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Thread: First Frame

  1. #1
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    First Frame

    Let me start off by saying that I have read all the links in the FAQ section, several of them twice!

    I have a solid design for a frame I want to build. I don't really have any questions, but I am looking for any needed constructive criticism.

    The goal is to make a steel fat bike frame, with a low CG, and that is stable at fast DH. Something that has short chain stays and a slack head angle. I am the type of rider that will do a wheelie drop, before I roll a drop, but at the same time love to get back behind the seat for fast single track downhills.

    This bike will have 26x4 tires on 100mm rims, and a 31.6 dropper seat post.
    The Fork will be a Salsa Enabler.

    I currently have 3 bikes that I ride and all of them have basically the same "cockpit size". I have copied those dimensions with this design.

    I also have been looking for a good excuse to buy a tubing roller, so this is it.

    I took the time to learn bike cad. Here is what Ive come up with:



    What have I missed?

  2. #2
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    The tires will sag so the BB is lower than the drawing shows. Though 318 isn't what I'd call low so maybe you got that already.

    65 head angle with a rigid fork of that offset seems waaayyy slack.

    427 stays are going to be challenging, depending on your component choices.

    Maybe it's BikeCad, but you probably want a little more clearance between the ends of your head tube and the outside of the tubes.

    And then the usual disclaimer that you might want to build something simpler for your first bike to get one under your belt. There's enough stuff to screw up on with your first bike that it sucks to throw away things like tubes that have been painstakingly rolled because you flipped a miter.

  3. #3
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    Hey;

    - I've designed down to about 419mm on the CS. It should be doable, but it does start getting really tight. The fewer rings and things you use up there under those circumstances, the better. Although I would like to try one just to see, I frankly think anything that short will limit the essence of a Fatbike; traction and stability on dodgy surfaces.

    - I'd try to get at least 20mm space from TT/DT to end of HT. You need to be quite accomplished as a welder to move much closer than that to the ends of the HT. Then, pay attention to how close the tubes get to each other. Too close and the whole front end VERY RAPIDLY loses strength and turns into a big liability.

    - A double curved DT is going to be quite a challenge. Prepare to waste some tubing trying to figure that one out, unless you are not concerned with smooth and seamless bends/transitions. I'm not sure a perfect one is possible with a roller. Lots of people have tried it, and are quite obviously not aware that they have missed. Getting those to look right is VERY difficult. Even some celebrated builders don't get them looking quite right. It's subtle, and you just have to have an eye for it. It's one of those things that if you get it right, it is smashing, but miss it by just a tad, and it ruins the entire look.

    - I too think those front end numbers are over the top. You are dropping HT angles back 5* and trail to 30%+ over what a normal Fatty is running. I can't say from experience that it won't work, but using downhill type numbers for a Fatbike will make it not much of a soft surface weapon. If you are looking at having a downhill Fatty, you will quickly learn the limitations of them on fast rough terrain. You can go really fast to a point, and then you get seriously out of control. I'm at 69* on one of my Fatties, and I've considered 68 as a trial. 65 seems a bit nuts.

    - I also agree with the Good Doctor that you might try something a bit simpler. Fatties are not impossible, but they present some challenges, for sure. Get fancy on #2.
    Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
    - John Hajny, a.k.a. TrailMaker

  4. #4
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    Thanks for the excellent replies!

    I originally picked a 65 degree head tube because my Yeti is 68.5 and feels steep to me, while my Intense is 64.5 and feels great.

    In the examples below I have adjusted head angle to 67 degrees. I don't want to go any steeper then that.

    I also changed the spacing of the tubes on the HT, I brought them in a little closer. Im not against having to run a single front ring on this bike. I really like the idea of super short chain stays. I have another fat bike that I can ride in the soft stuff. This one is for fun fast hardpack.

    Here are a couple new renderings:

    1. Steeper HT, TT and DT moved in on HT



    2. Same as above but with a single curved DT



    3. Same as above but BB is an inch lower



    Maybe I should do a few test bends in regular steel tubing. That way I can get the feel for bending down better, and maybe slap them together for a cruiser...

  5. #5
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    With the bigger tire you'll have a couple of things going on:

    1) It's taller, so for a given head angle you'll have more trail and therefore straight line stability
    2) It's heavier, so it will have more inertia and again more stability
    3) If you run it at fatbike pressures you'll have more pneumatic trail and a larger contact patch that will scrub on the ground, so yes, more stability*

    All of these means the bike will be more stable than just the head angle leads you to believe, so you can err on the steeper side. Plus if you're comparing against bikes with suspension forks you need to take sag into account. DH isn't my thing so maybe you are...

    On the other hand, if it feels too slow or floppy, you could always build a fork that's a little lower (if you have room to spare with the bottom bracket height) and/or increase the fork offset. Your intended use sounds like it's a little outside the norm so I see no harm in pushing the numbers a bit.

    * stability used loosely as the tendency of the bike to be unperturbed by a disturbance

  6. #6
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    Random bullet points. Welby and TM pretty much hit the main stuff already and I'm in agreement - make yourself a grocery getter or a jump bike first, where screwups won't ruin things as much.

    -You will be spending a lot of money on dies if you want to bend all that stuff, and as TM says, a roller will NOT make the downtube you want. Forget curving tubes for #1.

    -I will pile on the "fat bikes are not DH bikes" bit. Undamped suspension from tires is great on snow or smoother stuff. Not so much at speed running over rocks. Think human catapult. So designing a super slack front end is IMO kinda pointless, since the speeds where that geometry will work well are also the speeds where the tires/no suspension decidedly will not.

    -Unless you have a good reason for super short stays, you are just making things hard for yourself, probably. Double toptube plus fatbike plus short stays is going to mean serious knee clearance issues too unless you want to spend the rest of your life doing/trying to do compound bends on the tubes to get them out of the way.

    That all said, if this design is what you're psyched about, make it happen!

    _Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  7. #7
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    Again, Thanks for the constructive replies!

    I honestly had not thought about knee clearance. The reason for the super short stays is because I kind of want a wheelie machine, but I will admit that short stays and slack head tube are sort of counter productive.

    1 question I do have: Where can I get a 1:1 print? I think its a great idea...

  8. #8
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    What you call counterproductive, I have often called complimentary, ha!
    but Kinko's can get you a full size print, or any place like that, just make a PDF frome bikeCAD.

  9. #9
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    Hey;

    In terms of wheelies, you already have a pretty slack ST angle there. Most (many?) Fatties are up around 73. With the longer stays/WB, the steeper ST on a traditional layout gets more weight forward and centered where it is needed for stability on dodgy surfaces. With your slack ST and short stays, it should loft pretty easily. Besides, getting the wheel up super easily does not a good wheelie bike make, necessarily. Maybe it is easy for that, but it would suck for other things, potentially.

    And by the way, thinking that "you know what a Fatbike handles like," based on your "other" Fattie, is not very accurate, in my less than humble opinion. I'm just guessing, never having experienced one, but I don't imagine they are very representative of what a quality Fatbike is like. You've created something else beyond out of it, but it's still a sow's ear at heart, no? Hopefully, with this opportunity you will create something of more enduring value, and quality.

    I might even offer that you build this design out of generic mild steel to test your practices and theories. Transferring the geometry of an apple to an orange may not turn out like you think. Neither might your fabrication plans. Turning expensive ChroMo into scrap is not what you are looking to do, I don't imagine.

    I have used 329.5mm for BB height. Yes, I like to tractor over whatever is in my path, and this height allows a lot of that. I love it. I don't buy into this low CG crap. Out in the real world, having to pay CONSTANT attention to pedal strikes is f-ing ANNOYING. The one thing I truly HATE about my Niner. My Fatties will blast downhill and corner just about as fast as I care to go as they are.

    Yes, full size prints are very nice. It's what I use to check my curved/bent tubes against.
    Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
    - John Hajny, a.k.a. TrailMaker

  10. #10
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    Thanks for taking the time to write all that.

    I am starting to think that starting with mild steel may be a better idea. That way, worst case, I loose about $20, but gain the experience. And then I will know if my ideas will work, or if they will build a bike that isn't what I thought it would be. Plus then I can experiment with a double bent DT.

  11. #11
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    doing the HF roller with Swagoffroad dies?
    I'm thinking the continuous seatstay will need a tight bend around the tire to contact the seatstay on it's way to the head tube. A roller won't get you that so you'll need a bender also JD2d, protools, anvil. Anvil now has a clr of 5.375" but the others have tighter radius's.
    cheer
    andy walker
    Walker Bicycle Company | | Walker Bicycle Company

  12. #12
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    Quote Originally Posted by blown240 View Post
    I am starting to think that starting with mild steel may be a better idea. That way, worst case, I loose about $20.
    The thinnest mild steel tubing you'll find in bike dimensions will probably be .065. I could see using it to prototype bends, figuring out your radii and centers, but for a bike it's going to be really heavy and will roll easier. Bends that you had working in .065 mild steel might not transfer to thinner, stronger steel. And the cost savings is only $5-10 depending on the tubing size, since you are buying twice as much metal as you need.

  13. #13
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    Building most of it out of straight gauge 4130 would be fine too. I think mild steel would be pretty silly. If you are anything like me, even if your first frame really does suck, confirmation bias will make it the most fun bike you have ever ridden for at least a year.
    A straight gauge bike will be rideable, a mild steel bike will be wall art at best.

    You could build it pretty cheap from aircraft spruce, maybe get a HT, ST and CS's if you don't want to deal with those.

    Experimentation is sweet. So many bike people are really set on certain things that have worked for them. Also remember to take internet advice with a grain of salt. Great guys on here, but I have certainly been lead astray before.

    If you think you want it, I think you should just go for it. If you're going to spend the time making it, you should get exactly what you want.

  14. #14
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    I just did a nice test ride on a Specialized Fatboy. Great bike, but it definitely felt too steep. It has a 70.5 HTA, Im thinking 67 will be perfect for what I want.

  15. #15
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    Hey;

    Remember, true Fatbikes are meant to be ridden on snow, or other marginal, shifting surfaces. You need tons of built in stability to do that well. That equals long stays, roomy cockpits, and slower steering. You are building a dry bike, so you can indeed cheat toward dry geo numbers. I wouldn't go too wild, but I also am interested to try something slacker to see how it works. I'd love to try an On One fork first, which bumps the rake by 10mm.

    It's a good point about mild steel wall thicknesses. I am genetically opposed to waste. I love metal so much that wasting it makes me nauseous. I wasn't bold enough to try a double bend on such an expensive piece of tubing, even though I wanted to BADLY. I agree that handling MS compared to ChroMo is a far different, and might not be very instructive, but you can still learn whether your geo ideas work, and end up with a fun beater. Probably no worse than a Walgoose!
    Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
    - John Hajny, a.k.a. TrailMaker

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