Can my mountain bike frame be repaired?
September 17, 2012 8:31 PM   Subscribe

Can my mountain bike frame be repaired?

My frame broke recently on my mountain bike. There was a crack that I neglected to fix for a while, and it finally gave in. The break is all the way through. Images at

Is it feasible to have this repaired? If not, is it likely I'll be able to use most of my existing parts on a new frame? This bike is ~12 years old.
posted by Gomez_in_the_South to Sports, Hobbies, & Recreation (14 answers total)
It looks like an aluminum frame. I wouldn't trust it. It will be easy to find a replacement frame off a similar vintage used bike, on which most of your parts will fit. I fact, you mig be able to find a replacement frame for less than the cost of welding the current frame back together.

I'm sure you could get someone to weld it for you, if you really want. I wouldn't trust it not to break again, personally.
posted by tylerkaraszewski at 8:39 PM on September 17, 2012

If I were you, I'd probably get a new (or used) frame on e-bay or craigslist and swap the parts.
posted by M.C. Lo-Carb! at 8:41 PM on September 17, 2012

I'm not a mountain biker, but this does not look like an expensive frame. Whatever you save on the replacement (which might be nothing) is not worth the risk of the weld's failing during a technical descent. Plus, if you're careful about matching up the tube diameters, you should be able to move almost everything over to the new frame, even the fork.
posted by d. z. wang at 8:47 PM on September 17, 2012

Honestly, no, that's a frame on like a $150 bike... It would not be worth the time of a welder to attempt repairing it, and since it's likely an aluminum frame it cannot be re-welded anyways. You can buy a whole new "mountain bike" (ie: Not really a mountain bike, but shaped like one) for less than the cost of attempting to repair that.
posted by thewalrus at 8:50 PM on September 17, 2012

Your safety is pretty valuable. Time to say so long to that frame.
posted by 26.2 at 8:52 PM on September 17, 2012

since it's likely an aluminum frame it cannot be re-welded anyways.

Nothing stopping an aluminium frame being welded. After all, that's how they're made. But agreeing that it is likely not cost effective.
posted by Brockles at 9:00 PM on September 17, 2012 [2 favorites]

It broke there for a reason. Welding it will not be as strong as it was originally so the same strain that broke it the first time may do so again.
posted by empty vessel at 9:26 PM on September 17, 2012

Best answer: It's definitely aluminum, and definitely not worth repairing. That part of the bike has already been welded once, and a second time will weaken the tubing further. Don't waste your money.

If you're comfortable doing most of the work yourself, you could absolutely swap the parts on to a new frame, but in my experience buying a complete used bike rather than a bare frame is the best value, if keeping the cost down is your number one priority. Work done by a bike shop will add up quickly, so you should try to minimize the amount of work that you will need someone else to do. That said, this presents you with an outstanding opportunity to brush up on bicycle maintenance and repair!

Definitely keep the wheels, as long as the new bike uses 26" wheels and rim brakes: It's always good to have spares. The other critical dimension is the OLD (over locknut) distance, which is probably 135 mm, but you should measure it (easier on the frame, not the hub). if you're buying an aluminium frame, this dimension has to be an exact match, but steel frames can be bent a little to accommodate hubs which are slightly too large or too small.

You can also keep the seat, especially if you like it. Seatposts, however, come in a profusion of sizes, and it's not worth selecting a frame on this basis. The diameter will be stamped on the tube below the insertion point if you're curious. It will be a number in the neighborhood of 30 mm following ⌀, the diameter symbol. If your computer won't display it, this looks a little like the empty set symbol.

The handlebars, stem, brake levers and shifters (and the brakes and deraillers, though the front's clamp may not fit the new downtube, esp. if it's not aluminum) will also be fairly easy to swap so long as the new bike has a threadless headset of the same size, which will either be 1" or, more likely, 1 1/8". Inexpensive adapters which allow the use of a 1 1/8" stem on a 1" steerer exist. You should probably replace the housing and cables if they're original to the bike.

Almost all of the work above this point can be accomplished with nothing more than 4, 5 and 6 mm hex keys, assuming the shop will cut your new brake and shifter housing to length.

Pedals, too, are easy to swap, though they can be a bear to remove, especially after 12 years of uninterrupted tenancy in your cranks. You'll probably need a 15 mm wrench and a cheater bar, or a pedal wrench. All bikes with three-piece cranks use the same 9/16"/20 TPI thread.

Try to either keep the drivetrain together or replace it wholesale. Drivetrains cobbled together from mixed pieces often function less than perfectly. Most notably, if you're combining parts, you generally need to use parts from the same brand (Shimano) and gearing (probably 9, based on the age of the bike). Additionally, chains and cassettes "wear together" and combining a new chain with a worn cassette--or vice versa--will lead to both malfunction and accelerated wear on the new part.

And, at this point, you're looking at severely diminishing returns. The headset and bottom bracket/cranks require specialized tools to remove and reinstall, and may cause unforseen issues even if they fit. For the same reasons, these are also the parts most likely to be included with a frame.

If you need a good general reference, Sheldon Brown's pages on the Harris Cyclery website are a tremendous resource. Park Tool also provides useful videos, though with a bias toward using Park brand tools (natch).
posted by pullayup at 9:40 PM on September 17, 2012 [4 favorites]

Welding it will not be as strong as it was originally

A good weld will be stronger than the base metal. Still not worth repairing.
posted by mollymayhem at 9:54 PM on September 17, 2012

A good weld will be stronger than the base metal. Still not worth repairing.

This is way outside my experience (because it's virtually never done) but my understanding is that it depends on the particular alloy used in the frame--6000 series alloys used in aluminum bike tubing must be heat treated after welding, which is not practical (possible?) after a repair.
posted by pullayup at 10:19 PM on September 17, 2012

Response by poster: Considering what's been said above and the age of many of the parts, I've decided to go for a new bike and keep the old one around for spares.

Thanks for all the great advice.
posted by Gomez_in_the_South at 10:50 PM on September 17, 2012 [1 favorite]

Response by poster: pullayup - I appreciate the detailed info on the part compatibility. I'm not averse to getting my hands dirty, and will use your advice to see which parts are worth preserving from my existing bike.
posted by Gomez_in_the_South at 10:53 PM on September 17, 2012

Brockles, your automotive advice is usually dependable, but...

Nothing stopping an aluminium frame being welded. After all, that's how they're made.

Al bicycle frames must be annealed after welding in order to achieve their high strength-to-weight ratio. This can be performed cheaply in high-volume manufacturing, but is prohibitively expensive for after-market repairs. It's much cheaper to just buy a new frame.
posted by wutangclan at 2:47 PM on September 18, 2012 [1 favorite]

That doesn't stop it from being welded by any means, and heat treating is possible in small volume. It ain't easy, but that's not the same as saying "It can't be welded". It isn't realistic (which is perhaps what you were saying) but it certainly isn't impossible.
posted by Brockles at 3:26 PM on September 18, 2012 [1 favorite]

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