Dirt Jumper - Frame help- Mtbr.com
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  1. #1
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    Dirt Jumper - Frame help

    Hello,

    Starting my second frame soon and want to try a dirt jumper. My first frame was done at Dave Boehms school so this will be first on my own and figure I should keep as easy as possible. I haven't started on drawings yet as I don't know tubing size but do have general idea of geometry, which I will post up later.

    What size tubing for top tube and down tube is recommended for something like this. I am having a hard time finding anything by searching. Am I ok in assuming that .9/.6/.9 in 31.8mm and 35mm will be ok? Should I just use 35mm for both top and bottom tube? I weigh 175 pounds and more just concerned with strength over ride quality at this point, which I think strength should be more of a focus for dirt jumper anyways.

    Also, does anyone make a pre-ream/faced headtube in different lengths? I am trying to avoid having to buy a head tube reamer at this point so just seeing what other options are out there.

    Thanks!

  2. #2
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    Dirt jumpers aren't my forte, but front triangles are pretty easy. Your suggestions seem fine, given your total lack of ancillary information, and it's hard to endorse the huge TT w/o a specific reason to do so. I'd be inclined toward a thicker wall tube, but i'm only replying because your question implies no expertise.

    I've used paragon headtubes and while i chased/faced them, it likely wasn't necessary... but any serious shop is gonna have that tool available for a reasonable cost.


    Not enough information.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  3. #3
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    Hobby builder here, but I have built two dirt jumpers using 9/6/9 1.5" down tubes and 9/6/9 1.25" top tubes. Both bikes were ridden hard (large dirt jumps, flat landings, cased landings, skatepark abuse) for 2-3 years regularly with no issues. I am only 160lbs, but tend to break parts/damage parts & frames at a similar rate to my heavier riding buddies. Dent resistance is decent with these tubes, but I have dented one in the thinner butted area. Paragon head tubes are nice for the less skilled since their extra girth take a little more heat without warping.

  4. #4
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    Quote Originally Posted by scottzg View Post
    Dirt jumpers aren't my forte, but front triangles are pretty easy. Your suggestions seem fine, given your total lack of ancillary information, and it's hard to endorse the huge TT w/o a specific reason to do so. I'd be inclined toward a thicker wall tube, but i'm only replying because your question implies no expertise.

    I've used paragon headtubes and while i chased/faced them, it likely wasn't necessary... but any serious shop is gonna have that tool available for a reasonable cost.


    Not enough information.

    Thanks for the info and that's good to know about the Paragon headtubes. I am not sure what other information is needed at this point in order to get some recommendations on tube sizing and thickness for the specific application I mentioned. Since you seem like an expert and I am trying to learn here, what other info would you need in order to make tubing recommendations? And yes, I don't have any expertise, hence my comment about it being my second frame and needing advice.

  5. #5
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    Quote Originally Posted by tyler_mtb View Post
    Hobby builder here, but I have built two dirt jumpers using 9/6/9 1.5" down tubes and 9/6/9 1.25" top tubes. Both bikes were ridden hard (large dirt jumps, flat landings, cased landings, skatepark abuse) for 2-3 years regularly with no issues. I am only 160lbs, but tend to break parts/damage parts & frames at a similar rate to my heavier riding buddies. Dent resistance is decent with these tubes, but I have dented one in the thinner butted area. Paragon head tubes are nice for the less skilled since their extra girth take a little more heat without warping.
    Awesome, thank you. That helps a lot and makes me feel more comfortable going that route. I might be able to source some thicker straight gauge 4130 from a local steel supplier so might try that as well, but at least I know I am not wrong by going 9/6/9 either. I will definitely go the Paragon route for right now and keep things a simple as possible.

  6. #6
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    Straight gauge is a great place to start. Less complication as far as making sure you aren't mitering in the butted area and a tougher tube overall for the abuse dirt jumpers can get. I have several other bikes I built out of straight gauge and enjoy them just the same.

    I have been using straight gauge .035" 3/4" tubing for my seat and chainstays of late, but also used butted, pre bent stays in the past. Just depends on what clearances you need and what dropout type you are using.

    Watch out for the area between the tire and the chainring, this can be tight on a short chainstay bike. I have had to either dimple for the tire clearance or chainring clearance on every bike.

  7. #7
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    Hey, I can confirm that the paragon head tubes do in fact need to be reamed after welding. While they are thicker and distort little to none, they are made .020 undersized. The first frame I did I payed the local shop 50 bucks to do the job (15 min!). I recently bought the 100 dollar park cutter and made a handle setup. I only ream, and just use a file gingerly to remove paint off the faces of the head tube. If you find yourself up here in Santa Cruz your welcome to stop by and use the tool.

  8. #8
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    Quote Originally Posted by tyler_mtb View Post
    Straight gauge is a great place to start. Less complication as far as making sure you aren't mitering in the butted area and a tougher tube overall for the abuse dirt jumpers can get. I have several other bikes I built out of straight gauge and enjoy them just the same.

    I have been using straight gauge .035" 3/4" tubing for my seat and chainstays of late, but also used butted, pre bent stays in the past. Just depends on what clearances you need and what dropout type you are using.

    Watch out for the area between the tire and the chainring, this can be tight on a short chainstay bike. I have had to either dimple for the tire clearance or chainring clearance on every bike.
    I found some straight gauge 4130 locally that I will be using for front triangle. For this build I am planning on going with some pre bent stays just to make sure I am getting correct clearance and one less thing for me to screw up bending.

    What dropouts did you go with? I was planning on going with Paragons flat sliding dropouts. I am going to start on my drawings and post those up as I am sure I will need some help with making sure that was done correctly with the right spacing and clearance. Chainstays with tire and chainring clearance is definitely the one thing I am not confident in. Thanks for the help

  9. #9
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    Quote Originally Posted by Noah240t View Post
    Hey, I can confirm that the paragon head tubes do in fact need to be reamed after welding. While they are thicker and distort little to none, they are made .020 undersized. The first frame I did I payed the local shop 50 bucks to do the job (15 min!). I recently bought the 100 dollar park cutter and made a handle setup. I only ream, and just use a file gingerly to remove paint off the faces of the head tube. If you find yourself up here in Santa Cruz your welcome to stop by and use the tool.
    Cool, thanks for that info. I will call a couple of my local shops and see what they have as I will probably still go the paragon head tube route. If I cant find anything local then I might have to take you up on that offer to come use your tool. I live in Santa Barbara so not too far and I am always looking for excuse to go ride up there so might work out perfect. Thanks for that offer!

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