Anti-lock Braking System - ABS brakes help you steer the car out of trouble while applying the brakes in a "panic stop" – foot hard on the brake (which is the typical reaction). With non-ABS brakes, if you don't "pump the brakes", the wheels lock-up (i.e., they stop rotating) on slippery surfaces, causing a thin film of water to build under the tires resulting in a loss of traction. When this happens, there is a good chance that the car may go into an uncontrolled spin.

Because braking action is most effective at slowing a car at a point just before wheel lock up, an ABS system applies braking force and prevents wheel lock by monitoring the rotation (specifically, the rpm, or revolutions per minute) of all the wheels. By applying brake pressure in pulses controlled by the rpm of each wheel, the ABS system enables braking without allowing the wheels to lock up.

Interaction Between the ABS System and a Compact Spare - Because ABS brakes monitor wheel rpm, any difference in wheel rpm “confuses” the ABS system, causing it to activate even on dry roads. Because a compact (“donut”) spare rotates faster than a normal tire, this difference “kicks-in” the ABS system when you don’t expect it. Some car models disable the ABS by a connector attached to the spare. Be sure to reconnect this when replacing your spare.

Note that if you change tire size (such as smaller front tires), you may affect your car’s computers, such as ABS, fuel, traction control, etc. Check with your tire dealer to learn how tire size differences may impact your car.

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