30 t chainring size

foo606
foo606 Posts: 113
edited August 2018 in MTB workshop & tech
After lots of confusion over the size of a 30t chainring. I ordered a 28t raceface cinch DM for my atlas cranks .
Been trying to work out the size with home made templates .I have a 32t 134 mm a 28t 120mm but not a 30t 128mm ?.
I bought a Titus frame yeah another one of those .But hey at £60 worth a shot.
So 30t is what diameter .I need to know so I know how much clearance to frame I will have . Be running 1x12 eagle GX rear cassete .Thanks for any help . Paul .

Comments

  • steve_sordy
    steve_sordy Posts: 2,453
    They should be in proportion.

    Diameter and circumference are linked. As long as the teeth are the same size (which they will be on bikes), then the number of teeth will be directly proportional to the diameter.

    Therefore based upon your numbers of 32t = 134 and 28t = 120, then 30t = 127 mm diameter.
  • foo606
    foo606 Posts: 113
    Thanks I will draw up a template to see how much room I will have .Shame I couldnt find any online
  • foo606
    foo606 Posts: 113
    Well template made and its a bit tight for room doesnt hit the chainstay but damn close.
    I think in hindsight ordering the 28 was wise .
    I have heard of people putting 30t chainrings on the Titus frames
    I wasnt about to risk it since there not what you call ample room as it is .
  • steve_sordy
    steve_sordy Posts: 2,453
    foo606 wrote:
    Well template made and its a bit tight for room doesnt hit the chainstay but damn close.
    I think in hindsight ordering the 28 was wise .
    I have heard of people putting 30t chainrings on the Titus frames
    I wasnt about to risk it since there not what you call ample room as it is .

    It has just occurred to me that some teeth are not the same. Some of the narrow/wide teeth have a taller profile than normal chainrings, so the diameter would be bigger. But if the 30T ring is the same profile as the 32 and 28 you referred to then what I said will be correct.
  • 02gf74
    02gf74 Posts: 1,171
    ^^^^ the previous replies assume the chainring is circular, it's different in the case of an oval chainring,
  • foo606
    foo606 Posts: 113
    Theres not a massive difference between 28t and 32t laid over each over a few mms .
    It could be because its Sram nwc
  • foo606
    foo606 Posts: 113
    [img][https://m.facebook.com/story.php?story_fbid=1873719859377992&id=100002199878607img] These are the 32t and 28t on top of each other so the 30t cant be much bigger than the 28t .[/img]
  • steve_sordy
    steve_sordy Posts: 2,453
    foo606 wrote:
    [img][https://m.facebook.com/story.php?story_fbid=1873719859377992&id=100002199878607img] These are the 32t and 28t on top of each other so the 30t cant be much bigger than the 28t .[/img]

    (I'm not in facepuke, so I can't access the pics.)

    Correct, the 30T isn't much bigger than the 28T

    The diameters will look very close, a few mm as you say. comparing a 30T to a 28T with a 7mm difference in diameter is only 3.5mm on the radius, which is just over one eighth of an inch.

    All chainrings and gears have to engage with a half inch pitch chain that has circular rollers, so no matter what the tooth design, unless the teeth are made particularly tall (like some n/w rings), then the diameters are predicted by the numbers of teeth. "Ye canna beat the laws o' physics Cap'n!" :)
  • JBA
    JBA Posts: 2,852
    You need to host the image on line and then link to it to make it visible on here.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • foo606
    foo606 Posts: 113
    ok I will try sort it so everyone can benefit .Thanks
  • foo606
    foo606 Posts: 113
    https://www.instagram.com/p/BmMj8z_Ale6 ... pz4b3imrc6 Try this one on Instagrab .
  • foo606
    foo606 Posts: 113
    So according to the measurements chainring sizes go up in 7mm increments
    .
    For example 28 120 .30 127 32 134 .36 141 .
    So how is it they only look like they are 3.5 mm bigger or am I missing something here
  • JBA
    JBA Posts: 2,852
    7mm increase in diameter = 3.5mm increase in radius.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • 02gf74
    02gf74 Posts: 1,171
    JBA wrote:
    7mm increase in diameter = 3.5mm increase in radius.


    How do you work that out?

    Bicycle chain pitch is 1/2 inch so for each additional tooth, diameter increases by (25.4 /2)/3.142 in mm, where 3.142 is pi.

    About 4 mm as measured through centre of chain pins when on the chainring.
  • JBA
    JBA Posts: 2,852
    I divided 7 by 2. :wink:
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • 02gf74
    02gf74 Posts: 1,171
    JBA wrote:
    I divided 7 by 2. :wink:

    Genius, maybe you should apply for the position left vacant by Steven Hawking......
    .
    So where does the 7 come from.
  • steve_sordy
    steve_sordy Posts: 2,453
    02GF74 wrote:
    JBA wrote:
    7mm increase in diameter = 3.5mm increase in radius.


    How do you work that out?

    Bicycle chain pitch is 1/2 inch so for each additional tooth, diameter increases by (25.4 /2)/3.142 in mm, where 3.142 is pi.

    About 4 mm as measured through centre of chain pins when on the chainring.

    A piece of half-inch pitch chain wrapped around a 30t chain ring is a 30-sided polygon where the length of each side is half an inch. That makes it tricky to work out diameters of gears accurately without lots of complicated formulae. Unless you are prepared to approximate of course! 30t at half inch pitch is 15" long, which in mm is 15x25.4 = 381mm. If it was a pitch circle of that circumference, it would have a diameter of 381/PI (where PI = 3.14159)

    Therefore 381/3.14159 = 121.2. But that is not the diameter of the gear, just the pitch circle diameter! The teeth have to protrude beyond that to give the diameter of the gear.

    The chain roller has a diameter of 7.6mm and half of that sticks out each side. If the tooth tip sticks out as far as that, it would make the gear diameter 121.2 + 7.6 = 128.8 mm. The outer links of the chain are slightly larger and stick out above the teeth tips on normal chain by about 0.4mm, so that feels right.

    Based upon the information given by the OP previously, I said that the 30T chain ring would measure 129mm (half way between the diameters of the 32t and 28t rings). The above calculation of 128.8 mm isn't far away, given the assumptions I made about the pitch circle diameter of a 30-sided polygon with half inch sides. :)
  • JBA
    JBA Posts: 2,852
    02GF74 wrote:
    JBA wrote:
    I divided 7 by 2. :wink:

    Genius, maybe you should apply for the position left vacant by Steven Hawking......
    .
    So where does the 7 come from.

    From foo606’s statement that chainring suzes go up in 7mm increments.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • foo606
    foo606 Posts: 113
    Having Measured the actual chainrings I have
    .Raceface nw 28t and Sram 32 t nw both DM
    120 and 134 .Therefore I deduced they went up in 7 mm increments .
    It may not be 100% accurrate but close enough to see if it fouls the chain stay .
  • 02gf74
    02gf74 Posts: 1,171
    Nope, we're in agreement, I misread radius of 3.5 mm as diameter...

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