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  1. #1
    too many bikes
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    The DT-B project (Brompton design-around)

    I suggest the now inchoate but cognitively percolating Downtube design-around of a brommie ("DT-B") have two folding points (the primary inventive step of the Brompton design), be aluminum and titanium (Volvo survived by going premium), and be sold initially as a frameset with a threadless headset (a la Swift) and a 130mm OLD (road bike hubs) in the rear dropout. Two folding points = pivoting rear triangle AND hinged maintube (hinge near the handlepost). Use the existing, excellent "Chinese hinge" as found on the existing DTs, not the value-engineered copros on the small Dahons.

    For the full suspension model, use a plate or thin wall channel titanium swingarm with detachable shock. The biggest barrier to packability of the DT is the width of the swingarm. Offer various grades of shock and forks, as is done with high-end mountain and DH bikes. Let SB11, me, and the many other builders on the forum get our hands on the frameset as "product developers". If Yan can negotiate a Taiwanese (e.g., SA) or Chinese gear hub deal for a narrower OLD and BB, a second version becomes available with a 20mm narrower rear triangle, gear hub, and 10 mm shorter BB width .... but don't hold back the DT-B project waiting for a 100 mm gear hub.
    Last edited by maunakea; 04-12-07 at 03:31 AM.

  2. #2
    Life in Mono
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    This is a positive direction - dont just copy, Improve ! Excellent !! I would add: make the frame in the same way the latest Birdy is made (sort of welded out of 2 halves) this will give the freedom to update the frame. V, disc or even Hub brakes. Pantour front hub ?? or at least decent ergo grips (I find the Brommy front end hard on the hands). Quick action folding joints (but ideally not as big and ugly as the current Chinese Downtube - maybe more like the Dahon Joints (especially stem), but more robust. 8 speed hub (or single speed lightweight option).

    But keep the weight down ! Its small but can feel like carrying a cube of steel around ... so I'd lay off the mountain bike type suspension.

  3. #3
    too many bikes
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    SS, I envision the usual lineup, non-sus, hardtail, and full sus. The hardtail would have MEKS-level sus forks to get rid of the tingly digit syndrome known to afflict brommie riders.

  4. #4
    Life in Mono
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    tingly digit syndrome known to afflict brommie riders

    true !

  5. #5
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    Quote Originally Posted by Simple Simon
    This is a positive direction - dont just copy, Improve ! Excellent !! Quick action folding joints ((or single speed lightweight option).

    But keep the weight down ! I'd lay off the mountain bike type suspension.

    Wanted to echo what Simon said here. A low weight single speed that folds like a dream and stays folded. Maybe offer a few different gearing choices during purchase. Perhaps just the ability to change them out, not all will be happy with a partcular gearing setup.

  6. #6
    Bromptonaut 14R's Avatar
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    We should just name this thread "the perfect dream folding bike". I want 10 of those.

  7. #7
    Seņor Mambo
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    Hey MK: does having the bike in parts (e.g. framesets) still violate copyright laws? If not, users could easily assemble them themselves, or have an LBS do it for them; I think it's cheaper in the long run and much less of a headache to sell it this way as well. But I'm not sure what kind of liability a seller is encumbered with, especially for crappy builds for which he is technically not responsible.

    The reason why I ask is because I like the frameset idea, but if you've never assembled/disassembled a B., then it can be daunting if you don't know how the parts fit together (so a manual or some sort is needed, which is not a problem, just an overall consideration). Nevertheless, here is why I also think framesets are key:

    - Within this year alone, I'm seeing retailers stocking specific Brompton wheel solutions (ETRTO 349) that were never really offered in the US before. If one could also make a deal with Merc, their spare parts access would also be a boon. I had to custom make a single speed wheel, but now I see ready-made 2-speed options and Merc's 3-speed option. For front wheels, I've seen regular wheels and wheels with hub generators.

    - For the original design, a brake upgrade is mandatory anyway (specifically levers and pads), but B. uses a small custom part to keep the cable routing in place. A zip tie is not as elegant though it may work.

    - Some may want the option of different speeds, and can find cheaper solutions themselves or make their own wheels.

    Other observations:

    - Wheel size: I wouldn't mind it being able to use larger wheels. This would also mean the frame needs to be redesigned with the main frame arch having a larger curve. In the past, I would never have suggested 18" wheels to anyone, but with more and more Birdy dealers in the US, I think 18" is becoming a more realistic option. Hopefully this will also allow the user the option of installing wheels ranging from ETRTO 349 - 406 (or even 451).

    - Handlebar design: The handlebar post on the original B. is deliberately designed with a slight curve and exact height to neatly fit against the forks when folded. This also means you don't get great extension if you're tall because you can't install an extended stem and still be able to fold the machine in 15 sec. One way around this might be to install a two part stem with a quick-release that will allow you to rotate the top portion of the handlebars 180 degrees - ā la Dahon - before folding: it would make the fold wider, and I don't know how it will affect cable routing, but it should give better reach. Adjustable height, however, seems compromised because of the aforementioned bend in the handlebar post.

    - Chain tension: The rear chain tensioner can be noisy and I'd be in favor of junking it entirely, but this would also mean the fold must incorporate the bottom bracket as well. This also means a different location for the rear hinge, which is no small feat. Also, the Brompton Yahoo group has noted that the rear hinge bolts wear down to the point that the triangle can have lateral sway at its worst. This is because the rear triangle is not locked down due to the rear suspension.

    - Hinge Screws: These things can be completely screwed out and need some sort of retainer to stop that potential disaster from happening. For the frame hinge screw, you can add a nylock nut to the end. For the handlebar screw, a nut is too tight to fit so you're almost limited to using a longer screw.

    So who is going to do the designing?

  8. #8
    too many bikes
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    SB, great ideas ... I hear experience talking. As for the copyright issue with a user-assembled bike, if the bike violates another's copyright when assembled, the assembler is the infringer, along with the manufacturer and parties in parts distribution to the degree the infringing elements were sold as a unit (called "contributory infringement"). The manufacturer, wholesalers, and retailers become "contributory infringers". But the goal is to avoid infringement, and it can be easily done: don't use the same design for non-functional elements.

    I love the idea of wheel size ranging from ISO 349 to 451... this means disc brakes, I think, but I love disc brakes. They quickly stop a bike loaded for touring in the wet and on steep downhills at speed. Can't say the same for the alternatives.

    So ... by detaching the disc brake, handlepost, RD, and rear triangle/swing arm, the user can change the fork and rear triangle to mount different wheels ... why not 24" wheels so that Airnimals are fair game. I would love to travel with 24" wheels (whatever the ISO is), but switch back to smaller wheels at home (where there are road bikes to choose from).

    As for designing, Yan goes to his sources in China and says, "Please give me a quote on building this", and he hands an AutoCAD file over. As a math prof, Yan is automatically an AutoCAD guru, right Yan?
    Last edited by maunakea; 04-12-07 at 12:33 PM.

  9. #9
    Raleigh20 PugFixie, Merc LittlePixel's Avatar
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    +1 on the hinge going around the bb - that way it'd never drop the chain when folding AND be a perfect candidate for the worlds smallest fixie; This being with a 120mm track-width rear dropout. I'd have one of those.

    As for designing the finer details of the frame - I'll happily do it...

  10. #10
    Eschew Obfuscation SesameCrunch's Avatar
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    Some great ideas here. You guys know your stuff.

    Notice that Yan is suspiciously silent on this topic. Maybe he's thinking hard...

  11. #11
    Life in Mono
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    +1 on Options for bigger wheel sizes (as long as folded size stays as close).

    +1 on the hinge in BB - and say goodbye to the chain tensioner, (and hello to easier puncture fixes).

    - Optional rear triangle drop down (this is a great feature, when you want it ie parking but a royal pain when lifting bike up). Make it operate remotely from the seat or bars and it would be even better.

    - now if was say 8Kg / 18lbs or less ........ sorry that's really dreaming !

  12. #12
    Seņor Mambo
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    Thanks for the explanation, MK. "Contributory infringer," huh? You guys have a name for everything!

    LP: attached is a design you may be able to work with; the trick will be hinge placement(s). And please do something about that ugly handlebar post/stem. The more spare parts that can be used from Brompton/Merc, Dahon, etc., the better; that way you're not stuck re-designing everything. Also, since Dahon has a 5 speed hub in existence, that could be used as a base. In addition, a smaller front hub of 73-74mm can become a new standard since that's what Dahon and Brompton use.

    One potential issue with a folding bottom bracket - it may severely squeak if not locked down, not to mention the loss of efficiency problem.

    Longbikes also has changeable options for rear dropouts. These could accommodate difference wheel sizes, disc brakes, etc. It's a pretty slick design, but will add weight.
    Attached Images Attached Images
    Last edited by spambait11; 04-13-07 at 10:38 AM.

  13. #13
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    Quote Originally Posted by SesameCrunch
    Some great ideas here. You guys know your stuff.

    Notice that Yan is suspiciously silent on this topic. Maybe he's thinking hard...
    I think he's more likely rolling his eyes and chuckling at a bunch of enthusiasts trying to author his 2008 business plan.


    I do think it's a worthy topic for discussion though, and that DownTube is the company most likely to listen to this kind of input (for a bike at this price-point).

  14. #14
    Car free since 1995 pm124's Avatar
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    You could probably fit a Birdy's wheels on an existing Brompton. The tire selection is decent, as you say, and the ride is a touch better with minimal fold compromise. Many Birdy owners use 349s because you have a wider rim and tire selection. All you have to do is lower the brake pads.

  15. #15
    too many bikes
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    New DT-B variant to add.... the stretch DT-B with a maintube as long as a Swift to get the Swift magic ride.

  16. #16
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    Quote Originally Posted by LittlePixel
    +1 on the hinge going around the bb - that way it'd never drop the chain when folding AND be a perfect candidate for the worlds smallest fixie; This being with a 120mm track-width rear dropout. I'd have one of those.
    Wrong approach. The Brompton's rear hinge position is what allows a longish wheelbase but a short folded size. Your preferred approach would results in wheelies and a bigger folded size.

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