Coefficient of friction of a tire: with/without tread and wet/dry

[anthonylewington]

I'm new to the forum and after some help if anyone can explain. I've read a few articles now, which have stated that the average co-efficient of friction for a tire with tread will be between 0.4 in the wet and 0.7 in the dry. If instead the tire had no tread, the co-efficient of friction will be between 0.1 in the wet, and 0.9 in the dry.
I understand why a bald tire will have a lower co-efficient of friction in the wet, as the water won't be dispersed. However what I don't understand, particularly as the laws of friction state that friction is not normally dependent on the area of contact, why does the co-efficient of friction in the dry increase if there's no tread?

 

[PeroK]

I'm new to the forum and after some help if anyone can explain. I've read a few articles now, which have stated that the average co-efficient of friction for a tire with tread will be between 0.4 in the wet and 0.7 in the dry. If instead the tire had no tread, the co-efficient of friction will be between 0.1 in the wet, and 0.9 in the dry.
I understand why a bald tire will have a lower co-efficient of friction in the wet, as the water won't be dispersed. However what I don't understand, particularly as the laws of friction state that friction is not normally dependent on the area of contact, why does the co-efficient of friction in the dry increase if there's no tread?

https://en.wikipedia.org/wiki/Racing_slick

Simplistically, it seems, having no tread allows a softer, tackier rubber to be used. It's the same as walking boots (with a tread and hard rubber) and climbing shoes (no tread and soft sticky rubber).

 

[Stephen Tashi]

as the laws of friction state that friction is not normally dependent on the area of contact

When that is true, the complete explanation may be unknown. For example, see Feynman's remarks on the study of friction in section 12-2 of http://www.feynmanlectures.caltech.edu/I_12.html

It is quite difficult to do accurate quantitative experiments in friction, and the laws of friction are still not analyzed very well, in spite of the enormous engineering value of an accurate analysis. Although the law F=μN is fairly accurate once the surfaces are standardized, the reason for this form of the law is not really understood.

A government sponsored report from the 1960's describes the role of adhesion (tacky-ness, as mentioned by @PeroK): http://www.dtic.mil/dtic/tr/fulltext/u2/660927.pdf It would be interesting to know about modern updates to this research.

 

[JRMichler]

the laws of friction state that friction is not normally dependent on the area of contact,

The theory of friction for linear elastic materials (metals) says that friction force is a constant shear stress times actual contact area, and that the contact area is linearly proportional to contact stress. The result is that friction force is a constant (the coefficient of friction) times the contact stress.

The theory of friction for elastomers (tire rubber) is completely different. Friction force is a combination of adhesion and hysteresis. You can find some good references at scholar.google.com, using search the terms tribology of elastomers.

 

[Ranger Mike]

In racing , it all about tires, Tires, TIRES.Tires manufactured with tread pattern wet track cof = 0.4 dry = 0.7Tires manufactured with no tread for racing wet track = 0.1 dry track = 0.9Tires manufactured with tread but worn to wear bars ( no appreciable tread visible) = dangerNormal passenger car tires have tread that a large amount of non-contact open area so water and snow may squish out as the vehicle rolls over the road surface. By this very nature the tread tire will not have the best contact patch since most of it is open area. The racing slick has maximum contact area as no gaps exist. Conversely, this total contact means there is no way to displace rain water or snow so you have a maximum spin out condition = cof 0.1. We do race in the rain and have tread tires for this in various compounds. The tread depth is huge and a lot of area to displace the water.Both of these tires are available in many different compounds. The tire “hardness” is measured with a durometer. So the tackiness, grippiness, gumminess is a matter of choice (until the heat cycle comes into play) .

Low durometer reading tires (45) are very gummy, tacky and will wear out the quickest. We use these on race days where we have no sun light and cool track conditions. We use high durometer compounds (85) on hot sunny day with track temperatures over 110 degrees F. We record the cold Shore durometer reading , like 62 and it drops to 40 to 43 after the hot lap session. Again, both of these tires have significant tread depth. You can actually measure the depth of a slick as it is made with many depth holes present in the slicks surface. In both cases this tread depth acts like a spring and assists the tire surface compliance with the road surface.

In fact NASCAR has banned the use of tire dynometers that can measure the amount of springiness of tires as a cost cutting measure. The tread flexes as the vehicle changes direction in a turn. To not be able to do this we go into tire shear. Which is why the bald tire is the most dangerous. The tire was manufactured with tread that is no longer present. So we have no ability to flex. We also have a tire suffering many many heat cycles.A tire heats up when the vehicle is moving and cools to a steady state when it is stopped and parked. This is one heat cycle.

When the tire tread is completely worn off we have a whole lot of heat cycles and the tire compound has changed to harder durometer readings.Hope this helps a little

rm

 

[CWatters]

Students are frequently taught a simple model of friction where the coefficient of friction is constant and friction force is independent of contact area. These are gross simplifications. In the real world it's far more complex and if any accuracy is required there is no substitute for making your own tests and models.

 

[OldYat47]

It's not a simple question. It's not just surface friction in tires. It's also how much the tires sink into the irregularities of the pavement. This is dependent on how soft the tires are and how rough the pavement is. Softer tires sink in more but their surfaces also deflect easier. What's the happy medium for the required task? And that's just the beginning.