Problem with aluminium.

General cycling advice ( NOT technical ! )
Peter W
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Problem with aluminium.

Post by Peter W »

Apologies if, as a newcomer to the forum, the subject has already been covered in detail.

Practical experience of working with, and destroying, flexible alloy components in other sports does not inspire confidence in the material, unless it is built and assembled in such a way that continual flexing can be eliminated. In my experience (windsurfing with flexing alloy booms) constant flexing, even when not exceeding the limits, leads to brittleness and a shortened fatigue life of around a couple of years. (Salt water corrosion takes longer than that.)

I bought an alloy Boardman Comp road bike some years ago which I rode for 4 years before I started to worry about the structural integrity of those skinny (built in flex) alloy seat tubes. I imagined they must have been well into half life fatigue territory, so stopped using it.

Is it now accepted by cyclists that the modern generation of lightweight alloy bikes should be expected to have only a limited life span? Or is their something I'm failing to understand here?
hamster
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Re: Problem with aluminium.

Post by hamster »

Bike frames certainly had cracking problems early on from flex. As a result they were over-built (this was marketed to us as an advantage, stiff frames). Of course, aluminium has a finite fatigue life, unlike steel. However I would generally venture that quality frames have a life meatures in tens of thousands of miles, so beyond the expectation of the original purchaser. But you are right in principle - my experience of windsurfing booms was that they were extremely thin-walled and indeed poorly engineered (but at least not welded, which brings a host of other potential problems).

I have an 18 year old aluminium MTB, it gets a couple of thousand miles use annually. I give it a careful check at service time for cracking around the key weakness areas (head/down tube junction, bottom bracket). Other than that, ride and enjoy.
Tangled Metal
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Re: Problem with aluminium.

Post by Tangled Metal »

N+1 mate. You really should get more bikes then it's less of an issue. A cheap steel commuter and at least a Sunday best. Plus various types of mtb if you get into that too.

Seriously, I've not had aluminium for long. Think. My current bike is about 4 or 5 years now. An old steel is nearly 30 years. I've probably done a lot more hard miles on my newer aluminium bike though. No issues so far, including a few crashes / bad pothole strikes. It's not a top brand neither.

I reckon designers work on the right grades of alloys for their products. I don't know about wind surf booms but I'm guessing their requirements are sufficiently different to be considered as not comparable to bike aluminum frames. I'm guessing but the design requirements will be totally different if you looked deeply into it.
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al_yrpal
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Re: Problem with aluminium.

Post by al_yrpal »

The fatigue limit (the stress at which failure will never occur) of aluminium alloys is well known to aircraft airframe Engineers. Welding is largely avoided because welds are full of micro cracks. Rigourous quality contol is applied to everything. Vulnerable areas are regularly inspected to avoid failures. This knowledge is not shared by a lot of people who 'design' bikes.

Al
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Samuel D
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Re: Problem with aluminium.

Post by Samuel D »

Even the aviation industry gets it wrong sometimes. Look at that fan blade fatigue failure on the CFM56 the other day.

A lot of bicycle frames are designed and built with the knowledge that 90% of them won’t get ridden far enough to expose their known weaknesses. Some fail and are replaced by the shop or manufacturer at an acceptable cost. The rest outlast (a) the warranty, (b) the original owner’s interest, and (c) the company that made it (often only a few years in the high-churn world of bike brands).

So when it cracks as someone’s second-hand commuter five or ten years down the line, as is often reported on these pages and elsewhere, no-one involved with the design and manufacture has a reason to care.

Bicycle weight within the usual range does not matter much and frame weight matters even less. So I see no need to buy aluminium bicycles anyway.
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squeaker
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Re: Problem with aluminium.

Post by squeaker »

Samuel D wrote:So I see no need to buy aluminium bicycles anyway.
Unless you want a stiff frame for a full suspension bike, and don't want to risk / pay for carbon :?:
"42"
Brucey
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Re: Problem with aluminium.

Post by Brucey »

Peter W wrote: .... Practical experience of working with, and destroying, flexible alloy components in other sports does not inspire confidence in the material, unless it is built and assembled in such a way that continual flexing can be eliminated. In my experience (windsurfing with flexing alloy booms) constant flexing, even when not exceeding the limits, leads to brittleness and a shortened fatigue life of around a couple of years. (Salt water corrosion takes longer than that.)....


Just to clear up a few misconceptions;

continual flexing is the lot of any structure that is to do a job of work. Whether it is noticeable or not depends on geometry of parts as well as stress levels seen. For example a trivial case is that a tube of half the diameter flexes 'twice as much' before the same stresses are seen in the material. On the other hand a tube of twice the diameter (and the same wall thickness) is many times stiffer and sees smaller stresses when undergoing the same loads.

Fatigue does not cause 'brittleness' in aluminium, it causes crack to propagate. The two things are not the same thing at all. When the final failure occurs it may be brittle in nature but the properties of the material in the final fracture won't have been changed except by the advancement of a crack or cracks. The presence of salt water has the effect of greatly lowering the strength of the material in most cases, and fatigue under those conditions is accelerated and perhaps better described as 'corrosion fatigue'.

al_yrpal wrote: ....Welding is largely avoided because welds are full of micro cracks....


This is something that ( IIRC) you have repeated before. Having examined sections taken from thousands of welds in aluminium I can tell you without any hesitation that such welds are not 'full of micro-cracks'. What typical TIG welds do have are a load of nasty stress concentrations at the weld toe and (often) at the weld root. These are the things that limit the fatigue life of typical TIG welded components. TIG welding is not the only welding process, and it is possible to make welds that give similar properties to the parent material, if you put your mind to it. Obviously this would be impossible if 'the welds were full of micro-cracks'.

Regarding fatigue life of aluminium bike frames; best to inspect the frame regularly if you carry on riding it. There are a few places (like the steerer) where a failure in service will definitely cause a nasty accident. Most of the rest of the frame has some built-in redundancy, so an actual breakage usually isn't completely catastrophic and you are normally able to bring the bike to a halt OK without having a bad prang.

IME steel frames give a little bit of warning (e.g. by going floppy) before they break, but Alu frames and CF frames usually do not.

cheers
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NATURAL ANKLING
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Re: Problem with aluminium.

Post by NATURAL ANKLING »

Hi,
I have been using alu frames for several years now 5-7 I guess.
Two bikes but both have front suspension but always weigh up at about 25 kgs plus me.
Used on potholed tracks and cycle paths and I press high gears every ride three times a week.

I do give it the eye every other week say, and I will update on any failure.

Reason is because they are in abundance at the tip................real steel always demands the "Classic Racer" label from the tip opo's and loads of money.

Plenty of Raleigh's at the tip of late but mostly sixtys stuff when you do see it.

Reason for me and MTB frames is it always mostly has lugs for racks etc.
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
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al_yrpal
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Re: Problem with aluminium.

Post by al_yrpal »

Samuel D wrote:Even the aviation industry gets it wrong sometimes. Look at that fan blade fatigue failure on the CFM56 the other day.

A lot of bicycle frames are designed and built with the knowledge that 90% of them won’t get ridden far enough to expose their known weaknesses. Some fail and are replaced by the shop or manufacturer at an acceptable cost. The rest outlast (a) the warranty, (b) the original owner’s interest, and (c) the company that made it (often only a few years in the high-churn world of bike brands).

So when it cracks as someone’s second-hand commuter five or ten years down the line, as is often reported on these pages and elsewhere, no-one involved with the design and manufacture has a reason to care.

Bicycle weight within the usual range does not matter much and frame weight matters even less. So I see no need to buy aluminium bicycles anyway.


Was it a fatigue crack in that blade? Blades are regularly inspected tested and regularly changed. If it was a systemic failure there would have been many more. There may have been an earlier birdstrike which was not noticed and reported prompting a full inspection? What did fail was the engine casing which should have contained the failure and prevented damage to the aircraft. As you say, nothing is perfect. The first fatal US aircraft death in many years though.

Al
Reuse, recycle, thus do your bit to save the planet.... Get stuff at auctions, Dump, Charity Shops, Facebook Marketplace, Ebay, Car Boots. Choose an Old House, and a Banger ..... And cycle as often as you can......
Tangled Metal
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Re: Problem with aluminium.

Post by Tangled Metal »

The fan blade really should have been contained. These engine casings are designed to contain more than one blade failing catastrophically. In fact failure is not that uncommon (source of information was university professor who consults on aero engine metallurgy and failure). Not exactly everyday but does happen without serious consequences like deaths or loss of aircraft.

Interesting subject the metallurgy and manufacture of jet engine blades. Real exotic metallurgy going on in the modern engine blades. Single crystal structures and the like. Incredibly complex metallurgy involved. Definitely not aluminium too but I'm sure ppl know that.
firedfromthecircus
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Re: Problem with aluminium.

Post by firedfromthecircus »

I'm sure I read somewhere that aluminium, unlike other metals used in bike frames, also ages whether it is used or not, so an older aluminium frame will be weaker than it was when it was new, regardless of use. Is this true? :?:
Samuel D
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Re: Problem with aluminium.

Post by Samuel D »

I have only followed the mainstream news that usually gets technical details wrong, but they said the blade showed signs of fatigue failure.

Clearly the blade didn’t go through the fuselage. Something else broke that window several rows behind the fan-blade position. Perhaps the fan blade was contained but the engine cowling shed components in the impact, subsequent vibration, or uncoordinated flight (slipping/skidding). Lumps of the cowling were found on the ground below. The engine manufacturer does blade-off tests, but what happens when the aircraft builder (Boeing in this case) puts it in a cowling? Does it get tested in that state? I don’t think so but maybe that will happen in the future after this incident.

Jet airliners are mostly made of aluminium alloy and had many problems with fatigue in the early days (look at the De Havilland Comet story). The material is otherwise well-suited to flight so its use continues, but bicycles benefit from the wheel’s tremendous ‘lift/drag’ ratio (compared to any wing) and are lightweight anyway so have no need for the extreme weight-saving of aluminium frames … full-suspension mountain bikes aside.

Steel remains the best material for bicycle frames in my view.
Brucey
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Re: Problem with aluminium.

Post by Brucey »

firedfromthecircus wrote:I'm sure I read somewhere that aluminium, unlike other metals used in bike frames, also ages whether it is used or not, so an older aluminium frame will be weaker than it was when it was new, regardless of use. Is this true? :?:


some precipitation hardening grades continue to age even at room temperature. There are other grades that age-harden or (even coarsen) at elevated temperature. Needless to say the grades used in aero engines are chosen so that they are stable under the envisaged service conditions, but not all bike frames are made with similar diligence.

FWIW the CFM56 is no stranger to (sometimes uncontained) fan blade failures. The engine is tested and/or supplied with the shroud to the as-tested configuration. There have been several incidents over the years, including at least three subsequent to various changes that were made following the Kegworth crash. In this latest incident it may not have been the blade itself which caused the worst damage but in other incidents it has been. One of the factors which has caused problems is resonance in the blades under certain speed/load/altitude conditions. The engines in some variants have been 'remapped' so that these conditions are avoided. I suppose that it is possible that with slight variations in the way the engine is built and balanced, different resonance conditions may prevail.

If any engine is going to show this kind of problem, it is arguably the CFM56; it is one of the most common aero engines of that general type, and is used in many different variants and conditions. To what extent the maintenance schedules are at the discretion of the airline (rather than the manufacturer) is unknown to me. The conclusion of one incident was that it expected that the airline would carry out more frequent checks on the parts that failed. In others it was fairly clear that they had carried out checks at suitable intervals but has simply not done them well enough and had missed the cracks that caused the failure.

My understanding is that RR now offer 'engine on wing time' (together with real-time monitoring of engines in use world-wide) which means that their engines are subject to critical maintenance and inspection by RR personnel rather than airline personnel and the airline sees a fixed cost not a variable one. This and the real time monitoring may help to prevent problems arising through resonance effects since the users can be informed ahead of time if a problem or anomaly is detected.

cheers
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Peter W
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Re: Problem with aluminium.

Post by Peter W »

I posted a reasoned follow up earlier this evening on my experience with the apparent durability of carbon fibre composite v. aluminium alloy in other sports, (with some questions for Brucey) which I felt could reasonably be carried over into cycle construction and durability. It was added to the thread at the time, but has since disappeared, or been removed.

I'm at a loss to understand why?
Brucey
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Re: Problem with aluminium.

Post by Brucey »

dunno... I sometimes lose posts as they are posted and I don't know if it is this website (pages timing out etc) or my computer. I cut and paste text if I think it will take a while to type it again.

cheers
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