Question about aluminium frame challenge to carbon fibre frames?

[Pat "5mph"]

I can't answer people properly. My posts are all held back for some time waiting for mods approval. By the time they appear everything has moved on leaving me out on a limb. Sorry. I'll leave it at that.

I am prevented from answering, and being made to look stupid! The article I wrote in explanation yesterday is still being withheld awaiting mod approval. I don't wish to continue on such a basis. Thank you.

Mod Note:
Sorry about that @Rain drops, I have now changed the setting that was delaying your posts.

 

[Twilkes]

Wasn't there a thread a few months ago from someone who worked with aerospace carbon and didn't trust bicycle carbon? I'm sure they had a five year limit on it too...

 

[kingrollo]

Wasn't there a thread a few months ago from someone who worked with aerospace carbon and didn't trust bicycle carbon? I'm sure they had a five year limit on it too...

Probably was - but that's only another opinion. It doesn't make it true.

 

[Twilkes]

Probably was - but that's only another opinion. It doesn't make it true.

No I know, just showing that there are extreme views on both sides. It was this one: https://www.cyclechat.net/threads/secondhand-carbon-frames.283104/#post-6654312

 

[Peter Salt]

One reason for aluminium resurgence is weight. A few years back sweet spot mid-range CF frames were much lighter than they are today. A £1500-1800 complete build, kitted out with Ultegra, entry level wheels and saddle would weigh in at 8kg. You bought that, upgraded to a £1000 wheelset and a good quality saddle, voila: £3000 bike under 7.5kg. Flagship frames were much lighter then the 6.8kg UCI limit which led pro teams having to add ballast. So if you wanted to buy a bike for use in a UCI-sanctioned event there was no great reason to get the flagship - at least as far as weight is concerned.

We could debate why what happened happened, but the reality is that today flagships are at or very near the UCI limit. Naturally, a mid-range bike can't weigh the same - that would look bad. So a £3000 build now comes in at 9kg.

What gives a bicycle frame stiffness is the overall frame geometry, the material's Young's modulus and the cross section of the tubes (shape, diameter, wall thickness, etc). So if we assume that the overall geometry of two bikes is the same, the size and shape of the tubes as well, and they weigh the same but one of them is alloy and the other CF, then the CF one will be much stiffer. You can make the alloy one just as stiff as the CF but that would require thickening the walls and lead to a significant increase in weight. This is a great oversimplification but no point of getting in the details.

Alloy frames are, traditionally, regarded as less stiff/more compliant - this is because manufacturers tried very hard to make them not significantly heavier than carbon. Now, given what I said at the beginning about mid-range builds, this is less of an issue. I feel the industry is at a point where, if you're a mid-range consumer, the frame material stopped being meaningful.

 

[Jody]

I have heard a few stories about giant frames cracking around the junction of seat and top tube , i reckon its something to do with the sloping top tube in their design in the earlier models .
Heard of 2 replaced by giant under warranty and seen another personally which i found and stripped for parts

They were prone but apparently it was to do with running less than the minimum post insertion. Long post, less than 8cm in the frame and it caused flex which cracked the weld.

 

[wiggydiggy]

I have heard a few stories about giant frames cracking around the junction of seat and top tube , i reckon its something to do with the sloping top tube in their design in the earlier models .
Heard of 2 replaced by giant under warranty and seen another personally which i found and stripped for parts

Aluminium?

This was my Giant Toughroad (lol) SLR, this wasn't an instant break as I had a battery pack covering it and didn't discover it for a while.

The replacement frame (warranty) has been fine

 

[cyberknight]

Aluminium?

This was my Giant Toughroad (lol) SLR, this wasn't an instant break as I had a battery pack covering it and didn't discover it for a while.

The replacement frame (warranty) has been fine

yes aluminium

 

[wafter]

They were prone but apparently it was to do with running less than the minimum post insertion. Long post, less than 8cm in the frame and it caused flex which cracked the weld.

The rub is though that on some frames at least Giant used seat posts of significantly smaller OD than the seatpost tube's *EDIT* ID; separated with a fat moulded plastic shim / insert of all of around 80-90mm length, giving exactly the same effect as running insufficient seatpost insertion.

On top of that as you say the problem is made worst by running a lot of exposed post; but given the nature of the frame design this was necessary to get the correct seat height.

So, really two design issues that remain the responsibility of the manufacturer.

 

[the_mikey]

I've been riding alloy frames since 2005, without problems. Previously rode steel, whilst good to ride, eventually succomed to rust. Haven't tried carbon, as they always seem above my price point.

Do have any links to the new breed of alloy bikes?

I'm still using a 2010 Giant Defy 2, aluminium alloy frame, I also have an Aluminium Ribble GCR from 2017, and a Carbon Giant TCR from 2011, I currently like the looks of a Fairlite Strael (not aluminium) but time and money says no.

 

[Smokin Joe]

This makes a change from threads telling us that carbon frames fall to bits if you look at them too hard

 

[Jody]

The rub is though that on some frames at least Giant used seat posts of significantly smaller OD than the seatpost tube's OD; separated with a fat moulded plastic shim / insert of all of around 80-90mm length, giving exactly the same effect as running insufficient seatpost insertion.

I remember seeing some if those. Seemed an odd thing to do.

* Touch wood * both my Giant frames have proved reliable up to now. Although the MTB has had a lot of hammer in it's 10 years so I'm hoping it makes it to retirement.

 

[Dogtrousers]

This makes a change from threads telling us that carbon frames fall to bits if you look at them too hard

Oh, they do.

It's just that Aluminium ones do too.

 

[roubaixtuesday]

This makes a change from threads telling us that carbon frames fall to bits if you look at them too hard

It's hilarious. We've had steel, aluminium, ti, carbon frames between the family.

One steel and one ti have failed. No carbon or aluminium failures.

 

[Dogtrousers]

It's hilarious. We've had steel, aluminium, ti, carbon frames between the family.

One steel and one ti have failed. No carbon or aluminium failures.

Look, let's face it. We're doomed. Bike frames of any and all materials fail catastrophically all the time.