Why do my brakes keep going funny?

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  • Why do my brakes keep going funny?
  • bent_udder
    Member

    It’s unlikely to be rotor size, although most people are overbraked these days. can you borrow someone else’s discs and try that? What are you cleaning the discs with?

    rolfharris
    Member

    X-lite disc cleaner.

    billybob
    Member

    Are you dragging your brakes too much causing the pads to glaze over?

    rolfharris
    Member

    I wouldn’t have thought so- I tend to brake late and hard, though it did happen particularly badly after a trip to Helvellyn 2 weeks ago where, yes, I did drag the brakes. I guess I must’ve glazed them- how do I sort this? Put the pads in the oven for 20 minutes?

    billybob
    Member

    take out the pads, put them on a flat surface & gently sand off the top layer of glaze – in theory you should see that they are glazed over. Failing that just ride somewhere gritty/muddy & they’ll soon roughen up again.

    PeterPoddy
    Member

    although most people are overbraked these days

    😯

    I can safely say I’ve never wished for less powerful brakes…..

    rolfharris
    Member

    I took them out on Friday and they had a shiny black coating that I filed off, and they still didn’t work too well.

    Are there any other ways to sort it? Or could it be another issue?

    At 14.5st in my keks, I do find the ‘You’re overbraked’ comments rather quaint.

    rolfharris
    Member

    11 stone after a good feed, so I really am overbraked.

    PeterPoddy
    Member

    Yo’re not overbraked, you’re overslow….

    😉

    cynic-al
    Member

    A dirty file could contaminate the pads

    rolfharris
    Member

    Peterpoddy- I take offence to that, I finished 4th at an SXC yesterday and it was descending rather than fitness that got it me 😉

    Al- fair point. Spokes of St Andrews is not the cleanest place to keep a file.

    rolfharris
    Member

    At the start of every ride my brakes don’t work- they’re LX discs and at the minute I’m running Alligator pads but it happens with all brands.

    They squeal a bit and stop me slowly, but I’m used to one finger braking. If I go through essentially a bedding in process at the start of every ride they start to work, but with all the pads I’ve had over the past few months they all seem to just stop working at about half worn.

    I’ve cleaned everything with disc cleaner, filed the pads back and the rotor and it still does it so I’ve put it down to one of two things-

    1- running 203mm/180mm rotor combo- I think I don’t generate enough heat for the pads to work properly. This always used to work but I might’ve got better or something.

    2- keeping the bike in a shed this year while it always used to live in the house.

    Any other ideas?

    PeterPoddy
    Member

    If SXC is ‘Southern XC’ I doubt there’d have been many hills for you to descend that would have required brakes!

    😉
    🙄

    (I’m in the South. I know, I know….!)

    Stop using the brake cleaner. It really is not needed. Sounds to me like either contamination from the cleaner or glazed pads. Some brake cleaners can cause contamination as they do leave a residue – for cars its no problem as the heat generated burns the residue off

    I believe ( but without real evidence) that the big discs may be an issue – the brakes are never getting up to working temp. A set of pads should last thousands of miles anyway. Can you swap to smaller discs? I have some 160s if you want to borrow them to try.

    Skinny little runt like you riding xc shouldn’t need the big discs

    Sand the pads back and the discs then bed in again

    Premier Icon sweaman2
    Subscriber

    This has also been happening to me (Shimano XT 180mm on Shimano sintered pads) so I’m delighted to find someone else with the same problem; I keep my bike inside as well though. I had a long discussion with my LBS who recommended I do the following.

    Take the brake pads out of the caliper.
    Clean pads and rotor with Muc-off disc brake cleaner.
    File down the pads a bit with some wet n’ dry

    Clean inside the caliper (i.e behind the pads) with a wet rag and a bit of muc-off so everything is nice and new.

    Re-bed everything in.

    This has seemed to work on last ride. But I haven’t done the acid test of leaving the bike for a week and then seeing what happens. For me the longer I left the bike the worse it got. As the bike is sitting next to another bike that had zero problems I discounted the atmospheric contamination theory.

    although most people are overbraked these days

    Oh dear 🙄

    trial monkey – if you are only using a small % of the brakes capacity it will never get hot enough. Well set up 160s will exceed the grip of your tyres with single finger braking. Big discs used to a small % of their capability will glaze.

    Depends on many issues of course from riding style to weight to terrain to pad type etc

    druidh
    Member

    I’m with TJ on this. I’ve had exactly the same symptoms a described in the OP, but only since going up a rotor size on the front. Seems to me like the brakes work better once theres some heat in them. If I have a long uphill at the start of the day, I try to make sure I do some brake warm-ups before any serious descending (bike is kept in the garage – no obvious contamination).

    It also has to be said that LX brakes are a bit on-and-off compared with XT or XTR.

    TJ,

    I’m curious – how hot do you think brakes need to be made in order to sort the pads? Do you think this is a one off during bedding in, or needs to be ‘topped up’ during regular riding?

    SweetJumps
    Member

    Have you accidently sprayed GT85 on them. This is my favorite trick for stopping mine from working.

    If it’s and airlock try cable-tieing the levers into the bars overnight, this should remove any air. (As tight as possible)

    It sounds like you’ve glazed them to me, try getting them hot (riding with the brakes gentley on) them pouring water and some dust onto the pads (with the wheel in) this takes off the top surface without damaging anything.

    Science officer – I think its two part – you need the high temps to bed them in and you need high temps to prevent glazing in normal use.

    I had this issue with my motorcycle brakes as well as a result of my riding style of hardly using the brakes and then only using them gently – once a mate who has a hard braking style borrowed the bike for a weekend and the brakes were much sharper once he had used it! Burnt the glaze off.

    From the science stuff I have read its about the two forms of friction – abrasive and adherent. You need to get the pad / disc interface hot enough to produce a semi fluid boundary layer – this is hundreds of degrees hot. Brakes work best in a particular temperature range – this will vary depending on pad composition.

    I do get glazed pads on my mountainbikes as well – lots of gentle braking rather than short hard applications – even with small discs. getting them really hot removes the glazing and restores the performance. On the first big downhill of the day you can feel the brake performance increasing as they heat up.

    Dibbs
    Member

    Have you accidently sprayed GT85 on them. This is my favorite trick for stopping mine from working.

    That sounds reasonable to me, from the way I’ve seen some people use GT85 (orrible stuff wouldn’t touch it with a barge pole) at the start of a ride.

    When i had a 180 disc at the back it was eating pads like no tomorrow. I put it down to, like Luke, them not getting hot enough to bed in properly. 160mm rotor now and great braking

    Yes, the reading I’ve done suggests the same as you state here – I’m wondering if the relatively small magnitude braking forces on MTB’s (compared to cars and other motorised vehicles) and the addition of much dirtier conditions that road going vehicles never encounter, may alter the interaction of some of the processes as they’re described elsewhere for motorised vehicles. Dirt in the system will erode the transferred pads layer somewhat quicker – even if its removed after one revolution.

    Clearly, in the winter, when most folks pads disappear at an alarming rate, it will be much harder to get heat into the brake system due to ambient conditions and slower overall speeds.

    Next winter, it would be interesting to reduce my brake size, thereby increasing the heat in the system, and seeing if I get better life and braking. Its an easy enough experiment to do, with just a rotor change.

    Its a balancing act though, since the danger of over heating the system is greater too.

    This is quite a neat summary of the processes.

    <http://www.stoptech.com/tech_info/wp_bedintheory.shtml&gt;

    I’d venture in the winter, most of us are running abrasive friction processes, that’s why the pads disappear, since they’re the weaker of the two materials.

    When your brakes are hot and stinky at the bottom of a descent, there’s a good rationale for releasing the brake lever as soon as possible too.

    Part of what makes me believe in the “heat needed” theory is from the tandem – pads last many thousands of miles using in all conditions – with M4s and 30 stone all up weight the brakes get used hard ( 203 / 185 discs)- hard enough that the discs blue and we have overheated them – we just don’t get noticeable pad wear. I do glaze them from time to time however

    I can see how M4’s would get overheated by 30 stone!

    Premier Icon BigDummy
    Subscriber

    Is this definitely a pad problem? All of my intermittent braking problems seem to have been caused by air in the brake.

    rolfharris
    Member

    Definitely not air in the brake- it’s bled and works fine in terms of consistent lever pull. It’s all apparently pad/rotor related. The disc cleaner is mountain bike specific stuff we had knocking about so that’s hopefully not an issue.

    I’ll give everything a re-clean and will try the rotors off my race bike with new pads this weekend and hopefully it’ll behave.

    bent_udder
    Member

    How often are you using the disc brake cleaner?

    I use a bit of meths on a kitchen towel every now and then – maybe once a month, tops – but how often they’ll need a clean very much depends on where you’re riding. If you’re using cleaner after every ride, that might be the problem.

    B.A.Nana
    Member

    Just for the record, the problem I had recently (XTs) was caused by me bleeding the brakes (bottom up method with syringe) and forcing a bit of muck into the front caliper. The result was really poor braking but a nice solid feel in the lever. The braking improved as the brakes heated up, but you still needed to squeeze like hell all the time and often ended up in a heap at the bottom unable to stop with a strong smell of burning brakes.

    trail_rat
    Member

    luke , same problems on my old shape XT disks ..160 F & R with goodridge sintered pads …

    they were spot on before i went to nz … they squeeled like banshee when i got back , no leakage and no spongy lever … glazed to hell some how ..

    took em out , cleaned caliper and lubed up pistons , took a new sheet of sandpaper to the pads and sanded them till the glazed surface had gone ! then reassembled cleaned rotors and rebled and still they howl

    Ive spent plenty time fixing other peoples brakes but these xts are being a coont … it all started after i rode down ben nevis and boiled the life out of them :s …..definantly underbraked for that no questions asked. I think they may be **** ….

    Rich_s
    Member

    Same here with LX 203F 180R – 15 stone on a HT. Lots of 2 finger power, but no 1 finger bite. I thought it was because the bike lives in my car so never gets clean/always gets disassembled after each ride (greasy fingers?).

    I’ve been thinking about my braking recently and from following other people had thought I braked less often; turns out I probably brake for a longer time with less effort. Have sanded the pads and cleaned the discs but it’s (not much) better. Will sand the discs next…

    BANana – I use the bottom up method <insert punchline of your choice here> too – did you get dirt stuck behind the pistons in the bleed? How did you get rid of it?

    B.A.Nana
    Member

    Basically, the bleed nipple must have had the dirt in it (the cap had come off at some point I guess) and I forced it into the caliper with the syringe. After much messing around and pulling hair out, my last resort was to strip down the brakes and take everything apart and build up again. When I unscrewed the bleed nipple from the caliper and forced out some fluid a big bit of dirt came out, problem solved. It came out immediately, my guess is it couldn,t get any further into the system and was just stuck behind the nipple, but enough to impede the flow of fluid.

    Rich_s
    Member

    Ah, right. Will have a gander at that (both caps are missing off mine).

    Based upon what I’ve been reading, howling brakes, after cleaning and new pads can be down to uneven pad material coating on the disc.

    Using a harder disc compound can scrub off the coating after some heavy braking, since it can’t just be wiped off with a bit of meths. Gotta be worth a try?

    Premier Icon glenh
    Subscriber

    I used to have the same problem. Turns out it was a tiny (invisible) fluid leak from the caliper. I would burn off during a ride, but contaminate the pads again every time the bike was left for a few days.

    p.s. there is no need for brake cleaner. I haven’t cleaned the brakes on any of my 3 bikes (except with the garden hose) in 3 years. They all still work fine.

    rolfharris
    Member

    I just used the brake cleaner for the first time last week when the problem was at its worst.

    I do bleed bottom up- if my plan for this weekend doesn’t work I’ll rebuild it.

    Premier Icon sweaman2
    Subscriber

    glenh – how did you finally identify the problem? I’ve sort of had the assumption on my brakes that it is a tiny leak but have been unable to find it. Very frustrating.

    Premier Icon Stu
    Subscriber

    Did you fit the pads with a rock? 🙄

    Premier Icon stuartlangwilson
    Subscriber

    I would try avoiding cleaning the brakes with chemicals. If you use expensive car shampoo to clean your bike it has wax in it. Use water or a brush. Get mud on your brakes, drag them a bit, get them hot.

    Premier Icon glenh
    Subscriber

    glenh – how did you finally identify the problem? I’ve sort of had the assumption on my brakes that it is a tiny leak but have been unable to find it. Very frustrating.

    There was a very slight residue on the caliper occasionally, but nothing you would notice. The real reason I decided it was a leak was that I rebuilt the brake and it stopped going bad!

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