DOT 5.1 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.
A brake fluid's dry boiling point is the temperature at which the pure fluid will boil. Wet boiling point concerns the boiling temperature when the fluid contains water.
In the United States, all brake fluids must meet Standard No. 116; Motor vehicle brake fluids[1]. Under this standard there are three Department of Transportation (DOT) minimal specifications for brake fluid. They are DOT 3, DOT 4 and DOT 5.1.
DOT 5.1, like DOT 3 and DOT 4, is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based). Polyethylene glycol fluids are hygroscopic and will absorb water from the atmosphere, which is necessary to prevent sheer and undiluted water in the braking system, which is very corrosive, also water droplets can freeze in the pipes, thus blocking the system.
As of 2006[update], most cars produced in the U.S. use DOT 4 brake fluid.
Minimum boiling points for these specifications are as follows (wet boiling point defined as 3.7% water by volume):
| Dry boiling point | Wet boiling point | |
|---|---|---|
| DOT 3 | 205 °C (401 °F) | 140 °C (284 °F) |
| DOT 4 | 230 °C (446 °F) | 155 °C (311 °F) |
| DOT 5 | 260 °C (500 °F) | 180 °C (356 °F) |
| DOT 5.1 | 270 °C (518 °F) | 190 °C (374 °F) |
| DOT 5 | Brake fluids | None | ||
|---|---|---|---|---|