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When reading the question Front tire and downtube are extremely close, I realized that disc brakes (referring to the assembly of brake disc and caliper, not the brake levers) are always installed on the left side of the bicycle.

Is there a specific reason for this? For the rear brake, I imagine this is so they do not interfere with the derailleur mechanism (on bikes which have one), but it seems the front brake could just as well be on the right side.

Is there a technical reason why disc brakes are always on the left side? Or is it just a common convention?

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On the rear, the disk rotor is on the left side of the bike to keep it away from the contaminants around the chain and cassette. This is why QR skewers have their levers on the left side of the bike too.

As such the rear caliper is built to have its hose or cable enter from the "front".

Since caliper bodies are used on front and rear brakes, it makes sense to not have a dedicated design for front and rear caliper, and to just have two calipers. Otherwise mounting the front caliper on the right hand side of the bike would have to be mirrored.

In short, it minimises the inventory and merges two stock items into one.

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    Nice answer. This of course prompts the question of why the drive train is on the right side. However, there's already a question for that :-). Why do bikes have their drivetrains on the right?
    – sleske
    Commented Feb 16, 2023 at 12:10
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    In theory it would be possible to build an ambidextrous caliper, but given there is no benefit, why would the standard force the limitation on manufactures.
    – mattnz
    Commented Feb 16, 2023 at 18:43
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    Aside - this is not the case in cars, where the front and rear brakes are often different sizes to bias the brake toward the front, AND are definitely "handed" resulting in two mirrored pairs of brake calipers/cylinders. They may be made from the same castings but with different ports blocked off at the factory, but are still sold as left or right and this is not left to the mechanic to blank-off the wrong port.
    – Criggie
    Commented Feb 16, 2023 at 20:55

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