Road mechanical disk brake slips

[M_Chavez]

Hi All,

Just hoping to poke your brains on a road mechanical disk problem that I can't seem to figure out.
I've got a 160mm front disk brake that just won't work. If I apply the power lightly and then gently build it up, the brake slows the bike down - nothing to write home about - a bit like rim brakes in the wet. However, if I significantly increase the lever pressure, the disk squeals and starts slipping, almost completely losing power. So emergency stops are pretty much impossible and braking from the drops is very difficult. Very strange dynamic - the braking power builds up before letting go.

I've tried two different calipers (bb7 road and TRP Spyre) and several rotors. Done pad bedding procedures and even (reluctantly) put on 120 miles on the bike in hopes that the pads will just magically start working.
Never had a problem like this on MTB cable disk brakes, and I've managed to run an old Hayes MX1 for nearly 20 years - still getting adequate braking from it and just set it up on the Mrs' bike without too much fiddling.
I even managed to get better braking performance from cheap Chinese MTB jobbies that aren't supposed to work.

I am definitely matching STIs with road calipers.
This leads me to think that the problem might be in cable housing (Jagwire Universal) or STIs (microshift), but any suggestions are welcome. Poor cable routing? Something wrong with the hub?

Rear brake took a lot of fiddling to set up with BB7, but eventually started working as it should, and it worked with the Spyres pretty much out of the box. Using same cables/STIs.

Really looking forward to riding my summer bike with rim brakes as soon as it dries up.

Thanks!

 

[Moqi]

The usual faults are a) contaminated pads/ disc b) Incorrect cable routing at the caliper end c) cable outer - you really need compressionless cables. i suppose the problem could be the disc is worn below service limits and the caliper adjustment can't adjust for this ie you are going past the end of the adjustment and so it's resetting itself to zero . not describing it well - but think of it going past whatever cam is applying the pads..

 

[M_Chavez]

Thanks.

I've tried 3 sets of pads (BB7: lightly used metal, sanded clean & flat before use;BB7 resin (brand new); Spyres lightly used metal, sanded clean & flat before use). These were in combo with a used/sanded/cleaned Chinese rotor, brand new Chinese rotor and A2Z SP5.

Incorrect routing at caliper end? The cable comes off the drop bars and heads down straight along the fork leg. It's a euro set-up, so left hand lever for left hand caliper mount front brake, but the curve seems OK.

Also, if I gradually build up the power, performance seems to be better than if I slam the lever.

Just very strange - never seen anything like this before. It doesn't feel like I'm "going past the cam"...

 

[mdvineng]

Sounds a lot like misalignment

 

[Moqi]

re the incorrect routing at the caliper end - there was some problems with the spyres people were routing the cable wrong at the pinch bolt - see here for a discussion of poor spyre performance -https://singletrackworld.com/forum/topic/trp-spyre-discbrakes-poor-performance/.Cable needs to go at bottom of pinch bolt (nearest the caliper not the top.) Re the pads if they are contaminated a blowtorch works better in my experience especially if contaminated with brake fluid (which wont be the problem in your case). If you've not got compresionless cable i'd try them. Of course you've checked at the lever end that the cable ferrule hasn't split or deformed and is not deforming under load - or even is the right size - ie for brake cable not gear cable?

 

[M_Chavez]

Definitely right cables lol. And correct routing.

Spyres were a great improvement over BB7 - problem's not in the calipers it seems. Hub, cables and lever are the only 3 things that I haven't changed yet.

I'll try compressionless cables and take extra care facing them, see where it takes me.

 

[ectoplasmosis]

Sounds a lot like misalignment

Yes; have you had the fork caliper mount faced/squared?

 

[M_Chavez]

No I haven't, but tried concave/convex washers on the mount, that would have resolved any issues?
I'd hope that Kinesis checked the fork before shipping, but could be wishful thinking.

 

[ishaw]

Re:

Too much cable can be an issue. I always keep mine as short as I can while still allowing the bars to move freely. More cable means more flex and more places to lose power.

Seems a strange issue though.

I've only had 1 set of cable discs (bb7) and they worked pretty well.

Good luck finding the cure.