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What is the difference between dot3 and dot4 brake fluid?

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Brake fluid is a kind of hydraulic fluid commonly used in hydraulic clutch and hydraulic brake applications in motorcycles, cars, light trucks, and even some bicycles. The fluid is utilised to transfer force into pressure and amplify braking force. Brake fluid works on the principle of liquids not being appreciably compressible.

However, there are different types of brake fluid on offer, such as dot3 and dot4. The key difference between these two products concerns their boiling points, but in this blog post, we’ll take a close look at these brake fluids and when they are suitable for use.

Understanding the different types of brake fluid

There are four categories of brake fluid – dot3, dot4, dot5 and dot 5.1. The main differences between them relates to their dry and wet boiling points, but also their composition. Dot5 and dot5.1 have the highest dry and wet boiling points and are composed of silicone and glycol ether/borate ester respectively.

Dot5 brake fluid is designed not to absorb water and isn’t compatible with other brake fluids. It is mainly deployed in vintage vehicles that stay in storage for extended periods that require a non-water absorbent fluid. Dot5.1 is employed in heavy-duty and high-performance applications that require a higher boiling point and is compatible with both dot3 and dot4 fluids.

Dot3 brake fluids have a glycol ether composition and are the most common brake fluid type used today in trucks and cars. Their dry boiling point is 205°C and their wet boiling point is 140°C.

Dot4 brake fluid, however, is seeing increased use due to the widespread use of traction control and anti-lock braking systems, both of which benefit from the lower viscosity of the fluid. It has a glycol ether/borate ester composition and a dry boiling point of 230°C and a wet boiling point of 155°C.

Getting to grips with brake fluid boiling point

Aggressive braking action can result in intense heat. The process of braking generates high heat between the rotors and vehicle brake pads. Such temperatures can vaporise brake fluid, which makes it compressible, and this can lead to a spongy sensation when a driver steps on the pedal, applying the brakes.

The soft pedal is caused by compressible gas left in the line. In both performance driving and racing circles, this effect is referred to as “brake fade”, which is an issue that drivers always actively seek to avoid. To ensure they are driving safely and effectively, motorists must be confident that their vehicle’s brakes will perform consistently during use over time and can be relied upon when required.

The issue of brake fade can also originate from the rotor/brake pad interface. The brake pads release gasses, which decreases contact between the rotors and pads. For this reason, high-quality rotors are designed to be drilled and slotted, which allows them to release gasses faster, limiting brake fade.

While brake fade is common concern for racers seeking equal brake performance on lap one and lap ten, it isn’t just an issue for those on the track braking aggressively.

Vehicles driving down a steep hill can generate extreme heat when they ride and pump the brakes, especially when they are towing a trailer or hauling excessive loads. When the vehicle reaches the bottom of the slope, the driver may find they have pressed the pedal almost entirely to the floor.

Tackling the twists and turns of a country road can also result in brake fade. Stepping on brakes to corner can create enough heat to unleash brake fade.

Brake fluid boiling point refers to the temperature when the brake fluid vaporises. The higher the dot classification, the greater the boiling point, and as a result, the better the brake fluid can resist heat. For this reason, racers will always select dot4 brake fluid over dot3 brake fluid.

Dry and wet brake fluid boiling points

Boiling points of brake fluids are divided in dry and wet boiling points respectively. The dry boiling point is calculated using fresh fluid directly from a brand new container. However, the brake fluid’s wet boiling point is worked out using fluid that has been contaminated with 3.7% water. Consequently, the wet boiling point of a brake fluid is always lower than its dry boiling point. Water is added by test administrators to replicate what happens in real-world applications.

Most brake fluids (except silicone-based dot5) are hygroscopic, which means they absorb water. For instance, dot3 brake fluid can absorb up to 2% water per annum. Moisture enters the brake systems when the reservoir cap is removed to add fluid, but also via worn seals and through rubber brake lines. As a result, the wet boiling point of the fluid is the number that most accurately represents the processes at work.

As brake fluid will wear out, whether you are using dot3 or dot4, it is crucial that you change it periodically.

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