Single Wheel Bump Rate is the vertical stiffness felt at one wheel when that wheel alone moves upward, with the opposite wheel held stationary. It differs from the ride rate measured in two-wheel bump because a one-wheel input twists the anti-roll bar, so the bar’s contribution adds to the spring’s wheel rate. This is the rate that governs how the car reacts to a single-wheel impact such as a curb, a patch or a mid-corner bump, and a high single wheel rate causes the input to be fed into the chassis as roll and to unload the opposite tire rather than being absorbed at the wheel. It is the main reason large anti-roll bars degrade ride quality and mechanical grip on uneven surfaces, and the reason a chassis with a rear layout that isolates one-wheel inputs from the other side rides better at a given roll stiffness.