Larry Watanabe's Gearchart

Years ago I wrote a simple program for calculating the equivalent size wheel for a given set of gears. That piece of code worked it's way into a bicycle FAQ and will probably outlive anything I've ever written in terms of code or publications. You never know what you'll be known for :)

Here's the link (and the code). It took me about 5 minutes to write.

/*
The C Source for Larry Watanabe's gear chart program
Originally found at ftp://draco.acs.uci.edu/pub/rec.bicycles/gear.c

*/
/* gear.c - prints out a table of gear-inches.
*
* Example:
*
* Wheel outside diameter: 27
*
* Chainwheel teeth: 50 40 27
*
* Rear cog teeth: 13 15 17 19 21 27
*
*
* 50 40 27
* -------------------------------
* 13 103.85 83.08 56.08
* 15 90.00 72.00 48.60
* 17 79.41 63.53 42.88
* 19 71.05 56.84 38.37
* 21 64.29 51.43 34.71
* 27 50.00 40.00 27.00
*
* Larry Watanabe watanabe@cs.uiuc.edu 1994
*/

#include
#define MAXFRONT 3 /* max number of chainwheels */
#define MAXREAR 8 /* max number of rear cogs */
main()
{
int c;
int i, j;

double wheel_diameter; /* outside diameter of wheel */
int front[MAXFRONT]; /* number of teeth for front chainwheels */
int rear[MAXREAR]; /* number of teeth for rear cogs */
int frontn = 0; /* number of front sprockets */
int rearn = 0; /* number of rear sprockets */
double gear_inch; /* gear-inches of current combo */

printf("Wheel outside diameter: ");
scanf("%lf", &wheel_diameter);

printf("\nChainwheel teeth: ");

/* skip over junk to get to a digit */
for (c = getc(stdin); !isdigit(c); c = getc(stdin))
;
ungetc(c, stdin);

/* input chainwheel sizes */
for (;;) {
c = getc(stdin);
if (c == '\n')
break;
if (isdigit(c)) {
ungetc(c, stdin);
scanf("%d", &front[frontn++]);
}
}
/* input rear cog sizes */
printf("\nRear cog teeth: ");
for (;;) {
c = getc(stdin);
if (c == '\n')
break;
if (isdigit(c)) {
ungetc(c, stdin);
scanf("%d", &rear[rearn++]);
}
}
printf("\n\n ");
for (j = 0; j < frontn; j++)
printf("\t%4d", front[j]);
printf("\n-------------------------------\n");
for (i = 0; i < rearn; i++) {
printf("%5d", rear[i]);
for (j = 0; j < frontn; j++) {
gear_inch = wheel_diameter * front[j]/ rear[i];
printf("\t%3.1lf", gear_inch);
}
printf("\n");
}
}