Interesting article on frame durability

[LittleWheelsandBig]

Composites fail in a variety of ways - delamination, fibre breakage, matrix cracking and so on. Galvanic corrosion is just one of the causes of composite deterioration, like impact damage. Several of them take time and repeated loading, so design and production quality, fracture mechanics, load cycles and critical flaw size becomes interesting.

[Brucey]

when I was at school we were still using 'Nuffield Science' books which were a few (not that many) years old. They contained various examples which were held up as being ways in which science had (or was about to) benefit mankind.  Already several of these were obviously anachronistic or just plain wrong.  For example the guy that invented tetraethyl lead (who coincidentally was responsible for putting CFCs in refrigeration plant I believe)  was portrayed as a 'hero of chemistry'.

  The use of CF in RR fan blades had been under development when the books were published so that was cited as a wonderful example too.  Of course by then we all knew the truth; they didn't work as envisaged and this was a major factor in the subsequent bankruptcy/government bail out.

  CF is better understood now but that doesn't meant that there won't be problems. One of the main difficulties is 'where the buck stops'.  Manufacturers can put a lot of effort into their frames and then fit them with an off-the-peg CF  fork which comes from a sweatshop in China, just 'cause it is not seen as a major product differentiator, is just as difficult to do well as a whole frame, and OTP forks are cheap and they want to sell frames and make as much profit as possible.  I don't think there hasn't been a major manufacturer that hasn't been caught out this way, and price is no indicator of quality here; even Cervelo had to do a fork recall because they had been selling shonky rubbish of this kind....

cheers

[DuncanM]

There's an interesting Youtube channel run by someone who ultrasound scans and repairs carbon frames (he's been in the composites industry in a variety of roles outside bikes) here: https://www.youtube.com/channel/UCY9JUMYI54lLOHpb_zbIedQ/videos
He's really dry in his delivery, but he basically shows ultrasound scans and cut-up bikes and all the manufacturing defects and problems that he finds. Often the bikes are brought to him as a result of issues/crashes, so it's not a representative slice of the carbon bike market, but it's interesting to see the failure modes, the design issue, and the progress in how frames are made.

There's also crank, handlebar and wheel cut-ups.

[sojournermike]

There's an interesting Youtube channel run by someone who ultrasound scans and repairs carbon frames (he's been in the composites industry in a variety of roles outside bikes) here: https://www.youtube.com/channel/UCY9JUMYI54lLOHpb_zbIedQ/videos
He's really dry in his delivery, but he basically shows ultrasound scans and cut-up bikes and all the manufacturing defects and problems that he finds. Often the bikes are brought to him as a result of issues/crashes, so it's not a representative slice of the carbon bike market, but it's interesting to see the failure modes, the design issue, and the progress in how frames are made.

There's also crank, handlebar and wheel cut-ups.

‘You can buy a lot of KoolAid for that sort of money.’

Genius

[rogerzilla]

I’d suggest that many people on here are on an extreme end of a usage curve. Very many bikes, including expensive ones, often do only the few thousands of miles in their lifetime.

Logically, mileage is irrelevant for steel or Ti frames.  The fact that they do break shows that the fatigue limit* is not all that useful in the real world for a bike frame.

*just in case anyone is unaware, this is the stress below which the material can endure an infinite number of cycles - steel and titanium have such a limit, aluminium does not and will eventually break even at small stress cycles (it may take millennia for low stresses and well-specified parts)

[Brucey]

I’d suggest that many people on here are on an extreme end of a usage curve. Very many bikes, including expensive ones, often do only the few thousands of miles in their lifetime.

Logically, mileage is irrelevant for steel or Ti frames.  The fact that they do break shows that the fatigue limit* is not all that useful in the real world for a bike frame.

*just in case anyone is unaware, this is the stress below which the material can endure an infinite number of cycles - steel and titanium have such a limit, aluminium does not and will eventually break even at small stress cycles (it may take millennia for low stresses and well-specified parts)

I don't think that existence of a fatigue limit is at all 'irrelevant'  for real world bike frames.   

The reality is that many bike frames are manufactured complete with a) a mess of (unseen) residual stresses and/or b) (again often unseen) stress concentrations.  These conspire together to mean that in many cases the service stresses which will cause zero fatigue damage are  smaller than they might otherwise be.

Whenever I have seen a broken frame it is pretty clear that the failure originated with an a) or b) type problem and that this could have been avoided.   

The worst thing you can do to a (metal) bike frame is probably never to load it more than normal; this is potentially as bad as not stress-relieving wheels.  Occasional overloads can (in the right material) mitigate the effects of residual stresses and blunt nascent crack tips.

There are plenty of bike frames out there which have done 100k miles or more ( which is likely to be around 32000000 stress cycles from pedalling alone, where each stress cycle contains  + and - stresses, plus an unknown number of stress cycles from the bumps in the road) .  These frames would simply not have survived this mileage without either being massively overbuilt or getting benefit from a fatigue limit.

cheers

[rogerzilla]

I didn't say that.  I said mileage was 'irrelevant'.  In theory.  I said the fatigue limit was 'not all that useful'.  It is less important than design and construction, certainly.

i like steel because it's less dead than CF over bumps and it's easy to repaint or modify.  I don't expect it to last for ever.  My oldest frame amazes me by staying in one piece, given that the rear end (pencil stays) flexes by almost 10mm either way on steep hills.  I have to open the brake QR and it still rubs.  One day the chainstay will crack.

[Cudzoziemiec]

'port' is a posh word for a hole that some twit has made in a bike frame.  At best you will have a stress concentration of x3 with such a hole. At worst it will be a lot higher than that if the edges of the hole are not correctly radiused (and most commonly they are not correctly radiused on the inside).   Holes in frames are a very common point for fatigue cracks to initiate.  They also let water into the frame too.  All in all, holes in the main tube are probably  best avoided.

cheers

It’s not often I reach for the like button that isn’t there - spot on.

I was looking at the internal brake routing on a Ti frame today and pleased to see a nicely finished and radiused hole (top tube) under the insert. I still think external is better mind - with full outer if your riding in crap weather. The gears are external, so there’s only the welded on adjuster fittings to worry about...

All holes or only square ones? Or only over a certain size? Or something else? I'm thinking that no one seems to worry about bottle cage mount holes or those tiny little vent holes in fork legs and BB shells.

[LittleWheelsandBig]

Vent holes are very small and bidon mounts have a brazed in reinforcement. Sharp/ square corners are very many times worse than rounded corners.

https://www.fracturemechanics.org/hole.html goes into this stuff in great detail but the images and diagrams alone explain things quite well enough for a basic understanding.

[sojournermike]

'port' is a posh word for a hole that some twit has made in a bike frame.  At best you will have a stress concentration of x3 with such a hole. At worst it will be a lot higher than that if the edges of the hole are not correctly radiused (and most commonly they are not correctly radiused on the inside).   Holes in frames are a very common point for fatigue cracks to initiate.  They also let water into the frame too.  All in all, holes in the main tube are probably  best avoided.

cheers

It’s not often I reach for the like button that isn’t there - spot on.

I was looking at the internal brake routing on a Ti frame today and pleased to see a nicely finished and radiused hole (top tube) under the insert. I still think external is better mind - with full outer if your riding in crap weather. The gears are external, so there’s only the welded on adjuster fittings to worry about...

All holes or only square ones? Or only over a certain size? Or something else? I'm thinking that no one seems to worry about bottle cage mount holes or those tiny little vent holes in fork legs and BB shells.

I tend to the view that square or non radiused corners are bad and that bigger is worse than smaller. Vent holes and bottle mounts are small and round. I’ve avoided drilling new holes willy nilly in my frames - drillium can go too far.

I’m not convinced that any manufacturer really does stress analysis for bottle cage mounts and quite possibly not for the much bigger cable ports. It’s interesting that the internet lets us see frame failure at a far higher frequency than would otherwise be the case and these largely appear to be related to relatively big square holes in downtubes or to welded on items such as cable stops.

There’s a whole subculture of broken carbon that I haven’t investigated - scratching the surface is quite enough. As Duncan said earlier - steeper’s are what scare me the most. Overall I am more comfortable with carbon (monocoque) than bonded alloy steered, but they require care in assembly to ensure that you don’t create your own damage.

[Cudzoziemiec]

Vent holes are very small and bidon mounts have a brazed in reinforcement. Sharp/ square corners are very many times worse than rounded corners.

https://www.fracturemechanics.org/hole.html goes into this stuff in great detail but the images and diagrams alone explain things quite well enough for a basic understanding.

Thanks. I'm afraid even the diagrams are a bit beyond my level of maths  but I do (think I) understand stress risers as a concept and square holes seem to fit into that(?).

I have in any case always disliked internal cabling on aesthetic and practical grounds. 

[LittleWheelsandBig]

Vent holes are very small and bidon mounts have a brazed in reinforcement. Sharp/ square corners are very many times worse than rounded corners.

https://www.fracturemechanics.org/hole.html goes into this stuff in great detail but the images and diagrams alone explain things quite well enough for a basic understanding.

Thanks. I'm afraid even the diagrams are a bit beyond my level of maths  but I do (think I) understand stress risers as a concept and square holes seem to fit into that(?).

I have in any case always disliked internal cabling on aesthetic and practical grounds. 

Have another look at the pretty picture in the "Introduction" and the first and third diagrams in "Stress State Schematics". Most of the other stuff is fluff, if you are only considering the basic concept. The red is high stress (blue is low, green is average) and the tighter the radius, the more concentrated the stress. A sharp corner (crack = infinitely tight radius, sort of) into a material hugely concentrates stress compared to any sort of radius.

[FifeingEejit]

when I was at school we were still using 'Nuffield Science' books which were a few (not that many) years old. They contained various examples which were held up as being ways in which science had (or was about to) benefit mankind.  Already several of these were obviously anachronistic or just plain wrong. 

I have somewhere and old Geography book, which is of an age that pre-dates plate tectonics, think it was continental land bridges it professed.
MAybe not quite as (eventually) sinister as CFCs and Leaded petrol turned out to be, but...

[Brucey]

...All holes or only square ones? Or only over a certain size? Or something else? I'm thinking that no one seems to worry about bottle cage mount holes or those tiny little vent holes in fork legs and BB shells.

 the SCF of x3 arises from a consideration of the 2D in-plane stresses around a circular hole in a (infinite) flat plate.   It holds for all diameters of round holes (the amount of diverted stress balances against the effect of the hole radius on concentrating that stress). It is also close to reality for holes of a reasonable size in a thin-walled tube.   However there are complications such as

a) when the hole gets smaller and smaller diameter the stresses get 'more three dimensional' and whilst there isn't a sharp cutoff, holes smaller in diameter than the wall thickness of the tube are probably a special case.
b) as previously mentioned the edge radiuses of the hole can dramatically affect the likelihood of cracking, potentially  making the SCF in the region of x10 or more rather than x3
c) as others have mentioned proper bottle braze ons are reinforced. However ones done with Nutserts are not reinforced in the same way and these are by comparison a bit of a lash-up.
d) oval holes in main frame tubes are always about as bad (or worse) vs a circular hole of diameter equal to the long axis of the oval. The reason for this is that all the main tubes (but especially the down tube) are loaded in a mixture of torsion and bending; there is no 'favourable direction' for an oval hole to be oriented.
e) The primary loadings on stays are uniaxial and bending, and on fork blades bending. Breather holes in stays and fork blades are placed sufficiently far from the brazed joints that they don't see the highest bending loads.  In the case of fork blades they are  (over most of their length) made in such a way as they are about constant weight per unit length.  Unsurprisingly they are quite thick-walled near the smaller diameter ends and here the bending loads are lowest.  I don't remember ever seeing a crack at a breather hole in a steel frame, although I have seen a few that have been implicated in corrosion. Part of the reason they see low stresses is that they are commonly drilled orthogonal to the primary bending stress, so the breather hole lies on the neutral axis of the tube anyway.
f) if the hole diameter gets to be a significant fraction of the tube diameter the stresses will start to become larger than the simple SCF might predict.
g) there is usually plenty of meat that can come out of a steel BB shell quite safely; structurally speaking even quite large  cutouts in the bottom of the BB shell are unlikely to be problematic, except for the simple reason that they let water into the frame.  It doesn't look very 'trick' but a BB shell can be made a lot lighter in weight and still be weatherproof if cutouts are plated over with much thinner material.

cheers

[zigzag]

these big holes in downtubes should be really bad then, but somehow they have the passed safety tests?..

[LittleWheelsandBig]

Provided the wall thickness and profile varies appropriately around the opening, the stresses from specific loads can be matched to the structure's capacity at any particular location. That design approach might sound backwards and the structure’s capacity should be matched to the stresses resulting from applied loads. Actually, engineering needs to use both approaches simultaneously.

Any significantly large hole drastically drops member stiffness. Stiffer parts of a structure tend to ‘attract’ loads and therefore result in higher stresses. It can get quite complicated and unintuitive when working out how stresses change within a complex structure, so good FEM can greatly help the engineer optimise the frame.

[Efrogwr]

Re holes in tubes; I understand that bottle bosses might mitigate the stress concenrations due to the holes.
Does screwing bolts into the bosses have any effect?

[LittleWheelsandBig]

If high strength bolts are properly tightened, the clamping changes the stress distribution around the hole (like rivets do) but not your bog-standard bolts used to mount bidons. No difference there.

[Efrogwr]

Thanks.