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Old 12-22-2009, 12:10 PM   #84 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
A couple of thoughts:

1) The paper is almost 30 years old. The tires tested are a bit different than they are now - including the tire sizing methodology.

2) The traction part of the test is a "Braking" test - typically performed using a trailer specifically designed for this purpose. Unfortunately, lots of tire testing - including braking traction - are highly variable. The paper doesn't say, but usually there is a minimum of 3 different tires tested to get the data. I suspect they only did a single tire for each data point (The tires get destroyed during the test). I would take the "dips" as being based on real data (and not round off), but factor in the variable nature of collecting this data.

And one last thought to help the discussion:

Unlike the classical friction theory where there is a difference between the "static" friction force and the "sliding" friction force, tires develop their highest grip when sliding in the 10% to 20% range. Notice that the braking test data has "PEAK" values and not "Static".

So if we look back at the diagram posted earlier (the one with the rubber on top of the peaks), what happens is that the rubber is torn away by the peaks. That is what is generating the higher values and why it takes relative motion to generate the maximum grip in a tire.

Question: If you were to double the vertical load - and therefore the amount of penetration of the rubber below the peaks - what would happen to the amount of sliding force? Put another way, does the location of the tearing action change the force needed to tear the rubber off?

Barry
This is a valuable point that I don't think has been considered yet.

My input here is as follows:

The rubber's tear point will be at the same amount of force per unit area, but the more rubber that is being torn through, the more force per revolution it will take to tear the bits of rubber from the carcass of the tire, since the number of units changes, but the force does not increase.

That would serve to say that a vertical load does in fact increase tractive force, although it may not increase frictional capacity.

But does that even make sense? Isn't the tearing force generated by friction between the peaks and valleys of the road and the bits of rubber that fit inside them?

Most of us know that the best traction is usually achieved at the "scrub point", where the tires are just starting to make noise. The threshold is not a very comfortable place for many to be, though.
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