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Disc brake setup (drop bars)

godders1godders1 Posts: 750
edited December 2011 in The workshop
My Pompetamine versa came this week. Lovely bike but I've never worked with disc brakes before and am really struggling with the setup (Avid BB7).

I've followed the instructions to the letter and have looked at a couple of youtube vids etc but the braking just seems really weak (to the extent that I don't want to go out for a ride) and the levers travel pretty much all the way to the bars. If I try and dial the pads in to tighten things up I get rubbing on the rotors.

Any ideas?

Posts

  • bails87bails87 Posts: 12,998
    You'll have to live with a bit of rubbing at first, if they're anything like mine (BB5s with Apex. I switched to 105 levers and this helped).

    Also, make sure the caliper is dead straight. It can help to wind a lot of tension onto both pads (cable and 'static'), enough to grip the disc between the pads. Then tighten the caliper bolts, then back off the tension, and hey presto, perfectly alinged calipers.....unless your discs aren't straight. In which case, a bit of gentle tweaking can get them straight.
    MTB/CX

    "As I said last time, it won't happen again."
  • godders1godders1 Posts: 750
    Cheers, yeah the setup instructions I've been using involve loosening the calliper, dialling the pads right in so they clamp the rotor, tightening the calliper and then backing off the pads until the rubbing stops (with the rotor off centre and a bit nearer to the active pad). So the calliper should be perfectly aligned. The rotors look straight.

    Have just set up again (for about the tenth time today) using a few tips from an old BR thread I found and been out for 5 minutes and they are better but still not great. I'm taking it to Bike Science tomorrow for a fitting so they should be OK to get me over there (mostly off road) and I'll ask them to take a look.
  • bails87bails87 Posts: 12,998
    The pads also need to bed in a bit too, then the bite will improve.

    If the discs are straight, and the pads are touching the disc (hence the rubbing) then they should bite almost straight away.
    If you've got interrupter/cross top levers.....
    When you pull the main lever, look at where the brake cable leaves the bartape and joins the interrupter levers, can you see it flexing? This can apparently translate into sponginess in the brakes. See from about 2:10 here: http://www.youtube.com/watch?v=WtZ16XM2wSY
    MTB/CX

    "As I said last time, it won't happen again."
  • OuijaOuija Posts: 1,386
    bails87 wrote:
    The pads also need to bed in a bit too, then the bite will improve.

    If the discs are straight, and the pads are touching the disc (hence the rubbing) then they should bite almost straight away.
    If you've got interrupter/cross top levers.....
    When you pull the main lever, look at where the brake cable leaves the bartape and joins the interrupter levers, can you see it flexing? This can apparently translate into sponginess in the brakes. See from about 2:10 here: http://www.youtube.com/watch?v=WtZ16XM2wSY

    +1 on the bedding in. Put a new set of pads on my rear BB7 the other day and the grip is worse than the worn out pads it replaced (which could still catapult you over the handle bars when needed). You seem to be setting it up ok, so bedding it in is probably the problem. Also, try degreasing the rotors with a proper degreaser spray. And lastly, take a bit of sandpaper to the outer rim of the rotor as i've noticed this can help a little (did both before going out today and the brake seemed to be a tad better than yesterday, though still not "somersault over the handlebars" great.)
  • Negative Travel: Many cable operated disc calipers actually move the pad away from the rotor on the initial movement of the caliper's actuation arm.
    FCN16 - 1970 BSA Wayfarer

    FCN4 - Fixie Inc
  • The RookieThe Rookie Posts: 27,756
    Avid BB5 and BB7 are great cable disc brakes, in fact better than cheap hydro's, set up properly they shouldn't be doing what you say!

    Simon
  • godders1godders1 Posts: 750
    Thanks guys, Bike science sorted them today. Still not as "grabby" as I was anticipating but that's probably down to the new pads as a few of you have said. Cheers
  • pdwpdw Posts: 315
    I had similar issues when I fitted BB7s on my commuter.

    As others have said, bedding in can make a huge difference. Also, and I assume you've already done this, make sure that you've taken all slack out of the cable with the adjuster to the point that the arm starts to move if you tighten it any further.

    I now find my BB7s now work perfectly well, although they still feel pretty spongy compared to the hydraulics on my mountain bike. I think fundamentally the problem is that road levers don't pull very much cable, so any compression in the housing translates into a spongy feeling.

    Out of interest, what levers have you got? Mine are 5600 105s - apparently the more recent Shimano levers pull more cable.
  • godders1godders1 Posts: 750
    The bike has an Alfine 8 speed hub hence the levers are Versa 8 (afaik they're the only drop brifters that work with hub gears).
  • MichaelWMichaelW Posts: 2,164
    When fitting new pads I always scrub the rotor and then set off to the steepest hill I can find to bed in the new pads. You need to get the rotor burning hot from braking. Some people like to cool it down with a spray of water. Whatever you do, dont touch a rotor after bedding in, you will get burnt and the rotor will get greasy.
  • OuijaOuija Posts: 1,386
    Also make sure the cable is tight when the levers fully released. A lot of people have too much slack in the cable so that the pad doesn't really start to move until half way through the levers travel and doesn't seem to apply much pressure before you've got the lever all the way back. You then end up trying to compensate for this by simply putting the pads as close to the rotor as possible which still doesn't work very well. Instead tighten the cable at either the barrel adjuster or the break caliper end so that the brake lever is stiff to pull immediately. Then start notching the pads as close to the rotor before noise occurs. This way the pad will bite immediately as you touch the levers, feel like you can't pull the lever past the half way mark and when you do, apply enough pressure to lock up the wheel before the lever is anywhere close to your handle bars. If that makes any sense?
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