At what temperature do your brake pads start to work? At what temperature do they stop working? When was the last time you checked your brake temperatures? You are monitoring your brake temperatures aren’t you?
Temperature range is one of the primary considerations for brake pad selection. This is especially true if you run 30-minute sprint races where you won’t wear the friction material down to the backing plate. Everyone worries about high temperatures but your brake pads can also run too cool. If your pads don’t get up to their optimum temperature you’ll have uneven pad transfer.
The original brake pads that came with your production car operated best in the 100° to 650° range. Race pads operate best in the 600° to 1500° range. Race pads are just getting effective at roughly the same point where the OEM pads are becoming useless.
Core vs. Peak Temperatures: When you take brake disc temperatures on pit lane with an infrared scan gun you’re measuring core temperatures. These can be very different from peak temperatures that are reached on the track. Most engineers use an infrared scan gun but some such as Pagid (www.pagidracing.com) prefer to use a probe. The folks at Pagid worry about the light reflection from the shiny rotor affecting the scan gun.
These infrared thermometers are sold in all the big box stores but you don’t want them. The inexpensive units don’t read high enough. They’re designed for you home heating system, not the brakes on your racecar. Longacre Racing Products (www.longacreracing.com) makes one that goes to 1400-degrees and retails for $139.95. Pegasus (www.pegasusautoracing.com) carries this same product in stock.
On pit lane you have to record the temperatures quickly. Carl Bush, of Wilwood (www.wilwood.com), reminds us “using a heat gun or pyrometer can give a basic indication of balance and potentially identify a problem, but the temperatures they display are after the rotors have cooled to some degree. If you go around the car twice with the gun or pyrometer, you will get a new set of readings as the rotors continue to shed heat. Scan guns simply don’t show on-track peaks as well as the indicating paints do.”
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Temperature Indicating Paint: Carl Bush, of Wilwood, feels that “paints are the least expensive method and give the best indication of what happened while you were on the track. The technical name is thermochromic paint. This paint changes color as certain temperatures are reached. Each one of the three colors (Green, Orange and Red) has a heat range where it changes color. You can get a pretty good indication as to how hot your brake disc got while on the track by looking at how the colors have changed from the heat of braking.
Carl Bush “paints the outside edges of the rotor and between the cooling vanes to a depth of about ¼”. He paints two vanes with each color. He also puts a dab of each color on the outside edge of each brake pad.
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After a few fast laps the green paint (430°C / 806°F) should be completely oxidized (it turns white), the orange paint (560°C / 1040°F) should be starting to oxidize and the red paint (610°C / 1130°F) should be un-touched or changed only slightly. Andreas Boehm at Pagid “ likes to see the green paint totally gone with the orange gone at the rotor flanges and the red still intact.”
Are You Too Cool? Optimum is the critical word. Too cool is almost as bad as too hot. The different friction compounds have different operating ranges. You need to monitor your temperatures to see if you’re in the range of the pads on your car. You need to talk to the brake engineers about this if you see a problem.
Another type of cooling problem is an imbalance between the inner and outer faces on the rotor. Andreas Boehm of Pagid feels too many people have a brake duct that only directs air to the inboard side. You really want to “force cooling air into the eye of the rotor and out through the vanes. If the rotor temperatures are asymmetrical the brake paint will display this by being different on the inboard side of the disc from the outboard side of the disc.”
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Brake Caliper Temperature: The heat from the brake disc is passed on to the brake caliper, and in turn onto the fluid inside the brake caliper. Caliper temperatures can be monitored with temperature strips. These are small strips that record your caliper temperatures while on the track. They have an adhesive backing and are available in a variety of temperature ranges. Each of the little strips has a window that changes color as temperatures increase. Brembo calipers even have a little indentation designed for locating the Thermax temperature labels. Keep in mind that these labels are measuring the surface of your brake caliper, not the temperature of the brake fluid inside the caliper.
Once you've collected all of this data you can talk to the brake engineers about which friction material is best for you. You no longer have to walk around the paddock asking people which brake pad they like. You can actually buy a brake pad that meets your own particular situation. The brake engineers will like you a lot more and they may even think you know what you’re doing.
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