get_front_triangle_dims: Get front triangle dimensions
In ghuiber/bicycle: Design a bicycle frame

Description Usage Arguments Details Value Examples

The front triangle is a misnomer, because it's a quadrilateral made up by four tubes: the seat tube ST, top tube TT, head tube HT and down tube DT. Between these tubes there are four angles, and they must add up to 360 degrees. A bike frame designer aims for a top tube length (tt_length) and seat tube length (st_length) that fit a rider of a given height and torso length, and also for a seat tube angle (st_angle) head tube angle (ht_angle) and top tube angle (tt_angle) that give the bike its desired riding characteristics.

get_front_triangle_dims(
  st_length = 500,
  tt_length = 500,
  st_angle = 71,
  tt_angle = 0,
  ht_angle = 71,
  st_dt_angle = 60
)

`st_length`	The length of the seat tube ST in millimeters.
`tt_length`	The length of the top tube TT in millimeters.
`st_angle`	The angle between the seat tube and the horizontal.
`tt_angle`	The angle between the top tube and the horizontal.
`ht_angle`	Head tube angle with the horizontal, in degrees.
`st_dt_angle`	Angle between the seat tube and the down tube, in degrees.

The angle between the seat tube and the downtube (st_dt_angle) is pre-determined in lugged frame construction by how the bottom bracket shell was cast. With these six dimensions in hand we can get the full set of dimensions that describe the front triangle. This function returns them as a tibble of five columns and three rows. Each tube is described as a right triangle made up of the tube length as the hypotenuse and the tube's horizontal and vertical projections as the legs. The elements on each row are named accordingly. Why five columns for four tubes?

The fifth column, ett_triangle, helps calculate the effective top tube length when the top tube is slanted. For slanted top tubes the effective length is the horizontal distance between the front of the top tube and an imaginary extension of the seat tube equal to the vertical projection of the top tube times the sine of the seat tube angle in radians. In other words, this horizontal distance is equal to the horizontal projection of the top tube plus the vertical projection of the top tube times the tangent of the seat tube angle in radians. The second term of this addition is the third element of the ett_triangle vector.

A 3 x 5 tibble.