Nope - see above.
Yes - see above.
Steel does stretch, including guitar strings, but not as much as the wood moves. It is partly that ability that gives steel its tensile strenth.
But in a tire we’re taling about steel that’s constantly being flexed back & forth, constantly works, and also is subjected to various stresses including constant vibration. Like all materials, the way steel behaves under those conditions is not the same way it behaves in its static state. And the fact that it’s captive in the carcass probably affects its behavior as well.
When TPMS systems using the differences in the wheel speed were originally introduced, they did in fact trip erroneously due to the differences in wheel speeds when vehicles cornered or went around highway ramps. This was a common complaint. Subsequent systems required the differences to be over enough of a timeframe to allow vehicles to corner without tripping the TPMS light. The distance is in miles, long enough to also accomodate long highway ramps.
The feds only mandate TPMS systems. How it’s accomplished is up to the manufacturer. Attached is the actual standard.
http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.aspx?reg=571.138
Phillip “nailed” it. I’d add that the difference between the radius of the wheel between the axis and the ground, known as the “rolling radius”, and the difference in the radius between the axis and the outside surface of the rest of the tread becomes the “squirm” that I mentioned before. The energy of which is disspated as heat. Thus, the potential (and occasional reality) for underpressured tires failing at highway speeds.
@PhillipNeuman - It is not the diameter of the tire that matters, it’s the circumference. A flat tire that has gone from 12" radius to 10" radius does not roll 20% faster. The circumference of the tire has decreased by a very slight amount as the pressure reduction results in a slight reduction in the stretching of the tire cords.
One way to think of it is a tank tread with moveable rollers. No matter what the configuration of the rollers, the tank tread will make one complete revolution when the tank has moved a distance equal to the total length of the tread. Putting the tread in a circle (large ‘diameter’) is no different than having it close to the ground, in an oval (small ‘diameter’).
With respect, I believe he was referring to the “rolling radius” (If you’ll provide me the latitude), the distance from the turning axis to the closest point on the flat of the tire tread, from which the “rolling circumference” could derived. It may even be a more accurate way to determine the true “rolling circumference”. I;m comfortable with his definition.
Unless I don’t understand, I thought his point was that the “The radius of the wheel between the axle and the ground determines the rotational speed.of the wheel. When the tire pressure is low the radius is shortened.”
By that logic, if the tire was flat and that radius was reduced from, say, 12" to 10", then the rotational speed of the wheel would increase about 20%.
It doesn’t.
It will. That difference is exactly what the wheel speed sensors will “catch” to determine that the pressure is low (in those systems that use the wheel speed sensor).
The difference between that speed and the speed that the wheel should be spinning based on the speed of the car (the speed its colleagues, the other wheels, are spinning) is taken up by the squirm.
Think of it this way: If you remove the tire completely and ride on the rim, would the rotational speed of that rim have to be faster in order to cover the same distance in the same amount time? Yup, absolutely. The rotational speed of the wheel with the flat tire would therefore be somewhere between the speed of the properly filled tire and the speed of a rim without a tire.
Nope. A flat tire will turn at almost the same rpms as a full tire, except for the stretching that occurs because of the tension the tire is under. Think of my tank track example. It doesnt’ matter the configuration of the tire, if it takes 10 feet of tire cord to make it around the tire, that tire will roll 10 feet per revolution, flat or full (ignoring the stretching from inflation). Just like a 100’ tank tread will move 100’ in one complete revolution, whether the idler wheels hold it in a circle (like a full tire) or an oval (like a flat tire).
Disagree.
I’ll ask the question “would the bare rim turn at the same speed as the other wheels?”
In order for the tank tread analogy to work, one would have to ask “if one tank tread were shortened by a link, would it need to travel faster to achieve the same distance as the tread ib the opposing side in the same time?”
The fact is, the rolling circumference of a flat tire is less than in its nonflat state. Therefore, it MUST rotate faster to achieve the same distance in the same time. It’s circumference divided into distance. There’s no way around it.
A flat tire is very different that a wheel without a tire.
Yes, the rolling circumference of a flat tire is less, but only very slightly less, than a full tire. That 10 feet of tire cord doesn’t suddely shrink down to 8 feet, does it? This was discussed in detail in the Snapon scanner puzzler, where all this got its start:
http://community.cartalk.com/discussion/2282808/puzzler-question-monitoring-tire-pressure-with-a-snap-on-scanner
Slightly less means it turns slightly faster. A flat 215/70x15 would be significantlly less, therefore would turn significantly faster.
It is exactly the same as the bare rim analogy except for the magnitude.
Here’s more discussion (this was the one I was looking for):
http://community.cartalk.com/discussion/2282488/tires
It’s only a tiny fraction less, due to the lack of tension on the tread. Nothing to do with the difference in radius, flat vs. full.
If that were true, how could the TPMS systems that use only the wheel speed sensors work?
They sense the small increase in rotation speed resulting from the small reduction in circumference (from reduced stretching of the tire cords) when pressure drops.
A smaller radius wheel is going to turn faster than a larger radius wheel at a given speed.
A flat or low tire will result in a less-stiff sidewall, which will mean the sidewall flexes outward, lowering its height, which will decrease the effective radius (effective because it will only decrease the radius at the contact point, but that’s the point we’re worried about) of the wheel.
TSM is right.
I love Car Talk. At least half of the folks on here don’t know what they are talking about.
Nope, shadowfax. In the words of our resident tire engineer/expert (CapriRacer):
"Rolling diameter (or circumference) can NOT be measured in the normal manner. The ONLY way to do it is to mark the tire and roll it one revolution and measure the distance traveled on the ground. You can NOT get it with a formula involving PI! , - and the difference pressure and load makes makes in a tire’s rolling diameter is SMALL, but measureable. "
Just as I’ve said (about 32 times).
Philip and Mountain Bike: Just an FYI.
The distance between the center of the axle and the ground is called the “Static Loaded Radius” - and it can NOT be used to calculate the “Rolling Radius”. The only way to actually measure the rolling radius is to measure the distance traveled in one revolution. It turns out that if you measure the unloaded circumference (or diameter) of a tire, the rolling circumference (or diameter) is about 97% of that value.
I had a chance to ponder the problem last night, and have to say that I now better understand Texases tank tread analogy (which is actually pretty good) as well as the question itself. I’m persuaded. I now understand that Texases is right, and that the “length” of the tread is the major factor in determining wheel speed. As I understand it, the “tank tread” is meant to be abalogous to the “length” of the tire tread. I’m uncomfortable with the term “circumference” for a flat tire, because a flat tire sitting is not a circle, and it’s that fine point that misled me.
I recognize after having thought through the problem that the wheel actually turns slower than the distance traveled and the “rolling radius” would suggest.
I tip my hat to you all. The thread illustrates why “two heads are better than one”. Should I pursue my dream project after I retire (discussed in my other thread), I’ll look forward to encountering the countless challanges that I know I’ll hit and leaning on all of you to help me figure them out.
Life is good. Withe the help of y’all and the persistance and patience of Texases, I’ve learned something.
Enjoy.
PostScript: I was wrong. Both tank treads would have to travel the same speed even if a link were removed from one.
Philip did nail it by stating exactly what the article i linked to said. The tank analogy doesn’t work due to multiple wheels vs one. There is no central axle to compare speed against. The power to the tracks is independent and adjusted to maintain selected direction.