Automotive Friction force, Coefficient of friction &Traction

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1. Jun 21, 2017

EEristavi

I have few (more or less) general questions:

1. I know that: "The coefficients of friction are nearly independent of the area of contact
between the surfaces".
I understand this, but I know from everyday life, when car tire tread is worn off, braking distance increases and maximum acceleration & traction decreases (it's just my observation and assumption) which is caused by friction force. My explanation for this problem is following: I know tire has grooves/patterns, so not whole tire area is touched to the ground and when it's worn off grooves disappear, contact area increases and friction decreases.
Is nearly independent causing such a big change in everyday life, or am I far away from real answer?

2. As I understand friction coefficient depends on 2 surfaces (for ex. asphalt/rubber friction coef. is m1 and ice/rubber is m2).
Firstly, is there any parameter that characterizes smoothness/friction (or something like that, I think you'll understand what I mean) of single surface?
Secondly, if I know asphalt/rubber (m1) and ice/rubber (m2) friction coefficient, can I calculate asphalt/ice friction coefficient (and how)?

2. Jun 21, 2017

JBA

As to your comment about tread vs. smooth tires, your assumptions/observations are incorrect. Smooth tires are superior in traction as long as there is a clean dry surface, as evidenced by both track and drag racing usage. Tire treads are required for wet and soft surfaces. In theory, friction is not dependent upon area; however, in vehicle tires a larger contact patch will give more consistent grip because road surfaces are not perfectly smooth or clean and therefore do not have a consistent friction factor over the tire patch area. Larger patch area helps bridge the contaminated/irregular road spots and provides a larger area for imprinting of the irregular road surface into the elastic tire surface.

3. Jun 21, 2017

EEristavi

Good example and explanation, thank you.
As I understand you can't provide information about second topic.

4. Jun 21, 2017

jack action

No.
No.

Friction can be a very complex subject. To my knowledge, there is no theoretical way for finding the friction coefficient. We have to measure it experimentally with the given conditions.

A tire is a good example of that. Especially a drag tire. This type of tire is so «sticky» that it could be consider borderline glue at some point. (Which raise another question: What is the friction coefficient of duct tape?) Furthermore, when the tire is rotating under very high acceleration, the inertial effect of the tire «hitting» the ground increases the effective normal force and so the apparent friction coefficient is increased.

Imagine a rack and pinion gear arrangement with a fixed rack. What determines the force that this gear can take? Well, if the teeth are strong enough, it can be infinite. If the teeth break at a certain threshold, we can define the force easily. If the teeth breaks with a certain defined range, then some sort of average value can be estimated. If the range for the pinion teeth is different from the range of the rack teeth, the average can be more difficult to determine, especially if the ranges have common values. Now imagine some teeth are randomly missing in addition of everything else. Then imagine that the force range for breaking the rack teeth is dependent on the type of gear that is rolling on it, and vice versa. Now you begin to see the complexity of evaluating a friction coefficient between 2 surfaces.

Very complex subject.

5. Jun 22, 2017

EEristavi

Very nice example, makes me see whole new picture.
Can I imagine that teeth don't break, instead pinion jumps from rack's teeth?

6. Jun 22, 2017

jack action

I'm not sure what you mean, but friction is mostly that: Bonds that break.