I’ve replaced the disc brake rotors on my car twice due to distortion, and have finally trained everyone in the house to use the gears, rather than the brakes, on hills to prevent damage. Also, I was pleasantly surprised at the low cost of the new rotors. It was actually cheaper to replace them than to have them surfaced. Is this common?

– Jim L., San Jose, Calif.

You have brought up two valid points regarding automotive brakes that might be helpful to others. Brakes convert a vehicle’s kinetic energy to heat with each and every stop, and when using the brakes to maintain a safe speed on long descents.

Friction between the brake pads and rotors generates heat, and the heat is transferred to the brake caliper, wheel, nearby suspension components and surrounding air. Larger, beefier brake pads and calipers do this well, as do thick, ventilated brake rotors. Some vehicles, such as my Chevy Tahoe, in the quest for weight and/or cost reduction have rather skimpy brakes, which are prone to rotor warping and eating pads.

During normal city and highway driving, using the brakes exclusively to slow or stop the vehicle is no problem, but on hills a little help from the engine is a good idea. I’m referring to a descent of more than 20 or 30 seconds requiring braking to maintain desired speed. Slipping one’s automatic or manual transmission down a notch or two provides engine braking, as the engine resists running fast with a closed throttle.

Assuming a typical four- or five-speed transmission and axle ratio, one can safely drop to third gear at speeds up to 50 to 60 mph, and second gear up to 30 to 40 mph. Too low a gear for a given speed can cause engine damage, so watch your tachometer or consult the owner’s manual for safe maximum speeds in each gear.

As an experiment, I tested the brake temperature of my Tahoe’s front brake rotors during various driving conditions using a non-contact digital pyrometer.

Here are the results, listed in Fahrenheit, with an ambient temperature of 64 degrees, as I pulled over and measured the rotors through the wheel spokes:

• Highway driving/no brake use (coasted to a stop): 85 degrees;

• Around town stop/go driving: 152, 166, 137 degrees;

• Descending a winding 1,800-foot grade over six miles at 45 mph, using third gear to hold back, rarely braking: 161 degrees;

• The same grade repeated, this time using only the brakes to maintain 45 mph: 502 degrees and a noticeable burning odor!

As you can see, higher speeds and frequent braking tortures the brake pads and rotors, leading to pad damage and rotor distortion.

You’re right about the low cost of many brake rotors. According to my parts guru Tom Taylor, at www.rockauto.com, many replacement brake rotors cost less than the labor to resurface them. Taylor cites the widespread use of simpler, hub-less rotors and the influx of low-cost auto parts from China.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.