Are there devices to moderate speed on mountain descents without braking?

Question

In my neck of the woods there are some mountain descents that are pretty substantial: say 600 meters vertically averaging 7% grade, with lots of curves and other obstacles like potholes. Typically the surface is reasonable-quality asphalt; I have a touring bike.

I’m curious if there are techniques or gadgets to moderate my speed, in order to lessen the need for braking (e.g., in a car one can downshift).

As a back-of-the-envelope, computation, consider a 110kg rider/bicycle system coasting down a 7% grade at 50km/hour, or about 13 meters/second. Due to the grade, for every horizontal 13m, s/he descends 0.91m vertically; hence, every second, the potential energy of a 110kg mass dropping 0.91m is added to the bike & rider system and must be removed in order to not accelerate. That potential energy (U=mgh) is roughly 980 joules; i.e., the cyclist must dissipate about 1kW in order to not accelerate.

This surprises me; it seems like a lot of energy to be dumping into the environment. But it's on par with what can be produced by racers to reach similar speeds on the flat.

What I don't know how to compute is how much energy is consumed by air drag, mechanical friction in the bicycle, rolling resistance, etc., to know really what extra a slowing device would need to consume.

One thing I thought of was a dynamo hub; but those seem to consume less than 5 watts, which would make no real difference.

(I should add that I know I shouldn't brake continuously all the way down. My question is about whether there are additional things I can do or add to the bike to bleed off speed.)

Answer 1

There are two parts to your question, and two answers. The first part of your question is whether there are devices to moderate speed on descents. This is a common issue with tandem bikes (and occasionally with bikes designed for loaded touring). Many tandem-specific rear hubs have a threaded left side onto which can be mounted a "drag brake." Typically a drum brake, they can be adjusted for a slight amount of drag; the rim brakes are retained as the primary brake for stopping. The advantage of the drum brake is that it's away from the rim (since excessive heating at the rim can have dire consequences for tube or tire) and they have a high heat capacity. Although no longer manufactured, the venerable and venerated Arai Tandem Drum brake was an example of this.

Your second question is about how one would estimate the drag demand on a bicycle. This is a well-understood, though perhaps less well-known, problem, and is discussed in Section 2 of Wilson and Papadopoulos' Bicycling Science. As you surmised, the potential energy component has to be offset by drag elsewhere: either by aero drag, braking, or rolling resistance. Terminal velocity is achieved at the point where the potential energy loss exactly balances the drag forces generated elsewhere. The coefficient of rolling resistance (Crr) happens to scale exactly like slope, so a change in slope of 1% is exactly like a change in the Crr of .01. Sadly, for this purpose, you won't much be able to rely on Crr for appreciable extra drag -- commonly, Crr ranges from about .004 to perhaps .01. Thus, although in a theoretical sense you must take rolling resistance into account, in a practical sense it dissipates too little energy to matter. Deflating tires to increase Crr on a twisty descent is not just insufficient, it's a bad idea.

Aero drag is more easily manipulable but is also limited in efficacy. Drag area (typically denoted by the product of the frontal surface area, A, and the coefficient of aerodynamic drag, Cd) for a typical cyclist on a typical road bike will range from around .3 sq meters (roughly, 3 sq. ft) up to perhaps .5 sq meters or higher. Aerodynamic drag force varies with the square of airspeed (since the force varies with the square of airspeed, the power demanded to overcome that force varies with the cube of speed) so sitting up, unzipping a jacket, and widening your arms and legs can add some CdA; however, rarely more than ~ .2 sq meters.

That leaves braking, i.e., the conversion of potential energy via kinetic means into heat, and the place for a drag brake as described above.

Answer 2

You don't say what type of bike you are on, but there are some simple things that will help.

1. Sitting up and getting your body as wide as possible will add a significant amount of drag. You can also wear a jacket and unzip it 7/8 so it catches wind.
2. Slightly lower your tire pressure. Don't go so low that you lose stability, but the rolling resistance between 105psi and 120psi will make a difference
3. Find high rolling resistance tires. There are a number of sources for this data based on your bike type.
4. Not directly related, but make sure you use appropriate brake pads and rims/rotors. Some do better with long decants than others.

There are also the obvious comments like learn to descend at speed or switch to a slower bike.

Answer 3

Some electric bicycles have regenerative braking. That's the only thing I can think of that comes close to answering your question.

Theoretically, you could use something like a parachute to slow down, but this would create way too many problems (side winds, chute getting tangled in the back wheel, having to retract the chute when you don't need it anymore, etc).

Answer 4

If you're asking for a DEVICE to control the speed, some tandems (two-seat bikes) have a dedicated brake system called "Drag Brake". From the Sheldon Brown page on tandem brakes:

Friction Control Drag Brake

The most common, and most satisfactory system is to set up the rim brakes, one to each conventional brake lever, and operate the hub brake by a friction-type derailer shifter. This may be a "Barcon", a mountainbike-type thumb shifer, or a Sun Tour Command Shifter.

A friction-type shifter allows the captain to set the hub brake to apply the desired amount of drag, even once he or she has let go of the hub brake control. the captain then uses both hands on the rim brakes to modulate the bike's speed, and to stop if it becomes necessary.

While the most straightforward strategy would be to use a rear wheel with rim + hub brake, one for the handlebar lever and other to the friction-shifter, there are some dedicated hubs to use as drag brakes (although they are most probably cumbersome and difficult to find). One model I found on Google is the Arai Drum Brake (out of production).

Answer 5

I've never heard of a ready-made device to do what you want. The only thing I can think of is to get the fan or magnetic resistance device from a training stand or rollers and somehow attach it in a way that it can be switched on and off. Or even better would be to get a rig such as used in some exercise bikes where there is an aluminum flywheel with an electromagnet arranged next to it, such that resistance increases as the magnetic field increases. (In theory a deep aluminum rim could be used instead of the flywheel.)

But you'd have to salvage parts from somewhere to do any of these, and then do some machine shop work.

Otherwise probably the best solution in your case is a bike with disk brakes.

Answer 6

You could build or buy a drogue chute. It need not weigh very much, being less than a square metre of ripstop nylon and a couple of metres of thin cord. It may even be legal to do so, depending on how your country writes the "no sail powered vehicle" laws (or whether such a law exists). I'd be inclined to make it tear away by attaching it to my body with velcro.

I've done the opposite, and used a small sunshade/tent door as a sail when I had a strong tail wind and I was going uphill on my touring bike. It worked disturbingly well, in that I had to move the main attachment from my backpack waist belt to the seatpost as it was threatening to catapult me over the handlebars (the bike was quite heavy).

As a brake I suggest a cross-style drag chute as they're easy to make and more stable than a round or rectangular chute (rectangles are more controllable but unless you're used to parasailing or kite surfing the learning curve is steep).

The main problem I can see is that mountains often have wind, and specifically the wind can vary dramatically over short distances. For example, if you come out round a ridge you may move from still air behind the ridge to compressed wind on the exposed face, and the change in drag from a chute will be dramatic. In that case you have to decide whether you want to drop the chute onto the windscreen of the car that's following you, or slow suddenly in front of it. Either could be embarrassing.

Answer 7

You could always get a fixed gear and use your legs to slow you down. Getting back up the mountain would present a completely different problem though. Although with the right gear ratio it would probably be possible to go both up and down the mountain on a fixed gear.

Answer 8

Maybe use disk brakes, not rim brakes, if heating is a problem.

I'm used to the idea that people put about 1 KW into the environment just by existing; I'm not sure what the problem is, with losing one extra KW when descending.