Reducing Chain Ring Size

prince_madog

The first bike I ever bought had to be a do-it-all machine. I pottered around on it with the Good Princess, did Youth Hostel touring and even rode it in the national 100 mile TT champs. It's a Mercian Audax Special and has a braze-on front mech and 50-40-30 triple. The frame was built for this size of chainset and the mech locates right at the top of the braze-on. Having increased my stable of bikes by another five machines I'm thinking of changing the chainset for one more suited to general pottering. How many millimetres does the mech need to be lowered for each 2 teeth drop in chainring size?

andrewgturnbull

Hi there.

Just drop it down so it's about 1mm above the top of the outside ring.

Cheers, Andy

prince_madog

andrewgturnbull wrote:

Hi there.

Just drop it down so it's about 1mm above the top of the outside ring.

Cheers, Andy

Cheers Andy, but I'm trying to determine whether I can drop to 48 or 46 before I spend the money!

maddog 2

each chain link is 0.5 inch

so a 50 tooth ring has a circumference of 50x0.5 = 25 inches

converting inches to mm, we get 25 x 25.4 = 635mm (1 inch = 25.4mm)

hence the diameter of a 50t ring is 635/pi = 202.126mm

a 48t ring would have a circumference of 24 inches or 609.6mm

hence the diameter of a 48t ring is 609.6/pi = 194.04mm

so the difference between the two diameters is 8.085mm

which means the radius difference is half that, or 4.04mm


so.... for every 2 teeth you lose, you need to drop the mech 4mm 8)

prince_madog

Many thanks. I'd done a similar calculation myself and had come up with 2mm/tooth. Comfortable now that someone has confimed this for me.