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I Fluid Dynamics question -- compressibily of brake fluids "oil"

  1. Aug 23, 2016 #1
    As part of a racing community, there is a discussion going on that talks about compressibility of brake fluid. the Street car versions have a value of 6.5E-05 and the racing stuff is 4.5E-05 both at a pressure of near 5000psi. there is talk about the difference of the racing fluids which compare two for example , with a 25% difference between the two.. 4.6E-05 vs 4.5E-05.
    is it possible that someone could feel the difference in brake pedal pressure, in a car with the two different fluids in the system , comprised of fluid lines, calipers , and actuation systems (master cylinders made up of springs and seals along with brake pedals attached to long levers)?
    intuitively, we are talking about a very very small difference in compressibility for fluid in general, and the difference of the two , even if 25% greater is a very very small value as well. for those familiar with automobile braking systems there is a knowledge that the fluid also can absorb water which boils at above 100C and the fluid itself which can boil at temps in the 280C range certainly when it boils under extreme racing conditions, the pedal does feel much softer until the fluid is replaced or the water or brake fluid , condenses back to fluid again.

    as far as compressibility of the fluid,, could the compresibility of the fluid actually be felt before any of the heat based factors take effect? keep in mind that the braking system is comprised of :
    50ft of 2mm diameter brake lines, calipers with reserviors and pistons/seals, and a master cylinder with a plunger (like a 10mm syringe) and a foot long pedal

    thanks for reviewing my quesiton
  2. jcsd
  3. Aug 23, 2016 #2


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    If we assume that the system holds one liter of brake fluid in the high pressure circuit (that's high, but it's also a round number and since I'm doing the arithmetic in my head and .1 liter feels a bit low, we'll go with it) the the difference in compressibility works out to be ##2\times{10}^{-2}## cc. 10mm for the master cylinder diameter feels low (maybe not for a race application), but if we go with that value one centimeter of piston travel will displace about 3/4 of a cc. So the compressibility difference will yield an extra .3 millimeters or so of piston travel. Assume a 30ish to one multiplier from the pedal (that's high) and you might get something on the order of one centimeter of additional pedal travel.

    One centimeter? I can imagine a world-class driver looking for a thousandth of a second as he enters a turn noticing this. (Me, flopping around the autocross course? I have bigger problems - I just blew a full second trying to miss that cone and everyone is laughing at me).

    And note that every number in the back of the envelope calculation was chosen in the direction of maximum effect: high volume of fluid, small master cylinder, high leverage from the pedal. I wouldn't be surprised to find that in a real design we're only talking about a few millimeters of pedal travel from the extra compressibility.
  4. Aug 23, 2016 #3


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    A very well described and thought out reply.
  5. Aug 24, 2016 #4


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  6. Aug 24, 2016 #5


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    Your math needs checking here. There is only a 2% difference between 4.5E-5 and 4.6E-5.
  7. Aug 31, 2016 #6
    I was referring to the difference between the street fluid and the racing type being near 25%...so the real question is it plausible to tell the difference between the street and the two racing fluids.... 4.6 vs 6.4 or 4.6 vs 6.5E-5
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