Hydraulic discs & rotor rub

[GrahamG]

I've searched for a more generic thread along the lines of the tubeless one as my brakes are seriously getting on my wick so need some hints and tips.

Re. rotor rub, I've tried on half a dozen occasions to tweak my front icetech rotor so that there's no rub, but no joy. It's excruciatingly annoying, I swear the rub didn't stop on every turn off the wheel until the pads were near half worn. And they still rub whenever I get out the saddle in a big gear to muscle up some little rise.

This imperfection might also explain the fact that any microscopic grit particle that find itself into the brake caliper then makes an absolute racket until hard braking dislodges it.

So, I've two queries on this (plus would welcome any nuggets of wisdom you might have separately):

1. Is rotor trueing something that need very careful attention? Should I be getting a tool for it to allow fine tweaks instead of my bfo adjustable spanner?

2. Are larger rotors more prone to warping as you might anticipate, or is 20mm not going to make any meaningful difference? (I've 160 front and 140 rear).

Anyone else finding hydraulic discs annoying as hell in the noise department? I've always been one that's made sure wheels are 100% true and pads clear and clean so never had issues with rim brake in any big way but found myself with a bike equipped with 105/rs505 and feel as if there's definitely some compromise on the maintenance side I wasn't expecting (£30 pads!). Also contributing to my frustration is the fact that my wife has the same bike, but is on the stock crappy wheels with bolt on rotors and has had no such problems.

[vorsprung]

Anyone else finding hydraulic discs annoying as hell in the noise department? I've always been one that's made sure wheels are 100% true and pads clear and clean so never had issues with rim brake in any big way but found myself with a bike equipped with 105/rs505 and feel as if there's definitely some compromise on the maintenance side I wasn't expecting (£30 pads!). Also contributing to my frustration is the fact that my wife has the same bike, but is in the stock crappy wheels with bit in rotors and has had no such problems.

annoying?  no
the cost of the pads? yes that is annoying!

[frankly frankie]

If the hydraulic disc system is of the 'self-adjusting' type, then IMHO 'self-adjusting' is simply a euphemism for 'always rubs'.  This is OK on a car or a motorbike, but not on a pushbike, and it's the main reason that I gave up on hydraulic discs many years ago, preferring the less effective but much more manageable cable-operated type.

To work effectively as a brake, the rotor doesn't need to be 100% true, always provided of course that it is fitted the right way round, ie having regard to the direction of rotation marked on the disc.  The (Park tools) disc trueing tool that I have got doesn't really seem to offer any advantages over an adjustable spanner (and, actually, I've never used it).

[Brucey]

that the rub disappears when the pads are half-worn suggests that the pistons are not retracting in the usual way (when the brake is released) for some reason when newer pads are installed.

Top of the list of possible causes is that the brakes have a minor hydraulic fault, such as too much fluid in the MC reservoir, or a small amount of air trapped in the hydraulic circuit.

However it is also possible that one or both pistons are not retracting properly because there is crud in the fluid, and this makes most difference when the pistons are well back in the bore.

BTW rubbing whilst riding out the saddle is a different kettle of fish; it could mean that there is a small amount of free play in the hub bearings (which would be weird, usually they are set too tight...)  but it also might just be stuff flexing in combination with reduced running clearances.

Hydraulic disc brakes have their problems but they make sense on an MTB.  On bikes with dropped handlebars disc brakes make less sense anyway and hydraulic discs are appreciably more difficult to set up, needlessly expensive to buy and run, and far from immune from other difficulties.

cheers

[tiermat]

Sigh, you were doing so well, Brucey, until you brought your prejudices regarding discs on drop barred bikes into it...

In my experience, yes you do get rub (I have Ultegra 6800(??, I think) on the Jamis), but it tends to be short lived.

It being a CF bike, there does seem to be more flex when honking, which can cause a small amount of rub. FWIW the whole design of hydraulic discs is that they _don't_ fully retract, to allow for wear in the pads.  Compare this to cable discs where you have to re-set the gap ever X miles.

Guess it's one of those things.

[Si S]

£30 for pads?  I hope that's for both bikes. I've been using https://www.discobrakes.com/ for a good while now.

[chrisbainbridge]

Can I ask if the bike is thruaxle or QR?
I have had ultegra di2 hydraulics with both and the QR was a constant battle with rub.

I found that there was an optimum tightness which held the wheel but did not stress and warp the carbon.

If they are thruaxle then something is wrong with the system. Taking the wheel out and back in with thruaxle is a joy. No checking for alignment or fit. Once the axle is in they are centred and rub free. I would find a good bike shop.

I apologise to all the great mechanics on this site but I think some road bike shops are relatively inexperienced with the road hydraulics.

[GrahamG]

Thru axle, to be honest it's nothing major, just accentuates the other rub. Going to try more delicate rotor trying and check the front hub is all as it should be. The bike is only just over a year old with little use so only changed the front pads so far, made sure poisons were pushed well back and that's when I gave up on the rotor trueing and just left them to bed in a while.

[GrahamG]

£30 for pads?  I hope that's for both bikes. I've been using https://www.discobrakes.com/ for a good while now.

After market options don't yet appear to be available. That's £30 for two sets from the cheapest possible Euro supplier that I happened to be ordering a dynamo light from! £20 for one wheels worth rrp. "But you save on rims!"

[Brucey]

....Hydraulic disc brakes have their problems but they make sense on an MTB.  On bikes with dropped handlebars disc brakes make less sense anyway and hydraulic discs are appreciably more difficult to set up, needlessly expensive to buy and run, and far from immune from other difficulties.

Sigh, you were doing so well, Brucey, until you brought your prejudices regarding discs on drop barred bikes into it...

and you have no 'prejudices' then?

Read what I wrote carefully and you might see that your reaction is more to do with your prejudices than what I have written.

It is self-evident that disc brakes make less sense on bike with dropped bars, else more of them would be so equipped, eg in the pro peloton.

Ditto hydraulic brakes for other applications, e.g. touring.  It should be obvious that hydraulics are off-putting to many folk, else (say) Maguras would have been more popular over the last 30 years or so. 

I agree a lot of mechanics have yet to become 100% au fait with dropped bar hydraulics but (amongst other things)

a) I harbour a doubt about the fundamental layout of such MCs; I don't think that they are intrinsically self-bleeding in the same way as in some other arrangements

b) there is not enough flexibility in the system; once you have hydro STIs you are pretty much tied into using the gears and brakes that match them, and nothing else will do.  This limits choice and also sounds like a fast-track to built-in obsolescence.

c) there are not many spare parts available; if your hydro STIs go bad, chances are that you will have to replace them. Parts and labour for such is about the same cost as the cost to buy an entry level road bike outright.

d) you have several hundred pound's worth of kit strapped to the handlebars right where it is most likely to get clouted  in the event of a prang, or even a parking knock. Ordinary STIs are bad enough but hydro ones take it to a whole new level.

Many of the above (and more) may well be corrected in time. But right now, to a lot of folk, systems of this type look more like a load of faddy nonsense than anything else.

cheers

[DuncanM]

....Hydraulic disc brakes have their problems but they make sense on an MTB.  On bikes with dropped handlebars disc brakes make less sense anyway and hydraulic discs are appreciably more difficult to set up, needlessly expensive to buy and run, and far from immune from other difficulties.

Sigh, you were doing so well, Brucey, until you brought your prejudices regarding discs on drop barred bikes into it...

and you have no 'prejudices' then?

Read what I wrote carefully and you might see that your reaction is more to do with your prejudices than what I have written.

It is self-evident that disc brakes make less sense on bike with dropped bars, else more of them would be so equipped, eg in the pro peloton.

"drop bar" bikes with hydros are de rigeur in the cyclocross pro peloton.
What you mean is that they aren't often seen in road racing. The primary reasons for this are the difficulty of executing a speedy wheel change with through axles, the extra weight, the lack of wheel standards, and cultural conservatism. I'm not sure that any of these matter to us.
There are downsides that matter to enthusiasts - maintenance is different and more awkward, emergency repairs are unlikely, they are expensive to buy. Whether the upsides (consistency, extra power and modulation, much longer MTBF) are worth it is down to the individual.

[Si S]

£30 for pads?  I hope that's for both bikes. I've been using https://www.discobrakes.com/ for a good while now.

After market options don't yet appear to be available. That's £30 for two sets from the cheapest possible Euro supplier that I happened to be ordering a dynamo light from! £20 for one wheels worth rrp. "But you save on rims!"

What calipers?

[rogerzilla]

DOT fluid in a tiny volume absorbs water so fast that brakes will start dragging within months of a fluid change due to the increased volume in the system, unless there is an expansion reservoir.  I got seriously fed up of this.  Buy brakes that use mineral oil (Shimano).

Car brakes run with no real clearance other than that caused by slight disc runout.  The friction is insignificant in a motorised vehicle and they don't even get hot due to this slight rub.  On a bicycle some clearance is needed because power is much lower and thin bike discs make a silly "woo woo" sound when dragging.

[zakalwe]

d) you have several hundred pound's worth of kit strapped to the handlebars right where it is most likely to get clouted  in the event of a prang, or even a parking knock. Ordinary STIs are bad enough but hydro ones take it to a whole new level.

The same can be said of Di2, though, or any high-end shifter.  And at any rate, any fault in an STI shifter means outright replacement, in my experience.  Particularly with the gear ratchets, which are seem to be the most delicate part, whether with hydraulic brakes or not.

[GrahamG]

£30 for pads?  I hope that's for both bikes. I've been using https://www.discobrakes.com/ for a good while now.

After market options don't yet appear to be available. That's £30 for two sets from the cheapest possible Euro supplier that I happened to be ordering a dynamo light from! £20 for one wheels worth rrp. "But you save on rims!"

What calipers?

Rs505

[Brucey]

....Car brakes run with no real clearance other than that caused by slight disc runout.... 

that is not true. The seals are designed so that they are deformed when the brake is applied and as they recover their shape the pistons are withdrawn by a small amount. A similar approach is used in some bicycle brakes. Some positive piston retraction is obtained by this and the small 'dead spot' between when the MC's compensation port is covered and  when the pads hit the disc. 

It is truer to say that the maximum allowable disc runout in car brakes is often set by the amount of piston retraction that occurs.

cheers

[Brucey]

BR-RS505

http://si.shimano.com/pdfs/ev/EV-BR-RS505-3901.pdf

uses various pads including K04S (sintered, no fins).  They are £12.95 a set (for one brake) and there are alternatives, such as the K02S resin pads which are about £5 a go.

cheers

[rogerzilla]

....Car brakes run with no real clearance other than that caused by slight disc runout.... 

that is not true. The seals are designed so that they are deformed when the brake is applied

They were in the early days, then the makers realised it didn't matter.  A floating one-piston caliper as used on most cars can't retract the pistons like this anyway.  Go and jack one wheel of your car and spin it to confirm the pads are rubbing.

[Brucey]

you are right that single-piston calipers, and the pads in most car brakes are 'knocked back' once the brake is released, but the piston(s) are definitely retracted by deliberately (by about 0.5 to 1mm per piston depending on the seal design), and that is where the running clearance comes from.  If the retraction stops working properly, the tolerance to disc run out swiftly declines to near- zero and all kinds of horrors can ensue.

Simply because the rate at which the wheels are turning is so much higher and the discs are so much bigger (in relation to the wheel size), the lateral speeds at which the pads are knocked back in car calipers is quite high. During normal running there is stuff-all contact or rubbing in car brakes and (since the discs never run perfectly true) you would very quickly get runout-induced DTV if there were.

In a bicycle you might be doing ~10m/s on average and the pad contact centreline might be doing 2m/s past the pads.

In a car you might be doing 30m/s and the pad contact centreline might be doing about 13m/s. In other words the pads are (for any given amount of runout)  knocked back x6 or x7 faster (harder) in a car brake than a bicycle  brake, and because the seals have a much larger cross-section etc the retraction (and therefore the space into which the pads have to sit in) is larger anyway.

Most bicycle brakes have a barely adequate piston retraction anyway, and the pads are pushed back by springs (which eat into the usable pad thickness BTW). Any tiny fault with the brake hydraulics (such as those I mentioned upthread) means that you get reduced or uneven piston retraction and then you will get rubbing; there is nowhere for the pads to go, sprung or not.

I have seen plenty of bicycle brakes that have manifested uneven piston retraction; crud in the piston bores (from the outside, there are no dust boots like you find is decent car brakes), badly bled brakes, brake with pistons that bind because the caliper body was not cleaned properly during manufacture (lots of swarf inside the hydraulic circuit... ), corrosion in piston bores, water in the hydraulic circuit, worn seals, different amounts of oil on the outsides of the pistons, caliper body alignment wrt to the disc... the list goes on and on.

With some brake models, it is quite normal to have to check the pistons for even retraction part way through the pad life. Clues that all is not well in this regard include that the pad wear is (even without contamination on one side of the disc) rather uneven.

cheers

[mzjo]

you are right that single-piston calipers, and the pads in most car brakes are 'knocked back' once the brake is released, but the piston(s) are definitely retracted by deliberately (by about 0.5 to 1mm per piston depending on the seal design), and that is where the running clearance comes from.  If the retraction stops working properly, the tolerance to disc run out swiftly declines to near- zero and all kinds of horrors can ensue.

Simply because the rate at which the wheels are turning is so much higher and the discs are so much bigger (in relation to the wheel size), the lateral speeds at which the pads are knocked back in car calipers is quite high. During normal running there is stuff-all contact or rubbing in car brakes and (since the discs never run perfectly true) you would very quickly get runout-induced DTV if there were.

In a bicycle you might be doing ~10m/s on average and the pad contact centreline might be doing 2m/s past the pads.

In a car you might be doing 30m/s and the pad contact centreline might be doing about 13m/s. In other words the pads are (for any given amount of runout)  knocked back x6 or x7 faster (harder) in a car brake than a bicycle  brake, and because the seals have a much larger cross-section etc the retraction (and therefore the space into which the pads have to sit in) is larger anyway.

Most bicycle brakes have a barely adequate piston retraction anyway, and the pads are pushed back by springs (which eat into the usable pad thickness BTW). Any tiny fault with the brake hydraulics (such as those I mentioned upthread) means that you get reduced or uneven piston retraction and then you will get rubbing; there is nowhere for the pads to go, sprung or not.

I have seen plenty of bicycle brakes that have manifested uneven piston retraction; crud in the piston bores (from the outside, there are no dust boots like you find is decent car brakes), badly bled brakes, brake with pistons that bind because the caliper body was not cleaned properly during manufacture (lots of swarf inside the hydraulic circuit... ), corrosion in piston bores, water in the hydraulic circuit, worn seals, different amounts of oil on the outsides of the pistons, caliper body alignment wrt to the disc... the list goes on and on.

With some brake models, it is quite normal to have to check the pistons for even retraction part way through the pad life. Clues that all is not well in this regard include that the pad wear is (even without contamination on one side of the disc) rather uneven.

cheers

Add to that that car and motorbike rotors are much thicker and stiffer than cycle ones so the knocking back effect is more efficient (being done by an immovable object rather than a spring!)

[vorsprung]

£30 for pads?  I hope that's for both bikes. I've been using https://www.discobrakes.com/ for a good while now.

discobrakes did claim at one point to have some rs-505 pads so I got a few sets
sadly, they did not in fact fit in the caliper, so I sent them back

[GrahamG]

Thanks all, so it's a 'suck it up' or learn every little technical detail and maintenance process. In the meantime, I've just made an extra special effort to true the rotor better so will see how that holds up.

[Ben T]

Whilst on the subject, is there a simple solution to the issue of a very slight vibration under braking?
This manifests itself only under fairly light braking at fairly low speed.
I think it is more likely to be the front than the back but weirdly it seems more prevalent when both brakes are on at once, if that makes sense? The brakes are hope race mini and the front rotor is 180mm and rear 160mm. I have changed the front pads since it started and is still the same.

(sorry to thread hijack, but conveniently all the 'get cables they're more reliable blah blah blah' type responses have conveniently already been flushed out )

[LEE]

It is self-evident that disc brakes make less sense on bike with dropped bars, else more of them would be so equipped, eg in the pro peloton.

Team Trek were using Discs in the Giro I noticed.
They don't make sense if your team car has 20 spare wheels on the roof but, for me, they mean I can buy a really nice set of wheels and ride it every day, through all sorts of skog, without grinding my wheels away.

Occasionally I need to "ping" the brake lever to stop a rub but my full Hydraulics are much less prone to rubbing than my Avid BB7 cable-actuated ones, as they have minute clearances and are prone to "stiction" everywhere.

[Torslanda]

I'm going to stick my head above the trench wall and hopefully not get it shot off!

Rotors. I take it you've checked that they're not loose on the hub? Many's the time I've found centre lock rotors with play. Easily checked, put the brake on and rock the bike. If the wheel moves relative to the disc... Remove the lockring, remove the disc, clean both, lightly grease the mating surfaces, and tighten to specified torque.

Calipers. You'll have had yer tea? removed the pads, made sure there's absolutely no contamination on either face, cleaned the caliper with a proprietary brake cleaner and made sure the pistons were fully set back before refitting and centring the pads, making certain that both pistons are moving freely?

If, by any chance, the brakes have ever been bled without the pistons being fully retracted and a bleed block (no pads) or a wedge (pads in) inserted then the pistons won't retract the hairsbreadth they need not to rub.

Also I take it you've centred the caliper by undoing the fixing bolts and using the brake pressure to clamp the disc whilst retightening ?

If you've done all that and it still rubs then I think you have a faulty component somewhere.

HTH