Results 1 to 12 of 12
  1. #1
    MW
    MW is offline
    mtbr member
    Reputation: MW's Avatar
    Join Date
    Jan 2007
    Posts
    731

    Formula brakes: okay to run DOT5 or DOT5.1?

    Did a quick search and didn't find anything . . . if there's already a thread on this, just point me the way.

    Question: can I run DOT5 or DOT5.1 in my Formula K18 brakes? Formula specs DOT4 and their literature doesn't mention anything about other DOT standards. I believe 5 and 5.1 are both synthetic and have higher boiling points . . . would make sense to run one or the other if it won't hurt the brakes.

    Thanks for the feedback.

    --MW

  2. #2
    ...idios...
    Reputation: SteveUK's Avatar
    Join Date
    Apr 2006
    Posts
    5,662
    DOT 5 is silicone based and is not compatible with any bicycle disc brakesets. DOT 4 and 5.1 are glycol based. If your system has DOT 4 in it, just find a good quality (Motul, for example) fluid and use that. If you want to switch to a DOT 5.1 fluid you can do, but you should flush as much of the DOT 4 out as possible. It's OK to mix, although your brake fluid will only be as good as the lesser of the two fluids.

    What use is a philosopher who doesn't hurt anybody's feelings? -
    Diogenes


  3. #3
    Never trust a fart
    Reputation: frdfandc's Avatar
    Join Date
    Sep 2007
    Posts
    4,216
    Your not supposed to mix brake fluid. If it has DOT 4, put DOT 4. You can mix DOT 4 and DOT 3. Mixing fluids like DOT 4 and DOT 5.1 can cause items to corrode or break - lines/calipers.

    What I would do is get a bottle or 2 of DOT 4 synthetic fluid. Flush the brake system really well with one bottle, then use the other to fill the system.

    The synthetic brake fluid is less likely to boil during heavy braking as compared to the regular stuff.

    Also when picking a brake fluid, make sure you get the fluid with the highest wet boiling point. Something like Castrol SRF, or Motul Racing 600. There is probably others that I don't know about.

  4. #4
    ...idios...
    Reputation: SteveUK's Avatar
    Join Date
    Apr 2006
    Posts
    5,662
    "Mixing fluids like DOT 4 and DOT 5.1 can cause items to corrode or break - lines/calipers."

    This simply isn't true. As I said, you may reduce the effectiveness of whichever fluid has (had) the highest boiling point (not necessarily the 5.1), but you won't harm seals, lines or calipers/levers.

    What use is a philosopher who doesn't hurt anybody's feelings? -
    Diogenes


  5. #5
    Never trust a fart
    Reputation: frdfandc's Avatar
    Join Date
    Sep 2007
    Posts
    4,216
    Quote Originally Posted by SteveUK
    "Mixing fluids like DOT 4 and DOT 5.1 can cause items to corrode or break - lines/calipers."

    This simply isn't true. As I said, you may reduce the effectiveness of whichever fluid has (had) the highest boiling point (not necessarily the 5.1), but you won't harm seals, lines or calipers/levers.

    This is true. I have seen what mixing of brake fluid can do. As a automotive tech, I have been dealing with hydraulic brake systems and hydraulic clutch systems for over 11 years.

    Its not due to the compatibility of the actual fluid to each other, but the actual boiling point of each type of fluid. Dot 4 has different boiling points than DOT 5.1 That is why there could be component failure. Component failure comes from rust from water being in the system. Brake systems should be flushed every so often because of this.

    The lower the Wet boiling point, the more moisture the fluid draws in from the atmosphere.

    MINIMAL boiling points for these specifications are as follows:
    Boiling Point Ranges Dry Boiling Point Wet Boiling Point
    DOT 3 205C (401F) 140C (284F)
    DOT 4 230C (446F) 155C (311F)
    DOT 5 260C (500F) 180C (356F)
    DOT 5.1 270C (518F) 191C (375F)

  6. #6
    ...idios...
    Reputation: SteveUK's Avatar
    Join Date
    Apr 2006
    Posts
    5,662
    There's no discussion about the boiling points of the various DOT ratings, the point is that in a bicycle brakeset there is no risk to the brake components by mixing glycol-based fluids (DOT 3, 4 and 5.1).

    If a DOT 4 fluid with a lower BP is introduced to a DOT 5.1 fluid, the worst you've done is reduced the effectiveness of the higher rated fluid; that's it. This is why I said that's it's best to flush as much of the DOT 4 as possible. Bicycle brakes will typically not be exposed to the same temperatures or temperature fluctuations that automotive brakes will. It's not unusual for brake fluid to still perform adequately even after being in the lines for two or three years (although your brake system is more likely to feel at its best if you flush every one to two years, depending on your application).

    Again, there is no risk to brake components by mixing glycol-based fluids.
    Last edited by SteveUK; 05-30-2008 at 03:16 PM.

    What use is a philosopher who doesn't hurt anybody's feelings? -
    Diogenes


  7. #7
    mtbr member
    Reputation:
    Join Date
    Nov 2007
    Posts
    1,465
    Part of the DOT standard is the standard for miscibility with other DOT fluids. 5.1 standard specifies that it is miscible with DOT3 and DOT4.

  8. #8
    mtbr member
    Reputation: Raymo853's Avatar
    Join Date
    Jan 2004
    Posts
    743
    I was about to add 5.0 to my 5.1 systems and mixed a little together outside to see what is what. Well it is amazing how the two always separated. I suspect the different densities of the fluids might really cause problems. I do not know what anymore, it has been almost 20 years since I took a fluids college course, but I know having two liquids of different densities in the same hydraulic system can not be a good idea.

  9. #9
    mtbr member
    Reputation:
    Join Date
    Jul 2003
    Posts
    6,762
    Quote Originally Posted by Raymo853
    I was about to add 5.0 to my 5.1 systems and mixed a little together outside to see what is what. Well it is amazing how the two always separated. I suspect the different densities of the fluids might really cause problems. I do not know what anymore, it has been almost 20 years since I took a fluids college course, but I know having two liquids of different densities in the same hydraulic system can not be a good idea.
    The problem with mixing 5.0 and 5.1 is not the density of the fluids it's the chemical base. DOT 3, 4, and 5.1 are glycol based fluids. DOT 5.1 is silicone based. Silicone and glyco do not mix and will seperate. As previously noted part of the DOT standard is miscability. If you read the standards 5.1 is compatable with 3 and 4, DOT 5 is not. In addition any reputabel manufacturer will so state on the bottle of DOT 5 that it is not miscable with other DOT fluids. And it certainly causes some pretty weird stuff to happen to the rubber components in a brake system designed for glycol based fluids. Seals and rubber bits soften and expand and end up looking like sponge rubber then disintergrate. Not a good thing.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

  10. #10
    mtbr member
    Reputation:
    Join Date
    Aug 2008
    Posts
    4,932
    Quote Originally Posted by Squash
    The problem with mixing 5.0 and 5.1 is not the density of the fluids it's the chemical base. DOT 3, 4, and 5.1 are glycol based fluids. DOT 5 is silicone based. Silicone and glyco do not mix and will seperate. As previously noted part of the DOT standard is miscability. If you read the standards 5.1 is compatable with 3 and 4, DOT 5 is not. In addition any reputabel manufacturer will so state on the bottle of DOT 5 that it is not miscable with other DOT fluids. And it certainly causes some pretty weird stuff to happen to the rubber components in a brake system designed for glycol based fluids. Seals and rubber bits soften and expand and end up looking like sponge rubber then disintergrate. Not a good thing.

    Good Dirt

    Fixed a typo. You had 5.1 twice.

  11. #11
    Come see me after class
    Reputation:
    Join Date
    Dec 2005
    Posts
    1,431
    in a nutshell, DOT 3, 4, 5.1 are OK for your Formula brakes. DOT 5 is not OK.

    go to Autozone, buy a bottle of DOT 4 for $4, and be done with it. you don't need a $15/pint bottle of racing brake fluid. it's a freaking bicycle.

  12. #12
    Former Bike Wrench
    Reputation: mtnbiker72's Avatar
    Join Date
    Jan 2007
    Posts
    15,976
    To add...you can get high temp DOT 4 as well. It is not ALWAYS the case that DOT 5.1 has a higher boiling point than DOT 4. It is simply that DOT 5.1 has higher MINIMUM standard for the boiling point. For example, Amsoil DOT 4 has a dry boiling point of 304* C which exceeds the DOT 5.1 minimum of 270* C. It also exceeds the minimum wet boiling point of DOT 5.1.

    And mixing 3, 4, and 5.1 will do no harm to your brake system.

Members who have read this thread: 0

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

THE SITE

ABOUT MTBR

VISIT US AT

© Copyright 2019 VerticalScope Inc. All rights reserved.