Retro Grouch said:
Start with the concept that you can never change just one part on a bicycle. There is almost always something else that has to be changed or adjusted to make that new part work.
I've got a fair amount of experience with tandem bicycles and I have several tandeming friends who have done extensive touring. Tandem brakeing requirements, and I suspect yours also, are quite a bit different from mountain bike applications. On long downhills it's common to drag the rear brake for long periods of time which generates significant heat. Hub mounted drum brakes, or disc brakes are desired because they prevent the rim from overheating and causing the tire to blow off. Hydraulic disc brakes, for the most part, are not recommended for drag brake use on tandems because the heat generated can boil the hydraulic fluid or cause it to expand enough to lock up the brake.
Arai drum brakes are commonly used as drag brakes. To use one yu'll need a tandem rear hub that's threaded on both sides. All of the dual threaded hubs that I'm aware of are 145 over locknut dimension, so your frame will have to be able to accomodate that. You'll probably also need a 40 hole rim too. Actually, for a cross country tour, I'd recommend using 40 spokes at the front also but that's another story.
In practice, the Arai brakes are usually actuated by a friction shifter enough to set a drag on the rear wheel. Then the common rim brakes are used to sontrol your speed for curves etc.
There are very few photos of an installed Arai drag brake. You may see some :
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at Precision tandems
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on the Co-Motion website (top 1/3rd)
Since it is a third brake, one tricky item is how and where to mount the extra lever. There are a few philosophies and ways to solve the problem:
1. The Arai drum brake is not a powerful brake – i.e. unless you apply lots of brute force, you won't ever be able to lock a wheel with it – but it can be used for a long long time. So one favourite way to install it is to use a brake lever to activate it. Those who ride with drop bars and STI use a bar-end shifter; others use a clamp on shifter (an old Thumbshifter) or a downtube shifter, perhaps mounted on the stem.
Advantage of such a setup: you apply the drum brake by pulling on the shifter and let it drag (hence the name) all the way downhill; you may need to shift a bit more or less to let it drag more or less, but you do the extra braking (before turns, for example) with the two other brakes.
Drawback: you probably won't use it as much as you would otherwise.
2. You activate it with a brake lever, just like any other brake. You need to install three brake levers, which is what I have done on
my tandem (the lower left one is for the drag brake, and the "normal" left one is for the rear rim brake).
With such a setup, you use the drag brake like any other brake, and with large hands, it's very easy to apply both left brakes at ones.
Advantage: easier modulation, you'll use it more often, probably even regularly with the front rim brake, which means less wear on the brake pads.
Drawback: you can't apply the drag brake at the top of a hill and forget it; just like a regular brake, you need to apply it all along.
I forgot:
To install the drag brake, you need a special hub that is threaded on the left side, to screw the drum brake into. Phil Wood, Chris King and DTSwiss make some ($$), but Shimano also makes a tandem hub. Those are currently available in 140 and 145-mm width (145 is the current tandem width), but often 135 mm is also available (it used to be the tandem standard 15 years ago), so you should be able to find something. Tip: if you buy a new wheel, get 40 or 48 spokes.
On the frame, you don't need a special brazed-on clamp: there is a bolt-on clamp supplied with the Arai drum brake that you simply install on the left chainstay.
) it will make a HUGE difference in the amount of momentum and speed you carry. trust me. they can be had fairly cheaply also.
