To use DOT 4 or 5.1 brake fluid?

Discussion in 'The Workshop' started by ransom, Sep 10, 2009.

  1. b3rnard

    b3rnard Member

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    It will be impossible for the fluid to be be near the heat of the rotor. Check the piston, or better yet ask Avid what their pistons are made out of. It's a composite, designed to have very poor "thermal conductivity". Therefore it will not transmit the same amount of heat generated by the pads and rotors to the fluids.
     
  2. DirtymikeTDB

    DirtymikeTDB Guest

    Lee....Yes, Hayes is sitll using the composite.

    b3rnard......for as heat resistant as the composite pistons are, if they truly are not transferring heat to heat up the fluid, then can anyone explain why the fluid in the caliper gets sooo burnt and discolered far worse than the fluid in the rest of the system.


    Again, thats the very reason you dont see DoT3 in Bicycle braking systems.....Period, and the very reason Avid went to a 5.1...Because people were boiling the DoT4.

    Its not science fiction that brake fluid boils in bicycle systems, belive it doesnt all you want, But I have been victim to brake fluid boiling on a bicycle before myself, its nice when all of a sudden you dont have brakes on a gnarly descent.

    Crazy part about it.... when it cools, you get your brakes back.

    Fact, composite pistons ...Help... to insulate the fluid from heat...
    Fact, the heat still goes into the fluid. Feel the caliper, it shot, there is only one way the caliper gets hot, thats through the piston, into the fluid, then the caliper.
     
  3. gooseaholic

    gooseaholic Active Member

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    Hey Lee. If you remember Ford used Phenolic pistons at one time. The one car they never used them on was the Mustang????? Plus they dont use them any more. Has to be a reason for that.:-k
     
  4. DirtymikeTDB

    DirtymikeTDB Guest

    Because they have a horrible habit of litterally falling apart.
     
  5. thesmokingman

    thesmokingman Member

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    F1 teams use Castrol SRF. SRF is good for about 310C. If we go with your theory, how do you explain or rationalize the fact that F1 brake rotor temps far exceed 750C? Hell, you can pretty much expect current carbon/carbon brakes running about 950C, thats about 1,800F.
     
  6. thephat

    thephat Active Member

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    Just a theory here: Bigger heavier brakes have more fluid to heat up by a lot. Also Pistons, and caliper walls are also heavier and thicker.

    Also, how hot do you think bike rotors can get at the hottest? I have not measured, but have seen them start to glow red before.
     
  7. gooseaholic

    gooseaholic Active Member

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    Ok one post before I go. Race cars have Air directly vented to the brakes, and they still glow red. Look at the braking system they use. They spend more in one day on brakes than most of us make in a year. I can guarantee, the brake fluid, rotors and pads on the cars is changed after every race. Plus being professional drivers, they brake with purpose, and dont drag the brakes as someone stated above.:-k Not really a valid comparison.
    On a side note, F1 cars use a separated brake system and the master cylinders are located in the suspension. The front and rear have dedicated master cylinders. It takes many months to make and cure carbon discs properly for those cars. They reach temps of 1000c and dont really even work until they hit 600c.
     
  8. DirtymikeTDB

    DirtymikeTDB Guest



    They dont ride there brakes for 85 percent plus of there driving.


    seriously, next time your out riding, pay attention to how much your truly on the brakes while descending. If your on them at all, even just covering, if the pad is in contact with the rotor, your creating more heat. for the rotors to cool down a substantial amount, you have to be off the brakes completly, other words you end up either retaining the level, or continuing to increase the heat level.

    The cars hold heat in the rotos alot more than bucycles do because of this. Sure on hard stop can heat up the rotors, but then your off the brakes, allowing air flow<which is also massivly higher on f1 cars over Mtn bikes>to cool your calipers and rotors, but those air ventilation systems for brakes is more intended to cool the calipers, the rotor cooling is more of a side effect if taht makes sense.
     
  9. thesmokingman

    thesmokingman Member

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    You've missed the point, which is that fluid cannot or should not reach near rotor temps which if it did would render brakes pretty meek. There are a lot of other factors involved in saturating the wet boil points of fluid in mtn systems. Using a dry boil point is pretty much folly for mtn bikes. Wet boil is much more applicable for mtn bikes.
     
  10. gooseaholic

    gooseaholic Active Member

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    ^^^^^Details please.
     
  11. thesmokingman

    thesmokingman Member

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    Btw, you're missing the point like the other poster. Fluid doesn't reach 1000C, obviously right? That was his point, which was my counter. Let's not get into a "who knows" more about F1 stuffs, which isn't the point.

    What would you guarantee with that they change fluids after each race? Endless RF650 can be used for one whole year before changing. Many teams have switched over to RF650 for this very reason, especially lower ranked teams where they can actually save big money over SRF. Due to RF650's long wear it is the standard for endurance racing where they you know race for 24 hours at a time, w/o bleeding the brakes ya know. Check with any teams at Le Mans, it is the standard fluid.

    And the part about dragging brakes, cmon really? Comparing how taxing that is to actually stopping a car under anger on a track and hitting very high temps. Which do you think is more stressful?

    Try some logic? The fluid is only good to 650F and the rotors get to as high as 1800F. Obviously the fluid is not even reaching half the temps that the rotors are at.
     
  12. gooseaholic

    gooseaholic Active Member

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    Just asking . No need to get hostile. I have raced cars, and brakes are the last thing to neglect. I think the whole point is. Brake fluid needs to be changed regardless of empirical data. Oh and BTW if I were a lame guy who never drove a race car and was on the brakes around the whole track, it would have no effect on braking in the long haul? The brakes and fluid would not get hotter? Its all relative. Not trying to argue with ya,just having a conversation.
     
  13. thesmokingman

    thesmokingman Member

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    I hear ya, not being hostile here either. Dragging brakes is not good, sure, but I'd imagine that you wouldn't be going very fast either?

    Btw, someone said they've seen mtn bike rotors glow and if thats true, we're looking at temps over 750F. If we consider that all metal glows the same color at the same temp... That's pretty hot stuff indeed and well above DOT 5.1 specs dry. I've never measured bike rotor temps, but that seems really obscene.
     
  14. mfoga

    mfoga Intense Whore

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    Take some physics classes and a lot of this will make more sense.
     
  15. DirtymikeTDB

    DirtymikeTDB Guest


    That is actually more of my point. I brought up temps of the brake fluid for a bigger reason. That was that some had posted its better to use lower grade fluid to induce themselves to flush tehre brakes mroe often... My point was more that it doesnt matter if your running dot3, dot4 or dot 5.1... You still need to flush your brakes just as regular.


    And yes, brake temps on DH bikes gets quite obscene. Yes, you canboil the fluid in them. Yes heat transfer happens.

    This is yet another subject that has many variables that need to be looked at. Bottom line is, use what teh system calls for.
     

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