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Old 08-21-08, 09:09 PM   #1
adamlaw
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BB7 disk brake tightening while riding

I posted this on "tandem cycling" in May and only got one reply

I have a Bob Brown lugged steel tandem. I have been taking my 10 year old daughter out for rides. Initially after around 100 yards the resistance to cycling would increase and the BB7 disk brake would be slowing down the disk despite not activating the brake. After some messing around I concluded the exposed QR skewer was just not tight enough despite using the 90 degree rule. I purchased a Bob enclosed tandem QR skewer. With this I have been able to go on 20 mile bike rides. However, by the end of the ride on occasion the same problem occurs. Maybe I was just slight short of the 90 degree rule for tightening the QR skewer. I have since read some of the literature on standard forks and disk brakes. I was wondering if the torque of the disk brake is just causing the wheel to go out of alignment sufficiently to make the BB7 disk brake out of true and hence add friction. Unwinding the BB outer adjustment does improve the situation - as does releasing and then retightening the QR skewer. Any thoughts of a more robust solution? Should I go beyond the 90 degree tightening rule so the QR is biting even harder?
I didn't use disk brakes for the front!

I have tried a shimano Deor XT 145 QR skewer enclosed tandem QR skewer with no improved results. I have installed a longer tension screw into the actuating arm of the disk - this doesn't seem to have improved the problem. Today I road for more than 20 miles with my 13 year old. We are planning a 100 mile charity ride. After about 18 miles it rapidly became more difficult to pedal, we got off and the rear break was tight. I had to rotate the adjusting red nob about 4 clicks for it to become free again. 2 miles later the same thing happened again. I need to fix this before we get into more serious training and I don't have a clue as to what is happening. Please give me some troubleshooting ideas.
Technical information

Drop outs - comments from Bob Brown "I also chose to use Henry James Dropouts on this one, which is a departure for me. I typically don't care for them much, but they really fit this application well. The tandem chainstays are larger diameter than most, so the large touring dropouts fit the diameters really well. Most forged dropouts tend to look a little small in these larger stays"

Wheels: Velocity 36 rims
Hubs: Chris King
Davinci Drive
145 mm spacing
Attached Images
File Type: jpg rear drop out.jpg (35.3 KB, 18 views)
File Type: jpg rear wheel.jpg (88.7 KB, 24 views)
File Type: jpg rear BB disk brake medium.jpg (26.7 KB, 28 views)
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Old 08-21-08, 11:09 PM   #2
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The problem probably has nothing to do with you QR skewer. Is your BB7 caliper set pretty tight before the ride? Is it rubbing slightly before the ride? This may cause it to heat up and expand.
Have you tried simply switching the front and rear caliper and test riding it out for a while? Might be worth it too.
What about your cables and housing. Are they the proper length? Is the rear cable tightening when you turn the bars because the housing is too short? Is the adjustment screw on the brake lever loose? Are your cables lubed good?
What about the disc. Is it warped at all? Is it the standard 6 bolt mount or do you have one of the Chris King rotor adapters?
What kind of brake pads are you using? Sintered-metal or organic compound?
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Old 08-22-08, 08:47 AM   #3
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Good troubleshooting suggestions!

Quote:
Originally Posted by sheba View Post
The problem probably has nothing to do with you QR skewer. Is your BB7 caliper set pretty tight before the ride? Is it rubbing slightly before the ride? This may cause it to heat up and expand.
Have you tried simply switching the front and rear caliper and test riding it out for a while? Might be worth it too.
What about your cables and housing. Are they the proper length? Is the rear cable tightening when you turn the bars because the housing is too short? Is the adjustment screw on the brake lever loose? Are your cables lubed good?
What about the disc. Is it warped at all? Is it the standard 6 bolt mount or do you have one of the Chris King rotor adapters?
What kind of brake pads are you using? Sintered-metal or organic compound?
The BB7 caliper is 4 clicks from contacting the rotor - so free and clear. I don't think there is any friction or rubbing slightly. If it were heating up and expanding I would expect that when it cooled down would no longer contact the rotor. In fact, the rotor is tightly clasped on the rotor and it happens relatively fast, I back the adjustment red nob off another 4 clicks so it is free and then it happens again. I don't need to tighten it 8 clicks when I ride another day - it is permanently tighter. Strange.

I don't have a front caliper - I have regular side pool brake on the front. I might consider purchasing another BB7.

The cables are fine - this has happened since the bicycle was brand new and Bob Brown set it up himself - very carefully.

Standard 6 bolt mount for the rotor. The rotor is true. I have had a outstanding local bike mechanic look at it and he thinks the set up is fine. It is a difficult problem to reproduce in his shop - especially during the busy Summer months.

I am using the standard pads that came with the BB7s. Should I try something else?

I appreciate your suggestions.
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Old 08-22-08, 09:20 AM   #4
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It would appear that you have the right diagnosis. Your axle is moving during the ride. The force vectors should be pushing the frame down and slightly forward when you apply the brake lever. This should be pushing your axle further into the dropout. Have you checked that the wheel and axle is fully bottomed out in the dropout before you ride?
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Old 08-22-08, 01:37 PM   #5
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you'd know better than us, but is there any way that the brake cable tension is changing on the bike during riding? From that second picture you attached it looks like the bends in the housing are pretty tight. A bit of flex from the frame could maybe be causing that problem. You could let a little more slack into the cable and see if the problem changes at all.

I think the wheel moving around in the dropouts is another clear possibility. Can you tell if the wheel's moving at all? With a new frame like that you'd maybe be seeing marks on the dropout or in the paint (can't tell if it's painted or not)
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Old 08-23-08, 03:49 PM   #6
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Quote:
Originally Posted by maddmaxx View Post
It would appear that you have the right diagnosis. Your axle is moving during the ride. The force vectors should be pushing the frame down and slightly forward when you apply the brake lever. This should be pushing your axle further into the dropout. Have you checked that the wheel and axle is fully bottomed out in the dropout before you ride?
I think this is probably the right problem to pursue. The very first thing to check is if the ends of the QR axle are at least 1mm clear of the outside of the dropouts. If you hub has been overhauled or adjusted recently, this should be suspect. If the axle ends are flush with, or extend past, the outside face of the dropouts, that is no good and will not work. Axle ends should be 1.5-2.0mm short of the outside face of the dropouts.

The other thought that I'd look into is whether, or not, your cable is sliding easily through the housing. It's very easy to check. Just disconnect it at the caliper end, then grab on that end and pull. While pulling on the caliper end of the cable acutate the brake lever a few times and see if the cable is sliding easily back and forth. There should be very little resistance. If there is, you either have a kink somewhere in the cable, or it's badly in need of lubing.
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