Bicycle Mechanics - Help figuring out road vs. MTB disc brakes
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12-28-12, 10:34 PM
I recently got a deal on a new frame for a cyclocross/commuter build. It has disc tabs and canti studs. If I can get mechanical discs at a reasonable price I'd like to give them a shot. My problem is determining whether the brakes I buy will be suited for my drop-bar brake levers. I suppose I could buy V-brake levers but that is another expense, and I already have regular levers to use.
I know I need the right cable pull but I don't know which brakes will have that. I see lots of disc brakes listed as road/MTB which doesn't make sense to me. Any suggestions? Thanks.
12-28-12, 10:55 PM
avid bb5 road b7 road, hayes cx5. stuff from tektro and shimano too. i would consider the bb7 road or cx5
12-29-12, 02:46 AM
The Shimano versions are R505, for use with the previous generations of STI's, and R515/CX75 for use with the current (4600,5700,6700,7900) STI's
MTB discs won't work with road STI's due to the differences in cable pull, this applies to the the calipers, the rotors are the same for road, MTB, cable or hydraulic, just get the right size/adaptor for your needs. Wheels (hubs) come in 2 versions, 6 bolt, the most common, and centerlock (originally a Shimano only design, now being used my more brands), both work fine, but the rotors fittings to the hub are incompatible with each other.
12-29-12, 05:59 AM
Drop bar levers have an actuation ratio of about 6 : 1 (IIRC the actual number is 5.6), flat bar levers have about half that.
It's unusual to set up brakes with more than 30 mm of lever movement, so in practice you can count on about 5mm of cable pull for drop bar levers and about 10mm for flat bar. It's not hard to determine how much cable pull a given brake needs, if it's closer to 5mm than 10 your levers should be good to go.
12-29-12, 07:10 AM
If you want to use brifters, the road versions of the disc brakes recommended above are your obvious choice. If you are going to use barends or downtube shifters, Tektro's R520 V-brake levers work extremely well and are very low cost so that opens up other possible models.
Another possibility is Retroshifter (www.retroshift.com) which come in both road and V-brake versions with installed 9 or 10-speed shift levers or come just as the levers and mounts for your own downtube or barend levers. i have these on a Surly Cross Check using 8-speed downtube levers and they work extremely well. They aren't quite as convenient as brifters but beat barend or downtube shifters quite handily.
If you're buying new, the designation is on the box. You want road specific calipers to go with your road specific brake levers. If you're buying used and aren't sure, the orientation of the travel arm is different between mtb and road versions, due to the shorter travel of the road/canti brake levers. If in doubt, do a comparison on the SRAM site or with a known Shimano model.
12-29-12, 12:33 PM
The road caliper has a shorter cable pull to pad motion ratio.. to work with the higher MA in road levers..
Haven't torn one down,but the arm rotates a piece that looks like a crown ramps on on edge
turn rotation into sideways pressure, the slope of the ramp is probably steeper on the road version,
analysis, as I looked at the exploded parts diagram on SRAM's website
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