Straightening a section of bike wheel rim
October 1, 2022 2:00 PM   Subscribe

I’d like to take a section of a bike wheel rim and straighten it out. As in, I have a rim that I’ve cut in half so that it’s a big arc, and I want to end up with a long flat piece piece of metal. The rim is aluminum, box section, 700cc. How do I prevent it from buckling or cracking in on place under that pressure?

I know that this is physically possible. The aluminum is pretty flexible. I was thinking about constructing a frame that would hold the rim fast and then applying body weight or straps to pull it flat. Or straightening short sections at a time. But smaller sections I’ve experimented with have buckled, which I don’t want. Do I need to heat it? I’ve seen some discussion of annealing, which I could do but would avoid if possible.
posted by RandlePatrickMcMurphy to Sports, Hobbies, & Recreation (13 answers total) 2 users marked this as a favorite
Pack the interior with sand before you begin. Not sure how best to plug the ends.
posted by adamrice at 2:24 PM on October 1, 2022 [2 favorites]

I'm a bike mechanic, and I've been building wheels on-and-off since the 1980s. I've never tried to straighten a rim, mostly because my instincts say it won't work. I cannot imagine doing this without breaking the rim. I'm imagining a higher likelihood of success with a cheaper (single-wall, not anodized) rim but I still can't picture the inside of the rim stretching to be as long as the outer part without breaking.

I'll be curious to learn how it goes.
posted by workerant at 3:19 PM on October 1, 2022 [3 favorites]

Ask a local metalsmith? With wood, it would involve time, clamps, and wet sand from what I’ve seen. I would expect metal, even something softer like aluminum, due to the shape of the rim in this case (since it’s not just a flat strip) might need to have some heat applied, instead?
posted by eviemath at 3:47 PM on October 1, 2022

As far as I remember, these are manufactured in coil-like shapes. Or they were.

I honestly can't picture doing this without annealing. You'll need a pretty long heat treating oven, so... A blacksmith or other metal worker might anneal and later heat treat it for you? You can try just heating it with a torch but be prepared for it to almost work then crack. Sand packing is a smart move.

I've tried with a very small section, no annealing, just slooowly crushing in a big woodworking vise. It was a failure.
posted by Acari at 3:54 PM on October 1, 2022 [1 favorite]

Yeah you need heat. They’re heat treated after going from straight extrusion to round coils, so you need annealing to reverse that.
posted by supercres at 3:56 PM on October 1, 2022 [1 favorite]

Not really possible in my opinion.

The outer part of the half rim is, say ~1in further from the hub than the inside, so it’s pi times that extra distance longer than the inside of the half rim, or ~3in longer, which means you would have to stretch the inner rim and compress the outer an amount which added up to three inches.

I just don’t see that happening without remelting even in a more ductile metal than aluminum.
posted by jamjam at 4:40 PM on October 1, 2022 [4 favorites]

I think people are right about the geometry but also right about internal support. Thinking practically instead of theoretically: Cut it in 4-6 pieces and maybe you can get each one pretty straight. You can probably put them back together too, maybe with some reinforcement of some sort running through the interior of all of them. Also slower is always better than faster to avoid tearing and buckling.

Please post updates!
posted by SaltySalticid at 5:30 PM on October 1, 2022

I agree that this is unlikely to work. Aluminum is not that ductile a metal.
I also don't see anything lost by trying.
I'd put it on something heavy and rigid and tap it repeatedly with a big hammer. It might be best to put a small section at a time across something eight inches across - an anvil if you can figure any way to borrow one, a chunk of steel, the top of a concrete wall - and tap it in the middle.
Trying to do this in one shot will trap a huge amount of energy in the metal, and if it cracks there'll be shards of metal flying around. I wouldn't be happy in a room where this was happening. I think using straps is even more dangerous.
I don't think you can get one big deformation to get it straight, but you might be able to bend it a little at a time in different places. Put it so it's like an arch over something solid and hit it at the top of the curve. It'll bounce back, but you'll get a feel for how hard you have to hit it to deform it, hopefully without cracking it. Lots of small deformations are less likely to cause it to break than one major one.
People have mentioned annealing, but you'd have to do this a lot of times, because aluminum work hardens easily. You most likely can't heat and bend this because aluminum will most likely melt just as it softens. It's not like steel in this regard.
I'd suggest trying to involve someone with tools and experience doing this sort of thing. If you tell them you're prepared for it not to work they'll likely be happy to try. People with shop space don't like to get involved with projects that are obviously not going to work, but if you point out that you accept that and still want to try they'll probably be happy to help.
I assume that making something out of a rim is important to you, or I'd suggest buying a chunk of aluminum, which isn't that expensive.
posted by AugustusCrunch at 5:33 PM on October 1, 2022

You might be able to twist it into a straightish helix in which you accommodate the different lengths of the inner and outer rims by keeping the inner rim as close to the axis as possible.

I'm imagining some kind of tool with a projection which fits perpendicularly through a spoke hole and grips the sides that you would twist the rim with as you heat the section just behind with a torch, wait a bit for cooling, then move down a few holes and do it again — maybe two such tools and heat the section between them, and leapfrogging the tool behind over the one ahead and repeat. I don’t think very many full twists would be required and you’d probably want to make an estimated calculation so you didn’t get carried away.
posted by jamjam at 6:42 PM on October 1, 2022 [1 favorite]

I don't think it will work, but compared to straps, it's easier to try:

bolting it to a 2x4 or steel support through a few spoke holes, with standoffs (pipe sections cut to heights).

then hitting it with a torch,

then tightening a few bolts (or swapping standoff lengths), to force a new curvature

Iterate, repeat.

Might work. Anyway, the work-holding part is straightforward and doesn't get in the way of the torch.

Wear eye protection.
posted by sebastienbailard at 7:14 PM on October 1, 2022

If you just need a long, flat strip of aluminum (or steel), it might be easier to just buy one. You might even get lucky at a local Lowes or Home Depot. It's often sold where you'd also find things like threaded rods and bar stock. Aluminum rims seem to be extruded into a circular shape and welded at the butt joint. Cutting it and straightening would take a lot of effort and energy.
posted by 2N2222 at 7:23 PM on October 1, 2022 [2 favorites]

I doubt you'll succeed, aluminium is stronger than you realise. Nevetheless, more strength to your arm.

Not already suggested:
You could use a huge hydraulic press like they use to form many aluminium bike frames, which forces the metal to flow into the new shape. This lets the least amount of internal stresses remain in the metal without annealing it.

You could knock the braking rims flat first, because they're supposed to prevent exactly what you're trying to achieve (along with the spokes, they help the wheel remain a wheel).

Good luck!
posted by k3ninho at 3:07 PM on October 2, 2022 [1 favorite]

Response by poster: For what its worth this worked out quite well with a section that was about 1/3 of the rim circumference. I should have taken pictures but basically I made a form where that whole length of rim is trapped between two pieces of plywood just wide enough for the cross-section of rim, and then had a third piece hat was first cut into an arc with an effective radius of 28", so just slightly bigger than the original. I stuck a two pieces of pvc pipe into the hollow section instead of sand to discourage sections from just collapsing on me. I used ratcheting straps to pull the form together, then recut the form to a radius of 29", repeated, then 30", repeated, etc etc until I could just squeeze it between two parallel pieces of ply. It still bounced back so I gave it a bend in the opposite direction and its pretty much straight. There was some inward deformation of the cross-section, but not much, mostly I think the section actually got longer to accommodate the downward forces. I did look at videos on how rims are manufactured and from what I could tell heat was not used in the process. I'm planning to use this as the base of a custom rear rack, so there was some elegance in using a recycled bike rim for this. Plus its already designed to be incredibly strong and light which is what I needed.
posted by RandlePatrickMcMurphy at 6:00 PM on October 23, 2022 [3 favorites]

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