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I have recently purchased a hybrid bike with suspension (not sure if that is relevant to this question). It came with two pieces of plastic. Any ideas what they may be for?

flat yellow plastic part

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    Most people would throw them in the rubbish or recycling. You might want to keep them if you intend on travelling with your bike, with the wheels removed. – Criggie Jul 17 at 19:54
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    Make sure they are scrupulously clean before using and free of any grease, even oils from your fingers. Contaminating the brake pads reduces their effectiveness – Swifty Jul 18 at 10:01
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    @Swifty I didn't think about that! Used moderately clean blocks from the parts bin once or twice so far. Added your warning to my answer. Thanks! – gschenk Nov 9 at 21:46
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This appears to be a disc brake spacer (block).

Such a spacer may be inserted in the disc brake caliper between the brake pads. When the wheels are removed, and hence the brake rotor, an inadvertent pull on the brake lever might move the pistons* so that the rotor cannot be inserted any more. It is even conceivable that the pistons might be dislodged entirely – with rather messy consequences. A spacer block inserted in the caliper prevents such mishaps.

Swifty pointed out in the comments the blocks need to be scrupulously clean lest brake pads get contaminated with oils transferred during insertion.

*In a hydraulic disc brake the brake pads are pushed from either side into the rotor by hydraulic pistons. After releasing the levers the pistons do not move back entirely. This is due to a mechanism that automatically compensated for pads and rotor getting thinner from wear.

  • 5
    This is the correct answer. – Rider_X Jul 17 at 20:35

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