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Scratchy brakes - GPX250

Discussion in 'Technical and Troubleshooting Torque' started by Viator, Jan 7, 2009.

  1. I'm wondering if anyone would know why my brakes are scratching against the discs on my bike...

    It's happening on both the front wheel and back wheel but worse on the front. As I push the bike forward the sound changes from softer to louder and softer again, so it's obviously worse at one part of the disc than others, suggesting my disc is warped but as far as I can tell by looking at them both discs look fine.

    Looking at the brake pads I think they are cheapies ( I recently bought the bike), and have 2mm left on both the front and rear. The service limit in the manual is 1mm. Also, the pads on the back seem to be unevenly worn. One side of one pad has more left than the other end of the same pad, and it's the same with the second rear pad.

    I wonder if I need to replace the pads, replace the discs, bleed the brakes etc...

    Any advice would be appreciated :)

  2. - The rear caliper is not sliding (floating) properly on the shafts which hold it. You may need to regreaser them with high temperature grease. Only a little will do but just enough that the shaft is covered with a thin film. Obviously using too much and the grease may pick up grime and/or dribble onto your pads. ... which is a very bad thing. As long as it is cleaned from grime beforehand (and not bent aswell), you'll find that the pads will wear equally for such little maintenance.

    I had this very same problem, but i fixed mine with blue marine grease because I'm a cheap c&%t.

    - You need to lift the front wheel off the ground and give it a spin. Very minor warpage on the disk and you'll have brake drag. Although it is not a major issue if the warpage is within service manual tolerances *, it can vibrate the steering from the caliper moving side to side quickly with brakes applied. Another -very bad thing- is that the disk rotor bolts may come loose from excessive vibration. So, check these ASAP.
    * you need a dial gauge for out-of-true measurement. The wheel and disk still attached to the bike to get this number.

    - Cheap brake pads should work quite well but may produce more dust than higher quality types. It is quite possible that you may have grit/dust/dirt/crap/stones embedded in the pads themselves. You can pop them off and lap them against some 400grit to 600grit sandpaper to hopefully loosen the crap.
    NOTE: it will help if you use good quality sandpaper since you do not want grains of abrasive to embed themselves into the pad material. Water works well - wet everything before you commence.
    CAUTION: it would be better to get new pads allround and it would help immensely if you give your rotors and calipers a good clean as well. Remember to clean out every hole/slot on the rotor. Water and car-wash soap + stiff brush work well. ...just remember to return your girlfriend's toothbrush to the proper spot!
  3. Thanks for the tips mate, I'll get onto it over the weekend I think and see if things run smoother after that.

    I might be going on a 1500km ish ride soon so I think it's worth replacing the pads anyway. Last time I checked though my local bike shop was charging 90 bucks for both the front and rear pads for my bike. Very exxy considering the cheapies are about 30bucks for front and rear.

    I wouldn't have thought of cleaning out the calipers even though it seems fairly obvious. Does it matter what kind of grease it is? Can you get a tub of "multi purpose" grease that does everything on your bike for e.g. your axles and such...
  4. Don't use all -purpose grease in calipers, cos it rots the Rubber bits.

    There is a product called "rubber grease" . Use it.
  5. Went to the bike shop the other day asking for some grease for the caliper pistons, and was told that I shouldn't be greasing anything in the caliper unless I was taking it appart and doing a complete rebuild.

    I figured I'd just get the brake pads then and hope the scratching would go away, but the same guy came and had a look at my bike and said that there was no point in getting new pads as my old ones had "plenty" of pad left. He also rolled my bike back and forth to listen for the scratchy noise that I'd described and it appeared to have fixed itself.

    Gee did I feel like an idiot. I've gone into the bike shop thinking I know exactly what I want, and I'm pretty much told "You're bike's fine, go home".

    Bloody brakes though, as soon as I got home they were scratchy again. I'm worried that I'm going to do damage to the disks if I keep riding the bike the way it is. There are quite a few grooves in the front disk now, a couple of which are deeper than the others. Is this normal?

    Should I keep worrying about my bike or just ride it and see what happens?
  6. Is it just one side of the rotor with the added grooves or both?

    Given the wear on the pads it's possible a small stone or something may have gotten trapped between the pad and rotor. If you haven't already done so I'd recommend removing the pads completely and checking the surface of them - giving them and the rotor a spray with brake cleaner might help as well (can't hurt).