spoke_length.sh

#!/bin/sh

# Copyright (C) 2016 George Courtsunis, MIT license
#
# Script accepts -e -f -c -s -S -C arguments for calculating the spoke length
# based on the effective rim diameter, flange diameter, center to flange, spoke
# diameter, spoke number, and cross number. All measurements are expected in
# mm, however any unit as long as they are all consistent will work.
#
# --erd Effective rim diameter
# --fd Flange diameter
# --sd Spoke diameter
# --ctf Center to flange
# -s Number of spoke holes per hub (ie: 24, 32, 36, etc)
# -c Cross number of lacing pattern (ie: 2, 3, etc)
#
# Usage
# ~$ ./spoke_length --erd 566 --fd 56.9 --sd 2 --ctf 21.8 -s 32 -c 3
# ~$ 273.246980


while getopts ":s:c:-:" opt; do
  case $opt in
    -)
      case $OPTARG in
        erd )
          # The value is the index of the next option
          EFFECTIVE_RIM_DIAMETER=${!OPTIND}
          # Skip over the value of this option
          OPTIND=$(($OPTIND+1))
          ;;
        erd=*)
          EFFECTIVE_RIM_DIAMETER=${OPTARG#erd=}
          ;;
        erd*)
          EFFECTIVE_RIM_DIAMETER=${OPTARG#erd}
          ;;
        fd )
          # The value is the index of the next option
          FLANGE_DIAMETER=${!OPTIND}
          # Skip over the value of this option
          OPTIND=$(($OPTIND+1))
          ;;
        fd=*)
          FLANGE_DIAMETER=${OPTARG#fd=}
          ;;
        fd*)
          FLANGE_DIAMETER=${OPTARG#fd}
          ;;
        sd )
          # The value is the index of the next option
          SPOKE_DIAMETER=${!OPTIND}
          # Skip over the value of this option
          OPTIND=$(($OPTIND+1))
          ;;
        sd=*)
          SPOKE_DIAMETER=${OPTARG#sd=}
          ;;
        sd*)
          SPOKE_DIAMETER=${OPTARG#sd}
          ;;
        ctf )
          # The value is the index of the next option
          CENTER_TO_FLANGE=${!OPTIND}
          # Skip over the value of this option
          OPTIND=$(($OPTIND+1))
          ;;
        ctf=*)
          CENTER_TO_FLANGE=${OPTARG#ctf=}
          ;;
        ctf*)
          CENTER_TO_FLANGE=${OPTARG#ctf}
          ;;
        *)
          echo "HERE"
          ;;
      esac
      ;;
    s)
      SPOKES=$OPTARG
      ;;
    c)
      CROSS=$OPTARG
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      exit 1
      ;;
    :)
      echo "Option -$OPTARG requires an argument." >&2
      exit 1
      ;;
  esac
done

# Ensure we have correct number of arguments
if [ -z "$EFFECTIVE_RIM_DIAMETER" ]; then
    echo "Must enter effective rim diameter w/ --erd"
    exit 1
fi

if [ -z "$FLANGE_DIAMETER" ]; then
    echo "Must enter flange diameter w/ --fd"
    exit 1
fi

if [ -z "$SPOKE_DIAMETER" ]; then
    echo "Must enter spoke diameter w/ --sd"
    exit 1
fi

if [ -z "$CENTER_TO_FLANGE" ]; then
    echo "Must enter center to flange w/ --ctf"
    exit 1
fi

if [ -z "$SPOKES" ]; then
    echo "Must enter spoke number w/ -s"
    exit 1
fi

if [ -z "$CROSS" ]; then
    echo "Must enter cross number w/ -c"
    exit 1
fi

spoke_length() {
    echo "
    pi = 4*a(1)

    /* Theta from cross number */
    theta = 2*pi*$CROSS / ($SPOKES / 2)

    a = $EFFECTIVE_RIM_DIAMETER / 2
    b = $FLANGE_DIAMETER / 2

    /* Law of cosines */
    cs = a^2 + b^2 - 2*a*b*c(theta)

    /* Normalize for ctf distance */
    l = sqrt(cs + $CENTER_TO_FLANGE^2) - $SPOKE_DIAMETER / 2
    print l
    " | bc -l
}

echo `spoke_length`