[A_D_E_P_T]

> He ruled out magnesium, which is best per unit weight in compressive buckling but is brittle and difficult to extrude.

There's a fascinating, and very new, class of nano-laminate magnesium alloys called Long Period Stacking-Ordered (LPSO) alloys. These are very lean -- the standard version is 97% Mg + 1% Zn + 2% Y -- and they have outstanding mechanical properties. At an equal weight, they're much stronger and stiffer than 6061 aluminum, and the kicker is that this is generally true only if they're extruded. If they're not extruded, the laminate-like grain structure doesn't form properly.

Could make excellent bike frames.

Magnesium corrosion would still be a problem, though. I got some LPSO-Mg samples from Fuji Light Metals, in Japan, and they were quite badly degraded within weeks.

[twic]

There was a magnesium bike frame back in the '90s, made by Kirk:

https://www.elmycycles.co.uk/m21b0s365p4804/1992-Kirk-Revolu...

https://www.bikeforums.net/classic-vintage/1279777-kirk-prec...

https://www.independent.co.uk/news/uk/magnesium-in-frame-to-...

https://www.flickr.com/photos/11521783@N05/albums/7215764801...

A friend had one. It cracked.

[A_D_E_P_T]

> It cracked.

> "Kirk Revolution cast magnesium"

Cast magnesium is really weak/brittle compared to forgings and extrusions. Its use was not a great design decision on Kirk's part. I suppose they could have wrapped the casting in carbon fiber or something like that, to give it extra bending strength and spread out loads that might cause fractures, but then it would get expensive.

[_yb2s]

Carbon would cause galvinic corrosion in contact with magnesium, and would also visually hide dangerous cracks- I don’t think a carbon wrapped magnesium bicycle would be safe.

[A_D_E_P_T]

I was just spitballing, but it's possible:

First you coat or anodize the magnesium, which I imagine needs to be done in any case. Then you apply a layer of epoxy. Then you wrap in carbon/epoxy. Done properly, there's no direct contact between carbon and magnesium, and you're probably less likely to see corrosion in the Mg-CF composite than you are with magnesium by itself.

[_yb2s]

The epoxy barrier might work, but in general encapsulated metals are risky because they are impossible to inspect for corrosion and cracking so fail without warning, and the encapsulation can block surface oxide formation which causes crevice corrosion- especially if small amounts of salt and water get in there, which they will over time, even in epoxy.

I’m sure what you are saying could be done- especially to basically add stiffness to key regions of a carbon racing bicycle, but it would be experimental and I would not trust it to last a long time

[ndsipa_pomu]

> I’m sure what you are saying could be done- especially to basically add stiffness to key regions of a carbon racing bicycle

I reckon it'd be a lot easier to just increase the wall thickness if you want that section of a carbon frame to be stiffer

[impossiblefork]

If one wants to do that kind of thing it might be better to fill the CFRP with something else-- stiff foam, balsa etc.

[_yb2s]

Yes, that is how carbon fiber racing sailboats are made

[impossiblefork]

Yes, and it's a kind of standard thing 'sandwich construction'.

[ndsipa_pomu]

If you're thinking of wrapping a material in carbon fibre, why not just use carbon fibre composite in the first place? Is magnesium stronger than CF for a given weight?

[A_D_E_P_T]

Magnesium, especially a casting, can be something like an order of magnitude cheaper. Carbon fiber is an intrinsically more expensive material, and manufacturing complex engineering parts solely with CF, to high quality standards, is almost an artisanal process.

[ndsipa_pomu]

> Magnesium, especially a casting, can be something like an order of magnitude cheaper.

That doesn't seem to be borne out by bike prices - it's entirely possible to buy a very usable carbon fibre bike for approx £1000 but I can't recall seeing a magnesium framed bike for £100.

Edit: looking at cheap frames on AliExpress, you're not too far off. I saw a magnesium alloy frame for approx £80 and a carbon fibre frame for £350. Not quite an order of magnitude though.

[gravelc]

I had a Merida Magnesium 909 road bike back in the day. They were common in Australia. Was (wrongly) convinced magnesium was going to overtake carbon. Never had any issues in 10 years of ownership and a lot of kms. Welds looked shocking and it was very rigid and unforgiving though.

[giamma]

Vaast bikes [0] makes magnesium bikes today, I own one (A/1 gravel) and it's a very pleasant ride.

[0] https://www.vaastbikes.com/

[TomK32]

I have a Kirk Revolution frame sitting next to my desk waiting to be repainted as I don't like it's turquoise colour. Uncracked as many others out there. Looking at it as a very first puts the issue about cracking bottom brackets into a different light. How many other firsts in any tech do fail and show where the next iteration needs to happen? I think it's quite sad it didn't see any more iterations.

[3eb7988a1663]

Can modern material science model this computationally, or does everything have to be observed experimentally? This kind of insane just-so recipe - are researchers just iterating on hundreds of thousands of different alloy compositions and production techniques or are there strong theoretical principles on which some of this can be derived?

[choilive]

Its been some time detached from the mat sci folks deeply involved in the space but its both. There is a bunch of theoretical underpinnings but ultimately a lot of throwing darts on the board as well.

[A_D_E_P_T]

Yeah. It depends a lot on the type of material, too. Conventional metal alloys -- like LPSO-Mg -- are the toughest to model. Too many variables. Ceramics and intermetallics are a lot easier to model in principle, but they can have surprising properties on an atomic level, and there's really no predictive method for that sort of thing. Modeling does get you pretty far with high-entropy alloys -- because to a substantial extent their properties hinge on how a bunch of different atoms might fit together randomly, and that's something that can be computationally predicted. A lot of the recent interest in HEAs is because they're relatively easy to model.

[rjsw]

Computational materials science is a current research area.

[rich_sasha]

I'm looking forward to a sleugh of articles telling me carbon frames have a really harsh ride quality and lack a certain "je ne said quoi" compared to magnesium frames.

[philjohn]

Quite - the grass is always greener.

With modern understanding of composites, and complex layups with UD fibre, the "not comfortable, too stiff" is less and less true. The reduction in road buzz I got when I finally moved to CF handlebars was noticeable.

[alanbernstein]

I learned about this company's magnesium frames recently: https://www.vaastbikes.com/company/

[metal_am]

Interesting. I haven’t heard of them. Joining would still be a problem for a bike frame. Any idea on how well they work with other severe plastic deformation processes?

[A_D_E_P_T]

They appear to benefit from ECAP, if that's what you mean. For e.g.: https://www.sciencedirect.com/science/article/abs/pii/S09215...

Presumably they'd also benefit from SMAT and other forms of surface modification.

That said, they're really good as extruded, and they don't appear to benefit as much from SPD as some steel and aluminum alloys do.

[genter]

How would you fabricate a frame from extruded tubing? Welding would destroy the grain structure.

[A_D_E_P_T]

That looks like it's an active research problem: https://www.sciencedirect.com/science/article/pii/S266633092...

At a glance, though, the problem doesn't seem insurmountable. FSW appears to work.

[le-mark]

Brazing has a long tradition in building bicycle frames.

[Scoundreller]

And lugs

[etrautmann]

Aren’t truck chassis increasingly made with epoxy or other adhesive bonds?

[rfmoz]

Magnesium is common in suspension forks lower legs.

[m463]

I thought stiff was a problem with aluminum - stronger is better but is stiffer better?

That said, I wonder if there is a way to make frames more comfortable without having the flex absorb pedaling energy.

[ndsipa_pomu]

I don't believe that the frame material makes much difference to comfort. The part that can deflect/absorb bumps and vibration the best is the tyre. So the answer to your question is bigger tyres at lower pressures.

[loeg]

Tires and to some extent seat post -- more exposed post deflects more than very short posts (like classic steel bike shapes).

[david-gpu]

Yeah, whenever I hear people talk of the properties of bike frame materials it reminds me of audiophiles: plenty of strong opinions backed by a remarkable absence of data.

A typical bike frame follows a truss structure: stiff and unyielding by design. Vertical compliance is going to be found elsewhere: tires, exposed seatpost, chamois/saddle, fork, handlebar, tape.

Yet, how many roadies do you find talking about suspension seatposts? It's all because in that subculture emulating present and past pro racers is seen as cool, and anything else isn't.

If you are curious, a few people like CYCLINGABOUT [0] and Overbiked Randonneuring [1] have done some measurements and the data suggests that suspension seatposts provide even more reduction in vibrations than wide tires.

[0] https://www.youtube.com/watch?v=u1e3g8uqrJU

[1] https://www.youtube.com/watch?v=rAkDaHlZQow

[ndsipa_pomu]

Yeah, suspension seatposts can provide 20mm of vertical compliance (varies for different models) which is more than you'd get with 28mm width tyres. I have tried a split stem suspension seatpost, but found that that particular design wasn't great as it was difficult to set the saddle up so that it didn't end up tilting as the two parts of the seatpost moved within the frame. I currently use just a standard carbon fibre seatpost that provides some compliance, but isn't a "suspension" type.

[mikeryan]

How’s it compare to carbon which is the most common high end, lightweight bike material these days?

[A_D_E_P_T]

As of right now, nobody makes LPSO alloys in commercial quantities. Fuji Light Metals has a pilot plant that does small-scale production of extruded strips and plates, but their customers are all researchers and R&D labs.

That said, we can extrapolate from mechanical properties. If we assume that both materials are tubes with the same wall thickness, and that we're looking at T300 carbon fiber (by far the most common type) in epoxy resin vs a standard research grade of LPSO, then:

- The CF will be stiffer

- The CF composite will be slightly less dense (1.6 gm/cc vs. ~1.8 gm/cc)

- The CF composite will have a slightly higher tensile strength, but the difference is very small and could be nonexistent in practice.

- LPSO-Mg will be more damage tolerant -- with better resistance to abrasion and better capacity to flex in a recoverable way in response to extreme mechanical stress. (Cast Mg alloys are undoubtedly worse than CF, but LPSO-Mg is a lot more like an aluminum alloy in this respect. It's a pretty ductile material.)

- LPSO-Mg should in principle be cheaper, though this is likely not going to be the case for a long time.

- LPSO-Mg will have better mechanical damping properties, so might transmit fewer vibrations to the rider.