Breaking with Turning Wheels: The Science Behind Its Effectiveness

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Discussion Overview

The discussion centers on the effectiveness of braking with turning wheels compared to locked wheels, exploring the underlying physics of friction, tire dynamics, and braking systems. Participants examine concepts related to static and dynamic friction, the role of anti-lock braking systems (ABS), and the impact of tire width on grip.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that braking effectiveness is primarily due to the friction between the tire and the road, with static friction being greater than dynamic friction.
  • Others argue that while brakes can generate significant force, they can easily lock the wheels, which reduces steering control.
  • A participant mentions that ABS does not significantly reduce stopping power but allows for steering during braking, which is critical for avoiding obstacles.
  • There is a discussion about the coefficients of static and kinetic friction, with some participants expressing confusion about why static friction is greater.
  • One participant explains that static friction is higher because stationary surfaces settle against each other, while moving surfaces do not maintain the same contact.
  • Another participant raises a question about why wider tires provide more grip than narrower ones, linking it to stability and performance in different contexts.
  • Some participants challenge the notion that friction is independent of surface area, suggesting that practical factors such as material deformation and surface interaction complicate the friction equation.

Areas of Agreement / Disagreement

Participants generally agree on the importance of friction in braking effectiveness but express differing views on the implications of static versus dynamic friction, the role of ABS, and the influence of tire width. The discussion remains unresolved on several technical points, particularly regarding the relationship between friction and surface area.

Contextual Notes

Participants note that the friction equation's simplicity may not capture all practical factors, such as surface area effects and material properties, leading to ongoing questions about the mechanics involved.

madmike159
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Why is breaking with the wheels turning more effective than when the wheel locks up?
Is it because there is more friction between the breaks and the disks (or drums) than there is between the road and tyre, or am I missing something.
 
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No, the key is always the friction between the tire and the road: static friction is always greater than dynamic (sliding) friction.
 
The brakes on a car are usually stronger than the tire road friction as it is quite easy to lock the wheels in motion.

Anti-locking does not reduce the stopping power to any significant amount, but while the wheels are turning you can still steer the car instead of heading straight towards the object that made you brake in the first place. Usually locked wheels cannot steer even if you turn the steering wheel.

ABS will reduce braking power but in the range of instead of a 30m full stop, it might be 30.5 m. The added distance is small so the ability to steer comes first
 
russ_watters said:
No, the key is always the friction between the tire and the road: static friction is always greater than dynamic (sliding) friction.

I'll read the pf page on friction.

*Edit*
I read it but don't get why static friction > dynamic friction.
 
Friction is the normal force times the coefficient of friction.

The coefficient of static friction is always higher than the coefficient of kinetic friction (moving).

You can witness this by pushing a large block. It takes more force to get it moving than to keep it moving once it's going.
 
madmike159 said:
I read it but don't get why static friction > dynamic friction.
Essentially, when something is stationary, the rough surfaces settle against each other, but when the surfaces are moving with respect to each other, they don't. They'll skip from one bump to the next.
 
Abninfamy said:
Friction is the normal force times the coefficient of friction.

Yup, people are often suprised that it's independent of a surface area term.

So. Why do wider tyres give more grip than narrow tyres?
 
Why do wider tyres give more grip than narrow tyres?

Yeah that question was bugging me for quite a while.

In bikes it is worth it to have greater stability during turns...but for cars....don't know.
 
Friction is not independent on area, in practice. Small surface area will deform some materials, while to big and it might stick. The formula for friction is a bit more complicated than F=ymg. It depends on surface area (but it's mainly a small dependence if the surfaces are not at least on the order of 20x different) and speed of the two surfaces, this might explain why car tires should be wider, while on a bicycle nobody cares.
 
  • #10
The friction equation doesn't include surface area because it is a already part of the coefficient of friction. So that doesn't imply that friction is independent of surface area.
 

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