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

[squatch81]

Originally Posted by jamesqf

I have to wonder about that, though. If you look underneath most of these, you'll see that they have solid rear axles, which means that any increased ground clearance is purely a function of the larger tires. Jacking up the body just gives the illusion of more ground clearance while raising the center of gravity.

This is especially true of aftermarket lift kits, which elevate the body (often to absurd heights) while leaving the lowest point unchanged.

As an owner of off road vehicles for some 35 years, I have to disagree with the statement I highlighted in red.
Raising the body with suspension and/or body lift modifications serve several purposes.

1. Increased suspension travel to keep tires in contact with rough terrain.
2. Tire clearance. (Just try to stuff a pair of 38" mudders in the wheelwells of an unmodified truck)
3. Attack angle (to keep from implanting your bumper into the bottom of the hill you want to climb)
4. Ground clearance. It takes a lot less power to push just the tires and axles through deep mud than trying to force a path for the whole truck when traveling through 2 feet of gooey muck.

Please note that none of the above applies to the average yahoo that wants to jack up his truck because it looks KEWL.

[winkosmosis]

Originally Posted by thatguitarguy

Perhaps someone should take high school physics. More bumps do not equal more grip. If this were so, drag racers, NASCAR racers, IRL racers, Formula 1 racers, etc., would want bumpy race surfaces. The very opposite is true. They put smooth grippy tires on smooth track surfaces. The reason street tires are not smooth like race tires, is that they have to be able to displace water.

The concrete surface is rough, If you run your hand over it you will abrade your skin. The smooth tire has the most surface area in contact with the concrete, that's why they use them when it's dry-- wait, aren't you arguing that surface area doesn't matter??

Why do race cars have wide tires with no void space? And don't say for longer tread life, because they could accomplish that by using thicker rubber, without having the disadvantage of heavier wide wheels.

[winkosmosis]

Originally Posted by squatch81

As an owner of off road vehicles for some 35 years, I have to disagree with the statement I highlighted in red.
Raising the body with suspension and/or body lift modifications serve several purposes.

1. Increased suspension travel to keep tires in contact with rough terrain.
2. Tire clearance. (Just try to stuff a pair of 38" mudders in the wheelwells of an unmodified truck)
3. Attack angle (to keep from implanting your bumper into the bottom of the hill you want to climb)
4. Ground clearance. It takes a lot less power to push just the tires and axles through deep mud than trying to force a path for the whole truck when traveling through 2 feet of gooey muck.

Please note that none of the above applies to the average yahoo that wants to jack up his truck because it looks KEWL.

Even with the axles lower than the rest of the underside, breakover angle is improved. Also, solid axles move up and down with the tires. If you put a tire on a rock, the axle goes with it unlike the rest of the underside.

On a smooth surface (like a GOOD road), I'd rather have the solid axles. Look at what happens to the wheel located by independent suspension when the vehicle leans. It doesn't change camber enough to lean into the turn like a motorcycle tire, instead of it leans out of the turn. A tire on a solid axle stays vertical aside from the tilt due to tire deformation.

Hence I can take my Cherokee with 2" lift and 31" tires around corners like a sports car.

[thatguitarguy]

Originally Posted by winkosmosis

The concrete surface is rough, If you run your hand over it you will abrade your skin. The smooth tire has the most surface area in contact with the concrete, that's why they use them when it's dry-- wait, aren't you arguing that surface area doesn't matter??

Why do race cars have wide tires with no void space? And don't say for longer tread life, because they could accomplish that by using thicker rubber, without having the disadvantage of heavier wide wheels.

You should read more carefully, and don't try to manipulate my words to your own meaning. All I was saying is that higher tire pressure reduces the size of the contact patch. It's elemental physics.

It's hard to figure out what you are saying, because you are just asking questions. Are you being sarcastic? Ironic? Scientific?

Because a surface is rough does not mean that it that it has more traction.

Try this experiment: Get on your hands and knees on dry smooth pavement. Put as much pressure as you can on your right hand and slide it forward. Now look at the cuts and gouges and know that this didn't come because of traction, but because of lack of traction.

Now get on your hands and knees on an indoor maple basketball court, and try to slide your left hand forward. (The one not lubricated by hemoglobins.) You will find it much more difficult to move your hand forward. This is because of traction.

It's the reason you continually hear shoes squeaking on that basketball court during a game, but on an outdoor court or street, you do not hear the squeaks. This is because the glass smooth surface of the polished maple provides much more traction than the rough surface outside.

The caveat is the presence of moisture. The glass smooth surface with a tiny layer of moisture provides the perfect hydroplaning surface, while it takes more moisture outside. This is why the rough surface is better for driving puposes, because there will always be the chance of moisture outside, and the tiny layer of moisture will disperse to the bottom of the rough surface while the tiny peaks of the rough surface remain dry enough to provide some traction.

[winkosmosis]

Originally Posted by thatguitarguy

You should read more carefully, and don't try to manipulate my words to your own meaning. All I was saying is that higher tire pressure reduces the size of the contact patch. It's elemental physics.

It's hard to figure out what you are saying, because you are just asking questions. Are you being sarcastic? Ironic? Scientific?

Because a surface is rough does not mean that it that it has more traction.

Try this experiment: Get on your hands and knees on dry smooth pavement. Put as much pressure as you can on your right hand and slide it forward. Now look at the cuts and gouges and know that this didn't come because of traction, but because of lack of traction.

Now get on your hands and knees on an indoor maple basketball court, and try to slide your left hand forward. (The one not lubricated by hemoglobins.) You will find it much more difficult to move your hand forward. This is because of traction.

It's the reason you continually hear shoes squeaking on that basketball court during a game, but on an outdoor court or street, you do not hear the squeaks. This is because the glass smooth surface of the polished maple provides much more traction than the rough surface outside.

The caveat is the presence of moisture. The glass smooth surface with a tiny layer of moisture provides the perfect hydroplaning surface, while it takes more moisture outside. This is why the rough surface is better for driving puposes, because there will always be the chance of moisture outside, and the tiny layer of moisture will disperse to the bottom of the rough surface while the tiny peaks of the rough surface remain dry enough to provide some traction.

You said that friction stays the same regardless of contact patch size, which is incorrect. What I said about the rubber conforming to bumps in the road surface explains the reality that a bigger contact patch has more traction.

You're talking about race drivers wanting more bumps if it really helps. The surface is already rough. What race drivers want is WIDER TIRES.

Cars don't drive on basketball courts. They drive on pavement, which is a textured surface, both microscopically and macroscopically. If roads were made of basketball courts, you MIGHT be right about contact patch having no influence on traction.

[thatguitarguy]

Originally Posted by winkosmosis

You said that friction stays the same regardless of contact patch size, which is incorrect. What I said about the rubber conforming to bumps in the road surface explains the reality that a bigger contact patch has more traction.

You're talking about race drivers wanting more bumps if it really helps. The surface is already rough. What race drivers want is WIDER TIRES.

Cars don't drive on basketball courts. They drive on pavement, which is a textured surface, both microscopically and macroscopically. If roads were made of basketball courts, you MIGHT be right about contact patch having no influence on traction.

I can't tell if you are unable or unwilling to understand what I am saying, but it doesn't really matter because your return posts are confrontational and combative. You don't try to understand what is being said - you try to manipulate the words to suit your own purposes to give you something to argue against.

Race drivers don't want "WIDER TIRES", they want better traction. If they could get better traction with narrower tires, they would undoubtedly choose this option because the tires would be lighter and more aerodynamic.

Provide facts to support your argument, or just leave it alone.

[orange4boy]

Something not mentioned here is that racing tires use a very different compound than passenger tires. Racing tires are geneally quite sticky. You can't really compare the two.

The point of this thread was not to say that there is absolutely no difference in traction in relation to pressure (an oversimplification) but that the difference is insignificant for our purposes. There is not a big safety concern with running max sidewall compared with mfg suggested inflation pressure.

Data to prove or disprove this is always welcome.

The tread width issue is not particularly important to this thread because narrower is better for aero and an increased hydroplaning speed.

[orange4boy]

Found some interesting pages from a racing tire book. I have not had time to read the whole thing.

From excerpt#1

Adhesion is a property of rubber that causes it to stick to other materials, as we see with adhesive tape. Adhesion is generally thought to be the result of momentary molecular bonding between the two surfaces. If bond strength is the same at all the bond sites the force that resists sliding is proportional to the total of all the minute areas of contact. If the two surfaces were perfectly smooth the true area of contact would be the same as the observed area of contact, but this is not the case. Real surfaces are actually very rough on the molecular scale and contact is limited to the highest protuberances on each of the two surfaces. The true area of contact depends on the surface profiles, properties of the materials, and the contact pressure.

Tire Technology, excerpt from The Racing & High-Performance Tire

From excerpt #2

At higher slip angles portions of the tire patch are sliding, and you get less increase in lateral force with an increase of slip angle. This is called the transition region. As the curve tops out, more of the contact patch is sliding and the tire produces less lateral force. After the peak of the curve, lateral force can fall off 30% within a few degrees of extra slip angle. At these high slip angles most of the contact patch is sliding, producing a lot of heat and wear.

Tire Technology, excerpt no. 2 from The Racing & High-Performance Tire

Tire Technology, excerpt no. 3 from The Racing & High-Performance Tire

The following is a inference not a statement of fact.

What I took from this is that given a particular tire, if a higher inflation pressure shortens the contact patch,(I believe it does) it will reduce the proportion of slip to contact, increasing traction in cornering. The smaller patch will have a higher PSI, which would compensate for the lower square inch contact patch. The actual contact area may be roughly the same in both instances.

Good little primer on tires. The book would be a good one to have as a reference.

[Christ]

I gather that traction is a force generated on a surface by the weight imposed on a section of that surface.

I also gather that increasing tire pressure generally makes the contact patch of rubber to road smaller, but increases the downward force load (weight) per square inch.

In other words, if you put 100 lbs on a square inch of rubber, you've got 100 PSI of force pushing you to the ground.

If you increase the pressure of the tire sufficient to decrease the size of the contact patch, then you increase the point load on a smaller section of the tire, either maintaining the same PSI rating on that section, or increasing it. In all reality, since the change in contact patch is the only variable, the pressure per square inch should actually rise, in comparison to the amount that the contact section has shrunk, proportionately.

IOW - If you lose 15% of your contact patch, that's 15% less area to support the same weight, thus, the weight forces the remaining 85% into the ground harder, creating a higher friction coefficient between the tractive surfaces.

I guess that would be the definition of equilibrium...

The caveat is that the less tire that is in contact with the road at a time, the less hysteresis losses there are, and the cooler the tire can run, since the ratio of cooling rubber to heating rubber is changed positively.

[wdb]

I just read the entire thread. What jumped out at me:

• The article referenced in the initial post represents a very simplistic view of tire dynamics
• Tire dynamics is anything but simple

I sure am glad people here aren't afraid to ask questions and challenge assumptions.

I can offer some anecdotal, seat-of the-pants data. In the interests of maximizing rolling efficiency and fuel economy, I ran my Fit's tires up to sidewall max of 51psi. Wet traction suffered; the front of the car lost traction in turns much more easily than it had at the recommended pressure of 32 psi. So I lowered the pressures to 45psi, and wet traction improved. That pressure represents the traction/rolling resistance compromise I am happy with. YMMV.