Re: program to compute gears, with table

[Frank Krygowski <frkrygow@sbcglobal.net>]

On 9/8/2017 4:46 AM, Graham wrote:
> 
> "Emanuel Berg" <moasen@zoho.com> wrote in message news:86bmmlu54a.fsf@zoho.com...
>> 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/resources/Tire_size_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