# Bicycle Mechanics - diameter of a 38-tooth ring?

Bikeforums.net is a forum about nothing but bikes. Our community can help you find information about hard-to-find and localized information like bicycle tours, specialties like where in your area to have your recumbent bike serviced, or what are the best bicycle tires and seats for the activities you use your bike for.

View Full Version : diameter of a 38-tooth ring?

10-05-07, 07:49 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:!

MudPie
10-05-07, 08:21 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:!

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, 08:30 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"