chainring question.

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123malford
Posts: 100
Joined: 12 Nov 2011, 5:51pm

chainring question.

Postby 123malford » 28 Sep 2020, 1:59pm

Is the height difference, relative to the front mech., between a 50t and 46t chainring fractionally over 8mm ?

MikeF
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Joined: 11 Nov 2012, 9:24am
Location: On the borders of the four South East Counties

Re: chainring question.

Postby MikeF » 28 Sep 2020, 2:35pm

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
"It takes a genius to spot the obvious" - my old physics master

123malford
Posts: 100
Joined: 12 Nov 2011, 5:51pm

Re: chainring question.

Postby 123malford » 28 Sep 2020, 3:42pm

That's good. Many thanks.

MikeF
Posts: 3952
Joined: 11 Nov 2012, 9:24am
Location: On the borders of the four South East Counties

Re: chainring question.

Postby MikeF » 28 Sep 2020, 11:19pm

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.
"It takes a genius to spot the obvious" - my old physics master

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Chris Jeggo
Posts: 188
Joined: 3 Jul 2010, 9:44am
Location: Woking

Re: chainring question.

Postby Chris Jeggo » 29 Sep 2020, 12:29am

Quite right. 1/2 inch difference in circumference => 12.7mm/2pi = 2.02mm difference in radius.

rogerzilla
Posts: 1536
Joined: 9 Jun 2008, 8:06pm

Re: chainring question.

Postby rogerzilla » 29 Sep 2020, 9:12am

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.

Brucey
Posts: 41358
Joined: 4 Jan 2012, 6:25pm

Re: chainring question.

Postby Brucey » 29 Sep 2020, 11:24am

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
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