Traingle Calcs and stem rise

Pirahna
Pirahna Posts: 1,315
edited March 2008 in Workshop
Can somebody check this for me please as I haven't done geometry maths for about 25 years.

I have a bike with a 72 degree head angle and I run a 110mm stem with a -17 degree rise.

I change the stem to a 110mm 0 degree rise. How much higher are the bars?

I make it 13.5mm.

Comments

  • dennisn
    dennisn Posts: 10,601
    I make it 33.6

    Dennis Noward
  • Wooliferkins
    Wooliferkins Posts: 2,060
    I wish I hadn't thrown out that trig book 30 years ago when I left school
    Neil
    Help I'm Being Oppressed
  • aracer
    aracer Posts: 1,649
    Dennis is close enough to be within experimental error - I make it 32.0 (I think you've used tan rather than sine, Dennis - but the stem length is the hypotenuse of the triangle).
  • dennisn
    dennisn Posts: 10,601
    aracer wrote:
    Dennis is close enough to be within experimental error - I make it 32.0 (I think you've used tan rather than sine, Dennis - but the stem length is the hypotenuse of the triangle).

    I think both of us didn't quite get it. What you have is a triangle with 2 equal sides(110)
    with a 17 degree angle between them and 2 - 81.5 degree angles at the base(not a
    right triangle). The base(or added rise in the bar) is 32.51807. All that math kicks in
    at last.

    Dennis Noward
  • aracer
    aracer Posts: 1,649
    That's not right either - rise is normally measured vertically, and the number your calculation produces is the distance between the two positions of the bars with the different stems, but that has a horizontal component.

    My calcs:
    Original rise = 110 * sin(1) = 1.92
    New rise = 110 * sin(18) = 33.99
  • simbil1
    simbil1 Posts: 620
    That's right apart from the shades 8)

    Aracer's calcs:
    Original rise = 110 * sin( 1 ) = 1.92
    New rise = 110 * sin( 18 ) = 33.99
    http://picasaweb.google.com/thefirstsimbil/Bikes
  • dennisn
    dennisn Posts: 10,601
    aracer wrote:
    That's not right either - rise is normally measured vertically, and the number your calculation produces is the distance between the two positions of the bars with the different stems, but that has a horizontal component.

    My calcs:
    Original rise = 110 * sin(1) = 1.92
    New rise = 110 * sin(18) = 33.99

    Sorry to say but we are both still wrong. Pathetic but true. We have an isosceles
    triangle with sides of 110, 110, & 32.51807(@ 17 degrees). This 32.51807 is the
    sloping component of a right triangle with 90, 81.5, & 8.5 degree angles. This triangle
    has sides of 32.51807, 32.16089, & 4.806477. 32.16089 being the actual vertical change
    in the bars height. At least I think so. Well maybe. Whatever it works out to be it's been
    one easy problem that took some idiots like us way to long to figure. This is why I didn't
    even attempt to answer an earlier spoke length question.

    Dennis Noward
  • aracer
    aracer Posts: 1,649
    You're almost there, Dennis, but going a long way round to get to the same place I did a lot simpler. When you generate your second triangle you're forgetting that the original stem is 1 degree off horizontal (72 degree head angle, -17 degree stem) - hence your RA triangle actually has 90, 80.5 and 9.5 degree angles, and sides of 32.51807, 32.07210 and 5.36703.

    My calcs with extra (unnecessary except for this argument!) levels of precision:
    Original rise = 110 * sin( 1 ) = 1.91976
    New rise = 110 * sin( 18 ) = 33.99187
    Difference = 32.07210 (I made a rounding error in my first post).

    Hopefully the OP stopped reading ages ago, happy with "about 32"!
  • dennisn
    dennisn Posts: 10,601
    aracer wrote:
    You're almost there, Dennis, but going a long way round to get to the same place I did a lot simpler. When you generate your second triangle you're forgetting that the original stem is 1 degree off horizontal (72 degree head angle, -17 degree stem) - hence your RA triangle actually has 90, 80.5 and 9.5 degree angles, and sides of 32.51807, 32.07210 and 5.36703.

    My calcs with extra (unnecessary except for this argument!) levels of precision:
    Original rise = 110 * sin( 1 ) = 1.91976
    New rise = 110 * sin( 18 ) = 33.99187
    Difference = 32.07210 (I made a rounding error in my first post).

    Hopefully the OP stopped reading ages ago, happy with "about 32"!

    I quit. My brain has been damaged. You're right about the 72 head angle. That on got by me.

    dennis noward
  • Cajun
    Cajun Posts: 1,048
    Stem Calculators:
    http://www.zinncycles.com/stemFit.aspx
    http://www.habcycles.com/fitting.html

    (you guys have a much better education than I and you make my brain hurt :oops: )

    just in case you get curious again, here's some other softwares:
    http://www.machinehead-software.co.uk/cycling.html
    Cajun

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