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chainring question.

Posted: 28 Sep 2020, 1:59pm
by 123malford
Is the height difference, relative to the front mech., between a 50t and 46t chainring fractionally over 8mm ?

Re: chainring question.

Posted: 28 Sep 2020, 2:35pm
by MikeF
123malford wrote:Is the height difference, relative to the front mech., between a 50t and 46t chainring fractionally over 8mm ?
Maths indicate the difference in radius of the 2 rings should be 8.1mm assuming the tips of the teeth are spaced at 1/2" or 12.75mm

Re: chainring question.

Posted: 28 Sep 2020, 3:42pm
by 123malford
That's good. Many thanks.

Re: chainring question.

Posted: 28 Sep 2020, 11:19pm
by MikeF
123malford wrote:That's good. Many thanks.
Hope my maths is right! I haven't those sizes of chainrings to measure. However I've measured the difference between a 34 tooth and a 46 tooth and the difference in radius is close to 24mm - maybe 23.5mm, but not easy to measure. It's approximately 2mm radius difference per tooth eg 4 tooth difference is 8mm.
Maths is as follows circumferences are 50x12.75mm and 46x12.75mm. Divide by PI (3.142.......) to obtain the diameters. Actual diameter doesn't matter, but only the difference ie 4x12.75/PI high gives 16.2mm diameter. The actual diameter is slightly different on my rings.

Re: chainring question.

Posted: 29 Sep 2020, 12:29am
by Chris Jeggo
Quite right. 1/2 inch difference in circumference => 12.7mm/2pi = 2.02mm difference in radius.

Re: chainring question.

Posted: 29 Sep 2020, 9:12am
by rogerzilla
The tips are a bit more than 1/2" apart because 1/2" is the chain pitch, and the chain sits lower than the tips.

Re: chainring question.

Posted: 29 Sep 2020, 11:24am
by Brucey
in real chainring pairs for a double chainset the usual thing is that (for shifting purposes) the teeth on the big ring are shorter than those on the inside chainring. Hence it can appear to be (well, is actually) slightly less than 2mm per tooth.

cheers