5 years for an alloy frame?

Coopz

Likes Dirt
What I don't get is steel frame bikes cost more (a lot more) in the bike shops than alloy yet with kmart bikes the cheapest of the cheap are steel and their premium $180 bikes are alloy.
 

Calvin27

Eats Squid
Modern geo and suspension would have to be placing more stress on frames, now that bikes are more capable.
Better suspension that is less likely to bottom out, fatter tyres, could be line ball. Plus a lot of modern frames have got it figured out about where failures might occur. Early MTB frames had to figure this stuff out by trial and error effectively.

What I don't get is steel frame bikes cost more (a lot more) in the bike shops than alloy yet with kmart bikes the cheapest of the cheap are steel and their premium $180 bikes are alloy.
Economies of scale really. A lot of steel frames are relatively smaller batch. But in reality it's a bit of what the buyer will pay as well. There is no reason why a run of the mill steel frame should cost a lot more than alu (maybe more shipping weight?). If you look at wholesale markets like aliexpress, steel and alu frame prices are comparable.
 

The Duckmeister

Has a juicy midrange
Weird question, I was speaking to a local bike mechanic on the weekend. He's been in the industry for over 20 years.....
What a surprise, a 'bike mechanic' that doesn't know what he's talking about.
Sounds like a salesman rather than a mechanic. :rolleyes:

My 10-year-old aluminium Anthem - a vintage known for breaking at the stupidly small seat tube/top tube junction - is still going strong, although admittedly for a fair chunk of that time it has been on spare bike duty, so hasn't the use normally expected for that age.
 

safreek

*******
Dont make me post photos of my 25 year old carbon bike to prove carbon lasts forever, it has aluminium swingarm as well and not broke. also using a 20 year old fork that hasn't exploded as well.

Use by dates are only related to human bodies and food
 

Cardy George

Piercing rural members since 1981
Cost of replacement would be the reason Alloy has lifetime warranties and high end carbon or Ti have 5 year.
Unless it's a Trek. Two world cup level frames replaced no questions asked.

Fatigue stress depends on both the cycles and stress range. S-n curves are an approximation on this phenomenon, but ultimately big hits and lots of little hits are going to fatigue the material. How much is one big hit vs many smaller hits is the question. Interestingly theoretically carbon and steel (including titanium) has an endurance limit. That is under a certain level of stress these things should last forever. Aluminium does not. That means even for the smallest stresses, if given enough cycles they will crack. The reality of course is that steels will crack at the welds which have different properties as well as concentrated stresses, and carbon will have voids and other imperfections that mean their life is in fact limited.
I'm hoping Trek has accurately calculated the endurance limit on the flex stays. It does play on my mind the stays are always stressed and are designed to give the final 5mm of travel.

But then it's a Trek with a lifetime warranty.
 

Calvin27

Eats Squid
I'm hoping Trek has accurately calculated the endurance limit on the flex stays. It does play on my mind the stays are always stressed and are designed to give the final 5mm of travel.
Quite a few bikes adopt this philosophy. Giant and their cheaper stance (in fairness they made this simplification cheaper for the consumer) Spesh with their Epic evo (they just took the profits and still priced it high lol).

One of two thing can be designed into the flex 'pivot'. First they can reduce the stresses in the material, which usually means thicker and more material, but this in turn affects the stiffness of the member and the flexstays don't really work anymore. The other approach is to increase the stringency of heat treatment on the alloy. The point of this is to reduce the initial flaw sizes as much as possible and this has an order of magnitude impact on the propagation of cracks. Am I confident any of this gets done? Hell no.

The marketing and bean counting department probably runs the shop and just calculate how many people will actually get to the point of breaking them and then of that subset how many are original owners and then within that, how many will actually get a refund. Balance that against the profits and voila!

As a side note, be interested to see how many stress cycles a flex stay endures. The member would be under a fair bit of stress and every single bump is going to be one cycle. Most alloys are lookng at 10^6-10^8 fatigue limit depending on a range of factors, I can see that getting eatn up quickly for a bike that's ridden a fair bit.
 

Haakon

Keeps on digging
I bought one of these in 2000, and they apparently had a reputation for cracking. It was my primary mountain bike for about 15 years! Then it went to commuting duties with slicks for another few years. No idea how many thousand kms of abuse it had, but many.

Its fine. I get it out every now and then for some retro 26er hardtail love, still a great bike.
IMG_4807.jpeg
 

link1896

Mr Greenfield
From a purely academic point of view, and not incorporating bicycle manufacturing monkey business or operator error, carbon fibre composites are the more durable material.

Here is a pretty graph comparing Titanium, Steel, Aluminium and Carbon Fibre composites for strength vs load cycles taken from "Manufacturing Technology for Aerospace Structural Materials" by F. C. Campbell, page 275. link to full text click ------>>>> here




A real world example demonstrating the superior performance of CF composites are the rotor blades on the Leonardo AW101-612 helicopter (military and search and rescue helicopter). Notice how the Carbon/Epoxy line in our graph above has a very shallow downwards slope (demonstrating CF's ultimate strength but also very very slow degradation) vs the metals rapid decline.

The Aluminium blades on the AW101-612 are good for 6000-7000 hours of use. The CF blades will outlast the rest of the helicopter and will never need replacing, though many CF blade vendors so far have put a limit on service hours purely as an ass covering exercise, some as much as 18,000 hrs though.


BUT, and this is a bigger BUT than Moorey's bruised ass, any serious use of CF composites will incorporate more then just superficial visual quality control. Be it Aerospace, race cars/high end production cars, etc, all are performing proper non destructive testing on 100% parts before they leave the manufacturer.

Sporting goods such as bicycles and gold clubs? The end user is the QC tester. Rumours have floated around for years large bicycle manufacturers actually just accept a 50% warranty rate on CF composite frames as the cheapest QC option. Ultra sound scanning of CF composite parts isn't easy, it requires a significant amount of operator training and knowhow. And a CT image of parts? Fuck me sideways I've had my own parts scanned, get the full DICOM image pack back and am left wondering why I'm looking at the image of a pearl inside an oyster inside a drink thermos for 10 mins before I find which way is up and work out its the cross section of a CF rim. .
 

Cardy George

Piercing rural members since 1981
Quite a few bikes adopt this philosophy. Giant and their cheaper stance (in fairness they made this simplification cheaper for the consumer) Spesh with their Epic evo (they just took the profits and still priced it high lol).

One of two thing can be designed into the flex 'pivot'. First they can reduce the stresses in the material, which usually means thicker and more material, but this in turn affects the stiffness of the member and the flexstays don't really work anymore. The other approach is to increase the stringency of heat treatment on the alloy. The point of this is to reduce the initial flaw sizes as much as possible and this has an order of magnitude impact on the propagation of cracks. Am I confident any of this gets done? Hell no.

The marketing and bean counting department probably runs the shop and just calculate how many people will actually get to the point of breaking them and then of that subset how many are original owners and then within that, how many will actually get a refund. Balance that against the profits and voila!

As a side note, be interested to see how many stress cycles a flex stay endures. The member would be under a fair bit of stress and every single bump is going to be one cycle. Most alloys are lookng at 10^6-10^8 fatigue limit depending on a range of factors, I can see that getting eatn up quickly for a bike that's ridden a fair bit.
They are all carbon frames, but I understand your point.

Perhaps there's a silver lining to only getting on it once a month.
 
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