Trying to Decode Disc Brake Rub

[sethjs]

So I usually ride a new Venge in SF. It has 9170. CLX 50s. When torquing the bike / climbing standing, I get front pad rub.

I've experimented with different pads / discs. I've tried different torques. I've bled the system. I've lubed the pistons.

So I assume it's that the fork flexes.

So then I come on a trip to Majorca and rent an SL6. It has 9120. Corima 32s. No rub whatsoever no matter what I do.

So now I'm surprised / stumped.

Is it that the cylinder on 9120 in those massive hoods is bigger than the 9170 cylinder? Is it that the fork is not as aero and so latterally stiffer?

Anyone have a clue on this one? I'd come to take it for granted that there just will be rub. But then...on a rental bike...nothing but beautiful silence.

[dcorn]

My disc bike seems to do something different every time I ride it. One ride, I'll get pad rub when climbing/sprinting, the next ride nothing.

I'd say loosen the caliper bolts and try to center the rotor in the caliper as much as possible. It may ever so slightly be off to one side and rubbing when you torque the bars one way or the other.

[spdntrxi]

I'd do what dcorn says... mine too are different..well atleast my rear brake is.. My front is butter and hardly ever makes a peep day to day. My rear is tempormental... rubs after long descents sometimes..sometimes not. Rubs under power, sometimes not.. etc etc. Usually only for a short time so I get over it.

[Dan Gerous]

When it seems to behave differently all the time, it might be because one or both pistons are dirty, sometimes refered to as 'sticky pistons'. They don't move in and out as freely as they should so they sometimes don't retract enough after braking. It's kind of similar to mechanical shifting that seems out of tune randomly when cables/housing are dirty. It can usually be solved by cleaning the pistons...

But given how small the gaps are between pads and rotors, the only way to have a silent setup is to regularly clean the pistons and make sure the caliper is aligned perfectly. While it's easy to align one to be rub free in a bike stand, it has to be perfect, even space on each sides, true rotor, to stay silent if any small amount of flex happens when riding hard... then it might still rub after hard braking anyway, annoying but I think you have to accept that with discs.

[bilwit]

I used to get pinging all the time until I trued the wheels with the rotor+tube+tire installed to an absurd tolerance (well beyond what I would do for a rim brake wheel).. now I still get some rub on the rear but nothing really audible or noticeable anymore

[kode54]

Have you checked the pistons in the calipers? Lots of times, if you ride in rain or damp...the pistons get a bit sticky...and may not retract as much as they should...so you get a little rub.

[Lewn777]

Get yourself a feeler gage. It's the best tool I ever bought, try that. Works much better than trying to eyeball it with loosening and tightening the bolts with the brake held on.
If you've tried all other things try re-torquing all bolts/thru axle centerlock, brake caliper etc.

If all else fails you could try another rebleed, as possibly the system could be over filled with oil and therefore not allowing the pistons to retract fully, but that's clutching at straws.

Other than that I'd blame manufacturing tolerances. These first generation of road disks are essentially the same as MTB brakes, those tolerances don't matter in MTB much with some light rubbing, but with road, it's potentially robbing people of watts.

[marioc]

When it seems to behave differently all the time, it might be because one or both pistons are dirty, sometimes refered to as 'sticky pistons'. They don't move in and out as freely as they should so they sometimes don't retract enough after braking. It's kind of similar to mechanical shifting that seems out of tune randomly when cables/housing are dirty. It can usually be solved by cleaning the pistons...

But given how small the gaps are between pads and rotors, the only way to have a silent setup is to regularly clean the pistons and make sure the caliper is aligned perfectly. While it's easy to align one to be rub free in a bike stand, it has to be perfect, even space on each sides, true rotor, to stay silent if any small amount of flex happens when riding hard... then it might still rub after hard braking anyway, annoying but I think you have to accept that with discs.

^^ I had the same issue, out of the saddle pedalling and hard braking would cause front rotor rubbing. After thoroughly cleaning pistons the front brake is silent now in both cases.

[moyboy]

sounds like a sticky piston.... park tool has a good youtube video on it and how to clean it.

[WorkonSunday]

not yet mentioned, but does the bike wheel get taken off and re-installed during transport? also 100% sure disc rotor is true?

[sethjs]

So after more riding the (silent) Tarmac today - I've got a theory I'll test at home.

btw - to some of the suggestions - not an out of true rotor. Not an alignment issue. Unlikely sticky pistons - I've serviced them.

Net: I suspect the end caps on the Roval hubs allow for too much play. They're plastic, pressed in, and can be pulled by hand. Which is great for servicing - but definitely not great for wet weather sealing and potentially not for rigidity and therefore rubbing.

Backstory: I'd previously noticed that after a few wet rides, the rear hub gets "loud". Turns out sand / grime slowly works its way beyond the end cap and inbetween the end cap and the bearing seal. The solution is to pull the end cap, clean it out and lube it up.

It occurred to me today that the wheels I'm riding on the Tarmac are not Rovals, but rather Corimas. And it appears their end caps are much more solidly attached to the hub.

So I strongly suspect that's the weak point that allows for enough flex to get rubbing when torquing the bike heavily. I'll mess around with wheels on a different hub once I get back home.

[Lewn777]

So after more riding the (silent) Tarmac today - I've got a theory I'll test at home.

btw - to some of the suggestions - not an out of true rotor. Not an alignment issue. Unlikely sticky pistons - I've serviced them.

Net: I suspect the end caps on the Roval hubs allow for too much play. They're plastic, pressed in, and can be pulled by hand. Which is great for servicing - but definitely not great for wet weather sealing and potentially not for rigidity and therefore rubbing.

Backstory: I'd previously noticed that after a few wet rides, the rear hub gets "loud". Turns out sand / grime slowly works its way beyond the end cap and inbetween the end cap and the bearing seal. The solution is to pull the end cap, clean it out and lube it up.

It occurred to me today that the wheels I'm riding on the Tarmac are not Rovals, but rather Corimas. And it appears their end caps are much more solidly attached to the hub.

So I strongly suspect that's the weak point that allows for enough flex to get rubbing when torquing the bike heavily. I'll mess around with wheels on a different hub once I get back home.

Your own theory sounds like it's the best one!

[David84]

@sethjs: I have exactly the same issue with the S-Works Venge ETAP. When out of the saddle (and now even in longer curves) I have disc brake rub and your theory about the end caps of the hubs sounds absolutley plausbile to me. I have also observed that tightening the Axle can make it worse.

In my view, this is a serious issue Specialized should look into as well. If you promote these bikes with tons of technology going into frame (and probably fork) development to make the frame lighter and stiffer and then you equip these frames with your own (supposedly top in class) roval CLX 64 wheels with soft plastic endcaps on the hubs, that does not make much sense to me.

I wonder whether the usual workarounds (being (a) pushing back pistons and (b) bleeding some DOT out of the system to facillitate retracting of pistons) can bring you to a point where the end cap flexibility can be compensated or whether I need to intall new hubs.

Have you been able to test with other hubs/wheels? Would be very interested in your conslusions.

[alcatraz]

Try alloy endcaps perhaps.

Check hub for play.

See if it flexes more one way than the other and align the caliper accordingly.

True the rotor.

[andreas]

I also have the Venge and it seems prone to front disc brake rubbing with the Roval CL 50's. After I removed the wheels and re-tightened the Axle, the rub was permanent so I had to reset the caliper. The slightest hint of an out-of-true rotor will cause rub when I'm dishing out some power in the hills.

Where can aluminium endcaps for the CL 50's be found (DT 350 with custom Roval endcaps?) ?