"While i agree that width could be found ( assuming the tires arent toed in or out too much) the easiest, i believe tire diameter could also be found. If there is a repeating scuff, imperfection or identification (small nail in tire), that leaves a mark on the street and you see it again 62.8in away, then because diameter=circumference/pi, the tire has a diameter of 20in."
They "peeled out," which is why it left rubber from the tires on the pavement. When "burning rubber," the tires' rotation and the car's movement are not commensurate. A potential repeating mark in the tires could show up every half-inch, or quarter-inch, or eighth-inch, etc., depending on the RPM of the tires while they were "peeling out" (the RPM would vary throughout the "burnout," and it would be impossible to determine the tires' RPM just from examining the rubber they left behind). Your idea would only work if the tires were getting 100% traction, but when that happens, ~no rubber is left on the pavement.
So, if the tires were e.g., wet, and getting 100% traction, and they had a "repeating scuff, imperfection or identification," and someone took good pictures of the wet tire marks before they dried, then they could determine the tire circumference, not that that's relevant because that's not the scenario in the movie. Also, keep in mind that the circumference/diameter of the tires is never mentioned in the movie anyway, only the aspect ratio (75, which is meaningless without the section width), and the wheel diameter (R14, i.e., 14 inches). The wheel diameter is impossible to determine from examining tire marks, even if you could figure out the tire diameter. For example, a 225/75R14 tire has a diameter of 27.3" However, a 205/70R16 (mounts on a 16" wheel) also has a tire diameter of 27.3," and so does a 215/55R18 (mounts on an 18" wheel), among others. The list gets huge if you include tire sizes that are within say, 0.1" (27.2" or 27.4"), since that's well within manufacturing variance, and variance caused by tire wear.
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[deleted]
5 years ago
yes , you can get the "Rolling Radius" but not necessarily the wheel/tyre diameter.
Unless you know the sidewall size - which they seems pretty confident was 75, i dont know how they know that , but if they did they can use that with the rolling radius to get the wheel size.
maybe that brand of 14" tyre only came with 75 sides.
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MaximRecoil (4043)
5 years ago
"yes , you can get the "Rolling Radius" but not necessarily the wheel/tyre diameter."
There's no way to get the rolling radius from burnout marks. Rolling radius is the distance from the center of the wheel to the ground, and there's nothing whatsoever in burnout marks that can tell you that.
"Unless you know the sidewall size - which they seems pretty confident was 75, i dont know how they know that"
75 by itself isn't a sidewall size. It means that the sidewall height is 75 percent of the section width, but without knowing the section width, "75" tells you nothing.
"but if they did they can use that with the rolling radius to get the wheel size."
Like I said, there's no way they could know the rolling radius.
"maybe that brand of 14" tyre only came with 75 sides."
There's no way they could know it was a 14" tire in the first place, and the company was Michelin, which is one of the largest tire companies in the world. During the days when 14" tires were common, they had a huge selection of 14" tire sizes. Also, 75 has never been an aspect ratio commonly used on cars. During the time that 75 was typical for light trucks, 70 was typical for cars. 60 was sometimes used for high-performance cars, which allowed for a wider section width without making the tire diameter too large for the car's wheel openings.
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[deleted]
5 years ago
when you said
So, if the tires were e.g., wet, and getting 100% traction ...
you're correct in that that is when you get the extra details.
Im pretty sure , wet or not , when the wheels stop the burnout and start rotating normally there will be plenty of tracks on the ground to get not-spinning tread patterns , width etc. and rolling wotsit (circumference / radius)
The repeating "scar" on the tread you mentioned will give a circumference and therefore , the radius.
which could then be measured against the suspects car.
the rest of the stuff i said was based on them claiming to know the 75 figure . i didnt say i knew how they got it , in fact i expressely said i didnt. and ,yes, i know its a proportion of the width .
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MaximRecoil (4043)
5 years ago
"when you said
So, if the tires were e.g., wet, and getting 100% traction ...
you're correct in that that is when you get the extra details."
I also said that was irrelevant because that wasn't the scenario in the movie.
"Im pretty sure , wet or not , when the wheels stop the burnout and start rotating normally there will be plenty of tracks on the ground to get not-spinning tread patterns , width etc. and rolling wotsit (circumference / radius). The repeating "scar" on the tread you mentioned will give a circumference and therefore, the radius, which could then be measured against the suspects car."
When the tires stop spinning against the pavement they also stop leaving rubber on the pavement, i.e., when the burnout stops the tires stop leaving marks.
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[deleted]
5 years ago
im pretty sure the hot rubber would keep leaving tracks once traction regained
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MaximRecoil (4043)
5 years ago
That doesn't happen. If the tires were hot enough to continue leaving tracks after gaining 100% traction, they would be ruined; the tread would just be a molten mess. What actually happens is the rubber that's in contact with the pavement gets heated to an extremely shallow depth as the tires are spinning against the pavement, and that extremely thin layer that's heated comes off the tires and deposits onto the tar. The underlying rubber isn't hot enough to melt, which is why it remains on the tires. It's only getting heated to melting temperate at the contact patch between the tires and the pavement, so once the tires stop spinning against the pavement they instantly roll to rubber that isn't hot enough to melt, thus no more tracks. Go ahead and get in your car and try it yourself.
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