As an asphalt materials engineer, I’m sometimes asked what effects slamming on the brakes has on both vehicles and pavement. While we all try to avoid panic stops, understanding the physics and mechanics involved provides insightful background. In this article, I’ll break down what happens when a car rapidly decelerates on asphalt pavement, and how it impacts tires, vehicles, and road surfaces.
First, let’s briefly discuss tire friction fundamentals before looking at sudden braking. This will help explain how grip affects stopping forces.
Tire-to-Road Friction Forces
For a vehicle’s tires to accelerate, decelerate, or corner, they rely on grip and friction with the road surface. Asphalt provides great friction when dry. The small texture pores and aggregates on asphalt interact with the rubber to create a grip.
Friction force depends on factors like:
- Tire material properties and tread depth
- Road surface texture and condition
- Speed of the tire relative to the road
- Vertical pressure or weight on the tire
More friction force allows the tire to apply more horizontal acceleration, braking, or cornering without skidding. But exceed the friction limit, and the tires break free and slide.
Understanding these friction mechanics will help illustrate the effects of sudden braking. Now let’s look at that in more detail.
Locked Wheel Skidding on Asphalt
When a driver suddenly slams on the brakes, one of two things will occur depending on conditions:
1. A Controlled Hard Stop
If the car’s wheels don’t fully lock up, the tires rapidly decelerate but continue rolling with some forward speed while generating high friction with the road. The car comes to an aggressive but controlled stop.
2. Locked Wheel Skid
If the braking force exceeds the tires’ available traction on that surface, they completely lock up and cease rotating. The locked wheels skid and slide on the pavement. The car screeches to a halt.
So in an emergency stop, the best initial braking uses maximum tractive friction to decelerate quickly but retain steering control. Further braking past the lock-up point doesn’t shorten the stop but does eliminate steering until the car fully arrests.
For the rest of this article, we’ll focus on the locked wheel skidding scenario on asphalt. What exactly happens to the vehicle and road in this case?
Effects on the Vehicle
When the wheels lock and skid on asphalt pavement, the effects on the car include:
Flat Spots on Tires
With the rubber frozen in place but the car still moving, the locked tire belts abrade the asphalt leaving “flat spots”. These make the ride rougher until the treads even out again.
Loss of Steering Control
The front tires provide lateral cornering friction. Locked front wheels mean no ability to steer until fully stopped.
Reduced Braking Efficiency
Locked wheels can’t generate as much deceleration force. The vehicle slides with less control.
Risk of Hydroplaning
If the road is wet, tires skimming on a water layer provide even less friction than on a dry surface.
Stress on Components
The sudden deceleration places added loads on the car’s suspension, body, passengers, and unrestrained objects.
Increased Wear
The skidding action accelerates wear on tires, wheel bearings, and other chassis parts.
So while controlled hard braking is desired, locking the wheels has side effects on the vehicle – some minor like tire flat spots, others hazardous like loss of steering.
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Effects on the Asphalt Surface
In addition to vehicle impacts, locked wheel skidding also affects the asphalt pavement surface:
Polishing Action
The scrubbing action literally polishes the aggregates in the top layer of asphalt. This reduces macrotexture which provides wet-weather skid resistance.
Tire Rubber Deposits
The skidding tires smear rubber onto the pavement, reducing friction and making the surface slick when wet.
Rutting
With prolonged heavy braking in the same area, deformation and rutting can occur over time.
Thermal Damage
Excessive heat generated at the tire-pavement interface from friction and rubber friction can alter the asphalt binder properties if severe. This can make the pavement more prone to rutting or shoving.
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So in addition to tire damage, locked wheel skidding on asphalt can gradually degrade the road surface – something to avoid on public infrastructure.
Now that we’ve covered the effects, let’s go over some tips to minimize skid risks and damage.
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Techniques to Reduce Skidding
Here are some best practices and technology features In my experience that can help limit skidding from sudden braking:
- Leave ample distance for gradual controlled stops whenever possible.
- Scan ahead to anticipate stops; avoid the need for panic braking.
- ABS systems – Antilock brakes pulse the line pressure to prevent lockup.
- Modern stability control systems use ABS and selective braking to retain steering ability.
- Maintain proper tire tread depths and inflation pressures.
- Ensure tires have an adequate performance rating for your vehicle.
- Replace tires before they reach wear limits.
- Service brakes and suspension components routinely.
- On wet roads, reduce speed and leave added distance for braking.
Through proactive habits and taking advantage of modern vehicle technology, the risks and damage from locked wheel skids on asphalt can be greatly reduced both for vehicles and public roads.
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Your Questions Answered
Why do tires skid on asphalt when braking fast?
The friction forces between the tire and road surface exceed the available grip when braking too aggressively. This causes the wheels to lock up and skid on the pavement instead of rolling controlled.
Does slamming the brakes damage asphalt?
Repeated hard braking and skidding in the same area can gradually degrade asphalt through polishing, rubber deposits, rutting and overheating damage. But occasional controlled hard stops do not cause issues.
Is it bad to lock up tires on asphalt?
Yes, locking the tires results in skidding with reduced braking performance, loss of steering control, tire and vehicle damage, and road surface degradation. Always try to stop without fully losing traction.
What should you not do when the brakes lock up?
Never accelerate or steer sharply when brakes lock up. Stay off the gas and make smooth steering inputs as the car slows. Gently pump brakes to unwind once stopped.
How do race cars stop so fast without skidding?
Race cars use optimized tires, brakes, suspension, and aerodynamics for maximum deceleration before lockup. Drivers also expertly threshold brake up to the grip limit without fully locking the wheels. ABS assists in modulating brake pressure at the limit.
Understanding the friction dynamics between tires and asphalt can lead to better practices for controlled rapid stopping without pavement damage or loss of control. Let me know if you have any other road surface friction or braking questions!