Re: [ATM] OT: increasing mpg

["John Sherman" <atm@johnspics.com>]

Okay then,


Someone hinted that I am just a dumb flatlander who doesn't understand extreme driving, so please be aware that I have more driving experience than the vast majority of people on the planet - in number of miles, types of roads, types of vehicles, and yes, weather conditions. Perhaps I should mention a few things about tires, so that you can safely make it to your observing site without endangering the lives of everyone around you. I hope everyone will learn something or two from this post.


The ground contact area of a tire is dependent mostly on the weight on the tire, the relative pressure in the tire, and the tread pattern. Imagine a tire with no tread: if there is 900lbs of weight on the tire and 30psi of relative (above the ambient) pressure in the tire, then the part of the tire touching the ground is 30 square inches. Thirty pounds per square inch, times 30 square inches, equals 900 pounds supported. It does not matter if it is a wide tire or a narrow tire. The tread merely acts as spacers between the ground and the tire. The tire's 30sq.in. contact pad is at the top of the tread. If the tread has 50% air and 50% rubber, the pressure between the rubber and the ground is 60psi. 

When you overinflate a tire, you decrease the amount of rubber touching the ground. Not only does this decrease the friction with the road, and also forces the tire into a shape for which it is not designed, it causes the tire to run well over it's rated temperature. That is especially important here in California where the sun bakes the road surface - the first poster mentioned he had the a-c on while driving on the freeway in California. Driving on a hot roadway also heats the tire. Imagine what happens the very instant your first overheated tire blows out. When the tire blows, it takes a moment for the spring to push the wheel down to the ground. So now all the weight of your car is supported by only three over-heated tires. So there is a good likelyhood that one or two more tires will blow out nearly simultaneous with the first.


To answer some other questions...
A car's cornering speed is determined mainly by it's weight, the pressure in the tires, and the softness of the rubber the tires are made out of. Softer rubber grips the road better but wears out faster. 

A car's handling characteristics are independent of it's cornering speed. Handling describes how a car behaves when cornering. Wide tires usually have shorter and stiffer sidewalls, meaning there is less vertical and horizontal flex when cornering, so the car feels stiffer. Lowering the center of gravity, heavy-duty shock absorbers, anti-sway bars, and wide tires all serve to keep the car from leaning outwards when cornering, but do very little to enhance the cornering speed. 

For example, I used to have two Plymouth Satellite Sebrings. The red car was stock, with regular shock absorbers, tall narrow tires, and no anti-sway bars. The blue car was lowered, had low-and-wide profile high-performance tires, giant-sized anti-sway bars, urethane absorbers all around, and heavy-duty shock absorbers. When cornering at high speed the blue car barely leaned outwards, it always felt stable and in control. The red car leaned outwards greatly, as if threatening to roll over. But the red car could go around any corner at virtually the same speed as the blue car. Handling meant that when approaching maximum cornering speed, the red car was somewhat scary, the blue car was fun.


Now let me explain something that very few people understand, and that is the proper pressure to run in your tires. Indeed, most tire store employees don't know these things - if you want the correct answer, you need to ask the manufacturer. 

Now, do you think that when it says 35psi on the side of your tire, that is the proper pressure? On nearly every car, that is an illegal overinflation! Here's how it works: On your tire it says: Max load weight AT Max pressure (note the 'AT'). This means that if you are running the tire AT it's maximum load, you need to have the maximum pressure (measured when the tire is cool). But if you are running the tire at less than the maximum load, as most people do, then you must run less pressure. If you have 70% of the maximum load, and 100% of the maximum pressure, your tire is overinflated! If you have 70% of the maximum load, and 120% of the maximum pressure, you deserve to be in jail. Here in California it is called "Reckless Driving", and will normally get you some jail time. 

Do you know why new tires have those little "strings" of rubber sticking out all around them? It is so that you can determine the proper pressure to run in your tires. Say you have 70% of the Max Load on the tire, start with 75% - 80% of the Max Pressure in the tire. Carefully monitor those little wear indicators. If the ones in the center of the tire are wearing out first, the tire is overinflated. If the ones near both edges of the tire are wearing out first, the tire is underinflated. If the first ones to wear out are only near the inside edge or the outside edge, the wheel needs alignment. Because a wheel (rim and tire combination) has a certain stiffness when the pressure in it equals the ambient pressure, there is not a perfect proportion to Max Weight and Max Pressure. As soon as the strings are worn off, you will be left with little bumps. This provides you a second chance to monitor the wear of the tire and adjust the pressure. These strings which wear into bumps ar!
 e carefully designed for this purpose. The manufacturers use a special thermometer which measures the temperature of many points across the width of a tire after it has been driven, to see if they are all the same.



>buy a more efficient vehicle.  It is amazing 
>what kind of a scope you can get into a hatchback 

Yes, indeed! Steven Overholt once showed up in Arizona with a 22" and a 17" scope both in a Ford Festiva, one of the smallest cars around the USA. Later, he was carrying a 30" scope in that tiny car.


John




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