If you’ve spent any time in hilly areas, you’ve probably seen warning signs advising you not to ride your brakes on those lengthy descents. Some signs may even suggest using a lower gear. Those warning signs aren’t just for the big rigs, they also apply to passenger vehicles. You’ve probably heard that riding your brakes is bad but what’s really going on with your car, and what are the hidden dangers of riding your brakes?
The Modern Brake System
There are two types of brake designs found on today’s passenger vehicles, disc and drum. The same basic operational principles apply to both: when the brake pedal is pressed, fluid is sent to pressurize a mechanism (caliper or wheel cylinder), which in turn squeezes or pushes a friction material (pads or shoes) against a metal component attached to the axle (rotor or drum). A small amount of friction material is worn off of the pad or shoe and shed as brake dust. When enough pressure is applied, the vehicle slows and/or stops. A tremendous amount of friction is involved in stopping something as heavy as a moving vehicle, and the biggest byproduct of this process is heat.
Under normal driving conditions, this heat is able to dissipate as air moves over the rotors or drums whenever the brakes are not engaged. Most brake rotors and drums are finned to help dissipate heat. But riding your brakes during a long downhill drive puts the pads, or shoes, in constant contact with this metal surface. Even if the brakes aren’t fully applied, this constant friction builds heat around the surrounding components without getting a chance to cool down.
Short Term Risks
The biggest danger of riding your brakes is the possibility of generating so much heat the brake fluid actually boils. The reason fluid is used in the braking system in the first place is that it is not compressible, and thus transfers force directly from your foot. When it boils, however, gas bubbles form in the fluid, and the gas can be compressed. Under these conditions, pushing the pedal won’t build pressure in the system, and the brakes will fail. In fact, the brake pedal may go all the way to the floor. Fresh brake fluid has a very high boiling point, but old brake fluid can have a much lower boiling point due to water contamination. The more water in the brake fluid, the lower the boiling point. That’s why it is important to routinely test and replace brake fluid as instructed in the owner’s manual.
Brake fade can also occur when a large amount of heat causes a chemical reaction with the friction material itself, creating gas bubbles between pads and rotor, and these air pockets can reduce efficiency. Stepping on the brake pedal may still give a firm feel, but the vehicle won’t slow down due to the lack of contact between the friction material and the spinning rotor or drum.
Long Term Effects
The short term effects of brake fade or all out loss are extremely dangerous. But even if you are lucky enough to avoid this terrifying situation, the long term effects of riding your brakes can create substantial maintenance issues. For starters, the friction material on pads and shoes is meant to wear down over time, but riding the brakes speeds up this process significantly. The surfaces of the disc brake rotor or brake drum can become glazed and slippery. A glazed surface is very poor for friction material to grip enough to slow down. Glazed rotors or drums will need to be lightly machined or replaced.
Over time, the heat buildup discussed earlier can also damage components not meant to handle that sort of stress on a regular basis. Disc brake rotors can warp, impacting braking ability and unevenly wearing pads. Non-metallic parts such as caliper seals and dust boots can actually melt, causing leaks. Damage to these components require replacement immediately or else braking ability could be severely affected even after things cool off.
What To Do
On long downhill descents, back off the gas then downshift the transmission to let engine braking do the work. This is easy to do with a manual transmission, but even automatics have this option. On an automatic transmission there may be an “overdrive off” button, or there may be the ability to switch the shifter over into manual gear selection mode. Occasionally there are arguments made that this causes stress on your transmission, but the truth is that it’s not substantial wear and tear beyond what’s expected of a transmission in the first place, and it is most certainly a better option than one that could cause you to lose your brakes. Just don’t try to downshift from fourth to first at speed.
If you still need to use the brakes to slow the vehicle, opt for a punctuated, more forceful brake application instead of keeping your foot lightly on the pedal. Your ABS should prevent you from locking up the brakes, so don’t worry about standing on the brake pedal. These forceful short braking sessions give the brakes a better chance to cool off.
If you ever suspect that you have overheated your brakes, pull off the road to a safe area and let them cool off. Give everything a close inspection during your next brake job, or let your trusted shop know about the situation at your next brake service. That way any damaged components can be replaced and the brake fluid can be checked or possibly flushed. Nobody intends to overheat their brakes, but knowing what habits or driving situations can cause it helps prevent it from happing in the first place.
Check out all the brake system products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on your brakes, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
Photo courtesy Wikimedia Commons.
Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter. In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.