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Thread: my downtube 8H

  1. #1
    rhm
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    my downtube 8H

    I don't remember if I've ever made a detailed post about this bike, but it's reached a certain milestone and I thought some might find it interesting. I guess we could consider this an obituary, but it's not dead yet.

    I've had several folding bikes, prominently a Downtube Mini that I rode 2007-2010, replacing its frame early in 2009, I think. Just about five years ago the second frame broke, and I moved up to a downtube 8H. I rode the Mini about 9000 miles, wearing out countless tires, one cog, one chain ring, one rim, and many pairs of pedals.

    I immediately customized the 8H to suit my needs; changed the suspension fork to a non-suspension one; added full fenders; rebuilt the front wheel with a dynamo hub and built LED lights for it; and changed the handlepost arrangement to optimize my riding position and the bike's folded size. This involved cutting some metal and replacing the stem. The stem is clamped with a hose clamp.

    It has been an excellent bike. I never had any trouble with the Sturmey Archer 8 speed hub. A few years ago the frame (which was red) cracked. Mileage at that time was 5600. Yan was very helpful and found me a replacement frame. The new frame was silver, and was actually older, and took the smaller seat post size, so I had to find a super-long 27.2 post, but other than that I just transferred all the components over from the red one to the silver one.

    Being about six feet tall, I needed the longest seat post I could find, and eventually assembled one (photo below). the bottom of the post is a chromed steel 27.2 post, about half of which is inserted into an aluminum seat tube saved from a discarded mountain bike; another 27.2 mm seat post is inserted into the top of that. Both inner posts are held in place with clamps recycled from discarded Razor scooters.

    Here's the bike as seen this morning:


    I have worn out countless tires, and one rim; a few years ago I rebuilt both wheels with Sun CR18 rims. I have also worn out and replaced the rear cog, the chain ring, and several chains. I don't remember how many pedals. I've destroyed one fender, so the fenders on it now don't match.

    Detail shot showing the front dynamo hub and headlight (which is an LED in a housing made from copper plumbing pipe):

    Note that the quick release lever is on the right side; this makes the bike fold up a little smaller.

    From an electronics perspective, my LED lights are caveman simple. Nothing fancy, nothing that can break; no switch, no standlight function. If the wheel is turning, the lights are on. Detail showing the tail light:


    Here it is, folded:


    When I fold the bike, I take the seat post out. It's just easier to deal with that way. Note that the handlepost hinge is on the left side, so the handlepost comes down between the frame halves; but the stem extends the bar so far out that the handlebar actually sits on the left side of the frame.

    Mileage on the computer, at present, 10,554 miles.

    Alas, all good things come to an end. Detail shot of the frame this morning:


    The hairline crack, just above the seat tube weld, spells doom. This is how both my Mini frames, and my red 8H, ended their useful careers. Note also the spots where the paint is rubbed off on the seat stay and on the main frame tube. This is where the handlebar and shifter rest when the bike is folded.

    Again, Yan has been very helpful about finding a replacement frame, but at present it is not clear whether another frame can be found. If so, it will be the very last one. If it doesn't turn up this weekend, I'll be ordering a new 8H on Monday.
    Last edited by rhm; 07-24-15 at 11:49 AM.

  2. #2
    Part-time epistemologist invisiblehand's Avatar
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    It's safe to say that you got your money's worth RHM!

    Thanks for sharing.

  3. #3
    Nobody mconlonx's Avatar
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    I am not: an engineer, pro welder, bike frame builder, or anyone but some bike enthusiast yutz who has put some time into being a bike mechanic and riding bikes a lot.

    However, seems to me that is a pretty typical break I see on bikes where the seatpost is not inserted past the top tube and seatstays on a conventional bike. I assume that is not the case here, but it is worth mentioning.

    The other part of that is the physics of the whole deal, where the seatstay junction and seat tube could be functioning as a pivot and lever to induce cracks of that nature. I wonder if it would help at all if the seat stay junction to the seat tube was raised 2-3 cm on the seat tube, leaving the top tube junction where it is. In effect, raising the seat stay junction higher than the top tube junction with the seat tube.

    Although scoping out the current lineup, looks like it has been addressed instead by reinforcing supports where a crack like this might propagate.

    In any case, congrats rhm for wearing bikes out. Looks like a good excuse to try a different one!

    Have you documented your home-brew LED setup, like over on the electronics sub forum?
    I know next to nothing. I am frequently wrong.

  4. #4
    rhm
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    Quote Originally Posted by invisiblehand View Post
    It's safe to say that you got your money's worth RHM!
    Oh, yeah. No question.

    Quote Originally Posted by mconlonx View Post
    However, seems to me that is a pretty typical break I see on bikes where the seatpost is not inserted past the top tube and seatstays on a conventional bike. I assume that is not the case here, but it is worth mentioning.
    Correct. My seat post inserted about 16 cm into the frame. The bottom of the seat post is about 1 cm below the bottom of the horizontal tube (top tub). The crack is between 8.0 and 7.5 cm below the top of the seat tube.

    In my opinion, the solution to this particular problem would be to redesign the seat post clamping mechanism. The present system, pinching the top of the seat tube around the seat post, invariably allows the seat post to rock around a little; that rocking motion inevitably fatigues the frame. I'm not an engineer either, but I can think of many possible ways to either expand the seat post or constrict a longer section of the seat tube so there is no flex.

    But, you know... this is not my problem. I beat this poor frame to death and I'm not blaming anyone but myself.

    Quote Originally Posted by mconlonx View Post
    Have you documented your home-brew LED setup, like over on the electronics sub forum?
    Yeah, if you search my old posts on this forum, electronics, and C&V, you will find I've explained it many times, probably never the same way twice. This particular light setup has the two LED's wired in sequence. Basically the two dynamo wires go to a bridge rectifier; the output from the rectifier goes to one LED, then to the other, then back to the rectifier. Since then I've found even simpler ways to make my lights, not even bothering with the bridge rectifier.

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