Disc brake rub
 

I recently bought a Trek CrossRip that came with Avid BB5 mechanical disc brakes, 160mm front, 140mm rear for a commuter. When I am riding down the road, everything is fine until I come out of the saddle and put a little pressure on the front end. It seems that the rotors are rubbing the inside of the caliper. The sound it gives off is like something hitting the spokes, but it only happens on the front. The bike came stock with Bontrager Nebula 32 spoke wheels and Formula hubs.

I have tried to adjust the pads some and that seemed to have helped a little, but it still happens. The LBS also adjusted the brakes a bit....but it is still happening and it is bugging the crap out of me.

Is this a normal issue with disc? possible wheel flex issue?

I'm coming from a rim brake background and don't have the knowledge of issues with disc brakes and I cant really find anything really defining on this. I weigh in at 208 but as this is a commuter, I am also not putting a lot of torque on the bike, but what to see if I can eliminate this.

Any help would be great.

Brian

Mech disc brakes only move from one side, relying on the rotor to flex a little to pich from both sides, so they need to be set up tight.
What you're seeing is brake rub from the frame/fork flexing. In an equally flexy frame/fork using rim brakes, you'd seen the same with the brakes set up tight.

You can try widening the gap in the calipers, maybe going to "floating" rotors. Or switching to hydraulics, which move from both sides.
Or going for a stiffer frame/fork.

Or simply train youself to ignore it. Annoying as it might be, the good news is that brake rub from frame/fork flex doesn't cause enough drag to mean anything for your overall speed.

I have the 2014 CrossRip Comp. I had all of the same problems as you have now. I just kept pestering the LBS to adjust them and they did. Eventually these brakes will "wear in" and they'll work just fine.

+1 on both counts.

Fork flex allows the hub to twist slightly causing disc rub. if it only happens on one side (ie. right turns only), you can sometimes bias the brake to the other side a bit. But it's not worth opening it enough that braking suffers.

The incidental contact rubbing doesn't have any real force behind it so there's no meaningful friction involved. Car brakes often exhibit the same noise, and you can hear them chirping. If it were an issue the brakes would heat up and get cooked in short order.

Thanks for the info, that is good to know. Do you remember about how long it took for them to wear in and quiet down? Are you still running all the stock brakes and pads?

Typical of BB5 road brakes. In order to adjust them so there is not excessive lever pull, they have to be adjusted on the tighter side. Which doesn't allow for much, if any, imprecise adjustment or clearance between the disc and retracted pads. They could be adjusted looser for no noise, but you may not be happy with the lever throw at that point.

If that's the problem, the solution is easy. Switch the lever for one with greater cable pull. A pricey solution would be speed dials, but there are less expensive choices including some 2 position levers, or you could simply measure the fulcrum to cable head distance of a few and buy the longest.

It took a couple of months but I really think it was my first touring trip that did the trick. I was loaded with about 30lbs (all on the rear) and after coming down the first steep curvy hill, with much braking, the noise was gone. Incidentally, it was my front brake that was the problem.

I still use the stock brakes and pads. I had planned to install BB7's but I'm reconsidering. The BB5's do the job surprisingly well.

The CrossRip has two sets of brake levers. I was pretty "stern" with the LBS that the brakes be adjusted properly. On the third visit I finally got a mechanic who understood the problem. After adjusting the disc and rotor he adjusted the "chicken brakes" only and left the other levers where they were set. I don't know how he did this but this is what he said he did. No more brake drag.

A couple of more things to check. Hub tight? Wheel secure with quick release? Disc brake mounting holes flush and clear of paint? As said before, maybe some wheel flex. Spokes tensioned properly?

wheel flex as i understand it
meaning the rim and spokes flexing around the hub
has no effect on disk brakes
as they are not attached to the rim or spokes

the other things you said are good suggestions though

+1 the disc attaches to the hub, what happens beyond the hub - spokes & rim - has no effect on the disc.

I agree with you about checking those things but as monconlonx said it's typical of BB5's. They also screech, wail, creak and moan until they mellow out. The problem is that hard, prolonged braking when the brakes are new is the cause of these noises. Guess what I did when my bike was new.

Yea I removed the 2nd set of brake handles as I had no interest in them and I also replaced all the stock components with the Ultegra from another bike, but running 105 5600 series double STI. I've adjusted the front fairly tight and I noticed today, 175 miles in, the clicking noise, while still there is not nearly as prominent as it was. I have ordered some stiffer wheels, Boyd Altamont's with Disc hubs, so in the low chance of wheel flex, that should take care of that. Existing wheels look fairly solid,Hubs good, spokes feel good and wheel is true. Bike came with the allen key skewers and they are tight.

I am thinking about replacing the brakes with the TRP Spyres down the road if I don't like the vibe of the BB5's, but like I said, they seem to be less noisy today, so maybe that is a good sign and it is not like I am riding mountains here in NE Florida where the tallest thing around here is a 109 ft bridge.