On 9/8/2017 4:46 AM, Graham wrote:

"Emanuel Berg" wrote in message ...
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

Not sure about your code as I do not speak that language but a quick check gives:

622*3.142*50/25=3908.648mm

So if your definition of gear is roll out in mm then it looks close. Do not forget to include the tyre. See:

https://www.cateye.com/data/resource..._chart_ENG.pdf

for approximate circumferences. Taking a 23mm tyre the above example would be:
2096*50/25=4192mm

Graham.

Right, don't forget to include the tire. 622 is bead seat diameter, but
you want (effective) outside diameter instead.

I first did such a thing in the 1970s, using Fortran. But I formatted it
as a compact table in rows and columns. You could have one row for each
chainring, one column for each rear cog. A matrix, 2x8.

Another useful trick is to plot the gear development on a logarithmic
scale, so the change from one gear to the next is scaled as the
percentage change. Plotting using a separate row or a separate symbol
for each chainring makes clear which gear is "next" in your gear
progression.

These days, it's probably easiest to do all the above using a
spreadsheet, such as LibreOffice Calc. Or Micros**t Excel.

BTW, back when there were only five rear cogs and a person wanted a wide
range with uniform or other logical spacing, plotting gear development
(or "gear inches," in "inch" countries) was a useful tool for the art of
bike design. These days, with up to 11 cogs in back, the entire exercise
isn't as valuable as it once was.

--
- Frank Krygowski