Tandem Cycling - Rear disc brakes

I am looking at a used bike with rear discs. I have spoken to people who indicated that discs have problems such as dragging. Can anyone comment on this?

Unfortunately, it can vary depending on which disc caliper is being used, the size & type of rotor, and the way that it mounts to the frame.

If you can be more specific as to the brand, model and year of the tandem (e.g., '99 Santana Arriva) as well as the brand, year, and model of the disc caliper (e.g., Hope, Formula, Avid, WinZip, Hayes..) and the size of the rotor (e.g., 160mm, 180mm, 203mm). For example, we had a very old Hope mechanical disc with a 6" rotor on a '98 model-year custom tandem intended for use as a light duty drag brake. In no way could that brake ever replace a rear rim brake as a primary brake on a tandem. However, we had a '02 Avid BB7 Road model rear disc with a 203mm rotor on an '02 custom tandem that provided exceptional stopping power and added heat capacity for long challenging descents. However, we had to tweak it a bit to get rid of some minor disc rubbing and to improve the mechanical advantage / reduce brake lever travel.

In regard to the latter, most of the newer discs can be tweaked or dialed-in to preclude any annoying disc rubbing and to maximize their stopping power through a number of different ways, to include installation of a compression spring (http://www.bikeforums.net/showpost.php?p=4102517&postcount=2)between the reaction arm and the integrated cable stop as we've done on a number of tandems or through the the installation of an in-line Travel Agent (http://www.bikeforums.net/showthread.php?t=133131) or Side-Track BPB (Brake Power Booster).

Disc brake performance can also be degraded by contamination (e.g., oil from hands or lubes on the pads and/or rotor), bent rotors -- noting that rotors can and should be trued -- improper brake-in and glazing, or just poor brake pad material performance. Like rim brakes, sometimes just going to a better brake pad compound will improve performance and eliminate brake squeal.

It's an Avid mechanical on a Burley Rivasso frame. Not sure of the rotor size.

Thanks TG

It's an Avid mechanical on a Burley Rivasso frame.

Rivazza... nice tandem.

You should be in good shape as the Burley Rivazza's frames were designed using a solid I.S. mount and compatible with the Road version of the Avid BB7 and 203mm rotor.

Again, a rotor can get a little out of true and rub but that can be corrected by trueing the rotor, rotor drag with poor mechanical leverage with integrated shifters (Shimano STI or Campy Ergo) can be addressed by using a compression spring and/or an in-line Travel Agent + some fine tuning. The more true the rotor can be made, the better your rear disc will perform.

Avid's stock brake compounds were also pretty good and replacements are available from several suppliers that offer a more aggressive, sintered compound. Bloomingcyclist has posted regarding those in the past and you can easily find his updates by searching on the terms 'disc brake' and his user name.

There are Ultegra shifters.

TG -
I hope this isn't hijacking the OP's original post
-
Any input on truing rotors? One of ours has a slight wobble to it. I spun the wheel a few times to pinpoint the spot, then positioned that spot 180 degrees away from the caliper. Grabbed the high point and almost gave it an experimental shove, figuring I'd be able to tell after a few tries how much effort it would take to tweak it a bit, but chickened out.

Someone over on the Bike Mechanics forum stated that 1mm is acceptable runout? Does that sound right?

To the OP - grab onto the caliper's actuation arm (I don't know the actual term - by "actuation arm" I mean the arm that your brake cable attaches to) and see if it'll go back further. In other words, is there enuf drag in the cable so that the actuation arm isn't retracting all the way? I was having trouble with the above-mentioned wheel. It's the rear wheel, which of course means a longer cable run and that doesn't help. By the time I cranked the red puck adjusters far enuf back so that the pucks weren't grazing the slightly wobbly disc, there wasn't any power left at the lever. I realized that the actuation arm wasn't coming back all the way on its own, then noticed that there's a tiny coil spring between the actuation arm and the caliper body. That spring has a tiny adjuster screw built into it, and that adjuster screw was backed off all the way. The adjuster on the front caliper was cranked in all the way.

So I disassembled the cable run, oiled the parts that run in housing, and tightened the adjuster spring on the rear caliper. After using up about half the adjustment, the brake arm was snapping back all the way. I could set the red puck adjuster knobs to clear the disc and still have power at the levers.

I have found that runout needs to be close to zero to have the best performance. I use an Ultimate single sided truing stand which is quite nice for rotor truing, and I have the Park disc truing attachment for my TS-2.

Park and others make "forks" for actually bending the roters. You locate the area that needs bent, support it on each side with a fork, and bend the portion that needs it. You can also use adjustable "crescent" wrenches for this. Straighter is better.

In all cases, I clean the tools with alchohol or mineral spirits before touching the rotors. I'f I'm home, I wash my hands well and if I'm on site, I wear gloves to avoid contaminating the rotors.

I think 1mm is way too much runout.

Any input on truing rotors?

Park Tools covers it with their usual attention to detail:
http://www.parktool.com/repair/readhowto.asp?id=125

While it's nice to have the dail indicator set and a Park Truing Stand, they aren't essential to the process and neither is the first disc rotor adjusting tool. You can true it right in the brake caliper the same way you can true a bicycle wheel using rim brake calipers as a relative gauge. A small to medium size adjustable wrench can be used so long as you cover the flats with some vinyl electrician's tape to preclude maring the rotor. Most of the little wobbles can be corrected this way with just a little pressure applied right on the out-of-true section of the rotor. If the rotor is actually bent, the truing must be done at the rotor's spokes.

Someone over on the Bike Mechanics forum stated that 1mm is acceptable runout? Does that sound right?

1mm of runout -- the total amount of linear / side-to-side movement that a bicycle brake rotor exhibits -- is OK. If someone is using a Travel Agent they can get away with perhaps 2mm of run-out given the exaggerated amount of travel that's produced by the TA's cam.

...and tightened the adjuster spring on the rear caliper.

For tandems, the spring tension screw must always be all the way in. Someone has mentioned that a longer screw is being offered by someone to beef-up the action of the reaction arm. As noted earlier, I simply install a compression spring between the reaction arm and cable stop which, in essence, accomplishes the same thing. You can fiddle around with different springs to fine tune the action of the reaction arm.

I tried the bike and the discs were spectacular. Best brakes I've ever used.
Lifting the rear wheel and spinning it showed very little if any drag at all.
Braking pressure was perfect.
Now if I can just work on the communication..... :lol: