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I've been asking around for a while now but even workshop pros couldn't answer this (by citing exact figures):

How big are the brake cylinders of disk and rim brakes in terms of volume and surface area? Specifically the Shimano RS505/685, SRAM Force and Magura HS33 R (FIRM-tech).

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    I have the Magura HS33 manual, and it makes no statements about internal volume of any part of the system. The best I can say is the page on replacing the oil shows a 20-30 mililitre plastic disposable syringe, and that the 50 mL bottle is supposed to be enough for a refresh of front and back brakes, but that would be with no spills or loss. So you're looking at ~50 mL of fluid in a normal solo bike for a Magura. Sorry that's all the info I have. – Criggie Apr 6 '16 at 8:09
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    Just curious: why do you care? – Lee-Man Jun 13 '16 at 18:14
  • @Lee-Man I intended to build a full custom solution working around some of the limitations given by the brake manufacturers. I short, shifters from brand A on brakes of brand B. I went a different path so I'm not limited by this question any more. – lusitania Jun 15 '16 at 15:52
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    Shimano and Magura use Mineral Oils. SRAM (Avid) use DOT brake fluid. using the wrong fluid can cause seal failure and hence brake failure possibly leading to injury and/or death. – Emyr Jul 1 '16 at 15:37
  • You're asking an (IMHO) unanswerable question. The volumes inside the lever and the calliper are not constant and vary by pad-wear and lever-adjustment. Thats what the brake-fluid reservoir is for. You could of course go ahead and disassemble the components to measure the depth and diameter of the cylinders - but why? It's not the individual volumes of the cylinders that influence the brakes power, but the ratio between the lever- and the calliper-cylinders. Or is that ratio what you want to find out? If so - remember that the length of the brake lever is also important to this equation. – Marlon Mar 16 '17 at 9:42
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well the snarky answer is 0, a brake cylinder is used in a drum brake. What you are referring to is a caliper. Drum brakes have a cylinder. Disc brakes and the HS33 use a caliper.

In terms of volume it is a varying amount I hope you understand that - volume at rest =/= volume at full activation and the function of volume inside the caliper at any given time is continuous between rest and full activation.

  • Do you find your answer particularly useful? en.m.wikipedia.org/wiki/Hydraulic_cylinder – gschenk Jan 26 '18 at 16:45
  • no - but its the correct answer. Asking for exact values of a continuous function is not possible. It depends on the length of the hose, the setting on the brake lever, and how much the brake lever is pushing the fluid into the cylinder. The question cannot be answered with an exact figure because it is not an exact figure. – mbillion Jan 26 '18 at 17:12
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    It can be answered. One might take the brakes apart, measure the inner diameter of the cylinder, calculate the area, and determine the travel of the piston to determine the volume. It is certainly not a correct answer (in a double sense!). Packing a truism in sciency sounding language in the second paragraph doesn't make your lack of understanding a common engineering term in the first paragraph better. – gschenk Jan 26 '18 at 19:02
  • If you want to bring engineering into this I actually know quite a bit about this. It really boils down to engineering being a very specific detail oriented thing. The reason its called a caliper and not a cylinder is the action it takes in the system. A caliper's job is to act like a pincer and squeeze something while a cylinders job is to exert force away from its base. its specific. Furthermore, the volume inside the cylinder is a continuous function dependent on the fluid used, the amount of pressure applied, the master cylinder size, and the stroke length. more unknowns than equations – mbillion Jan 26 '18 at 19:39
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    @mbillion it does not matter if it is in a caliper or in a hydraulic press. The system that transfers the movement and pressure change to movement and change of force, respectively, is a hydraulic cylinder. Ridiculing someone for using the term, while being pedantic about terminology, ist not only rude, but may backfire. Your line of argument about a continues linear function is (a) trivial, and (b) not relevant. (a) we are all aware that the piston does not teleport from one extreme to the other. (b) it can be characterised perfectly well by the volume as described above, using justa caliper. – gschenk Jan 27 '18 at 12:54

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