That's a pretty interesting theory. I like it...and if it's true (or remotely close), there would be some digital gadget somewhere in the car that keeps track of that sort of thing?
depends. most cars, it wouldn't be a digital gadget. some cars now have digial odometers, run by the car's onboard computer. those are gradually becoming more common.
still, most cars have an actual physical odometer. that would not be converted by a microchip. instead, it would be converted by mechanical means.
one way you could do that would be to use a gear train. let's say the circumference of the tires actually was 3 feet. in that case, 3/5280 = 1/1720. so, the dial that registers 1/10ths of a mile should turn (going up one number, not a full rotation) once every time the car's wheels turned 172 times. it should do a full rotation once every 1720 times the car's wheels turned.
now, the way a gear train works is this:
you take a big gear and a small gear and mesh them together. the small gear has to turn several times to get the big gear to turn once (number of teeth that the big gear has devided by number of teeth that the small gear has). otoh, if you put two gears on the same axle, they turn at the same rate. so, you put a small gear on one axle and connect it to a big gear on another axle. on the same axle as you have the big gear, you put another small gear. mesh that one with a big gear, and so on. you can set it up so that it takes exactly 1720 rotations of the first gear to turn the last gear once.
there are ways to do the same thing with an electronic signal that involve resistors and capacitors and such. that's most likely the way it's done in cars. you have a sensor reading the number of times the car's axle turns, then wires that go up to the odometer. those wires carry an electronic signal which would be processed mechanically, by running it through resistors and capacitors until you got the right signal strength. use those wires to activate a motor, and have that motor turn the dial on the odometer.
you can set up the odometer like one of those hand counter devices (you click it, and it counts the number of times you've clicked). the clicker turns the first dial, and the first dial is geared so that the one next to it turns once for every 10 rotatios of the first one. and so on down the line.
am i being clear? a bit behind on sleep right now, but i'm hoping i'm making sense.
The one that gets me though, is our car measures TIRE PRESSURE. That still amazes me, since there is no way it can actually measure the air pressure inside the tire.
yeah, i was wondering about those, too. the thought i had was this: put a strain gague on the inside of the wheel rim. run the signal up to an onboard computer. calibrate it with the properties of the rim (material properties, shape, etc). the strain on a given point on the rim will fluctuate due to tire rotation (i.e. there will be more strain when that point is near the bottom than when it's near the top), but if you keep track of something like the minimum or the average, you should be able to back out the pressure exerted by the tire. you won't get the most accurate reading, but it should be enough to tell you when the tire is going flat.
actually, you could set up multiple gagues. put four of the things on each wheel, spaced out around the rim, then average the input from each. that should help smooth out the variations.
of course, i'm not sure how tire changes and tire rotations and such would affect that. the strain gagues might become unreliable.
it's a thought, anyway. makes a lot more sense than trying to read the strain on the rubber with some kind of optical sensor.
i do like your level idea, but what happens if you're driving on an incline or something?
what i really want to understand is traction control. i guess you'd put a tachometer on each wheel, to see if one is spinning faster than the other? but then how do you correct for that? you'd need the left and right tires to be on seperate axles, wouldn't you? each with its own computer-controlled rapid-response gearbox or something. doesn't make much sense to me.
Paul
