My XT brakes have been great for years, but recently, the friction levels and braking power have dropped off. The rotors have been cleaned within an inch of their lives and the pads changed – I've tried three different brands and types all with crap results. So just removed the front rotor and put on a cheapo from a Quad disc brake that I had handy. Bingo! Full power! Despite being spotless, it seems the XT rotor just doesn't want to be gripped. Has anyone had similar experiences or got any useful comments to add? 😕

Like anything, rotors wear out.

I read somewhere the suggestion that you should change rotors every time you get through 3 sets of brake pads, or something like that. Sounds like BS but I'm sure they will wear out eventually.

More likely IMO it is polished – sand it to roughen the surface and bed in again.

They wear by getting thinner – this should not affect braking performance – just heat dissipation and eventually they might shatter

What have you cleaned the rotors with?

I normally use acetone, but have been considering soldering flux (phosphoric acid), followed by warm water

You can get glazed rotors IME – have you tried some wet'n'dry on the rotors in the braking direction i.e. circular sanding?

http://www.ehow.com/how_4421355_clean-glaze-off-brake-rotors.html – Car content sorry 🙂

I keep mine in the fridge.

I keep mine in the fridge.

I hope you store them next to the mature cheddar or that tip doesn't work.

The fridge! 😆 I knew there'd be a logical answer!

i might or might not have put a couple of discs through the dishwasher when the wife wasn't looking…

i might or might not have put a couple of discs through the dishwasher when the wife wasn't looking…

i once put a chain in the dish washer (with the pots i might add) made one hell of a mess

i was naive and didn't realise that dishwasher's recycle water whilst washing

More likely IMO it is polished – sand it to roughen the surface and bed in again.

….. 😯

no no no?

its not abrasive friction, like sandpaper on wood (or anything else)
that involves 2 rough surfaces "hooking up" on each other.

its adhesive/cohesive friciton, whereby 2 smooth surfaces rub on each other and generate heat, therby removing energy (kinetic -> heat)

IIRC!!

its likely that they have glazed?

bedding in the brakes compressess the brake material creating a much denser harder compound, which is why PROPERLY bedded in brake pads should last months if not years (not minutes, all the clever clogs who were whinging about superstar pads after putting fresh pads in and then dragging them around a filthy course, shock horror, pads shredded)

it also leaves a residue on the rotor.

which is why, when you clean a rotor, it needs re bedding in…..

SO, if youve glazed a set of pads, you will compact "glaze" onto both the pad, AND the rotor.

sanding is the easiest and quickest way to get it off both parts of the setup, but be careful you dont score the rotor itself too badly!
your not sanding the rotor, but removing the glaze from the surface (as per foxy riders link)

glaze from the surface (as per foxy riders link)

Yeh sorry – thats why I would use a very fine wet'n'dry so you remove teh debris not the smooth surface 🙂

Correct Olly – me guilty of gross over simplification.
IIRC there are two different processes that lead to a shiny surface and inneffective brakes – true glazing is a high temperature process but something very similar happens at low temp.

Its a mix of abrasive and adherent friction in disc brakes. Its all a bit black magic with semi fluid boundary layers and such – that most STWers deny occurs

"that most STWers deny occurs "

…or don't give a shit about…

The rotors have been cleaned within an inch of their lives

I suspect this might have something to do with your issues, go get them muddy

Eventually they seem to get "dished" – the braking surface becomes concave. Braking performance drops off pretty rapidly at that point!

Have you checked the thickness of the rotors? I have managed to wear rotors out in the past to the point where there was a detectable by touch step between the spider and braking surface. Braking was rubbish by that point. New Rotor sorted that problem out though.

My XT brakes have been great for years

Yeh may have a point about rotos which are really really worn 🙂

I have managed to wear rotors out in the past to the point where there was a detectable by touch step between the spider and braking surface. Braking was rubbish by that point.

I have a rotor that has a significant step due to wear. It works absolutely fine. The only reason* a rotor will work less effectively is if you make some chemical/structural change to the surface, either through overheating or getting something on them.

*unless it gets so thin it isn't physically strong enough any more of course.

p.s. in my experience cleaning the rotors only ever has one effect and it's a negative one!

Will a thinner rotor not be more prone to overheating though? My worn one was a 160 and it wasn't very effective at the bottom of long descents

Buy some meths

Remove rotors and hang them from a wire

Cover rotors in meths

Set them alight

Repeat.

Never fails. Yes, seriously. Was the only way I could ever get my Mini Monos working proerply, until I gave up and replaced them with Maguras
🙂

I would say yes:

(as achieved by TB while "testing" his brakes 🙂

Was that rotor installed the correct way round? 😯

Was that rotor installed the correct way round?

TB replies:
Of coarse!!
I hope you reminded them I work in a reputable bicycle emporium ;-);-)

[however, despite going to a well respected grammar school his spelling leaves something to be desired…]

Will a thinner rotor not be more prone to overheating though? My worn one was a 160 and it wasn't very effective at the bottom of long descents

Wouldn't have thought it would make much difference- the change in surface area is negligible isn't it?

A badly worn rotor i.e. uneven with new pads in maybe a poorer braking surface i.e. curved as said above but if worn and flat ? Maybe one would just ahev to llok at the wear and braking surface to make that assesment – I still reckon a bit of wet'n'dry and some heavy braking would prob work a treat 🙂

Yes I suppose so, my evidence is anecdotal only but if you're applying the same friction to a rotor 2 mm thinner than new then I would expect it to heat more quickly – by how much I don't know and it was not as if I had anything to compare it to in the situations mentioned

Was that rotor installed the correct way round?

not according to wikipedia

http://en.wikipedia.org/wiki/Disc_brake
4th image down.
tut tut tut whoever wrote that.

not according to wikipedia

http://en.wikipedia.org/wiki/Disc_brake
4th image down.
tut tut tut whoever wrote that.

Fairly sure my hope discs have their direction arrow set for mounting that way round.

Seems to me thatthings would be better if spokes in tension – but usually mfrs recommend mounting in compression.

They should be the way round as shown in that picture.

Hmm, well I cleaned my brakes with acetone last night, and they work much better now.

I have a sneaking suspicion that the bit about bedding in pads leaving a layer of pad material on the rotor might only apply to organic pads rather than sintered. Half way through my first set of organic pads and the rotors have this strange sort of greasy tacky feel to them which I've never noticed before – I've always used sintered pads.

And pads will never last years when you are in a certain kind of wet gritty mud btw. Ever – no matter what you do to them.

And pads will never last years when you are in a certain kind of wet gritty mud btw. Ever – no matter what you do to them.

I'm on my second set of pads in 5 years riding in alpine trails and scottish mud.

It's not where the mud is, it's the exact mineral composition and grade, and the exact amount of water spraying about during the ride.

I've had pads on my bike for like two years getting through half the pad, and lost the other half of the same pad in a single ride in a different location in the rain. Did they become un-bedded in somehow?

It's when you've got coarse grit and enough water to form a film suspended between pad and rotor. The only way to save your pads in this situation is to stop periodically and either chuck some water on the caliper or just blow it out. Don't burn your lips on the rotor tho…

I'm on my second set of pads in 5 years riding in alpine trails and scottish mud.

Only ridden the bike 10 miles during all that time though 😆