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7005 Alu frame welding- Worth trying?

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    7005 Alu frame welding- Worth trying?

    Hi All,
    I am trying to build up a BBSHD 52V full suspension bike. I have an ST3 ESR 3500 to use as the base bike http://www.a2xtreme.com/2000/0019.htm . I went for it as it has a monoshock raised chainstay design so no chainstay/ BBSHD fit issues, and because the (almost unused) bike was significantly cheaper than its 20 year old but immaculate Mavic/ Shimano wheelset and Avid Arch Rival brakes will sell for.
    Due to the thickness of the downtube (Deep ovalised tube runs an inch or so lower than the BB), the BBSHD will not rotate forward at all leading to a very poor ground clearance. (just under 8 inches)

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    Now, the frame, being 20 years old and a non big name brand, is I guess no longer a high value item compared with the components that were on it. I'm wondering if I can cut an inch deep by 6 inches or so section off the bottom side of the downtube near the bottom bracket, and get a plate welded on to reinforce the area? Thus allowing the BBSHD to swing up nicely away from the ground- as designed. Or even just make the cut indicated and tape up the hole with carbon, and epoxy and hope for the best???!!
    Being an ebike, I won't be putting exactly the same stresses on it that hard pedalling would.

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    I could then shotblast and lacquer the yellow frame so it has a nice retro frame look and to get rid of all the burnt paint after welding.

    Anyway, I'm thinking ahead a few weeks. My first move will be to fit the BBSHD as it is and use it for a bit as is, (still waiting for the battery which is on a slow boat from China) just firetrail stuff not use on extreme off road due to ground clearance issue). See if I like the bike's handling with the motor. If it's a dog anyway, it's not worth doing, but I did ride the bike a few times as a conventional MTB last week before I started stripping it and I did quite like it, but who knows what it will be like with all the new extra weight.

    Anyone know If getting 7005 aluminum bike frame welded is a realistic prospect? Some things I have read suggest 7005 is the easier of the two bike aluminums to weld, due to post heat treatment not being required. Obviously my concerns are the frame breaking the moment I try to ride it, or the alteration being uneconomically expensive. Or just ruining the frame, which does have some value.

    I do have a backup plan in the shape of a 2006 (Skinny tubed cromolly) Kona Lavadome, which I know for a fact takes the BBSHD with motor rotating forward properly as designed, as there is a forum post somewhere with pics of one someone else has done. But that's a hardtail of course, and has the worst forks I have ever ridden so is still my Plan B at the moment.

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    Eagle eyed viewers will spot that I'm currently also short of back wheels. They are both away being trued/ rebuilt for discs.
    Attached Files

    #2
    Really hard to say. Are you planning to do the welding yourself? Local welder? Some other source? If you're planning to have it done, guess I'd start by calling around, explaining what you want, seeing if it is doable by the shop you choose and what they'd charge. You may find no one wants to touch it, or cost prohibitive which sort of shuts down the show right out of the gate.

    Seems like most any way you go, you're going to be cutting out a substantial portion of the structure. When you pedal, or run the motor, you're essentially pulling 'forward' on the chain, meaning the pedals/motor want to move backward. Right now, you have a fairly thick structural member of the downtube preventing that...but once you cut along the scissors line, you reduce that strength. Will it be reduced too much? ...essentially impossible to say.

    I guess there is no possibility the motor fits on the 'back' side? Seems like that stubby yellow pipe isn't really doing anything and could be removed for extra clearance? Though suppose it is pretty tight with the rear wheel.

    Other option might be to sell every thing you can't use, consider what you would have spent for a weld, and apply that money to a frame which works. You might come out ahead in the long run.

    Comment


      #3
      Must be a very special frame to go to the trouble of altering it. Sounds like a good project if you want to learn welding. Otherwise there area slew of URT bikes out there. Did you know that URT's compress their suspension travel when you apply the brakes? When you apply drive line torque the suspension expands. Some people like that combination. Others like fully active 4 bar linkage which can be tuned for fully active suspension.

      I tried to fit a BBs2 to a hard tail that had an open flanged down-tube junction with the bb (This was the internal cable routing down-tube exit point.) It put the motor in a low position, kind of like the way your motor sits. I considered cutting the flange but ended up putting the motor on a bike with a traditional bb down-tube junction and built a nice grave / x-country rig with the hard tail frame. The danger with my method is that you end up with more bikes than originally planned for. Good luck. Be careful riding around with that motor hanging low. Keep us posted on what you end up doing.

      Comment


        #4
        Originally posted by Jakey555 View Post
        Hi All,
        I do have a backup plan in the shape of a 2006 (Skinny tubed cromolly) Kona Lavadome, which I know for a fact takes the BBSHD with motor rotating forward properly as designed, as there is a forum post somewhere with pics of one someone else has done. But that's a hardtail of course, and has the worst forks I have ever ridden so is still my Plan B at the moment.

        Click image for larger version

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        Eagle eyed viewers will spot that I'm currently also short of back wheels. They are both away being trued/ rebuilt for discs.
        Understand that I know virtually nothing about welding, so please take my comments with that in mind. However, if it were me, I just wouldn't take the chance of cutting up that frame. A catastrophic failure at speed could be life altering. Yes there are risks with any level of cycling - electric or not. I would just err on the side of safety and try to mitigate those variables. I would consider another frame if you really want full suspension. Me? I would find a quality used suspension fork for the Kona and build that bad boy up ... especially if others have already done a build with success. You can always add a suspension seatpost later if you find the hardtail too jarring. I've hit the dance floor at speed, and I'm honestly lucky to be here typing right now. And you may not be saving any money based on the cost of the chop and weld. Good luck either way you decide, but please just give it some thought.

        Comment


          #5
          7075 Aluminum is the most common 7000 series.
          7075 is NOT weldable.
          I would NOT use any 7000 Alumnium.

          I have done Heliarc and Tig for 50 years.

          Don

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