How to determine chainring tooth-count from diameter?

[spinningmagnets]

I am swapping-in a different crankset to a frame I have, and it has a 104-BCD 4-arm spider (don't ask, just bear with me). I made a cardboard aided design (CAD) disc, and it turns out the max diameter before hitting the front of the right-side chainstay is 110mm radius, so a chainring with 220mm OD at the peak of the teeth would be my max tooth-count.

There's gotta be a resource on the web with a "punch in diameter/spit out result" function. Sorry that my Google-fu is weak today, couldn't find the right search terms. Any help appreciated.

Also, anybody have a pile old 104-BCD chainrings collecting dust? I prefer to verify that something works before buying new parts, whenever possible.

edit: looks like I could go up to a 54T if they existed. Looked around and the largest 104-BCD I can find is 48T, so I guess that's what I'll use, no danger that it will hit the chain stay.

 

[amberwolf]

opposite of waht u need, maybe flip th math?

http://forums.roadbikereview.com/fixed-single-speed/max-teeth-front-chain-ring-96612.html#post1071429

http://bicycles.stackexchange.com/questions/5264/how-do-i-calculate-the-diameter-of-a-chainring-from-the-number-of-teeth

https://www.rbracing-rsr.com/calcsprocketdiam.html

 

[spinningmagnets]

Thanks, Amberwolf.

You can get a rough idea of overall chainring diameter in mm with this formula: tooth count multiplied by 13.2 divided by 3.14

Second link looks to be very accurate, but also very complex. Third link looks to be easy to use, but doesn't specify if it has a result that measures the center of the tooth, or the tip. However, I'll give it a try, and compare to existing sprockets to see how they compare. (all hundredths rounded up to the next tenth of an inch, pitch is .500)

Teeth, OD in inches, in mm

60T 9.6 243
59T 9.4 238
58T 9.2 233
57T 9.1 231
56T 8.9 226
55T 8.8 223
54T 8.6 218
53T 8.4 213
52T 8.3 210
51T 8.1 206
50T 8.0 203
49T 7.8 198
48T 7.6 193
47T 7.5 191
46T 7.3 185
45T 7.2 183
44T 7.0 178
43T 6.8 173
42T 6.7 170
41T 6.5 165
40T 6.4 163
39T 6.2 157
38T 6.1 155
37T 5.9 150
36T 5.7 147
35T 5.6 142
34T 5.4 137
33T 5.3 135
32T 5.1 130
31T 4.9 124
30T 4.8 122
29T 4.6 117
28T 4.5 114