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Introduction: Bicycle Design
Why Upright Bikes?
Why Recumbents Cost More
Types of Recumbents
Seat Height
Handlebars
How Much To Spend?
The Importance of Wheels
Types of Wheels
The Importance of Weight
Frame Materials
Frame Alignment
Parts, Derailleurs etc
Brakes
Suspension
Are Recumbents Invisible?
Acclimatizing to a Recumbent
Hills on a Recumbent
Clip-On Pedals
Racks & Mudguards
Bicycle Trailers
Trikes
How to Ride a Tandem
Credit Card Touring
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Brakes

On any wheeled vehicle, the brakes convert momentum and the pull of gravity into heat. Coincidentally, the bicycle wheel has a large surface area to dissipate braking heat into the air. The rim, tube and tire also act as thermal sinks to absorb the heat produced by braking.

Single bicycles do not usually have problems with heat buildup but a tandem can release twice as much heat energy in the same braking distance as a single bike. Momentum is the product of mass and velocity and two people on one bicycle double the momentum at any given speed.

Amazingly, the rim brakes on a tandem can safely absorb and dissipate the heat of normal braking activity and many tandemists never use a supplementary braking system. The exception is when a heavy (or heavily loaded) team brakes down a long hill and the rims build up heat so quickly that the rim temperature rises to the point where the tire becomes hot enough to be damaged. In that circumstance, the team must slow down or glide without braking or stop and enjoy the scenery while the rims cool.

Wind resistance can be utilized as a brake to avoid the conversion of potential energy to rim heat on a long downhill but most people are averse to approaching terminal velocity on a bicycle. The alternative to stopping, slowing or blasting down hills at freeway speeds is to install a supplementary braking system.

A separate brake lever for the supplementary brake (a.k.a. drag brake) can be routed to the stokers compartment. The stoker just has to be careful not to apply the drag brake without warning the driver

Rear tandem hubs are usually threaded to accept the Arai drum brake. The Arai brake is not a particularly glamorous bicycle accoutrement but it is excellent for getting rid of heat buildup because it has a very large thermal mass to absorb braking heat and a finned surface area to dissipate heat into the air.

A supplementary rear disc brake could, in theory, produce braking performance equal to the Arai brake but disc brakes tend to have a much smaller thermal mass to absorb heat and they are no better at dissipating heat than the Arai brake. Tandemists who have been using disc brakes have had to exercise caution to make sure the disc brake is not damaged and incapacitated by excessive heat buildup.

Disc brakes on road forks have had mixed results. One result is that disc braking tends to send uneven forces to forks. Road forks are designed to flex and because the disc brake mount is only attached to one leg of the fork, some people have reported that they are not too fond of the sensation of applying a disc brake on a road fork. The issue is a different one on the inflexible legs of a mountain bike suspension fork.

Some mountain bike fork manufacturers are learning, by trial and error, how to install disc brake parts to avoid having the disc brake apply forces to the wheel in the same direction as the dropout openings. It has recently become apparent that front disc placement and hub skewers on some suspension forks must be designed to be a lot more idiot-proof than they were previously.

Disc brakes have introduced a new type of complexity to bicycles and consumers must understand some of the physics of disc brakes to avoid getting into trouble. Do not expect manufacturers to supply idiot-proof disc brakes that do not require routine vigilance. There is no such brake, especially on a tandem.

Disc brakes are appealing because the wheel rims do not have to be designed to include a heavy braking surface. Additional weight on the outer perimeter of the wheel has a larger effect than weight near the wheel axle. Weight on the rim will add rotational inertia that will tend to slow the acceleration of the wheel. Conversely, a wheel with a lot of rotational inertia tends to act like a flywheel and may have a positive effect on performance at steady speeds.

Disc brakes offer the potential for less fussy brake maintenance. Disc brakes are also excellent candidates for hydraulic brake cables, which offer much better performance, especially on tandems and recumbents, where long cable runs are involved. On the other hand, disc brakes add torsional stress to the wheel and may affect the longevity of the wheel.

Rim brakes are not a beautiful thing but they are light and they conveniently wear through the rim walls at about the same time the wheel should be rebuilt anyway.

More About Disc Brakes

Discs brakes on bicycles are not new technology, they just became more popular when mountain bikers started using them in the last few years. Disc brakes on mountain bikes give better performance on off-road racecourses that coat the wheel rims with slippery mud that would interfere with rim brakes. However, recumbents (and most upright tandems) are not used for off-road racing. Discs brakes are not an advantage on pavement.

Tandemists who are using the current crop of mountain bike disc brakes on their tandem should keep in mind that none of the current disc brakes are designed for tandem use. All of the current disc rotors are too small and light for tandem loads on big hills with heavy teams. Some of the rotors are too light even for light teams on medium hills. In the right conditions, the rotor will overheat too quickly because of its low mass (and therefore low capacity to absorb braking heat).

Certain people are successfully using existing single-bike disc brakes on tandems, without risk of brake failure. Those people ride in flat places with no big hills. The next question those people may wish to ask themselves is why they have bothered with the expense, complexity and extra weight of disc brakes when rim brakes would have offered perfectly adequate performance.

Rim brakes on a 700c or 26" wheel have a mechanical advantage over a typical 8" disc brake because the radius of the braking surface is much smaller on a disc rotor than on the wheel rim. Much more braking power can be applied with the rim brake (about 10 times more, thanks to the miracle of leverage). On a 20" wheel the relative mechanical disadvantage of a disc brake is smaller than it would be on the larger radius wheel on an upright tandem.

In some ways the 20" front wheel of a recumbent tandem may be a better candidate for a disc brake than the bigger front wheel of an upright tandem. However, there are reasons such a plan may not be attractive. A heavy disc brake on one side of the front wheel will affect steering symmetry and therefore steering feel. Besides one-sided weight bias (and the aforementioned one-sided braking), the issue of overheating and brake-fade remains. Under hard braking the front brake on a tandem supplies up to 80% of the braking force. That means the front brake is more subject to heat buildup. Front brake failure is especially important to avoid because the bike will not make a very quick emergency stop without the all-important front brake.

It should also be noted that an overheated disc brake can completely cease to function. An overheated rim brake may glaze the rim but it will continue to work on a limited basis.