Bicycle Mechanics - diameter of a 38-tooth ring?

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erader
10-05-07, 06:49 PM
does anyone know how much bigger a 38-tooth ring is compared to 36-tooth ring?

i'm looking at replacing the 36-tooth ring on my 1 x 9 with the larger ring and i'm trying see if i have enough clearance.

i have an MTB frame and i run the ring in the middle position and i believe the stays are bent to optimize a 34-tooth middle ring (i already had to move the BB over to get the 36 to clear).

TIA :eek:!

ed rader

MudPie
10-05-07, 07:21 PM
does anyone know how much bigger a 38-tooth ring is compared to 36-tooth ring?

i'm looking at replacing the 36-tooth ring on my 1 x 9 with the larger ring and i'm trying see if i have enough clearance.

i have an MTB an mtb frame and i run the ring in the middle position and i believe the stays are bent to optimize a 34-tooth middle ring (i already had to move the BB over to get the 36 to clear).

TIA :eek:!

ed rader

I believe the diameters are directly proportional to the number of teeth. For example, the diameter of a 32T is twice as large as a 16T.

If you measure the diam of your 36T, you should be able to calculate the diam of a 38T.

The ratio is constant: 36/38 = diam of 36/diam of 38.
waterrockets
10-05-07, 07:30 PM
c=2*PI*r
c = 38/2 inches = 19 inches (teeth are 1/2 inch apart)
2 * PI * r = 19
r = 19 / (2 * PI) = 3.02 inches
d = 2 * r = 2 * 3.02 = 6.04 inches

That's along the chain centerline, and I'll bet the teeth are 1/16 of an inch proud of the centerline, so I'd say 6 1/8"

erader
10-05-07, 10:36 PM
c=2*PI*r
c = 38/2 inches = 19 inches (teeth are 1/2 inch apart)
2 * PI * r = 19
r = 19 / (2 * PI) = 3.02 inches
d = 2 * r = 2 * 3.02 = 6.04 inches

That's along the chain centerline, and I'll bet the teeth are 1/16 of an inch proud of the centerline, so I'd say 6 1/8"


wow! that's some pretty fancy math :eek:! i get 5 7/8" for the 36 so that would 1/4" difference. thanx to the both of you!

ed rader