bending aluminum frames

Discussion in 'Motorized Bicycle Welding, Fabrication and Paintin' started by Earbiter, Feb 20, 2015.

  1. Earbiter

    Earbiter New Member

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    My mountain bike has a strange design for the seat stays and I am trying to put a larger sprocket on it but the chain is rubbing. So I was wondering if anyone has had success bending an aluminum frame, or are there any other methods out there? I think I may need to use a shim or two as well to move the sprocket towards the center of the wheel, but that only goes so far. I'm talking about 1/4 of an inch total that the chain needs to move.
     
  2. Chainreaction

    Chainreaction New Member

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    As a general rule it's not a great idea to bend aluminum. Steel generally handles bending well but aluminum fatigues and breaks sometimes with not much bending at all. I am no expert just personal experience. If you do you decide to bend you need to bend the opposite side too to maintain alignment. You might get away with it if its a gentle bend spread over a large area. Really need pictures to see what you have going on.
     
  3. maniac57

    maniac57 Old, Fat, and still faster than you

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    I simply use an old axle to hold the dropouts in the proper position and use a small car jack to spread the frame where needed for clearance. Add a couple boards to spread the loads and avoid kinking the frame tubing.
    The small amount you need should be no problem at all.
    But remember aluminum does not bend like steel and will work harden and crack if bent too much.
    Only do it once.
     
  4. Davezilla

    Davezilla New Member

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    Aluminum can be bent a little, but not a lot... trying to get 1/4" bend in a 1 foot or larger span should be no problem, just don't heat the aluminum to get teh bend because these alloys are also heat treated in order to keep their hardness and strength, it's this same hardness that makes bending the aluminum a bit on the risky side. I'd say no more than about 15 to 20 degrees for a bend and keep the bend radius as loose as possible.

    Most aluminum bike frames are made from 6061 T6 for it's strength and corrosion resistance, 6061 can also be welded on but needs to be re heat treated again to bring back the hardness and strength.

    If you need to bend out the rear forks to fit a wider tire or for sprocket clearance, it can be done but do it a little at a time to prevent cracking, and also, you don't want to stress the aluminum by drawing it in with the bolts such as widening the rear for more clearance but going wider than needed then using the wheel nuts to bring it back in during assembly, you can use washers to take up this space or bend it back just a little closer so it's not under any stress after things are assembled etc.

    One nice tool for bending rear sections is an exhaust manifold spreader for a small block Chevy... like this one... you can sometimes find these at auto parts stores like O'Reily's or Pep Boys etc... http://www.ebay.com/itm/Lisle-13000...471?pt=LH_DefaultDomain_0&hash=item35d960636f or you can order one from ebay or Amazon if needed, but these will do most frame bends when you need to spread something out since these are nice and controlable.
     
  5. Earbiter

    Earbiter New Member

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    Thank you all for the tips. I will post pictures as soon as I get a chance, this game just has a funky shape where it curves inward. Looks good but no so good for mbs, I won't do anything till i get pics up and an opinion on trying it.
     
  6. maniac57

    maniac57 Old, Fat, and still faster than you

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    I know exactly what you mean. I have had this coke bottle shape on several steel frames and had to do what you are asking about for chain clearance.
     
  7. 2door

    2door Moderator
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    1/4" isn't much. Have you considered a dished sprocket. The teeth are off-set from the center so you might be able to get close to your 1/4" goal by using one.

    So if I understand you correctly, the chain is hitting the seat stay, not the chain stay? Your engine must be mounted high in the frame?

    Some photos would certainly help.

    Tom
     
  8. Earbiter

    Earbiter New Member

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    Yes a coke bottle describes it perfectly. The sprocket is dished and I have it inward. The bike is a 29er and therefore a strange shape. So the motor is somewhat high in the frame, I have just shaved down the rear motor mount 3 mm to bring it down some. I plan on using the metal pieces (rag joint) that came with the kit to shim the sprocket more. I'm at work so I can't do anything now, but I can post pics. BTW I'm using a top hat adapter and a 48t sprocket.
     

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  9. maniac57

    maniac57 Old, Fat, and still faster than you

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    EXACTLY the same shape as the pair I had to mod. Hitting in the same place and all.
    Simple fix, but again, be careful since it's aluminum.
     
  10. Earbiter

    Earbiter New Member

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    Good to know I'm not the first person to have this problem
     
  11. Earbiter

    Earbiter New Member

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    I have made some progress. The sprocket will now sit another 1/8th of an inch inward and I think the engine should sit about a 1/2 an inch Lower in the frame, since I removed 5 mm from the rear motor mount and bent the front mount plate a little to help it sit flush. It's good that I had to do all this because in the process I found out I had the motor mounted wrong the whole time. The rear mount was not flush with the seat tube. Now solved (I hope). If all these adjustments don't solve my clearance issue, I will go ahead with bending the frame. Just want to make sure I've exhausted all other options first. Thank you all for the advice.
     

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  12. Davezilla

    Davezilla New Member

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    That's one of the things I had to do to my rear mount but I had to remove about 1/2" from it before it would put my engine down low enough in my frame. You can shave off a lot from the rear mount quickly if you have a belt sander or a small mill. I milled mine down on my lathe by mounting it to the tool rest and used a 1/2" end mill for most teh cutting, then chucked in a fly cutter to finish it off nice and flat.

    A belt sander will cut it down really fast too, but you'll need some welding gloves to hold it or some really good channel locks because that little piece of aluminum will heat up in your hand rather quickly trying to hold it down on a belt sander.
     
  13. Earbiter

    Earbiter New Member

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    I used a bench grinder on the side of the stone went pretty quick. I would go lower in the frame but have some clearance issues there too with the clutch arm and carb hitting the seat tube.
     
  14. Earbiter

    Earbiter New Member

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    Well I bent the frame, slowly, got the clearance I need. Ty all for the help. That stock 44 was just too high for the 29 and all the hills around here.
     

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