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Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

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Old 03-25-07, 08:14 AM   #1
ramalone1
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Disc Brake Drag

My wife and I ride a 2006 Cannondale Road tandem with Avid mechanical disc brakes on both front and rear wheels. The bike rides fine but an annoying problem is disc brake drag especially on the rear wheels. The LBS told me that after the brakes "wear in" it will disappear but that has not happened after owning the bike for 5 months and ridden about 400 miles. I have played around with adjusting the two red knobs on either side of the caliper but that has not helped much. Can someone advise me how to go about adjusting these brakes so that there is no or minimal drag (I know the clearance is only 1mm to play with and I dont think my rotor is warped).
PS: I am sure that this has been dealt with on previous threads but I could not find the information perhaps someone could direct me to a link.

Thanks for your help.
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Old 03-25-07, 11:50 AM   #2
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Right off the bat I'd suggest:

1. Follow Avid's set-up instructions: http://sram.com/_media/techdocs/bbdb...60-install.pdf

2. Make sure the internal spring is set to it's highest tension.

3. Make sure your rotor isn't bent and runs true between the pads. If is a little out of true you can usually straighten small wobbles with hand pressure.

4. If it still drags install a compression spring between the actuator arm and cable stop. In fact, I'd probably suggest that you do this anyway as it goes a long way towards improving the performance of the Avid mechanical disc brakes. I found that a 5/16" x 3/4" spring makes for a nice fit that does not require the use of any washers to preclude the spring from travelling up the return arm. Interestingly enough, the spring I selected came from an assortment pack sold at Home Depot for just under $4. I've since installed several of these on our friends Avid equipped tandems.
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Last edited by TandemGeek; 06-24-07 at 09:57 AM.
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Old 03-25-07, 08:27 PM   #3
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Originally Posted by TandemGeek
4. If it still drags install a compression spring between the actuator arm and cable stop. In fact, I'd probably suggest that you do this anyway as it goes a long way towards improving the performance of the Avid mechanical disc brakes. I found that a 5/16" x 3/4" spring makes for a nice fit that does not require the use of any washers to preclude the spring from travelling up the return arm. Interestingly enough, the spring I selected came from an assortment pack sold at Home Depot for just under $4. I've since installed several of these on our friends Avid equipped tandems.
Can you be more specific as to how the spring improves performance? From what I see, it should give system a little bit of "snap" as the brakes are released, and help those brakes that tend to drag a little bit, but what else it might do, I'm not sure.
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Old 03-25-07, 08:30 PM   #4
ramalone1
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Tandem Geek

Thank you very much sir ! I followed your instructions, readjusted the brakes and installed the compression spring (coudn't find the correct size at Home Depot so used 3/8" by 1 1/8" and cut it) on the rear brake. I also removed any slack due to cable stretch. The drag was eliminated and my disc brakes now work very well.

Sincerely
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Old 03-25-07, 09:45 PM   #5
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Originally Posted by twilkins9076
Can you be more specific as to how the spring improves performance? From what I see, it should give system a little bit of "snap" as the brakes are released, and help those brakes that tend to drag a little bit, but what else it might do, I'm not sure.
What else would you want it to do?

Seriously though, the Avid ball bearing mechanical disc brakes were not designed for use on tandems and the internal return spring is not strong enough to deal with the added static and dynamic drag of the significantly longer brake cable run leading from the brake lever to the caliper's torque arm. Using "slick wire" and Avid Flak Jacket or Aztec cable housing can eliminate some of the dynamic drag, but they don't resolve the static drag that comes from the extra 2.5' - 3' of brake cable that runs the full length of a tandem.

The use of a compression spring sized to fit between the Avid caliper's integrated cable guide and the torque arm provides just enough additional pre-loading on the caliper's torque arm to remove excess slack from the brake cable while, at the same time, helping the torque arm to return to it's fully open position.

What this means to you when operating the brake is that you're no longer losing the first few mm's of brake lever movement to take up the slack in the brake cable which provides a more positive lever feel and linear brake action from the caliper. As you modulate your rear brake the action is also more positive and it allows the torque arm to uniformly return to its fully open position, hence the elimination of the slight dragging noise that is often found on tandems with rear discs.

There are some dealers and owners who have opted to install in-line QBP / World Class Travel Agents or SideKick BPBs (brake pull boosters) on tandems equipped with the Avid BB discs to increase the mechanical advantage of the road brake levers, similar to how linear-pull or V-brakes are integrated with STI and Ergo road levers on tandems. Personally, I think that it's overkill and an ineligant solution that yields a somewhat odd lever feel; however, many teams with the 'brake boosters' report good results. The brake boosters simply pick-up the excess slack that still remains in the brake cable run (that is, unless you also install the compression spring) with the roller-cam's higher mechanical advantage... so much so that the Avid's brake pads don't need to be set as close to the rotor as they do on a "stock" installation to preclude brake drag. The increased mechanical advantage also reduces the intervals between brake pad adjustment which may a be good or a bad thing. If you don't keep after your brake pad adjustment with the built-in red knobs you may start hearing a metallic sound as you operate your rear disc. It's not that the pads have worn down so far that they need to be replaced; rather, they've just worn down enough on the inboard /fixed side to where the outboard / actuating side is pushing the rotor up against the caliper body instead of the brake pad. A turn on the large red, inboard adjusting knob will quickly resolve that problem.

So, at least for me, when I'm asked how to "tweak" an Avid I'll always suggest the compression spring as the first "mod" beyond making the most out of the internal adjustment features. If that still doesn't provide an owner with the right feel or performance -- noting that it really does take about 20 good, hard braking events to start bedding in the pads and rotor -- then I'll offer up the brake booster mods, but only as a last resort. IMHO, simple solutions are always the best.

Last edited by TandemGeek; 03-25-07 at 10:31 PM.
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Old 03-25-07, 10:21 PM   #6
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Originally Posted by ramalone1
(coudn't find the correct size at Home Depot so used 3/8" by 1 1/8" and cut it) on the rear brake. I also removed any slack due to cable stretch. The drag was eliminated and my disc brakes now work very well.
Glad to hear that it worked out for you. As for the springs, for future reference I've attached a photo of the package. Also in the photo are some of the 'other' springs that come in the assortment, excluding the good ones that I've cherry picked and either installed on brakes or that are in my tool box. I found these in the aisle at Home Depot that has all the sliding screen door hardware, furniture leg protectors, and other miscellaneous stuff and not in the hardware aisle or the multi-drawer cabinets with the really pricey, individually packaged odd-ball hardware. I would expect that Lowes would also have them.
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Old 03-26-07, 06:39 AM   #7
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If I recall correctly, Pep Boys has springs as well in assortment boxes where you can buy just the one you need.

Rick
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Old 03-26-07, 06:45 AM   #8
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My Ace Hardware, which is a small one on the corner had a huge selection of various springs available for individual sale. 50c each.

Dave Bohm
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Old 03-26-07, 07:05 AM   #9
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I want to say that there were enough of the correct size springs in this box to take care of 8 or 9 Avid brakes...

I've also surprised myself by finding various other uses for some of these springs, e.g., replacing broken springs in my squirrel-resistant bird feeders and "boosting" the latch mechanisms on my pick-up truck camper shell's rear door.

Not a bad deal for $4.00 as I can't image that I'll ever need to make a special trip to the store for another spring.
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Old 03-26-07, 07:02 PM   #10
ramalone1
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I didn't see that particular package in my My Home Depot (in Pearland, TX) but I will have a nothe rlook when I visit the store again. I will also try Ace's.Thanks for all of the suggestions.
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Old 04-06-07, 10:47 AM   #11
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I don't remember who made them - but we picked up a longer adjusting screw for the Avid disc brake caliper at the 2004 Northwest Tandem Rally. It allows for more tension in the spring. It allowed me to ditch the helper spring. I also use a travel agent in the rear and it helps considerably by allowing me to similar feel to the brakes both front (XTR V-brakes) and rear in terms of lever pull.
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Old 04-10-07, 09:47 AM   #12
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I had the same problem

I had light rubbing on the rear brake. I find Avid mechanicals difficult to keep adjusted. Just removing and re-installing the wheel seems to throw it off. The tolerances between the pad & rotor are so close for Avid's road mechanical brakes, that any little thing like removing / reinstalling the wheel seems to throw it off. Any little frame or fork flex causes some dragging & rubbing as well. But here's what I've found that helps:
1. Adding a booster spring to tension the cable, as discussed in this thread.
2. Following Avid's set up instructions to ensure that the caliper is properly aligned with the rotor makes a big difference too.

Rich
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Old 04-10-07, 10:01 AM   #13
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Originally Posted by Richbiker
I had light rubbing on the rear brake. I find Avid mechanicals difficult to keep adjusted. Just removing and re-installing the wheel seems to throw it off. The tolerances between the pad & rotor are so close for Avid's road mechanical brakes, that any little thing like removing / reinstalling the wheel seems to throw it off.
Rich
We have not experinced any of those issues.
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Old 04-10-07, 11:16 AM   #14
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I can't wait to try this, our 2006 Cannondale suffers from the same problems. Thanks for the tip about the extra spring, this should save me from wanting to switch to rim brakes.
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Old 04-10-07, 05:11 PM   #15
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We have not experinced any of those issues.
We also do not have problems with the brakes rubbing. They just need to be properly adjusted and have the right cables. We do not use the springs mentioned here. We have another set of wheels and with shims do not need to adjust the brakes when changing wheels.
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