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Old 12-10-09, 01:15 PM   #1
flesh_pile
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Modifying/welding custom cranks

I've recently made the switch to custom 200mm cranks on my daily rider to fit my long legs 'properly' and would like to make the switch on all my bikes. Rather than pay $450 to $600+ per set for low tech square taper cranks that only come in 130bcd, I was thinking of welding extensions onto standard cranks that I pick up dirt cheap on eBay. I discussed this with my uncle who is a supervisor at a machine shop and he thought this would be a good and cost effective way to go about doing this. He doesn't have any bike experience though and as I'm sure some of you framebuilders out there have aluminum welding experience, I'd like to hear your opinions as well.

What would be better:

1. Cutting the pedal threading off, welding on one piece, and drilling/tap a new hole for the pedal threads?
or;
2. Cut the crank in between the spindle and pedal hole, then weld the extension in the middle.

What are the pros and cons of both aspects as far as cost and strength/durability? One weld but with drilling and tapping or 2 welds with no drilling? Are cranks usually 6xxx or 7xxx series aluminum? Does it matter when welding and are cranks sometimes made from different alloys? How many hours of labor per crank do you estimate this would take? What type of general labor rates would I be looking at most likely if I went to a framebuilder? Or would a general welder/machinist with aluminum experience be a better bet?
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Old 12-10-09, 02:11 PM   #2
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Welding extension into a forged aluminum crank sound like a bad idea. I doubt it would be anywhere near as strong as the original forging. I would suggest you start with a tubular CrMo steel crank (such as bullseye ). Chromoly tubing could be sleeved and extended much easier than forged Alu.
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Old 12-10-09, 08:44 PM   #3
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If you have access to a machinist (your uncle), I'd look into having him make a copy of your existing crank.

Welding extensions on cheap (???) ebay cranks would likely end up costing more than materials and labour for a good welder to add extentions, tap them for pedal threads, and heat-treat the crank so it's strong. I also don't know anything about how easy it is to weld a piece of alu to a forged crank. On top of that, since you're shelling out money for the welding, you'd want it ground down nicely and possibly finished... so yeah, I think a cheaper and better solution would be machined custom cranks by your uncle. You can have them made to whatever BCD pattern you'd like.
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Old 12-10-09, 09:30 PM   #4
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If I were to extend a crankset I would probally avoid a process that required redrilling and tapping, the problem is the angle needed is too much set up time to be worth it, we all know how wonky a pedal is if its bent a little, right? If you get that hole a little off youre screwed. I would be apt to pick up two cheap sets and make one set out of them, one joint, maybe cut at a long angle so there is more weld surface and alignment is easier, you could make a 300mm set out of two pairs of 170's if you wanted this way.
heck, give it a try, It may work out great or it may seem like a custom set is cheap. Try anyway, you never know.
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Old 12-11-09, 04:44 AM   #5
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Forged aluminum cranks are often 7075 or some other unweldable alloy of aluminum. I think tubular steel, like you see on BMX cranks, is a more feasible method of construction for a custom project. Or CNC machined as suggested above.

Last edited by zzyzx_xyzzy; 12-11-09 at 05:12 AM.
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Old 12-12-09, 02:00 AM   #6
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Hmmmm.... I forgot that the cranks are usually forged. I'm not sure how welding to forged pieces works, especially as forged materials are usually much stronger than cast or CNC-ed pieces which probably would not be a problem welding too. Perhaps welding two cranks together could work as suggested below? I think I'll check in with either a materials and/or a welding forum to see what people's opinions are on welding forged pieces together. I guess I could always purchase some KCNC cranks as they are not forged.
As far as the cranks themselves go, I think KNEEL brought up a good point about the precision needed in the pedal threading angle. As far as CNC-ing cranks, that was my original idea that I had initially discussed with my uncle and he had suggested welding instead as a cheaper, better, and less time consuming option. I have CAD skills (I'm an architect) as well as the specs and diagrams for both ISIS and BB30 interfaces so I could do the design work, but I would have to spend a while drafting and designing something that I have no real idea will actually come out properly. It would be a bummer to spend hours designing something, then hundreds of dollars getting it CNCed just to find out something is off and I have to start over. I was thinking I could bring it to a welder and have them do the work. If they don't do it properly, I could just chop the shoddy work off and start over (with a new welder!)
I didn't mean 'cheap' cranks, but rather 'inexpensive' nice cranks found used on eBay. I was thinking a BB30 set of the aluminum Cannondale SI cranks for my race bike as they are quite nice, light, and have a good profile to them that would be conducive to welding a new piece on there (it's oval shaped like a velodrome - it doesn't have the slight indentation on the insides of the crankarms like most cranks). For my other bikes, any cranks would do, Deore/LX level or Sora/Tiagra would be fine.
I'm not familiar with BMX stuff, so I'm not sure whether I could fit different spiders on those tubular steel cranks to fit my different needs (or what type of bottom brackets they use and if they would fit in my BB shells). For my touring, mountain, and even track bike, I wouldn't be too concerned about weight but I'd rather not have a huge beastly crank on my racing bike. I'll look into it...
I'll also get back with info and opinions from a welding/materials forum...
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Old 12-12-09, 02:29 AM   #7
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Here's some info on the Cannondale SI cranks:

Crankarms are constructed from a 2000-series aluminum that has twice the fatigue life of 7000-series aluminum.

Crankarms are mated to a thin-walled, hollow, oversized 30mm aluminum bottom bracket axle for superior balance of strength, weight and stiffness.

The two-piece, hollow crankarms are CNC-machined for optimum strength and light weight


So it looks like the crank arms themselves aren't forged, even though the stock that they are CNCed out of could be. If forged aluminum can indeed be welded, I could just use a chunk of forged aluminum cut and shaped down to the extension size needed to be welded on.

2024 series aluminum is unweldable, other 2xxx series are weldable. I guess I'd have to finagle the info out of Cannondale to be certain what they're made out of before I drop any cash if I decide to go this route.
http://en.wikipedia.org/wiki/Aluminium_alloy
http://en.wikipedia.org/wiki/2024_aluminium
http://en.wikipedia.org/wiki/Duralumin
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Old 12-12-09, 09:35 AM   #8
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Seems the cheapest though not too elegant method would be to machine something like these crank shorteners, but extend it past the end of the cranks.
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Old 12-12-09, 02:05 PM   #9
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Hmmmm.... I forgot that the cranks are usually forged.
Good cranks are usually forged. Cheap ones are cast (aka "melt-forged" in marketing parlance).
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Old 12-12-09, 06:43 PM   #10
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I don`t know about the weldability of diferent aluminums- sometimes we do aluminum repairs at work and we just kind of cross our fingers and stick it together, so as you can guess we talke a pretty "Flintstone" approach that wouldn`t cut it for critical applications like that. Still, with the hollow s in the "good stuff", I don`t think you`d find somebody who knew what he was doing to accept that job for less than what you`d pay for custom arms.

I didn`t know there were cromo arms out there. If anything, I`d guess the best chance for "stretching" would be that route.

Machining a set from scratch wouldn`t be any easy affair either and I still doubt you`d end up with hollow arms or fancy BBs. Drilling and tapping the pedal hole is the least of the problems- you`d almost have to buy a custom broach for the taper interface, which you by itself would run a lot more than the $450 - $600 you`re looking at from Zinn or wherever. For Isis or Hollowtech, it would be a fortune- the only advantage you`d see is getting the BCD you wanted.
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Old 12-15-09, 07:55 PM   #11
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Try here http://www.nettally.com/palmk/crtips.html or just google "custom/long bicycle crank arms". Not sure if you'll find cheap + custom from the same place but worth a try.
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Old 12-16-09, 03:45 AM   #12
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An aluminium crank is usually pretty darn thick, and it would take a really skilled welder on a powerful machine to get enough penetration to safely weld on it. I'm not skilled, but I have a bit of experience, and anything above 1/4 inch makes me a bit nervous about weld quality. Cranks are usually much thicker.
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Old 12-16-09, 06:35 PM   #13
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An aluminium crank is usually pretty darn thick, and it would take a really skilled welder on a powerful machine to get enough penetration to safely weld on it. I'm not skilled, but I have a bit of experience, and anything above 1/4 inch makes me a bit nervous about weld quality. Cranks are usually much thicker.
Google up Marschall framework, they have a set of cranks they make out to 220mm i think, check them out.
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Old 12-23-09, 04:43 PM   #14
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You could buy one of your ebay cranks,weld the piece in one side.Put it in a press and stress it until it fails,then do the same to the one you didn't touch and see what the difference is.
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Old 12-23-09, 08:05 PM   #15
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Sounds like there is no inexpensive way, other than making some sort of bolt on pedal extender attachment like the ones shown. Having seen the results of welding 2 different alum alloys, and the resulting failures I don't think welding is a option unless you are sure of the exact alloy used. Do alloy research on the net, your welder might not know (mine sure didn't)

Is there a lack of chainrings for 130? Maybe we can go low tech, work in the opposite end, how about some sort of raised pedal platform that extends the length of the pedal. It might be easier to weld the pedal mech to a extended spindle. I can picture it, but having a hard time writing it.

That way you can use any crank.
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