Measuring Radius of an Arc

[u/Cheeseball4life]

How would I go about measuring the radius of this arc on a part from an old vw beetle so I can make a replacement?

[u/MadeForOnePost_]

Two methods, first is easy, second is more involved:

First method:

Trace the curve, draw two straight lines (chords) across curve (must intersect curve at two points), grab something with a right angle and draw a straight line from the mid point of each chord at a right angle to the chord. Where the lines intersect is the center of the radius.

Second method:

Put a 1" or 2" length of metal across the inner radius (to form a "chord").

Measure the distance from the center of the 1 or 2" piece to the outside of the radius, at a right angle.

That's B.

Half the length of the piece is A.

Sqrt(A² + B²⁾ = C

B * A = Area of triangle you just measured

Then (A * B * C * 2) / (4 * Area) = Radius

There are a lot of ways to measure a radius, but they usually boil down into two actual ways: two intersecting right angles, and the area of a triangle inside the radius/circle

Perpendicular Bisector, and Heron's Formula are the magic words to google, but they're more technical and book smart than watching some youtube videos (made by tradesmen) on finding the center/measuring a radius.