"Inflation pressure does not affect grip": Autospeed article.

[dwtaylorpdx]

My wifes 318 is a "typical passenger car" <grin> and it gets almost un-drivable with over inflated tires. I think I could accurately paraphrase what you said and we'd be agreeing.

Hows this,
Within the design of the vehicles suspension air pressure has more effect on traction with tightly managed suspension and less on a more simplistic suspension with softer springs as related to the GVW of the vehicle.

Of course the speed you travel at and your driving envelope make a huge difference on results.

I think a lot of the stuff I do at the track is like those tests when you make stuff wear out faster by exceeding its duty cycle. a tire that's consistently over inflated by 2 lbs can last < 1/4 the expected life. Resulting in catastrophic failure of the case. On the street because heat is not as intense this is far less likely. But lose one front tire at 120 and for the rest of your life your torquing your lug bolts twice and checking the air pressure on your street cars constantly.... No matter what your technical voice says...

Dave

[orange4boy]

Hows this,
Within the design of the vehicles suspension air pressure has more effect on traction with tightly managed suspension and less on a more simplistic suspension with softer springs as related to the GVW of the vehicle.

Of course the speed you travel at and your driving envelope make a huge difference on results.

100% agreed.

[CapriRacer]

Originally Posted by Old Tele man

...would be nice (better!) if those charts had X- and Y-gridlines to make it easier to "eyeball integrate" the straightness/curviness of those plots.

...and, since those were only "single-point" data collections, those "dips" might actually be from data "round-off" errors, etc. and not really "dips" per se.

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

[Christ]

Originally Posted by CapriRacer

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.

[natefish]

Here is a slightly more up to date study (scroll about half-way down):
FEA chapter III. tire pressure survey and test results

I don't think the tearing of the rubber off a tire actually provides more stopping force, it just means the force of the grip with the ground has exceeded the bond of the rubber to the rubber so the bond fails. If the bond could hold on longer, you'd stop faster.

@CapriRacer: Do you have a source on 10-20% sliding providing more stopping force? I'd be willing to bet that if it's true, it has more to do with the heat changing the tire's grip than the static vs. kinetic friction coefficient.

Ultimately, the mathematical model should reflect real world observation...I'm guessing with tires that model can be pretty complex. There are rarely exceptions to the laws of physics, just unaccounted for variables ;-)

[CapriRacer]

I was attempting to distinguish between the force generated by friction and the force generated by the tearing action - and I'm going to continue with that distinction:

If it doesn't matter how much the vertical load is in the force generated by the trearing action - except, of course, on the amount of area in contact with the macrotexture of the pavement generating that tearing force, then:

Larger footprints would generate more force.

[dremd]

Not sure why everyone doubts such basic physics.
Easy test.

You need
Scale (spring for measuring force),
Wooden block,
various bits of tire (my high school and college physics teachers both spent more time telling us no to cut ourselves on steel cords than the experiment),
String
Some surface to frag on (floor, table, pavement, etc)

Basic test, drag a mass along on the same surface with each tire type, cut tire piece in half repeat (with scrap tire on top of mass), cut the remaining piece in half repeat (again scrap on top).

The various sizes of tire represent the contact patch with different inflation pressures.
You will quickly realize that contact patch has extremely little to do with friction (traction)

[Christ]

Originally Posted by CapriRacer

I was attempting to distinguish between the force generated by friction and the force generated by the tearing action - and I'm going to continue with that distinction:

If it doesn't matter how much the vertical load is in the force generated by the trearing action - except, of course, on the amount of area in contact with the macrotexture of the pavement generating that tearing force, then:

Larger footprints would generate more force.

This is still outside the realm of normal driving, though. Is it not? The scope of the paper, as I understood it, was only applicable to passenger tires within a specific range of pressures and load limits.

These tend to be harder compound tires, meant for a compromise between tread life and traction... Therefore, they probably don't experience as much "interlocking" with the bumps in the road surface, not to mention that they also usually don't experience the forces necessary to do what you suggest.

(Except in the case of bad drivers and ricers, I suppose.)

[CapriRacer]

Originally Posted by dremd

Not sure why everyone doubts such basic physics.
Easy test.....

It is indeed unfortunate when we don't understand the experiences of others - in this case, tire test engineers. If the traction properties of a tire could be described by a simple laboratory test, then that would be the standard test. Why go through all the trouble of conducting a traction test with the tire mounted on a trailer being pulled by truck on an outside certified test surface (subject to the vagarities of the weather) where you have to replicate the test 3 times in order to get reliable results?

There is nothing wrong with classical physics, but it is important to know when to apply them. Newtonian physics works extremely well, but sometimes Einsteinian physics is the correct applicable theory.

Originally Posted by Christ

This is still outside the realm of normal driving, though. Is it not? ....

Why would it be? Is there something about harder rubber compounds that means they don't penetrate the macrotexture of the road? After all, even very hard rubber compounds are softer than stone.

Try this: Take your fingernail and press it into the tread rubber of your tire. Did you indent the rubber when you were applying the pressure? Doesn't this demonstrate that the principle is applicable to street tires?

And you have to ask yourself: Why do drag racers use such wide tires? Why do road racers use such wide tires? Why do rally cars use such wide tires - except for certain surfaces such as snow, and mud? Since there are penalties for using wide tires, wouldn't these folks have abandoned them long ago if they didn't work?

But to be fair: The only time this becomes applicable is when you push the tire towards the limit of adhesion. Are there situations where a normal driver would experience these limits - particularly when dealing with a small contact patch? wdb reports above that he has experienced such a situation.

[gone-ot]

PHYSICS -- simply pick the correct equation (from many) and apply the correct data (from too few) and get an almost wrong answer 100 percent of the 'standardly deviated' (Sigmoid) time...and usually having "ƒurlongs-per-ƒortnights" units.