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Concave disk brake rotor

tgotbtgotb Posts: 4,714
edited July 2015 in Commuting chat
Finally figured out that the poor front braking on my commuter was due to a concave rotor. Plenty of information on the web to suggest that concave rotors cause poor braking; braking's great with a new rotor, lever throw much reduced, all good. It's had a lot of use, so I'm neither surprised nor disappointed that it's worn out.

However, I can't quite figure out *why* the braking should have been so poor. The pads had worn to a matching convex shape, so the contact area was presumably the same. After I sanded the pads, they quickly developed wear patterns across the entire surface, again suggesting their shape conformed to the shape of the rotor. The lever throw was much greater with the concave rotor, which may be a clue, though I also can't understand what was causing this. Brakes are hydraulic. Pistons are the full width of the pad, so I don't think the pad was flexing.

Any ideas?
Pannier, 120rpm.

Posts

  • meanredspidermeanredspider Posts: 12,550
    Radial movement of the pad relative to the rotor could cause uneven wear and, potentially poorer braking. The relative positioning of the pad to the rotor could also have a similar effect.
    ROAD < Scott Foil HMX Di2, Volagi Liscio Di2, Jamis Renegade Elite Di2, Cube Reaction Race > ROUGH
  • rower63rower63 Posts: 1,991
    if the plane of the pad/rotor mating surface isn't exactly perpendicular to the axis of the slave piston/cylinder, perhaps the piston could partially jam?
    Dolan Titanium ADX 2016
    Ridley Noah FAST 2013
    Bottecchia/Campagnolo 1990
    Carrera Parva Hybrid 2016
    Hoy Sa Calobra 002 2014 [off duty]
    Storck Absolutist 2011 [off duty]
    http://www.slidingseat.net/cycling/cycling.html
  • The RookieThe Rookie Posts: 27,748
    Disc gets hot, brake surface cools and contracts first and forces the concave shape onto itself, happens on cars as well, new disc and pads.
  • nicklousenicklouse Posts: 81,520 Lives Here
    twin or single moving piston?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • tgotbtgotb Posts: 4,714
    twin or single moving piston?
    One piston each side - a big round one, diameter similar to the width of the pad. The part bearing onto the pad is effectively the end of a tube; contact is purely around the circumference of the tube.

    Brakes are TRP Parabox.
    Pannier, 120rpm.
  • roger_merrimanroger_merriman Posts: 6,155
    Finally figured out that the poor front braking on my commuter was due to a concave rotor. Plenty of information on the web to suggest that concave rotors cause poor braking; braking's great with a new rotor, lever throw much reduced, all good. It's had a lot of use, so I'm neither surprised nor disappointed that it's worn out.

    However, I can't quite figure out *why* the braking should have been so poor. The pads had worn to a matching convex shape, so the contact area was presumably the same. After I sanded the pads, they quickly developed wear patterns across the entire surface, again suggesting their shape conformed to the shape of the rotor. The lever throw was much greater with the concave rotor, which may be a clue, though I also can't understand what was causing this. Brakes are hydraulic. Pistons are the full width of the pad, so I don't think the pad was flexing.

    Any ideas?

    Out of intrest how many miles did this take?
  • tgotbtgotb Posts: 4,714
    Out of intrest how many miles did this take?
    Not sure. The wheel's probably done 6,000-7,000 commuting miles with this rotor attached, but I can't remember where the rotor came from when I built the wheel. It could well have done 1 or 2 cyclocross seasons before that.

    I'm trying to remember how many commuting miles I used to get out of a wheel with rim brakes; can't have been more than 10-15,000 miles, possibly less, so I'm pretty chuffed to be able to spend a couple of minutes fitting a £5 rotor, rather than an hour or more replacing a £40 rim.
    Pannier, 120rpm.
  • jds_1981jds_1981 Posts: 1,858
    Didn't you also have the wrong size adapter on a bike? Was this the same one?
    FCN 9 || FCN 5
  • roger_merrimanroger_merriman Posts: 6,155
    Out of intrest how many miles did this take?
    Not sure. The wheel's probably done 6,000-7,000 commuting miles with this rotor attached, but I can't remember where the rotor came from when I built the wheel. It could well have done 1 or 2 cyclocross seasons before that.

    I'm trying to remember how many commuting miles I used to get out of a wheel with rim brakes; can't have been more than 10-15,000 miles, possibly less, so I'm pretty chuffed to be able to spend a couple of minutes fitting a £5 rotor, rather than an hour or more replacing a £40 rim.

    I have started the last few years to use Stava's components, so i can see just how quickly Bushy Parks gravel paths chew though chains/cassettes, ie not long! though 9 speed chain lasts a little longer than the 1/8 inch chain. still talking only just 1000 miles though!

    I only do 2k/3k a year and split over 3 bikes most on the old MTB so probably a good few years before rotors become a issue!
  • rower63rower63 Posts: 1,991
    I have started the last few years to use Stava's components, so i can see just how quickly Bushy Parks gravel paths chew though chains/cassettes...
    Didn't know what you were talking about, so just looked this up ... very useful feature. Up until now I've used a spreadsheet to log component changes with a view to combining it with Veloviewer's data to get the same info, but this seems much easier.
    Dolan Titanium ADX 2016
    Ridley Noah FAST 2013
    Bottecchia/Campagnolo 1990
    Carrera Parva Hybrid 2016
    Hoy Sa Calobra 002 2014 [off duty]
    Storck Absolutist 2011 [off duty]
    http://www.slidingseat.net/cycling/cycling.html
  • InitialisedInitialised Posts: 3,047
    twin or single moving piston?
    One piston each side - a big round one, diameter similar to the width of the pad. The part bearing onto the pad is effectively the end of a tube; contact is purely around the circumference of the tube.

    Brakes are TRP Parabox.

    Chances are that the calipers were out of alignment. Mine occasionally get knocked out of alignment after sudden or sustained heavy braking.

    I tend to align my calipers by eye as the calipers can move as you tightening them up the with the brake lever pressed and you wont notice until the brakes start singing or you see uneven wear on the pads.

    Has anyone tried fitting Shimano IceTech pads to parabox yet?
    I used to just ride my bike to work but now I find myself going out looking for bigger and bigger hills.
  • tgotbtgotb Posts: 4,714
    Chances are that the calipers were out of alignment. Mine occasionally get knocked out of alignment after sudden or sustained heavy braking.

    I tend to align my calipers by eye as the calipers can move as you tightening them up the with the brake lever pressed and you wont notice until the brakes start singing or you see uneven wear on the pads.
    I don't think it was an alignment issue. I'd been through multiple iterations of realigning everything, replacing pads etc, and nothing was making a difference. I finally replaced the rotor, and everything started working immediately with no further realignment. Agree with your point about aligning them by eye though; takes a bit more finesse than tightening them with the brake levers closed, but the end result is better (probably because the rotors aren't very stiff).
    Didn't you also have the wrong size adapter on a bike? Was this the same one?
    Ha! Yes. I somehow managed to fit a 140mm rotor to a 160mm setup. Worked surprisingly well until the pads wore down to the point where the parts outside the rotor could touch each other; at that point, as you might imagine, the brake stopped working pretty much immediately. It actually was this bike, but on the other wheel (pads and rotor have obviously both been replaced since then).
    Pannier, 120rpm.
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