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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.

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> 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.

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.

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.

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

> 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

Not really- even very thick carbon is quite flexible…. It works great for applications where you want that like bendy sailboat masts and front forks on bikes, but it should be cored or replaced with something else if you are looking for stiffness

That doesn't match with my experience. I've got a carbon fibre road bike and some parts of the frame are remarkably stiff whereas other areas such as the handlebars have noticeable flex.

It can be surprising to people just how tough/strong carbon fibre parts can be - here's Danny MacAskill's destructive testing of some CF wheels: https://www.youtube.com/watch?v=VfjjiHGuHoc

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

Yes, that is how carbon fiber racing sailboats are made

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

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?

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.

> 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.

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.

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/

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.

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