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Old 09-13-09, 03:11 PM   #1
abecrabt
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broken aluminium frame - back wheel mount

There is another thread describing a broken plate on which the wheel axle is fixed, which bridges the chain and saddle stay - that was for a steel frame wherein the plate is brazed.

I have this problem where the frame is aluminium, or at least some aluminium alloy. The bike is fairly ordinary, nothing flashy.

http://abecrab.jalbum.net/bike-frame.../IMG_0641.html



can one simply weld the ends together with a MIG, and is that a bike shop or a car garage?

Or must an entire piece for the ends of the stays replace the one there? That seems difficult in that it's not a plate that drops out.

I can't believe I have to replace the frame.

Thanks for any help!
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Old 09-13-09, 03:17 PM   #2
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Aluminum has a finite life span. If you are having cracking problems now it will only get worse.
Saying that, if can be welded.
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Old 09-13-09, 03:30 PM   #3
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you'll have to replace the entire plate.

I'm not too sure of the cost for such a thing, but if that piece is cracked, you might as well replace the frame.
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Old 09-13-09, 04:37 PM   #4
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Aluminum has a finite life span. If you are having cracking problems now it will only get worse.
Saying that, if can be welded.
Would you mind telling us all what the "finite life span" of aluminum is? I've never heard this be proven.
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Old 09-13-09, 04:39 PM   #5
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Would you mind telling us all what the "finite life span" of aluminum is? I've never heard this be proven.
My Felt F-80 has a five year warranty if that helps.
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Old 09-13-09, 04:41 PM   #6
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My Trek 7.5 FX has a lifetime warranty on the frame. I'm looking for conclusive proof. How long your warranty is for doesn't provide that.
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Old 09-13-09, 04:43 PM   #7
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My Trek 7.5 FX has a lifetime warranty on the frame. I'm looking for conclusive proof. How long your warranty is for doesn't provide that.
How many Treks last one lifetime?
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Old 09-13-09, 05:02 PM   #8
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You're not helping. But this has been hashed over before. An aluminum-frame has not been shown to have a 'finite' lifespan. Up until the turn of the 19th century to the 20th, aluminum was a very precious metal - extremely rare in it's pure form. It wasn't possible to refine aluminum without a major supply of electricity. But in the 1880's the cap was made for the Washington Monument - and it was pure aluminum. People came from all over to gaze at this extremely expensive display.

That cap is still in place. Despite frequent lightning-strikes - it hasn't fallen apart. But it does get cleaned now and then. And with plenty of electrical-power available, there is an abundance of aluminum.
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Old 09-13-09, 05:03 PM   #9
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start looking for a steel frame
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Old 09-13-09, 05:21 PM   #10
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That cap is still in place. Despite frequent lightning-strikes - it hasn't fallen apart. But it does get cleaned now and then. And with plenty of electrical-power available, there is an abundance of aluminum.
I'd have to say that they're definitely a difference between metal used as ornamentation versus metal subjected to stresses such as a bicycle frame.

Beyond that considering the bike market isn't like other markets where materials are subjected to extensive metallurgy experimentation I'd say they last until they break and you avail yourself of a warranty if it's to be had or you buy a new one. There's a reason replacing them is done under warranty, because the issues caused by fixing them can be extensive.
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Old 09-13-09, 05:26 PM   #11
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Yup - aluminum is very costly to work on if it needs brazing. Inert-gas and all. I wouldn't want a racing-bike with an aluminum-frame. But I'm very please with them on hybrids or mountain-bikes. But I wouldn't suggest jumping them off curbs or other abusive riding-styles. Aluminum OR steel.
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Old 09-13-09, 05:48 PM   #12
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Would you mind telling us all what the "finite life span" of aluminum is? I've never heard this be proven.
metal fatigue from load/unload cycles.
aluminum doesn't have a fatigue limit, which means any load will reduce it's life span. (Why is English is tricky?)

Airplanes are a good example of this. The hull is constantly going through pressurization and depressurization. Some of the skin and other parts are replaced or patched after a certain number of flights to reduce the risk of catastrophic failure of the hull.
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Old 09-13-09, 07:13 PM   #13
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Aluminum fatigues? Who would have known....


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Old 09-13-09, 07:21 PM   #14
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Metals have what's called "fatigue cycle".It the amount of stress/release cycles a material can absorb before breakage can occur at any time.Aluminum has no fatigue cycle.None.Zero.Zip.It can break on the first stress/release.If you really wish to know the reasons and whatfers buy this paper and read it.You'll need a dictionary and some understanding of metallurgy to truly understand it.Or you could listen to what many here have said.Up to you.

And to the OP you may have it welded.But I'd bet as soon as you begin preheating it for the weld many other cracks will become visible.Time for a new frame IMO.
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Old 09-13-09, 09:51 PM   #15
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Why bother replacing the frame? It's just going to fall apart, right? Get some boots. Meanwhile - I'll ride my hybrid made out of Al. Seems they fall apart about as often as steel (leave carbon alone). In fact, I've read more and older cases of steel breaking.

Yup. Get some boots. And an SUV.
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Old 09-13-09, 10:13 PM   #16
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the only difference between bike and airplane is that bike really doesn't go through the same sort of stresses a plane does.
looking at a complete bike, rims are probably the most stressed, and those are made of aluminum.
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Old 09-13-09, 10:40 PM   #17
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Aluminum frames are (current production) made thicker to address their lack of "fatigue-cycle" or, simply put - ability to hold up despite it's being a brittle metal compared to steel. And good ones have butted tubes these days based on tests - rather than marketing suggestions.
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Old 09-14-09, 02:47 PM   #18
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Thanks Everyone!

Very interesting about the metallurgy.

Oh dear - looks like it might be curtains for my bike.

I'll ring round a few shops, but this is NOT a posh bike, so I think I'll keep it for parts and get a new one.

Thanks again everyone for your speedy replies.
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Old 09-15-09, 04:24 AM   #19
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Aluminium frames aren't ever gonna "wear out" from fatigue. I heard from the guys at Trek when I worked at a shop that they engineered it to last 150-years under normal usage before fatigue becomes a factor. The vast majority of failures are from overcoming the ultimate-strength of the material. A crash or impact of some sort; which would destroy steel or CF frames as well.

For the OP's bike, it's definitely a overloading problem due to design. Breaks like this are much more common nowadays due to the stupid hole placed in the dropout for the derailleur hanger. Although Cannondale probably started this trend with their ill-fated cantilevered rear dropouts.
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Old 09-15-09, 08:29 AM   #20
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Aluminum is also not very ductile, either. Steel may bend or dent if jumped hard or run into a tree. Aluminum might just crack or snap in half. Granted that would take a bit, but still.

Read a lot of frame warranties. Most include not riding the bike/frame in severe conditions or jumping, hard off road riding, etc. So basically if you use that mountain bike as a mountain bike, you just voided your warranty.

I had a cheap Mongoose aluminum frame (frame was actually off a walmart bike) that I had built up with quality components. The headtube has two hairline cracks in it now, I believe from a loose headset but I'm not sure, either. Frame is trash. I wont have it welded because SOME aluminums require heat treating after welding.

Aluminum frames will likely not last as long as steel, given proper care. Steel will take a lot more stress and fatigue. Why do you think most car frames have been made of steel?

By the way, the aluminum cap on the washington monument just SITS there. It isnt being flexed up and down over potholes, roots, jumping downed trees offroad, etc. You couldnt have picked a worse comparison to a bike frame in the world. Really, if ya dont know what your talking about, stay out of it.

OP: go buy a new bike, or frame. Whichever you decide, but being aluminum I wouldnt mess with that one. Even steel unless it was a real high end handbuilt frame. Aluminum frames are fine, but when they break, they are broke and its best to move on. Same for any frame material. The bike gods want you to buy a new bike
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Old 09-15-09, 09:03 AM   #21
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Aluminum frames will likely not last as long as steel, given proper care.
This is not necessarily true.

THe actual material is not as important as the way the frame is designed and constructed. An extremely light steel frame (do these still exist?) ridden hard might eventually break, while a burly aluminum frame (possibly even one lighter than said super light steel frame) might be able to tolerate the same use for decades.

As someone said above, some companies intend for their Al frames to be used for indefinitely without failure.
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Old 09-15-09, 11:27 AM   #22
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Ok.

Dont think anyone said what it would need to repair the bike.

The bike would need to be stripped down. Then cleaned. The frames Black. That would have to be removed on the drop out. Since its a part of the frame that holds the wheel and the gear mech. It has to be held in alignment. As if the drop outs are squint, you might have trouble getting the wheel in. Or trouble breaking axles. Or trouble with gear selection.
Then the welding would need to be done. Ive heard of a product called Lumiweld. I dont know if its suitable for bike frames. I dont think normal hobby type ARC or MIG welding machines are able to weld alloy. So you may need to get a TIG machine. Or some one thats got one and is capable of welding alloy.
Then when the weldings done. The welds would have to be dressed back acurately. So that theres still enough metal to be strong. And so the gear hanger and the wheel fits in properly.
Then the frame would need repainting in the area.

It would be a lot of work. By skilled craftsmen.
If this was a week old frame, that was under a guarentee. It might get fixed. But I think they would just replace it.
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Old 09-21-09, 03:27 PM   #23
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Thanks for the further information you four - much appreciated.
You're confirming that the frame really is not worth the trouble - I've taken off all the components to keep as spares.

DannoXYZ - I think you have a definite point - that bit of the frame seems too sparse, cut into and machined not to be steel, and half-thickness to accomadate the derailleur too(!) ; I reckon that's very plausible it's a weakness.

The bike's 6 years old, done good service for daily commutes on London's roads, which are like ploughed fields here and there - I'm going to get a new one.

Thanks all!
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Old 09-21-09, 11:10 PM   #24
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Aluminum frames will likely not last as long as steel, given proper care. Steel will take a lot more stress and fatigue. Why do you think most car frames have been made of steel?
Uh, auto frames are more about tooling costs more than anything else. The highest-performance autos have aluminium bodies on street models and monocoque carbon-fibre on the race cars (who in their right mind would pay $20mil for a CF street car anyway?).

Rather than talking abstractly about irrelevant materials properties without considering design and engineering, take a look at the final results in a real-world test of steel versus aluminium versus titanium versus carbon-fibre:

http://www.sheldonbrown.com/rinard/E...tigue_test.htm

After you've digested the results table. Answer these questions:

1. Which frame materials lasted through the entire 200,000 stress-cycle without failure?
2. Which frame material had the most number of failures at the lowest number of cycles?
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Old 09-22-09, 07:36 AM   #25
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Danno there's a huge qualifier early in the article.To wit:
Quote:
Steel frames are not competitive in this weight class; the 1.500 gram threshold can be undercut with the steel Nemo tube set from Columbus, but only if small tube diameters are selected (different outside diameters can be selected from the Nemo tube set).
Draw your own conclusions.
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