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Drilled frame rails-is there a formula? Thoughts?

Discussion in 'The Hokey Ass Message Board' started by hillbillydeluxe, May 1, 2006.

  1. I was looking through some builds for frame rails and noticed some areas where the frame rails would (or could) be drilled completely through to make it appear to lighten the frame. That technique has been used in everything from racing to zeppelin structures. I have a few questions, though. I've heard there is a formula for the diameter of the hole related to the frame rail height as well as the distance between centers. This is supposed to be based on shear strength of the material and the stress paths around the hole openings. Anyone know for sure?

    The second question I have is that if you pop enough holes in the frame, what's the best way to finish them off without sleeving them and adding the weight you just took out?

    Any thoughts?
  2. Damned if I know where my Machinery's handbook is; bet it has some info. I tried a few min's of google; found which has some interesting stuff and may pan out (but it's ten o'clock and I wanna' go to bed).

    There's probably engineers on here who know this stuff; a long-winded BTTT for ya'.

  3. Andy P
    Joined: Oct 1, 2004
    Posts: 27

    Andy P
    from UK

    There was a thread on here last year on this subject, a guy posted an Autocad drawing showing a series of crosses along the rail that set the diameter and pitch of the holes....
  4. Yeah there's some formulas in a couple aircraft const. books I'll have to look it up , but generally it's half the width of the hole size to the edge of the next hole.

  5. burger
    Joined: Sep 19, 2002
    Posts: 2,347

    from burbs

    I don't know if this specifically applies to frame rails, but I've been taught the following guidelines:

    1.5 x dia from the center of the hole to any edge

    2 x dia between hole centers

    Which means.... for a 3" tall frame rail, you're looking at 1" dia holes spaced 2" apart. I guess nobody follows the rules!


    PS- I just noticed the second part of your question about finishing the holes. What you're looking for is called a "dimple die," which will bell the edge of the hole. Here are some links:
  6. That was what I had bookmarked, but can't locate anything specific. I'll have to try the engineers here at work and let you know what I find out.
  7. Seems like most of it is an aesthetic thing. I thought sure as hell there was a formula taking these measurements into consideration. Hmmm.

    Attached Files:

  8. evltwin
    Joined: May 11, 2006
    Posts: 73

    from Milford MI

    It looks like a truss with the holes so large and spaced so close.

    I would look at truss structures. Remember the top and bottom chords should not be cut or modified in a truss.

    There is a formula for load on a truss that involves the height of the truss, chord and the length. The loads are (guessing) for still - dead weight. Once you start bumping down the road everything changes.

    I have not seen aftermarket frames offer this as an option to reference. Boxing the frame does not look like an option.
  9. enginebloch
    Joined: Feb 23, 2006
    Posts: 114

    from norfolk va

    Keep in mind that a square foot of 11 guage steel only weighs 5 pounds, thats the equivalent of 20 3 inch holes. IMHO, there are better ways to cut weight than cutting large holes your frame rails. Smaller holes are safer and still give it a bitchin look. I would follow the machinist handbook recomendations for the main rails. You can probably get away with more on some of your supporting members.

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