Hardly any other topic divides the MTB community as reliably as the choice of brake fluid. DOT or mineral oil? Both systems work - both have clear strengths and weaknesses. The most important rule up front: Never mix. What comes next is a question of priorities.
Of course, both brake fluids have pros and cons on their side. But I think the disadvantages of DOT clearly outweigh the pros. The mere fact that the fluid corrodes and is harmful to health is an exclusion criterion for me. - Max Fuchs, BIKE test editor
Both fluids transfer the force applied to the brake lever to the pistons in the brake calliper. Both are incompressible - this is the basic requirement for a stable pressure point. The decisive difference lies in how they deal with heat and moisture.
DOT: High boiling point, but hygroscopic
DOT stands for the American Department of Transportation and refers to a class of glycol-based brake fluids. DOT 4 and DOT 5.1 are used in MTB applications - with boiling points of 230 °C (DOT 4) and 260 °C (DOT 5.1). These are clear advantages over mineral oil, which boils at around 190 °C.
The disadvantage: DOT is hygroscopic - it draws moisture from the ambient air. Water that gets into the system lowers the boiling point and thus impairs braking performance as soon as there is a lot of heat in the system. In addition, DOT mixed with water is no longer a homogeneous medium. Even small inhomogeneities react slightly more elastically under pressure than pure DOT fluid. Ergo: spongy pressure point. In practice, this means that DOT should be changed every one to two years, otherwise the braking properties will deteriorate in the long term. DOT also has a corrosive effect on painted surfaces and skin. Anyone working with DOT should therefore wear gloves.
Mineral oil: Easy to clean, but boils earlier
Mineral oil is not hygroscopic - it does not bind water. This has two practical advantages: The oil hardly ages and needs to be changed much less frequently. An opened bottle of Shimano mineral oil lasts for years. Mineral oil is also non-toxic and does not damage paint or skin.
The disadvantage lies in the boiling point: at around 190 °C, mineral oil is below the level of DOT 4 or 5.1. In practice, this is irrelevant for most riders - brake fluid practically never boils in normal trail use.
Important for survival: Never mix!
DOT and mineral oil must not be mixed under any circumstances. The seals in brake levers and brake callipers are only designed for one of the two media - the wrong fluid destroys the rubber seals. This usually happens gradually. However, once the brake is leaking, the braking effect can suddenly collapse. This can have fatal consequences for such a safety-relevant component. Caution is also advised with mineral oils from different manufacturers, as they may contain different additives. If in doubt, always check whether the brake fluid used is approved for the respective brake.
Max Fuchs
Editor
