I've installed an Arai drum brake on our Burley Duet. I've been following the instructions found here, on the Precision Tandems site.
Installation has been smooth and straightforward, so far. But now that the brake is on the hub, and now that the wheel is re-attached to the frame in the rear dropouts, and with the bike currently suspended in a repair stand, I notice a slight amount of drum-brake actuation while the rear wheel is free spinning.
As I turn the pedal and set the rear wheel in motion, the drum brake emits a scraping sound for approximately 180 degrees of each complete turn of the wheel. It is a light sound, but definitely noticeable, the kind of noise that would drive any stoker crazy within 1 minute. The scraping does appear to scrub small amounts of speed from the wheel, but the braking action is not dramatic.
The cable has not yet been attached to the brake's lever.
I've moved the actuation lever back and forth with my hand, but there does not appear to be a location in the lever's extent of travel at which the scraping noise stops. There appear to be only two modes: full braking, or nearly-free rotation marred by the 180 degrees of the aforementioned pad-to-drum contact.
The installation instructions are silent on this matter. Only at this step do they appear to contemplate the precision of the fit:
6) Thread or slide on the outer spacers, locknuts or axle ends but do not tighten at this point. Next move and hold the actuation arm in the "brake on" position while tightening the outer locknut or axle end. This step will help the shoes center up by the tolerances available between the backing plate and the axle.
I followed these instructions. Or at least, I believe I did, meaning that I did hold the actuation arm in the "brake on" position while tightening the nut. Did I do it right? Did I do it enough? I can't say.
Perhaps the most telling clue I can find: With the rear wheel on the workbench, I threaded the brake drum onto the hub. No problem. Then I placed the brake-shoe assembly into the drum. No problem there, either. Then, I held the wheel stationary and began to rotate the brake-shoe assembly within the drum. I did not hear any scraping sound, or feel any resistance, but I did notice a slight off-center eccentricity to the orbit of the brake-shoe assembly within the bounds of the drum's inner cavity.
Is this the sort of characteristic that works itself out after a few (hundred) miles? Or the sort of thing that requires the attention of a trained bike mechanic? Or something else?