Looking for input on steel FS 29er design/build- Mtbr.com
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  1. #1
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    Looking for input on steel FS 29er design/build


    I'm designing and plan on building a mid travel 29er this winter. A few months ago I built a couple of hardtails (
    https://forums.mtbr.com/frame-buildi...a-1092402.html) and that inspired me to embark on this project. I'm shooting for something along the lines of a 160/140mm travel trail bike for the front range of Colorado. I already have a 160mm Fox 36 and a 215x63mm Fox DPX2 for this build and I'm aiming to have it clear 29x2.5" tires. The drivetrain will be 1x12 SRAM GX or NX Eagle and will be 148mm boost and use a 73mm BSA bottom bracket. Here are some geometry details (all dimensions are with the bike unsagged):

    Rider height: 6' 2" (188 cm)
    Rider weight: 150 lbs (68 kg)
    Rider ape index: +3" (+7.6 cm)
    Rider saddle height: 815 mm

    HT Angle: 65˚
    ST Angle: 76˚
    Stack: 653 mm
    Reach: 490 mm
    Chainstay Length: 435 mm
    Front-Center: 839 mm
    Wheelbase: 1270 mm
    BB Drop: 25 mm
    BB Height: 350 mm (w/ Ý755 mm tires)
    ST Length: 450 mm
    HT Length: 130 mm (plus 20 mm headset)
    Fork Axle-Crown: 567 mm (160 mm Fox 36)
    Fork Offset: 51 mm
    Stem Length: 35 mm


    My primary questions at the moment are about the kinematics. I want this bike to pedal very well as I enjoy climbing. I'm under the impression that I want to aim for just over 100% anti-sag at the sag point. The design as I've shown it below is around 130% anti-sag at 25% sag. Will this result in an overly insensitive rear end when climbing? And my leverage ratio goes from 2.75 to 1.9. Will this be too progressive for an air shock?

    I'd appreciate any input you can add and any suggestions or comments are more than welcome.

















    Last edited by CallumRD1; 01-01-2019 at 08:03 AM. Reason: Added hyperlink to previous build

  2. #2
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    My guess is that youíre close enough to the realm of normal that itíll ride just fine. However, Iíll mention that doing a faux-bar instead of a Horst link can simplify things a bit for yourself. It also looks like your shock mount is buried inside your downtube. If this is the case, you might consider if the couple $hundred towards a different shock length (e.g. 200x57) may simplify your life enough to be worth it. You could get the same travel with a slightly different rocker geometry.

    I was also going to question if you left enough room for dropper insertion, but then I looked at your previous post and youíve got super long legs so more than likely not an issue.

    Keep posting on this, Iím excited to see your progress!
    I make bikes. www.feldybikes.com

  3. #3
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    Quote Originally Posted by Feldybikes View Post
    My guess is that youíre close enough to the realm of normal that itíll ride just fine. However, Iíll mention that doing a faux-bar instead of a Horst link can simplify things a bit for yourself. It also looks like your shock mount is buried inside your downtube. If this is the case, you might consider if the couple $hundred towards a different shock length (e.g. 200x57) may simplify your life enough to be worth it. You could get the same travel with a slightly different rocker geometry.

    I was also going to question if you left enough room for dropper insertion, but then I looked at your previous post and youíve got super long legs so more than likely not an issue.

    Keep posting on this, Iím excited to see your progress!
    Thanks for the response. I havenít measured up my dropper to confirm fit yet, but I donít anticipate any issues at my saddle height. I havenít finished making a proper cad drawing of the bike to confirm fit yet, but I am planning on using a 15į bent down tube to provide more space around the BB. Hopefully that will be enough space for the shock. If not then Iíll probably get a different length shock. I got a great deal on this one so flipping it shouldnít be hard.

  4. #4
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    Your CG in that model looks like it's too low, when you compare your model in Linkage to other bike your numbers are not going to be comparable unless they both have close CG.

    I like lots of anti-squat so those numbers look good to me, but it's personal. Also a reminder to set the front chainring and rear cassette to what you plan to use.

    Your LR looks very coil friendly, will be fine with air if you're not looking for a plush couch. If you want it to be plush deeper in the travel I'd look at flattening out the LR a bit.

  5. #5
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    Thanks for the suggestions. I confirmed that my 170mm dropper will fit with ~2 inches to spare. I updated the drawing to illustrate where my saddle will be and where I plan on putting the seat post clamp.

    I did end up finding it difficult to fit the shock in and a good deal on a 200x57mm DPX2 came around so I sprung for it. (I don't really have any plans to try a coil any time soon and I like a more progressive rear, so I'm thinking that the falling leverage ratio won't be too much of an issue for me.) This change is represented in the current iteration of the design. It is considerably easier to fit the shock above the down tube now. The redesign gives me 143mm of travel which is right around what I'm looking for.













  6. #6
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    Clearance between the shock and seat tube looks tight. Probably okay with a regular air shock maybe not with a large can shock (e.g. fox X2 or Cane Creek) or coil. But Iím obviously just eyeballing from the drawing and could easily be wrong.
    I make bikes. www.feldybikes.com

  7. #7
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    Quote Originally Posted by Feldybikes View Post
    Clearance between the shock and seat tube looks tight. Probably okay with a regular air shock maybe not with a large can shock (e.g. fox X2 or Cane Creek) or coil. But Iím obviously just eyeballing from the drawing and could easily be wrong.
    Thanks for the reminder. I created a reasonably accurate 2D CAD model of the proposed frame with my DPX2 and had to tweak the geometry to get it all to fit. It's tight, but it does fit now. This also allowed me to tweak the leverage ratio curve, flattening it out a bit which should work well with my air shock.






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