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  1. #51
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 1 Day Ago Has been a member for 9-10 years. Has a total post count of 2,147. Received thanks 236 times, giving thanks to others 5 times.
    I get a radius of 20.207.

    I'd explain fully, but I'd need to do a drawing.
    It involves the constant that all the angles within a triangle add up to 180deg, and a bit trigonometry.

  2. #52
    ahh... i get it now, im with m_c on this one 20.07mm radius will intersect 3 points 35mm apart

    took me ages to work that out

  3. #53
    i can find the formula for 4,5,6 but not for 3 :confused:
    Click image for larger version. 

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    PCD Calculation Formula
    3 stud PCD = S / 0.866
    4 Stud PCD = S / 0.7071
    5 Stud PCD = S / 0.5278
    6 Stud PCD = S / 0.5
    Last edited by h4ppy-chris; 02-01-2011 at 10:30 AM.

  4. #54
    3 stud PCD= S/0.866


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  6. #55
    thanks peter star man, added it to the post :-)

  7. #56
    Ahh, now I understand the question! You wanted *diameter* not circumference at stated in your post.

    It's just cos(30)...

    Let x be side length of the triangle the three points form (i.e 35mm).

    (see diagram)

    Draw in lines on triangle that bisect the angles. So we've now got a smaller triangle with one side length x/2, and we know the angle indicated is 30 degrees, since it's an equilateral triangle and we've bisected the corner angle.

    r=17.5/cos(30) = 20.2072594...
    (since d=2r, d=X/cos(30))

    Incidentally to help remember it cos(30) is just sqrt(3)/2 = 0.866025...
    Hope that helps. You can get the other PCDs using similar methods. 4 is half root 2.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  9. #57
    thanks jonathan ;-)

  10. #58
    Ahh just realised. My method can easily be extended to the general case for n bolts. You just need to know half the angle interior angle of the particular polygon. Then do cosine of that to get the factor...

    3 = Triangle, 60/2 = 30, so cos(30)
    4 = Square, 90/2 = 45, so cos(45)
    5 = pentagon, 108/2 = 54, so cos(54) = 0.5878 (!)
    6 = hexagon, 120/2 = 60, so cos(60) = 0.5

    The formula to find the interior angle is 180(n-2)/n.
    So that means, given n bolts with spacing x.

    PCD = x/(2*cos(90(n-2)/n))

    Last edited by Jonathan; 02-01-2011 at 03:49 PM.

  11. #59
    like the formulae but...
    er where is x used??
    there are 10 sorts of people in the world
    them as understand binary
    and thems as doesn't

  12. #60
    x=bolt spacing(centre to centre)

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