Bicycle Mechanics - Specialized TriCross - what's inside the top tube?
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02-22-12, 11:51 AM
I had the following thing happen on my Specialized TriCross with disc brakes yesterday: when I applied hard pressure on my rear brake lever, something snapped with a relatively loud bang and my rear brake instantly went dead. (I was intentionally trying to lock-up my rear wheel to test the brakes).
Initially I thought that my cable snapped or detached. However, on further inspection I discovered that the cable was perfectly intact, yet it developed about 6 mm of slack. External inspection revealed no changes in the exterior routing of the cable - everything's as it was before. This makes me assume that something happened inside the top tube, through which the rear brake cable is routed on this bike.
I load-tested the cable and took up the slack, which fully restored the functionality of my rear brake.
I always assumed that the cable is simply openly stretched inside the top tube from one point to the other, with no extra guides or anything like that. (That's actually exactly how it was on some bikes I had previous experience with.) But now I suspect that in this bike there's some internal structure in there that can break under load.
Can anyone shed some light on this?
On my morning ride today the brake was working fine. However there was a loud rattling sound coming from the front portion of the top tube. (Or at least that appears to be the point the noise is coming from.) I checked all external equipment - noting rattles on the outside - and now I'm sure that the rattle is produced by some loose object inside the tube, which further makes me believe that something snapped in there.
If there is some cable routing part broken then it is unlikely you would be able to get the brakes to function simply by tightening - you would likely have to take up enough slack to bring the two ends of the housing segments into contact with one-another... approximately the length of the toptube worth of slack would need to be removed, and it is quite unlikely that you have that much extra housing to make this possible.
The most likely explanation is simply the cable slipping through the pinch bolt on the brakes. Also possible is the ferrule (cable-housing end) failed and the inner steel coil pulled through and your brakes are only supported by the plastic jacket. Less slikely but also possible is that most of the strands of cable have snapped and the cable is stretching before final failure.
Is this a new bike? If so, you should bring it back tot he shop where you bought it to find out what is going on.
One last possiblity - maybe the brake caliper had something that was not quite 'seated' and your hard pull on the lever snapped it into place.
02-22-12, 01:06 PM
Well, if the bike routed a bare cable inside the top tube, then breaking some [hypothetical] cable guide in there could have easily introduced cable slack. That was my first thought. However, this bike seems to route continuous housing through the tube: if I pull the housing out at the entrance hole (front) it pulls in at the exit hole (rear) and vice versa. So the rear brake cable housing seems to be continuous. Under these circumstance it is certainly not clear where these 6mm of slack could come from (aside from pinch bold slippage, as you noted, or something snapping into place in the brake lever mechanism).
At the same time, I'm fairly certain that it wasn't caused by the slippage at the pinch bolt. Firstly, I recently replaced my brakes (installed Avid BB7) and measured the distance from the pinch bolt to the cable end. It was about 20mm. That length didn't change after the event. Losing 6mm from 20mm would immediately be noticeable. Now, after taking up the slack, I have about 25mm of the cable sticking past the pinch bolt. Secondly, when I installed the new brakes I put some light grease on the cable as it passed through the cable guide arm on BB7 (per SRAM BB7 installation instructions). When I retensioned the cable, about 6mm of the greased portion of the cable pulled out of the BB7 rubber boot, indicating that I'm really taking up some slack that developed somewhere along the frame, not at the pinch bolt.
I tried to wiggle the cable housing at the point where it enters the top tube (front), and I hear a distinctive sound of some part moving inside and hitting the tube from inside. I'm pretty sure that's the source of the rattle I heard on my ride this morning. I can't see the part, but judging by the sound it is relatively large (could be an illusion, of course). I wonder what it is, how I can take a look at it.
02-22-12, 01:39 PM
The answer here is clearly to bring the bike to your friendly neighborhood gastrointerologist and see if you can talk him into letting you use the endoscope to have a look inside. :lol:
I recently replaced my brakes (installed Avid BB7)
This is a big clue, I htink.
I tried to wiggle the cable housing at the point where it enters the top tube (front), and I hear a distinctive sound of some part moving inside and hitting the tube from inside. I'm pretty sure that's the source of the rattle I heard on my ride this morning.
Sounds like the noise is the cable housing hitting the inside of the frame tube.
If it is a full run of cable housing (which is likely) then there is nothing that could have broken on or in the frame that could introduce any measurable amount of slack.
Another possibility: The end of the brake calbe was not properly seated in the stop at either the brake caliper or lever - likely the edge of the ferrule was hung up on the edge of the cable stop or barrel adjuster, and the 'snap' noise was the end of the cable slipping off the edge of the sto and landing in its proper position. THis will give you about 5 or 6mm of slack.
Cable often get hung up like this if you don't pay attention when you tighten it after it is allowed to go slack. You can ride for miles without noticing. Sometimes the brakes work fine and sometimes the cable feels rough or damaged.
02-22-12, 04:36 PM
blown out shifter housing?
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