Bicycle Mechanics - Disc brake nearly caused crash due to sudden grabbing
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I have a road/commuter bike with Tektro Lyra disc brakes. Every 30-40 miles or so, if I am applying moderate brake pressure it will brake smoothly for a second or two and then make a *BANG* which sometimes locks the wheel (both wheels do it but front is more prone) and it has nearly caused me to go out of control a few times. I feel I can't trust them at any speed above 25mph or so. I have checked the following:
Pads are not worn
Rotor surface is free of contamination
Caliper is aligned to rotor
Gap between pads and rotor is within spec
Rotor is true
Pads are not hitting the rotor arms (by this I mean the arms coming from the hub of the rotor that support the braking surface).
I've taken it to an LBS and they can't seem to find the problem because it occurs so rarely, and it can't be replicated on the repair stand.
Anyone? Can't find anything on BF or elsewhere.
11-10-11, 02:38 PM
6 bolt or centrelock rotor?
is there any play in the rotor. Is the rotor flat?
Did you bed in the pads ?
Have you removed and inspected the pads ?
6 bolt rotor.
Bolts are tightened to spec.
Rotor is flat against the hub attachment points.
Pads were bedded per BF recommendations (several long stops, enough to heat the pads and rotor)
Pads were removed, no contamination noted, a lot of thickness left.
11-10-11, 03:41 PM
If you use quick-release hubs, are the skewers kept tight?
I've removed the wheel skewers multiple times since it started doing this and they've always been tight, never loose to get off.
11-10-11, 03:45 PM
Maybe you looking at it on the wrong end? Have you checked everything on the brake lever?
I will check everything over on the brifter, I haven't checked there before.
I have noted that the deceleration after the BANG stays constant at the higher level even though the pressure on the brake lever did not change. I don't believe I feel anything in the brake lever when it does the BANG but mostly I'm too startled to remember what the brake lever felt like before and after the BANG.
Looked at the brifters, didn't see anything.
I did however make sure that the skewers were really really tight, and have put another 100 miles on the bike in the last couple days and the problem hasn't recurred. In googling around I did find that some people have had problems with disc brakes and QR skewers torquing and banging around under braking (even to the point of the wheel popping off!). I'll keep an eye on the skewer tension and report if the problem comes back.
11-14-11, 12:46 PM
There is the working, but better than you expected, that may be the issue.
I have Avid BB7 160 discs on my 406 wheel bike friday.
I just have to be mindful that at slower speeds it really grabs,
and will stop just the bike, short, leaving the rider still in motion.
yours may just be a problem inside the caliper. toss it aside,
fixing a price point chosen part is a timewaster, put something else on..
You might find installing an Avid caliper set, BB7 better,
easy to adjust for pad wear, and is made in a short pull road version.
11-18-11, 10:29 PM
I was going to answer earlier but forgot my password.
I work as a bike mechanic, and one model we sold had these same Tektro mechanical brakes.
Some of these bikes had a similar issue, that the brakes would sometimes get stuck on when braking hard, they also made some kind of click noise at the same time. On one bike I was able to fix this by changing new brake cable outers and new brake cable installed with plenty of grease in the casing - the return spring in the brake caliper was too weak to overcome the drag in the original (crappy, full length) cable housing. I also disassembled the spring from swingarm, but could not find anything else wrong with them, it was simply the draggy cable causing the brake to seize on when cable was pulled far enough, with enough force.
That might not be the only reason it happens, but I consider these poor brakes in any case (and it would probably be easiest to switch to some decent model from say, Shimano's better mechanicals or the ever popular Avid BBs). The moving arm to which the cable attaches, isn't reaffirmingly firm but has all kinds of wobble sideways, the return spring also is not strong enough imho.
In one set of Avid BB mechanical breaks, couple of years old model, they had an issue with the washer that the brake cable attaches under would sometimes be too close to caliper body, and the washer would get stuck on the body at times, causing similar trouble. That was sorted in their new model for the next year, and could be fixed by bending / filing the washer so there was no interference.
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