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  1. #1
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    Full suspension belt-driven bike project

    For a while I've been thinking about what would be the ultimate bike for myself. I was intrigued by the idea of a full suspension bike which uses a belt instead of chain. Since I also do a fair amount of commuting, I wanted it to be easy to swap in some road tires, which rules out the gear hubs. After finding this bike (Ram URT), I felt it would combine well with a pinion gearbox. After quite some time, this is the resulting design:


    Full suspension belt-driven bike project-bike3.jpg


    Full suspension belt-driven bike project-bikedimensions.jpg
    I decided on making this as a carbon-fiber frame, using wet layup over a 3d printed ABS core, with the idea that I can flush the ABS out later using acetone if I feel the need to shed a couple pounds. The suspension linkage will be made from aluminum. The tires are 27.5", which happen to have a similar diameter to 700c / 35. This bike will be quite heavy, with 33lbs being the optimistic estimate. I hope update this post with progress, and appreciate any comments / criticisms.

  2. #2
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    Well, a URT is certainly a way to make the belt work with suspension. But, um, have you ever ridden one?

    -Walt

  3. #3
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    Quote Originally Posted by Walt View Post
    Well, a URT is certainly a way to make the belt work with suspension. But, um, have you ever ridden one?

    -Walt
    I haven't, and have seen some articles about the Klein Mantra. The virtual pivot point on this design is close to the bottom bracket, so hopefully it will feel closer to a concentric rear suspension. Part of this project is my curiosity on how this bike will feel.

  4. #4
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    Well, good luck. IMO designing suspension around a drivetrain component is cray-cray but building stuff is fun even if the end result is silly. Keep us updated on your progress!

    -Walt

  5. #5
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    Quote Originally Posted by cluh View Post
    so hopefully it will feel closer to a concentric rear suspension.
    FWIW, concentric pivots which are effectively single pivots (i.e. not like a Specialized Demo) aren't particularly well regarded for their pedaling characteristics either.

    But +1 for keep us updated on the build.

  6. #6
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    Quote Originally Posted by cluh View Post
    Since I also do a fair amount of commuting, I wanted it to be easy to swap in some road tires, which rules out the gear hubs.
    I haven't priced a pinion lately but with all the hassle doesn't it come close to 2 Rohloffs? And there's the Products - Kindernay coming soon hopefully

    And if you're making everything yourself and complicated why bother with a belt*) when you could make a proper chaincase?

    *) ok I'm biased, for me Gates is on the same shitlist of companies life's too short to deal with as Pinion

  7. #7
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    Thanks for the feedback! Yes, in the end this may not ride the best, it may even ride terribly, but part of the fun is finding out.

    Quote Originally Posted by m-gineering View Post
    I haven't priced a pinion lately but with all the hassle doesn't it come close to 2 Rohloffs? And there's the Products - Kindernay coming soon hopefully
    It's closer to 1 Rohloff than 2, though I purchased the entry level Pinion for this project since it is an experimental build. And in your defense, Rohloff is possible to find used.

    Quote Originally Posted by m-gineering View Post
    And if you're making everything yourself and complicated why bother with a belt*) when you could make a proper chaincase?
    I think this one comes down to personal taste.

  8. #8
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    This is a 1/5 scale model printed out to find any small design flaws before I print full scale.
    Full suspension belt-driven bike project-img_20171127_170522.jpg
    I found that I needed to fix crank arm clearance on the chain stays, but other than that, everything is looking good.

    I've since printed out the full scale parts, in addition to receiving the aluminum linkage pieces, and should hopefully have a full scale mock-up this weekend.

  9. #9
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    Zerode Taniwha
    Attached Thumbnails Attached Thumbnails Full suspension belt-driven bike project-fullsizeoutput_549.jpg  


  10. #10
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    The position of the pivot is critical, as you know. The Mantra was known to be subject to brake jack. Other URT designs are supposedly susceptible as well, but depending on the placement of the pivot, it can be managed.

    Check with John Castellano about his SweetSpot designs. I ride one and like it very much as a single speed.
    "And crawling on the planet's face, some insects called, The Human Race..."

  11. #11
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    Quote Originally Posted by ladljon View Post
    Zerode Taniwha
    Yep. Since the Taniwha uses the same gearbox, I looked to it for dimensions, weight, and other design choices. I also got my hands on a single speed rear hub made by them, which I plan to use.

    Quote Originally Posted by chuckha62 View Post
    The position of the pivot is critical, as you know. The Mantra was known to be subject to brake jack. Other URT designs are supposedly susceptible as well, but depending on the placement of the pivot, it can be managed.

    Check with John Castellano about his SweetSpot designs. I ride one and like it very much as a single speed.
    This design is a bit different from the single pivot URT designs, so it may have a different sweet spot. I found this video, where the guy who created the original design explains how it functions.

  12. #12
    will rant for food
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    If I recall correctly - and this is like third or fourth hand info so take it as you will - the price for a Pinion used to be one and a half Rohloffs, but Pinion scaled up to achieve the one-ish Rohloff price.
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  13. #13
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    Here's the assembled bike.
    Full suspension belt-driven bike project-img_20180121_035353981.jpg

    It may look a little silly with road tires, but I haven't laced up a rear wheel for the mountain set yet.

    The frame is a combination of black and white printed plastic, along with some tape. It's a little ugly now, but I plan to cover it up with carbon fiber. The fit of everything is good, however the strength is a little suspect. The seat stays like to bow out when trying to compress the rear suspension, likely because they are only 1/4" thick. Carbon fiber will be layered on over the seat stays, but that will only add about 1/8" to the thickness. I'm planning to increase thickness at least to 3/8", possibly 1/2" before moving forward with the build. I tested 1/2" thick, and the improvement is substantial. Here is a comparison:
    Full suspension belt-driven bike project-img_20180123_033249.jpg

  14. #14
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    Don't you want to have pretty stiff seat stays for the high tension required by the belt drive?

  15. #15
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    Whoa, 2 crazy projects making awesome progress this week! What happened to the days of people showing up with zany MS Paint drawings only to inevitably disappear into the ether?

    /snark (sorry)

    Seriously, I've very impressed at the progress here. Two questions:

    1. Do you have a massive 3D printer or are you splicing together a bunch of pieces?

    2. I can't quite tell how the suspension is supposed to work. It may be the not-great resolution of the picture, but I can't tell where all of the pivots are. Not including the linkage, there's one concentric with the BB, 1 (or is it 2?) near the dropout, and 1 on the downtube (I'm referring to the pivot that's lower and further back, the higher, forward one appears to be just the linkage) What am I missing?

  16. #16
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    Quote Originally Posted by formula4speed View Post
    Don't you want to have pretty stiff seat stays for the high tension required by the belt drive?
    Definitely, it was on oversight with my design. It also showed itself with the 1/5 scale bike, which I ignored at the time. Luckily, it'll be easy to fix at the current stage.

    Quote Originally Posted by Feldybikes View Post
    1. Do you have a massive 3D printer or are you splicing together a bunch of pieces?
    I'm splicing it together with pieces printed from a mid-size printer(10" x 10" bed). I looked into sourcing out the print so it could all be 1 piece, but the cheapest I found was $1300. I'm using ABS, which can easily be "welded" together using acetone.

    Quote Originally Posted by Feldybikes View Post
    2. I can't quite tell how the suspension is supposed to work. It may be the not-great resolution of the picture, but I can't tell where all of the pivots are. Not including the linkage, there's one concentric with the BB, 1 (or is it 2?) near the dropout, and 1 on the downtube (I'm referring to the pivot that's lower and further back, the higher, forward one appears to be just the linkage) What am I missing?
    The are no pivots other than the linkage. The holes near the dropout are for the swivel dropout and the breakaway. I've made this animation to help clarify how the suspension works.

  17. #17
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    Quote Originally Posted by cluh View Post
    The are no pivots other than the linkage. The holes near the dropout are for the swivel dropout and the breakaway. I've made this animation to help clarify how the suspension works.
    I got very confused somewhere along the line. That clears it up. Thanks.

  18. #18
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    I'm long overdue for an update so here's where I'm at. The chain stays and seat stays are now twice as thick. I decided it was easier to add onto the existing frame rather than reprint it.

    Full suspension belt-driven bike project-img_20180303_231926.jpg
    You can see the black piece glued on to thicken the chain stay.

    The aluminum additions proved to be more difficult. I got my previous parts cut by waterjet, but there weren't enough new parts to justify the cost a new order. It took me a few weeks to cut them out with a hacksaw in my free time. The pieces with varying thickness were most difficult. It took many cuts, but the result is satisfactory. It still has a few deep scratches where I overcut, but it will be covered by carbon fiber.
    Full suspension belt-driven bike project-img_20180218_191822.jpgFull suspension belt-driven bike project-img_20180303_233043.jpg

    I also took the time to try and lower the weight of these parts since they are nearly 3 pounds. I drilled many holes on the backside, and was able to shave off a little more than half a pound.
    Full suspension belt-driven bike project-img_20180303_170524.jpg
    Full suspension belt-driven bike project-img_20180205_165701839.jpg
    Since then I have sent these parts off to get anodized.

    The next step is will be laying up carbon fiber. I haven't done this before, so I'm taking the process slowly. This week I practiced using burlap, since it has similar bending properties to carbon fiber. After wrapping, I unwrap and trace the fabric shapes onto paper, so I can use them as a pattern when laying up the real thing.
    Full suspension belt-driven bike project-img_20180303_170235.jpg
    Full suspension belt-driven bike project-img_20180303_165534.jpg

    What's next? I'm sanding down the upper frame before wrapping it with burlap. I learned with the lower frame that the rougher areas are more difficult to remove tacky spray from. From there I will make the patterns for the upper, and research a bit more to calculate the amount of carbon fiber to order.
    Last edited by cluh; 03-04-2018 at 03:59 AM.

  19. #19
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    Are you vacuum bagging the carbon? I would recommend doing a test run or two over another piece that has some similarities in shape to your frame pieces. That'll give you a good idea of how the carbon behaves once wet out and how the bagging goes.

  20. #20
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    My hope is that I can get it done with a combination of heatshrink tape and open cell foam. I do plan on doing a test or multiple to get a feeling for if that approach can be successful. If not, I can buy the equipment for vacuum bagging, and and do some test runs. I expect that part of the process to be a significant challenge, especially with the complex shape of the rear triangle.

  21. #21
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    Here's the straight dope on heat shrink tape: plain unchanging profile tubing only. The heat shrink tape does not warp without wrinkling.

    Hate to be the bearer of bad news. But I've been down that road before and could not make a good go of it on tubing profiles that changed shape along a length. However... I never used foam as an insulator layer between the surface carbon and the shrink tape. So, try it, and post up the results. Maybe you'll have more fortune than I.

    Your project is still cool as hell. I'd just look into vac bagging if I were you as a reliable plan B.
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  22. #22
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    Hi Cluh, interesting project.
    I am undertaking a full suspension carbon build myself HERE.

    Careful with heat shrink tape and even vacuum bagging. With the tight/complex shapes of a bike frame (positive curves to negative curves, small details, interior corners, dramatically changing cross sections) I found both of those methods were very challenging to apply predictable, even, and constant compaction.

    Are you cooking pre-pregs or using wet layup? Avoid heat shrink with wet-layups. PITA.

    I ended up using polyester elastic waistband material wrapped around my wet-layup frame, and it worked incredibly well. Good de-bulk, no sliding or wrinkling, it soaks up the excess resin, and it pulls off easy. The only challenge is flat areas or spots with negative curvature. I used clamps and blocks to compress there, but vacuum bagging could be supplemented too.

  23. #23
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    Quote Originally Posted by Drew Diller View Post
    Here's the straight dope on heat shrink tape: plain unchanging profile tubing only. The heat shrink tape does not warp without wrinkling.
    +1 on avoiding heat shrink.

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