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DOT Fluid vs Mineral Oil: What's the Difference?

Miles Garage, Tech Tips -

DOT Fluid vs Mineral Oil: What's the Difference?

Most areas of your bike's brake system can be customized, including rotor size, pad compound, reach adjust, and other variables. But there is one thing you will never want to experiment with on your brakes - the type of hydraulic fluid being used in your brake system. 

There are two main types: Mineral Oil and DOT fluid. On the surface, both of these are fluids that are used under compression in a closed system. As you squeeze your brake lever, the hydraulic fluid is pushed through your brake lines. Because fluid is generally incompressible, it results in the pistons of your caliper being pushed out, pressing the pads against your rotor. 

These hydraulic principles hold true for all brake systems, but that doesn’t mean DOT fluid and Mineral Oil are interchangeable. Manufacturers have designed their brakes to only utilize a certain type, and attempting to use something else will result in damaged seals, corrosion, erratic braking performance, and a voided warranty if any other issues were to arise.

Why would one brake company choose Mineral Oil over DOT? Or the other way around? And why does swapping fluids cause so many problems?

It has to do with the characteristics of these fluid types and what the brake manufacturer is trying to achieve.  

It’s also due to the difference in the seals being used as DOT systems use EPDM rubber while mineral oil systems use nitrile (NBR). If an incompatible fluid is used in your brakes, it will cause the seals to swell, stick, and ultimately fail over time as the rubber weakens. 

Key Differences to Consider

To understand the difference between these two fluid types, we can start by looking at their two main purposes: to transfer the input from your brake lever to the caliper and to resist the high heat created by the friction of the pads on the rotor. 

The variables that allow those two functions to happen are:

Boiling Point: The higher the boiling point, the more heat the system can handle before you experience brake fade. Brake fade occurs when the fluid gets hot enough to turn into a gas. Because gas is compressible, you’ll no longer have a firm feel when you squeeze your brake lever. Once this type of brake fade happens, and the fluid turns to a gas, your brake system will need to be bled. Brake fade is also dependent on the type of compound your brake pads are made of, but a breakdown of the Mineral Oil and DOT fluid in your system, due to exceeding it’s boiling point, will also result in fade. 

Water Absorption: The boiling point of your brake fluid can actually change over time due to water being absorbed in the fluid. This is why periodically bleeding your brake lines is an important maintenance task. As water is absorbed into your hydraulic fluid, the lowered boiling point makes your brakes more prone to fade. 

We’ll go into the specifics of how each fluid absorbs water in the detailed explanation of the fluid types below.  

About DOT Fluid

DOT fluid is one of the most common types of brake fluids in use today and is widely used in automobiles and other applications. In fact, the acronym DOT stands for Department of Transportation because it is regulated by that organization and must meet the standards set by the Society of Automotive Engineers (SAE). 

This allows DOT to be fairly compatible across brands (as long as you stay with the same classification) and means all DOT types have a set minimum boiling point that the product must adhere to.   

DOT brake fluid for use in mountain bike brake systems

Water Absorption of DOT

One of the major differences in DOT fluid versus Mineral Oil is in the manner in which the fluid absorbs, or does not absorb water. In the case of DOT, it does absorb the water and essentially makes any water in the system a part of the hydraulic fluid. SRAM actually notes this as one of the major reasons they have chosen to go with DOT over Mineral Oil in their brake systems. This is because while the absorption of water does lower the boiling point of the fluid, it does so in a manageable way.  

One important consideration with DOT fluid, knowing that it absorbs water, is how it is stored. If left in an unsealed container it will absorb water at a much higher rate. Even in a closed container it will become contaminated over time and should be used quickly after a new bottle is opened. If you are completing a brake bleed on your bike, go for smaller bottles and purchase them as necessary, versus getting a larger bottle with the intent of using it over time. 

Boiling Point of DOT

All DOT fluids have a dry boiling point and a wet boiling point. A dry boiling point is the rating for new fluid, with no water in the system. A wet boiling point is the rating for fluid that has absorbed water over time. More specifically, a wet boiling point is the boiling temperature for a brake system that has absorbed 3.7% water by volume, and this is expected to occur after about 2 years in a sealed braking system.

Below are the dry and wet boiling points for the different classifications of DOT fluid.  

DOT Fluid

Dry Boiling Point

Wet Boiling Point

Primary Constituent

DOT 3

205 ℃ (401 ℉)

140 ℃ (284 ℉)

Glycol ether

DOT 4

230 ℃ (446 ℉)

155 ℃ (311 ℉)

Glycol ether/borate ester

DOT 5

260 ℃ (500 ℉)

205 ℃ (356 ℉)

silicone

DOT 5.1

260 ℃ (500 ℉)

205 ℃ (356 ℉)

Glycol ether/borate ester


You’ll notice there are two DOT ‘5’ fluids, with the difference between them being the primary constituent. In bicycle brake systems, only the glycol based DOT fluids are used and DOT 5 should not be substituted for DOT 5.1 (which is what SRAM uses in their brake systems). 

If a brake system is not properly maintained those wet boiling points will continue to decline, resulting in decreased performance and an inability of the brake system to mitigate heat. SRAM does state that “[their] tests showed that [in] even old, “wet” DOT fluid the boiling point never gets below 180C, while a bike brake system rarely sees temperatures over this point. The result: DOT fluid offers more consistent braking performance.” However, it is important to always maintain your brakes so that their performance never drops beyond a given level and to prevent the boiling of any water that has migrated into your fluid. 

About Mineral Oil

Mineral oil is a distillate of petroleum, and is often seen on the shelves of drugstores. However, that mineral oil should not be confused for nor used as the fluid in your brake lines. The mineral oil we’re talking about here has had various additives mixed in to increase the viscosity of the fluid and change its properties to be more conducive to high heat applications. Shimano’s offering is a mineral oil based hydraulic fluid that is much different than the perfume smelling bottle of mineral oil from your local drug store. 

Shimano Mineral Oil for use in mountain bike brake systems

One benefit of mineral oil over DOT fluid is that it is non-toxic. If you happen to spill a little on your bike while working on it, the mineral oil is less likely to cause any damage to other parts. This is one of the reasons Shimano mentions that they have chosen to use mineral oil. They say, “Although Shimano made DOT fluid brakes in the 1960's and '70's, our modern brakes use mineral oil. Mineral oil won't harm painted finishes, and is less damaging to your skin and the environment.” That being said, the volume of DOT fluid or mineral oil that might be spilled is quite small, so wipe it up as soon as you can and try not to worry about it too much. 

Another benefit of mineral oil is in its ability to not absorb water. 

Water Absorption in Mineral Oil

Mineral oil is hydrophobic and this means it does not absorb water. Unlike DOT fluid, its consistency will not change and any water that enters the system does not become a part of the fluid. 

Besides the obvious benefit of your brake fluid not absorbing water, being hydrophobic also makes mineral oil much easier to store. You can buy a large bottle of it and not worry about it going bad over the course of a few months. 

But hydrophobia is not all that it is cracked up to be. 

Boiling Point of Mineral Oil

Because mineral oil cannot absorb water, it maintains its boiling point over time. DOT fluid has a dry and a wet boiling point, but when it comes to mineral oil there is no such thing. Assuming the brake system is sealed and water in the lines is minimized, it should hold a consistent boiling point and most mineral oil hydraulic fluids are rated around 260 ℃. 

The elephant in the room is that if there is enough water in your brake lines, it will ultimately result in the boiling temperature of the fluid being the same as water at 100 ℃. Why does this happen? Because the water is not absorbed, it falls to the lower part of the brake system as water is denser than mineral oil. Typically the brake caliper is the lowest point of the braking system and susceptible to the most heat. The caliper will heat up the water, boil it, and you’ll experience brake fade. 

What Manufacturers Use Mineral Oil vs DOT

Mineral Oil

  • Shimano
  • Magura
  • Tektro
  • Gatorbrake

DOT

  • Hayes
  • Avid/SRAM
  • Formula
  • Hope

Other

  • Trickstuff - uses Bionol, a hydraulic oil based on vegetable oil

Which is better - DOT or Mineral Oil

Which one is better? Whatever one your bicycle brake system uses is the best. You’ll need to service your brakes no matter what, and if you follow the manufacturers recommended service intervals you shouldn’t notice any degradation in performance from either fluid type. 

All of the issues we’ve covered in regards to water absorption and boiling points tend to arise after long periods of time. DOT absorbs 3.7% of water after 2 years and we can assume a similar timeframe for mineral oil to have that much water in the brake lines, too. 

You should be bleeding your bike's brakes at least once a season, if not more, depending on how often you ride (we recommend every time you replace your brake pads). In doing so your brakes will always feel like they should and you can ride any trail with confidence that they will work as intended and you won’t have to worry about fade caused by the fluid overheating. Fade caused by the compound type you’ve chosen is another story. 

Important Takeaways

  • DOT and Mineral Oil cannot be interchanged. Doing so will result in damage to your brakes
  • Both fluid types are generally incompressible
  • Both fluid types have initial boiling points above 200 ℃, which is over 400 ℉ 
  • DOT
    • DOT fluids are hydrophilic and absorb water
    • DOT fluids have a range of boiling points based on how much water has been absorbed
  • Mineral Oil
    • Mineral Oil is hydrophobic and does not absorb water
    • Mineral Oil boiling points are constant, until there is enough water in the system to bring it to the boiling point of water

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