# Epitrochoids

Epitrochoids
Field: Geometry
Image Created By: Kevin Liu

Epitrochoids

An epitrochoid is a roulette made from a circle going around another circle. A roulette is a curve that is created by tracing a point attached to a rolling figure.

# Basic Description

The basic concept of this is: a circle that can't move and a circle that can. The circle that can't move is fixed onto a surface (like paper). Within the circle that can move is a line that will create a line once this circle is moving around the fixed circle. The exterior circle will not slip away from the big circle because they are attached together by a point where both circles will always touch. Think of it as a rabbit that runs around its hole (that's hopefully on the ground) holding a marker while staying in the same position(i.e. a rabbit with its left hand on its head,would keep its left hand on its head), but twirling. The rabbit and whole are connected and cannot leave each other when this occurs. In a more detailed explanation, this page will go more in depth on the mathematical concept, the equations required for an epitrochoid, and its special forms.

# A More Mathematical Explanation

Note: understanding of this explanation requires: *Roulettes

## Deriving the Parametric

Parametric Equations for an Epitrochoid:

http://www.math.hmc.edu/~gu/cu [...]

## Deriving the Parametric

Parametric Equations for an Epitrochoid:

1: As you can see, in the picture, there are 2 circles. We’ll call the radius of the bigger circle: a ; and the radius of the smaller circle: b.

2: To find the coordinates for point P, we see that the y-value is 0 and the x-value is line h - (a+b). a+b is the sum of the 2 radii, and h is the line spinning.  P0= (h-(a+b), 0)

3: On the picture to the right, you can see that beta is really the sum of the 2 mini arcs, t and t1 so by the Arc Addition Postulate, beta= t + t1 .

4: s = r(theta), -> a t = b t[1]

5: To solve this, solve for t1 first, thus you divide both sides by b to get: t1 = (a t)/b

6: Substitute this into the equation in step 3 to get: beta= t + ((a t)/b)

7: So if we look at the parametric equations for an epitrochoid, by definitions of a polar graph, sine is always the y-coordinate and cosine is always the x-coordinate. Variable h is the length of the line SP, while a and b represent their appropriate radii.

8: Now we can see that P= a+b [cos(t), sin(t)] - h [cos(beta), sin(beta)]. Sine and Cosine of (t) comes from theta; the first part of this determines where the outer circle is located. Sine and Cosine of beta (in which this case, we have found that beta= t + ((a t)/b) so…) we see in the second part of the parametric equations and because of the (h), this part determines the location of line segment h. By finding the place of h and the radii, we can get the position of point P.

Note: Due to some technical issues... "b" represents "beta"; "A" represents "a" and "B" represents "b" in the photos; [] represents subscript. For all of these above equations: "h" is the distance from the point being traced to the center of the rotating/rolling circle.

The radius of the rolling circle is "b".

The radius of the stationary circle, that the smaller circle is rolling around, is "a".

Furthermore, the ratio for radii of the circles must be rational for the curve to be periodic. If it is irrational. then the epitrochoid will not repeat itself. The ratio is also important in determing the number of curves the epitrochoid will produce in one rotation around the circle.

## Other Significant Equations

(These equations were found at http://mathworld.wolfram.com/Epitrochoid.html)

This is the equation for the velocity of the rotating circle:

This is for the acceleration of the rotating circle:

The Equation for the Arc Length:

This is the equation for the amount of curvature:

## Special Epitrochoids

Limaçon: When the value of "a" is equivalent to the value of "b".

Epicycloid: When the value of "h" is equivalent to the value of "b".

## Explanation on the Position of the Point

The amount of times the point being traced goes back to where it started on the revolving circle is based on the two different circumferences. If the circumference of the revolving circle is a, and the stationary circle is 2a, that means the point on the revolving circle will end up where it started twice (2a/a=2). In other words, it takes the rotating circle two full rotations in order to return to the place where it first started.

Where the point is does not affect how many times arcs will be traced, as long as it is attached to the revolving circle. As demonstrated by this image, there are 2 curves no matter where the point is. If the point is on the edge of the circle, the curves will touch the edge of the stationary circle (red). If the point is inside the rotating circle, the curves will not touch the edge of the stationary circle (blue). If the point is outside of the rotating circle, the curves will loop into the stationary circle (purple).

However, the positioning of the point being traced does affect how curved the line is when the point goes back to its starting point. right

There is an equation for the number of bumps the rotating circle takes as it rolls around the stationary circle once (from starting point back to starting point).

If N is the number of bumps when the rotating circle cycles once from starting point to starting point, and R1 is the radius of the stationary circle and R2 is the radius of the rotating circle,

then N=R1/R2

This equation is demonstrated in the image to the right; the radius of the stationary circle is 2 and the radius of the rotating circle is 1.

N=(2)/(1) N=2

As you can see, there are two bumps created.

# Why It's Interesting

It creates interesting shapes and can be applied to uses of art and engineering. Two examples of usages of an epitrochoids include the Wankel Rotary Engine and the Spirograph.

## Wankel Rotary

The Wankel Rotary Engine was created by Felix Wankel in the 1920s that is used in cars and vehicles as engines. At the time, it was a new type of gasoline engine. The rotor itself is an equilateral triangle while the bore is made of an eptitrochoid curve. By following the path that the rotor tip creates, you can see the traces produced. For another explanation, you can go on: Involute

## Spirograph

The Spirograph is a geometric tool used in manually drawing roulettes. It can produce epitrochoids. It can also produce hypocycloids, hypotrochoids, and epicycloid.

## Ptolematic System

In Ptolemy's concept for his view of the universe, the Earth was the center of the universe and everything else revolved around it. It was not until the Heliocentric theory by Copernicus that this theory became challenged. Today, modern technology has shown that the Heliocentric throury is correct. However, Ptolemy's astronomical system is still acknowledged today. The orbits in this system are epicycloids, a special form of epitrochoids.

# How the Main Image Relates

The concept of epitrochoids is explained.

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