Tire Physics Questions

[Superman]

First: I had a rather robust blowout the other day. It sounded like a helicopter was landing on my roof, then my right rear tire came apart. All that's left is the beads and some ragged sidewall material. The entire tread part of the tire, complete with casing, went somewhere else. As near as I can tell, I must have picked up a nail and had a slow leak. I had checked my tire pressure within the past week or two. Anyway, I suspect this sort of complete sidewall failure can only happen if the tire is WAY low on air, the sidewall heats up over time and BOOM. Am I correct?

Second: I have been led to understand that the weight of your car divided by your tire pressure(s) equals the number of square inches of tire contact patch you have. If so, then what good does are wider wheels and tires? If they don't increase the size of your contact patch.

And besides, why would size matter anyway? In my hometown, a logging town, it is widely known that wide tires perform poorly in the woods. Tall, narrow tires are what they use. They grip better.

So I guess I'm wondering about the relationship between tire width and things like overall adhesion.....and contact patch size.

[jcge]

The Essence of Grip | Technology | Racecar Engineering

[Bill Verburg]

Originally Posted by Superman

...... Anyway, I suspect this sort of complete sidewall failure can only happen if the tire is WAY low on air, the sidewall heats up over time and BOOM. Am I correct?

I suspect that is correct

Originally Posted by Superman

Second: I have been led to understand that the weight of your car divided by your tire pressure(s) equals the number of square inches of tire contact patch you have. If so, then what good does are wider wheels and tires? If they don't increase the size of your contact patch.

Not quite, the tire carcass also contributes.

It's the combination of lower stiffer sidewall and the shape of the contact patch that is important. The sidewall stiffness is fairly obvious and intuitive, the shape not so much. The shape is important because of the way grip is developed by the contact patch. Grip is developed by the contribution on each little piece of tire compound along and just behind the leading edge of the contact patch and the twist it develops, The sum of all the little twists is called the slip angle, the more little pieces lined up the more lateral grip is added. The greater the slip angle to a point the greater the lateral grip. The down side of the twist is heat in the tire, you want to twist enough to generate grip within the tires design range but limit the heat generated as a by product.

In general the wider the tire the lower the slip angle needed for a given level of grip and the cooler the tire will run. wider shorter side walls run at lower slip angles than taller narrower tires. Contributing factors are wider wheel widths, within design parameters, which support the tire better and suspensions which reduce unwanted extraneous wheel and tire excursions.

Originally Posted by Superman

And besides, why would size matter anyway? In my hometown, a logging town, it is widely known that wide tires perform poorly in the woods.Tall, narrow tires are what they use. They grip better.

different design goals, they are not generating lateral grip, they are generating longitudinal grip, the narrower tires tends to cut down through the softer surface layers and grip the more solid under layers, this is particularly true for snow where there is usually nice hard pavement underneath, offroading in soft bottomless sand requires a wider lower pressure flotation type tire, probably w/ traction devices well up the side walls.

[Dr J]

On the first topic:

Driving on low tire pressure will fatigue the sidewalls. This is why it is important to know our tire pressure. If you have a leaking air valve or a slow leak, you can have this condition and not even notice. Our tires are pretty stiff so it can be difficult to tell that they are low without measurement. Even if you fill it up to normal pressure after detecting the low pressure condition, the damage is already done. That fatigued sidewall can rupture clean off and the tire plus sidewall detaches. You may be left with part of the sidewall on the rim. If you find you drive a few miles on a flat tire, it should be replaced. Don't count on your common tire place to diagnose it for you.

[DanielDudley]

On a narrow tire, the contact patch is long and thin, and the tire deforms more, scrunching the tread, On a wide tire, the contact patch is shorter and wider. In high lateral stress, narrow tires would roll under, making a thinner band of contact. Wide tires tend to create a more triangular contact patch. Long narrow contact patches also have more time to dissipate water, and less water to get rid of. Wide tires have a greater floatation effect.

[Superman]

Thanks, guys.

So......when we move from a narrower tire to a wider tire, tire pressures being equal, we're not appreciably changing or enlarging the contact patch. Instead, we're changing the SHAPE of the contact patch. Right?

[RWebb]

right (to a 1o approximation) - and there are pics of this in some old R&T's or maybe C&D magazines

when wide tires first came out they ran these diagrams to explain it -- the shape changes from nearly round to an elongated oval w/ thin section F to R and fat section side to side