tire sizes
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The formula to figure the rolling radius of the tire is stupid because it mixes standard and metric units. Basically, the sidewall height is the AR * the nominal width. This figure is in millimeters. Then you add the radius of the wheel (half the diameter). This figure is in inches.
For the case above: 205/55R16
55% of 205 == .55 * 205 == 112.75mm
half of a 16" rim is 8". 1in = 25.4mm, so 8in is 203.2mm.
The radius of the tire is then 112.75 + 203.2 == 315.95mm
The diameter of the tire is 2x the radius, or 631.9mm.
The rolling distance (rd) of the tire is pi*d, where pi is however many digits you want to use, starting with 3.141592653
rd = 3.141592653 * 631.9 == 1985.17mm
for silly US units, that's a tire+wheel radius of 12.43", or a tire diameter of 24.8", and a rolling distance of 78 inches (6'6")
This level of "accuracy" is irrelevant, since inflation pressure will affect the amount of deformation during rolling. Additionally, speedometers have inaccuracy built in, usually between 3 and 10% @ 60mph. Finally, the wear of your tire will contirubte -- a new tire may have an 11/32nds tread depth, which is 8mm, while a worn tire could be be down to essentially 0mm. This constitutes a 16mm reduction in diameter, and thus a ~50mm reduction in rolling distance, which is just about 2 inches. A reduction 2 inches on a stated rolling distance of 72 inches constittues a 2% difference...just from tire wear.
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yep tired reading it have 0 idea what the hell was said
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wtf... do they still teach math in school ?
the distance a tire rolls in one turn is roughly the same as the circumference (distance around) of the tire. the circumference of a circle is pi * the diameter. The diameter is the distance across the center of the circle from edge to edge. The radius of a circle is the distance from its center to its edge, and is always half of its diameter.
To figure out the diameter of a tire, and thus the distance it rolls in one turn, its easiest to first figure out the radius.
The radius of the tire is made up of three components - the tread depth, the sidewall height, and half the empty space where the wheel goes
You can calculate the sidewall height and the empty space by knowing the tire size. The tread depth measurement you'd need to measure, and i'm not sure what depth is used when the tires sidewall height is calculated, so i don't know of the tread depth is additive or subtractive.to calculate the sidewall height you just multiply the tire width (205mm) by the aspect ratio (55%). To get the distance of "empty space" you just cut the wheel diameter in half (we're talking about radius, remember). Since the wheel diameter is in inches and the section width of the tire is in millimeters, you need to do a unit version between millimeters and inches, depending on what units you want the answer to be in. I chose millimeters because the metric system is awesome and the US system is stupid
once the sidewall height has been added to the wheel radius, can you double that value to get the total diameter, and then multiple the diameter by pi to get the circumference, which is the approximate linear distance the tire will travel in one revolution along the ground.
what would make this more clear? pictures?
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I think what he meant was TMI. Like I stated earlier, most people don't need nor will use most of the info that has been posted. Alot of times, just the basics will do. Hell, if half of my customers knew just that... my job would be a helluva lot easier.
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http://www.rims-n-tires.com/rt_specs.jsp
plug and play what you are looking for, then scroll down for some pretty descriptive details of your selections...
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again all these things are helpful but still dosent explain the reason that the spedometer would change if i put a 235/45/17 on instead of a 245/45/17
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killer69penguin;180542 wrote:
again all these things are helpful but still dosent explain the reason that the spedometer would change if i put a 235/45/17 on instead of a 245/45/17Your fucking kidding right???
What do you not understand???
Going from a 245/45 to a 235/45 the tire diameter is smaller. Le'ts say you drive 1,000 rpm's in drive(1:1) tranny ratio. and you gears are 1:1 your tire will spin 1,000 rpm's too. So to make it easier to see let say the tire diameter is 24". Then at 1,000 rpm your tire will travel 1,000PID = 1,0003.142=6280' in one minute.
Now lets say you put some 4' diameter tires on there.
1,000PiD= 1,0003.144=12,560' in one minute. Your speedo is not measuring the the distance your tire travels but it measures the speed of the tranny output shaft(or whatever on rice??) that is why on some you change the gears on the speedo to correct gear or tire size changes.......
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