|MadSci Network: Physics|
While a car moves along a road, the contact surface of the tire temporarily has a static contact with the road surface. This is the same, whether the car is moving fast or slow. However, there are differences with speed, duration of contact and rate of loading. For this reason, a tire-road surface, when wet, is much more likely to produce a skid as the vehicle speed increases. It does not really matter what loads the tire contact. It could be from the brakes, the steering, or even a wind gust. When the tire-road contact is dry, many variables influence skidding. You asked specifically about a tap on the brake pedal. For most brake linings, the friction will be lower at high speeds--for the same brake pedal tap. Thus, the tires will have a bit less loading at higher speeds. I looked at data from proving ground tests on cars to get some specific data. These tests were performed to stop the car as quickly as possible, without skidding. Brake applications were made at 30, 60, 80, and 100 mph (about 50, 100, 130, and 160 kph). A small difference with vehicle speed did appear. However, no human would be able to reliably detect this small difference. You should be aware that there are exceptional conditions, where things would be different. For example, some racecars have wings that increase the down force on the tires as speed increases. Some passenger cars and light trucks may have the opposite effect--there can be some aerodynamic lift. In addition, the braking action also can vary with speed, depending on the type of brake (disc, leading-trailing drum, duo-servo drum), and some brake specifics (brake lining type, rubbing surface runout). To answer your question simply--when road conditions and brakes are good, there is no significant speed effect.
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