Center Cap vs Tire Diameter Rotation

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Discussion Overview

The discussion revolves around the relationship between the RPMs of a tire and a center cap on a car traveling at 30 MPH. Participants explore the implications of diameter differences on rotational speed and tangential velocity, examining whether the RPMs are the same for both components.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant inquires about the RPMs of a 22-inch tire compared to a 2.5-inch center cap, questioning if they are the same or different.
  • Another participant suggests that the RPMs must be the same if the tire and center cap are aligned, as differing RPMs would indicate a mechanical issue.
  • A third participant provides a formula for tangential velocity and angular frequency, detailing the calculations for RPM based on the car's speed and tire radius.
  • One participant confirms that while the RPMs are the same, the distance traveled by points on the tire and center cap differs due to their respective circumferences.
  • Another participant emphasizes that a point on the outer diameter moves through a greater circumference than a point on the inner diameter, leading to different tangential velocities.

Areas of Agreement / Disagreement

Participants generally agree that the RPMs of the tire and center cap are the same, but there is a discussion about the implications of radius on distance traveled and tangential velocity, indicating some nuances and differing interpretations of the relationship between these quantities.

Contextual Notes

The discussion involves assumptions about alignment and mechanical integrity, as well as the need for unit conversions in calculations. There are also unresolved aspects regarding the implications of differing circumferences on motion.

element80
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Hello all,

Just trying to find a formula and need some help.

If a tire is 22 inches in diameter, what are the RPMs of the outer diameter of a 22 inch tire versus the RPMs of the outer diameter of a 2.5 inch center cap, on a car traveling 30 MPH?

Are they the same, or are the rotations of the diameter of center cap different than the diameter of the tire?

Any help would be appreciated.
 
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element80 said:
Hello all,

Just trying to find a formula and need some help.

If a tire is 22 inches in diameter, what are the RPMs of the outer diameter of a 22 inch tire versus the RPMs of the outer diameter of a 2.5 inch center cap, on a car traveling 30 MPH?

Are they the same, or are the rotations of the diameter of center cap different than the diameter of the tire?

Any help would be appreciated.

If a point somewhere on the circumference of the cap revolved a different amount of times than a point somewhere on the circumference of the tire during a minute of rotations, then they would no longer be aligned, and your tire would probably have something wrong with it (if the tire slipped around the hub or if the hub twisted). So you can assume the RPMs are the same, which is a scalar quantity. What is not the same is the distance traveled by each point and the tangential velocity.
 
Last edited:
Here is a little more help:

Tangential velocity = [itex]V_{tangent}[/itex]
Angular frequency = [itex]\omega[/itex] in revolutions/minute, this must be converted to radians/minute for units to work correctly, which 1 rpm = 6.28 rads/min.
Radius = r in inches
speed of car = 30mph, so to make units agree, 30mph = 31680 inches/minute

And we use the formula:
[itex]V_{tangent} = \omega * r[/itex]

If you consider the tangential velocity of the point on the tire that contacts the road, this velocity vector will tell you how fast the car is moving. So,

[itex]V_{tangent} = 31680 \frac{inches}{minute} = 11in\;*\;\omega[/itex]

So now, solve for [itex]\omega[/itex]:

[itex]\omega = 31680\frac{inches}{minute} * \frac{1}{11\;inches} = 2880 \frac{rad}{min}[/itex]

This is in units of radians/minute, so convert to RPM now:

[itex]\frac{2880\;rads}{min}\;*\;\frac{1\;revolution}{6.28\;rads} = 458.6 RPM[/itex]There is also a much simpler way of computing this, if you consider that when a tire travels distance of its circumference, it has completed 1 revolution, and then you can calculate how many of those circumferences (revolutions) must be traveled in an hour to get 30 miles, and then convert to minutes.
 
Last edited:
Thank you for your reply DragonPeter. So the rotations are the same for both , it is just the distance that is effected by the radius.
 
...it is just the distance that is effected by the radius.

the circumference gets bigger as the radius gets bigger...so an outer point moves thru
a greater circumference during one rotation than does an inner point...hence it has to go faster [v = wr] than an inner point...
 

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