BIKE reader Nico Schweiger had a problem with his disc brake that every biker knows: the brake pads were rubbing against the disc. Without further ado, he helped himself and raised an interesting question.
"I recently replaced the brake pads on my MTB (Avid Elixir X0 organic brake pads). When fitting the new brake pads with the pistons pushed back, I always have the problem that the gap is too small and I can't fit the pads or they still rub badly. As a result, I have ground down the brake pads. Now the real question: Is it allowed to grind the brake pads? Is there a health risk due to the ingredients? And if so, do you handle the abrasion in the workshop or during the test in a special way?"
Grinding brakes and toxic dust: BIKE answers
Organic brake pads consist of a variety of ingredients, the exact composition is always different and is usually an unknown trade secret because it is closely guarded by the manufacturer. In addition to the metals iron or copper, the components also include carbon fibres (PAN), graphite, petroleum coke or the "adhesive" epoxy resin. [Breuer, Bill (ed.): Bremsenhandbuch - Grundlagen, Komponenten, Systeme, Fahrdynamik. 4th edition. Springer Vieweg. 2013]
Although organic brake pads are no longer permitted to contain asbestos, which has been proven to be carcinogenic, the brake dust is almost certainly hazardous to health in large quantities.
A fundamentally hazardous effect on health can therefore be assumed. Of course, this also applies here: The dose makes the poison, and not just with all kinds of dust. In this case, i.e. a single grinding of one or two pairs of brake pads in the workshop, the amount of dust inhaled will hopefully not have been so great.
Here are a few links on the possible health hazards, in this case for brake linings of motor vehicles.
If you want to protect yourself from brake dust, for example in the workshop at home, the same applies as for other abrasive dusts. If there is a risk of the dust being swirled into the air, you should wear gloves, safety goggles and a suitable filter mask for fine dust. Strictly speaking, the same also applies to cleaning work on dry components.
Dust adhering to the surface of components is less critical, as it is usually bound in thin layers of grease and cannot be inhaled as easily. When rinsing with water, only very small amounts should be released into the air. However, do not wipe off the dust with your bare hands and wash your hands thoroughly afterwards.
What to do if the disc brake grinds?
But I would like to address the core problem of the brake pistons not being able to be fully reset or the gap being too small. There are several possible causes for this.
- The new brake pads could of course be slightly thicker than the previous pads. Admittedly, this is more of a theoretical problem, but with the small gap, just a few hundredths more can be enough to cause the brake to grind. In addition, there should of course be no dirt between the brake piston and the pad.
- It is more likely that the brake is simply overfilled. Again, this can have various causes. It may have been vented or refilled in the meantime if the pistons are not fully reset or the pads are not new.
- It is also possible that the brake fluid has increased in volume over time. DOT brake fluid is hygroscopic and ages by absorbing water. In addition, dust, metal and rubber abrasion collect in the brake fluid during driving. Both can increase the volume so that the brake grinds or even blocks completely, e.g. if the temperature in the system increases significantly.
- In older brakes, the rubber seals that pull back the brake pistons can stick to the pistons. One indication of this is if the pistons are not pushed out evenly from both sides, for example. The same also applies in the other direction, so that the pistons can no longer be pushed fully back into the seals.
- The first step to remedy stuck brake pistons is to clean them (see linked tips below), otherwise replace the pistons and seals. However, the latter is relatively expensive if you don't do it yourself, as the brake has to be dismantled for this. The spare parts themselves only cost a few euros at Sram.
In any case, if the brake has not been serviced for a long time, the brake fluid should definitely be changed and the brake bled. This should be done with the pistons completely pushed back. Sram recommends changing the DOT brake fluid at least once a year. To be on the safe side, follow the manufacturer's instructions.
Important: Always use the correct brake fluid! DOT 5.1 is used in Sram/Avid brakes, other brake manufacturers (e.g. Shimano or Magura) use mineral oil. DOT and mineral oil must never be mixed or confused.
If the DOT brake fluid is less than a year old, it would be sufficient to drain a tiny amount of brake fluid in a controlled manner. If you are unsure about this, please contact a specialist workshop. But now the pads are already worn down. Next time then :-)
In our article "Sram/Avid brake workshop for mechanics" you will find useful tips on servicing the Avid Elixir X0. In our workshop section you will find much more interesting information about MTB brakes and brake pads >>
Sebastian Brust
Editor
