get_bbsc_offset: Get the bottom bracket shell offset 'bbsc_offset'
In ghuiber/bicycle: Design a bicycle frame

Description Usage Arguments Details Value Examples

In the plane that is perpendicular onto the bike, the chain stay is part of a trapezoid that is as wide as half the OLD spacing at the dropout and half the bottom bracket shell center offset at the bottom bracket. The OLD spacing is standard (130 mm for road bikes, 135 mm for old-school mountain bikes, 142 mm for newer mountain bikes, etc.). So is the width of the bottom bracket (BB) shell (68 mm for road bikes, 73 mm for old-school mountain bikes, etc.).

1	get_bbsc_offset(old_spacing, angle_btw_css, cs_length, bbs_width = 68)

`old_spacing`	Over-locknut-dimension (hub width) in millimeters.
`angle_btw_css`	Angle between chain stays, in degrees.
`cs_length`	Chain stay length in millimeters.
`bbs_width`	Bottom bracket shell width.

Given a target OLD and a bottom bracket shell of a given width, given angle between chain stays, and a chosen chain stay length, you can derive the bbsc_offset that suits. When you build a lugged frame, this function is just a sanity check, because your bottom bracket shell lug will come with a given bbsc_offset).

When you build a welded frame and you have a choice of bbsc_offset, you must set it subject to the constraint that you must leave some room between the centers of the chain stays and the faces of the BB shell. This function forces you to leave 19 mm, which should be enough for chain stays of typical outer diameter at the BB shell end.

A scalar equal to the bottom bracket shell offset in millimeters.