The forward seat post must be clamped in very firmly, since the rear handlebars are usually attached to it: you don’t want panic reactions of the stoker to twist the captain’s seat.

Tandem Wheels

Tandem problem number one is the fact that the wheels are so heavily loaded, especially dynamically. Bent or dented rims and lots of broken spokes are typical headaches on the tandem. With the advent of the mountain bike and the hybrid, these problems should have receded, providing the wheels are treated properly.

More than any other part of the tandem, the wheels suffer anytime the bike takes an obstacle in the road surface. High tire pressures and properly tightened spokes are the clues to making tandem wheels last, providing they are sturdy enough in the first place. That means they should not be selected for low weight: considering you only have two wheels for two riders, even the strongest wheels will weigh less than the equivalent on two single bikes, so the effect of the rotating mass is not so serious as might seem at first.

The tires should be relatively wide and must withstand a high pressure, since the double weight will tend to push them in more than on a single bike. For touring use on regular roads, I prefer 32mm wide tires inflated to 6bar (85psi) or more, although 28mm is adequate for continuously smooth road surfaces. Unloaded tandems may be used with 25mm wide tires on smooth roads. On rough surfaces on or off road, only mountain bike wheels with matching highly inflated tires should be considered. Aramid (Kevlar) belted, puncture-resistant designs are of little additional help, since tandems don’t puncture in the tread area but by pinching between rim and road, against which no reinforcing helps, high tire pressure being the only answer.

16.10. Jesper Soling’s Copenhagen Pederson tandem. A unique feature of this tandem is that the frame can be folded up like an umbrella for easy transportability and storage.

Rims and Spokes

Also the (aluminum) rims should be pretty strong — that means heavy. The introduction of the hybrid bike is a real boon to the road tandem, since it has popularized wheel components that are superbly suited to tandem use as well. Installing mountain bike wheels with their smaller diameter wheels, on the other hand, does not work on a tandem even if clearances are adequate, since the brakes will not reach the rims.

It has long been customary to install wheels with more than the standard number of 36 spokes, requiring special rims and hubs with the appropriate number of spoke holes. If you use 44 or even 48 spokes per wheel, these should be spoked in at least a 5-cross pattern, while a 4-cross pattern is the best you can do for 36- and 40-spoke wheels. The use of thicker spokes does not seem to lead to reduced wheel problems, but one simple trick does: place a small washer under the head of every spoke to fill the distance between it and the hub flange, as shown in Fig. 16.12. This reduces the movement of the spoke bend relative to the flange, thus greatly reducing the likelihood of fatigue-induced spoke breakages.

Wide hubs with solid axles are recommended, rather than models with quick-release. Once more, the mountain bike has provided new sources for tandem components, since there are now suitable solid-axle hubs that are wider as well as stronger. These solid axle hubs should be used with axle nuts with integral washers to reduce the friction resistance against the fork-ends and drop-outs when tightening them.

The Tandem Drivetrain

Two bottom brackets, two cranksets, four pedals and two chains are needed to transfer the riders’ effort to the rear wheel. That allows a number of different con figurations, the most common of which are shown in Fig. 16.14. At a time when it seems the only thing most cyclists think of is more gears, the call for 18- and 21- speed tandem gearing is not unreasonable. In part, this is due to the more pronounced difference between easy going arid hard riding on a tandem: the extremes are more noticeable than on a regular bike: climbing is harder, and cruising is easier.

To maximize the number of available gears, the connecting chain between the two cranksets should be run on the LH side, since no more than two chain-rings are effectively available for the derailleur gearing when one of those on the right is sacrificed for this purpose. On the other hand, if you can live with 10-, 12- or 14- speed gearing (e.g. in level terrain), the latter solution offers the advantage of simplicity and easily available standard bicycle parts. Real tandem cranksets de signed for the connecting chain on the LH side are harder to find and quite expensive. For touristic purposes, TA is the only manufacturer whose range is reasonably widely distributed. Campagnolo’s excellent tandem cranksets are only intended for racing use and are not available with three chainrings on the RH side.

There are other solutions. As can be seen from Fig. 16.14, it is not necessary to connect the rear crankset directly to the rear wheel: when the front one is used, it is easier to shift gears and many experienced touring tandem riders prefer this configuration. Whatever way they are connected, the chainrings over which the connecting chain runs should be relatively large, preferably 36 teeth, which minimizes wear and offers smooth running. That’s not to say it can’t be done differently: the German frame builder Gunther Sattler actually runs a small-pitch connecting chain over tiny chain wheels inside the very large diameter horizontal bottom tube that connects the two cranksets.

The connecting chain should be kept adequately tightened, allowing no more than 12mm (½”) vertical movement. Although this can be achieved by means of a sprung chain tensioner, the preferred method is to adjust the crankset locations so that their distance tensions the chain. This is done by means of an eccentrically held bottom bracket bearing for one of the crank-sets, allowing about 2 cm of forward adjustment.

It is not necessary to keep the cranks of front and rear cranksets in line. Many touring tandem riders prefer to offset them relative to one another by 90’, as shown in Fig. 16.20. The advantage of this configuration lies in the fact that both riders’ cranks are not in the dead center position at the same time, resulting in an overall smoother flow of output. This is achieved simply by undoing one of the pairs of cranks and installing them on the square axle of the cotterless spindle 90’ offset. On one-piece and cottered cranks, the same can be achieved by undoing the connecting chain and Installing it again when the cranks are in the desired orientation relative to each other. In fact, when the latter method is followed, you are not even stuck with 90’ offset but can choose almost any angle.

To accommodate a child stoker on a standard tandem, the rear crankset has to be installed higher. This can be done by means of a special adaptor known as Kiddy Cranks, shown in Fig. 16.21. Although the traditional manufacturer of these items, Andrew Hague, has gone out of business, something similar can be made by any slightly inventive bicycle mechanic using the bottom bracket shell of an old bike and a welded or brazed-on bracket. A custom-built tandem frame could even be ordered with a perfectly aligned adjustable bracket.

In a pinch, one can use standard components to make a tandem drivetrain with triple derailleur chainrings. To do this, use RH cranks on the left as well as on the right. Then the pedal hole of the one installed on the left must be drilled out and a Helicoil insert for a LH pedal installed there. Don’t do what seems obvious, namely use RH pedals on both sides, keeping the threaded pedal hole the way it is, since the RH pedal with its RH screw thread will most likely come undone if installed on the left. Besides, pedals are sold in pairs, while cranks can be bought singly and Helicoil inserts are cheap.

16.11. Something different: Simultaneous hand and foot cranking for the rider in the front (actually the stoker in this case) on the Counterpoint Opus III (photo Steven Kodish).

16.12. Place a washer under the spoke head to minimize spoke breakages due to fatigue on all but the thickest hub flanges.

16.13.Gary Fisher’s Gemini mountain bike tandem. The upward sloping rear diagonal bracing tube (referred to as ‘uptube’) pro vides more vertical than torsional rigidity.

16.14. Tandem drivetrain options. The most common one is the cross-over drive in the middle. Easiest shifting is the one shown at the top. The one-side drive shown at the bottom is least demanding in terms of loading, but limits the number of available gears.

16.15. Probably the first off-road tandem. This Mountain Goat by Jeff Lindsey dates back to 1982.

16.16. Eccentric front bottom bracket

Tandem Gearing

For serious tandem use, only derailleur gearing is suitable, certainly if loaded touring in hard terrain is involved. Three- and five speed hubs have hollow axles which are just not strong enough for this kind of use, regardless of their relative merits from a gearing standpoint.

The need for 18- and 21-speed gearing systems is by no means unjustified. Yet that does not mean that conventional mountain bike systems with the same number of gears are suitable for tandem use. Unlike the mountain bike, the tandem also needs very high gears, with developments of 8.80m or more (110-inch gear), due to the phenomenal speeds that can be reached on level ground.

However, those restrictions really only apply to the choice of chainrings and sprockets. The derailleurs and shifters developed for the mountain bike, on the other hand, lend themselves very well for tandem use. The modern index shifters, unfortunately, only work well with their pre-cut cables, which may not have the right length for tandem use. The one way to get around this is to install the shifters on the front of the stoker’s handlebars, where they are accessible to both riders and work with the normal cable lengths.

16.17. Typical rear bottom bracket detail shows crossover drive and oval bottom tube (also referred to as ‘drain pipe’).

Tandem Brakes

Trying to stop or slow down a machine weighing 150kg (330 lbs) with standard bicycle brakes is a risky affair, especially if one considers the fact that this machine is likely to travel faster in the first place. Downhill, the problem is an even bigger one. Yet tandem brakes can be quite adequate if a combination of measures are taken to make the most out of their properties.

One advantage from a braking standpoint is that the tandem’s weight distribution over the long wheel base does not lead to the tipping forward effect that limits braking on regular bikes. You can brake in the rear and in the front with about equal effect and with very little risk — assuming the brakes themselves are powerful enough.

Probably the most suitable brake arrangement for the tandem is to use cantilever brakes in combination with an additional drum or disk brake on the rear wheel. Amongst suitable tandem drum brakes, the Maxicar is the finest available. With its large drum, it offers superb brake mantle cooling, and its construction is so superior that it assures light rolling as a hub and reliable stopping power as a brake, combined with a ‘buttonhole’ spoke hole design assuring easy spoke re placement. Some tandems are actually used with hub brakes both front and rear, doing without the usual rim brakes altogether. This solution is not recommended, since it places a very high strain on the front fork when braking.

For the normal configuration with two rim brakes and one drum brake, the accepted standard operation is by means of one special lever with double cables that control both rim brakes together and a second lever that controls only the drum brake. Another method is to use the drum brake only as a drag brake and connect each rim brake to its own lever, installing a little ratchet lever (from a derailleur gearing system) for the drum brake.

16.18 and 16.19. Above: Eccentric bottom bracket Below: Drum brake on rear wheel.

Tandem Maintenance

Due mainly to their heavy use, tandems do require a little more attention to maintenance than most regular bicycles. Consequently, here is a brief list of tandem specific maintenance suggestions, most of which apply to normal bikes as well, but are more critical in this case.

Tire pressure:

Check and inflate if necessary before every ride. The pressure should be at least 1 bar (15psi) higher than for the same tires used on a single bike.

Brake adjustment:

Check the brake performance before every ride, adjusting if necessary, to make sure the full braking force can be applied with at least 2cm (¾) clearance between the brake lever and the handlebars.

Spoke tension:

Check and tension the spokes at least once a month, to make sure they are really tight and all equally tensioned, which avoids both wheel warp and spoke breakages.

Crank attachment:

Check, and if necessary tighten, the crank attachment bolts once a month — before every ride during the first 100 miles.

Chain tension:

Check and adjust (by reorienting the eccentric bottom bracket) once every three months. If the eccentric does not allow tightening the chain any further, it is better to replace the chain than to remove two links, since the worn (and consequently seemingly ‘stretched’) chain links run less smoothly and cause wear of the chainrings.

16.20. Aligned and offset tandem crank sets. The offset method evens out the flow of power.

16.21. Kiddy crank for child stoker. Note: install on rear bottom bracket