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Discussion Starter · #1 · (Edited)
Steel FS 29er design and 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-building/steel-29er-hardtail-v2-0-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/ ø755 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.














 

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My guess is that you’re close enough to the realm of normal that it’ll ride just fine. However, I’ll 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 you’ve got super long legs so more than likely not an issue.

Keep posting on this, I’m excited to see your progress!
 

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Discussion Starter · #3 ·
My guess is that you're close enough to the realm of normal that it'll ride just fine. However, I'll 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 you've got super long legs so more than likely not an issue.

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

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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.
 

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Discussion Starter · #5 ·
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.







 

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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 I’m obviously just eyeballing from the drawing and could easily be wrong.
 

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Discussion Starter · #7 ·
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 I'm 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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Discussion Starter · #8 ·
Just a quick update:

I've been very busy so this project has stagnated more than I'd have liked, but it's finally making proper progress. I jigged and tacked up the seat tube a couple of weeks ago and mitered in the front triangle yesterday. The chain stay yoke/main pivot parts are mostly done, I just have to check chainring clearance before I weld it up properly and then add the chain stays. Progress must halt now while I wait for some free time to machine the two rocker plates. Once they're done I'll be able to locate the lower shock mount, check shock clearance to the down tube, and then install the seat stays. At that point I can weld everything.


IMG_2245 by Callum Douglass, on Flickr


IMG_2210 by Callum Douglass, on Flickr
 

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Discussion Starter · #10 ·
Final update before powder coating: I've finished welding everything and gave it a quick test (just rolling off a few curbs because I haven't installed the bearings with retaining compound yet and I don't have a derailleur on hand). All appears to be well and I'll be moving forward finishing the project this week. I can't wait to ride it!



(Forgive the poor picture. It was just a quick snapshot I took as I was cleaning up this afternoon.)
 

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Wow! That looks awesome! As someone who's been putting the pieces to making my first full suspension frame, I'm really curious about your suspension hardware. Did you make all of the pivot parts yourself? I guess you made the rocker, so I'm assuming you made the other parts too, but would love to know if there's another source for that type of thing that I just don't know about!

Thanks, and thanks for sharing!

-Brendon
 

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Discussion Starter · #13 ·
I did make all the hardware. Screws and raw material were purchased from McMaster-Carr and I machined it all myself. I know there are a bunch of online services that will make parts if you provide specs and drawings, but I've never explored that option. I really enjoyed making all the parts myself!
 

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Very cool. I would make them, but am limited to a drill press as far as tooling, so I think I'll have to find another way. Hopefully one of these days I'll be able to make my own bits though!

Thanks much for the quick reply! Most appreciated.

-Brendon
 
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