Re: program to compute gears, with table

[Skip Montanaro]

Something's fishy. Is the result mm of forward travel per crank revolution?
I'm used to seeing gear inches. Your 50x12 should be about 110 gear inches.
If your output is in millimeters, I get about 320 gear inches, so either
I've mistaken your intended units, or there's a mistake in there somewhere.

A couple nits which won't change anything:

* Why the 1.0 divisor when computing gear?

* You can skip the radius and use wheel (diameter) directly in computing
the circumference.

* It never occurred to me to do this in Lisp. I always just use an online
calculator, like:

http://www.gear-calculator.com/?GR=DERS&KB=34,50&RZ=12,13,15,17,19,21,23,25&UF=2150&TF=90&SL=2.6&UN=KMH

Skip

P.S. My latest acquisition (and oldest bike):
https://www.flickr.com/photos/49705339@N00/albums/72157686279443084


On Fri, Sep 8, 2017 at 12:33 AM, Emanuel Berg <moasen@zoho.com> wrote:

> Hey guys, does this look right to you?
>
> ;; (print-gears '(34 50) '(12 13 15 17 19 21 23 25) 622)
> ;;
> ;;                          =>
> ;;
> ;; 622 wheel
> ;;
> ;; chainring   sprocket   gear
> ;;
> ;;    34          25      2657.5360544880004
> ;;    34          23      2888.626146182609
> ;;    34          21      3163.7333982000005
> ;;    34          19      3496.7579664315795
> ;;    50          25      3908.1412566000004
> ;;    34          17      3908.1412566000004
> ;;    50          23      4247.97962673913
> ;;    34          15      4429.226757480001
> ;;    50          21      4652.549115000001
> ;;    34          13      5110.64625863077
> ;;    50          19      5142.291127105264
> ;;    34          12      5536.5334468500005
> ;;    50          17      5747.266553823531
> ;;    50          15      6513.568761
> ;;    50          13      7515.656262692309
> ;;    50          12      8141.960951250001
>
> (require 'cl-lib)
>
> (defun compute-gear (chainring sprocket wheel)
>   (let*((pi     3.14159265)
>         (radius (/ wheel 2.0))
>         (circum (* 2 radius pi))
>         (gear   (* (/ chainring sprocket 1.0) circum)))
>     (list chainring sprocket gear)))
>
> (defun gear (chainrings sprockets wheel)
>   (let*((gears
>          (cl-loop for c in chainrings
>                   append (cl-loop for s in sprockets
>                                   collect (compute-gear c s wheel) )
>                   )))
>         (cl-sort gears #'<= :key #'cl-caddr)))
>
> (defun print-gears (chainrings sprockets wheel)
>   (let ((out-buffer (get-buffer-create "*Gears*")))
>     (with-current-buffer out-buffer
>       (erase-buffer)
>       (insert (format "%s wheel\n\n" wheel))
>       (insert "chainring   sprocket   gear\n\n")
>       (let ((gears (gear chainrings sprockets wheel)))
>         (cl-loop for g in gears
>                  do (let ((c (   car   g))
>                           (s (   cadr  g))
>                           (d (cl-caddr g)))
>                       (insert (format "   %s          %s      %s\n" c s
> d)) ))))
>     (pop-to-buffer out-buffer) ))
>
> --
> underground experts united
> http://user.it.uu.se/~embe8573
>