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  1. #1
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    downtube/fork clearance

    Am I overthinking this? I am designing a steel brazed 29er. fox 51mm offset fork. I am concerned about the downtube fork clearance. I drew it out on paper considering the stack height, fork dimensions etc. it looks close, should I just leave the headtube a little long, and trim to fit once done?

    thanks

    Sean

    sxconway@yahoo.com

  2. #2
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    You certainly *could* do that, but it will leave your geometry a big question mark until you're done. Better to just get it right in the first place (maybe that's obvious). Do you have any room to move the downtube up the head tube to hedge a bit?

  3. #3
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    Cutting the head tube after the bike is built seems like a bad idea. You will have all sorts of trouble making a clean cut plus as Feldy pointed out it will impact your geometry. With a milling machine you could make the cut pretty clean but if you are working with hand tools or cutting your head tube on a lathe it would be pretty hard.

    I think the best approach is to really aim to get it right the first time. If you miss by a couple of mm you can correct later by aggressive facing with a hand facing tool. If you miss in the other direction you can have a fork crown race made to hide the mistake.

    In any case your frame needs to be within a couple of mm of what you are aiming for when you get the frame out of the jig so it is good that you are thinking about it now.
    Mark Farnsworth, Raleigh, NC
    http://farnsworthbikes.com

  4. #4
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    One of the nice features of bikecad pro is the fork crown clearance which is grayed out in the java version. Know anyone with the pro that can model it for you? If you plan of building several bikes, I highly recommend the purchase. It keeps getting better with Brent's hard work, and future upgrades are free
    cheers
    andy walker

  5. #5
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    I plan on getting it right, by preparing everything. but, in event I am off a c hair, I was going to leave a little on the bottom of the head tube, just in case. I will look into the bikecadpro software, using rattle cad now.

    I guess I can also get the tubes mocked up on jig, and eyeball the fork take some measurements to see where I'm at before I start brazing.

    fyi, I am trying to avoid using a bent downtube, i hate they way they look.

    thanks for the input, more brains are better than just mine.

    sean

  6. #6
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    For the 1st couple frames. I'd hand draw the frame on the back of wrapping paper. Takes about 10 minutes and I always did it anyways for reference during construction.
    Then lay the fork in it's place to see actual clearance. Rebound and compression "knobs" can be awkward and this gives an actual view.

  7. #7
    RCP Fabrication
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    These are the critical measurements for a White Brothers Loop 29er. How this correlates to the Fox, I don't know, but the Loop's have some pretty chunky knobs.

    Hopefully it helps.

    Attachment 838711

  8. #8
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    ^ RCP FAB,
    thanks that helps me, I made a bike for the Loop 29r and it was pretty tight. Those numbers are the height below the headset to the top of the knob? And 44 deg angle is what?
    cheers
    andy walker

  9. #9
    RCP Fabrication
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    Andy,

    44mm is the diameter of the circle. The other two measurements are where to locate the circle. 24.7mm down from the crown and 66.4mm out, perpendicular to the fork.

    Ryan

  10. #10
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    Save the conjecture. Measure the real parts and draw it out. That's what "they" always told me. It works.
    Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
    - John Hajny, a.k.a. TrailMaker

  11. #11
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    You should be able to get the critical fork measurements from Fox. I got the original Reba measurements from SRAM when I was drafting up frames for a friend of mine. I did basically what RCP FAB did, and drew the fork in rotated 90 deg to simulate a full bar turn. I was able to emulate the knobs/critical points on the fork, and from there I set 3-5mm as the minimum fork clearance as a good guideline to have a tiny bit of wiggle room. From there, you can determine the minimum distance up the head tube that you need to be to provide your needed clearance to the down tube. You will also need to know your headset lower stack height, as this plays into the equation.

    If you want, I could probably draft your bike up in AutoCad for you and send you a pdf. It may take me a few days to get to it, but once I have all of the critical numbers, it doesn't really take me long to draft up a bike.

  12. #12
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    Quote Originally Posted by Francis Buxton View Post
    You should be able to get the critical fork measurements from Fox. I got the original Reba measurements from SRAM when I was drafting up frames for a friend of mine. I did basically what RCP FAB did, and drew the fork in rotated 90 deg to simulate a full bar turn. I was able to emulate the knobs/critical points on the fork, and from there I set 3-5mm as the minimum fork clearance as a good guideline to have a tiny bit of wiggle room. From there, you can determine the minimum distance up the head tube that you need to be to provide your needed clearance to the down tube. You will also need to know your headset lower stack height, as this plays into the equation.

    If you want, I could probably draft your bike up in AutoCad for you and send you a pdf. It may take me a few days to get to it, but once I have all of the critical numbers, it doesn't really take me long to draft up a bike.
    As I was riding today, I thought it would be helpful to have a to scale drawing of the fork I am going to use. then I can draw it out. I have a similar drawing as seen in this thread, and the clearance is close. I might take you up on the cad mock up. I have a file from rattlecad I can send. thanks in advance for all the help.

    beer
    sean

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