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Framebuilders Thinking about a custom frame? Lugged vs Fillet Brazed. Different Frame materials? Newvex or Pacenti Lugs? why get a custom Road, Mountain, or Track Frame? Got a question about framebuilding? Lets discuss framebuilding at it's finest.

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Old 05-06-11, 11:23 AM   #1
Lawrence08648
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Magnesium frame over titanium?

I want to order a custom titanium frame. After riding steel, alum, ti, and carbon, I like the ride titanium has. It has better road absorption than aluminum and in my opinion, rides better than carbon. I don't like the feel of a carbon bike. Plus I like the durability of Ti.

I understand you get get a custom frame in magnesium. The one site I visited said magnesium frames are 1/2 the weight of Ti. and better road absorption of vibrations than, alum, Ti, and carbon, though it did say it was not as durable as Ti. The mix in a magnesium frame is Mg, Alum, and zinc.

Anyone have any more info on a magnesium frame, or mag compared to Ti?
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Old 05-06-11, 09:15 PM   #2
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I thought it had been tried in the past and abandoned. I don't know of any builders using it, do you have any links?
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Old 05-06-11, 10:21 PM   #3
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Don't get it wet or near a fire!
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Old 05-06-11, 10:27 PM   #4
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I thought it had been tried in the past and abandoned. I don't know of any builders using it, do you have any links?
http://www.paketabikes.com/

http://www.zinncycles.com/customMg.php
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Old 05-08-11, 01:53 AM   #5
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I think I read that mg was largely abandoned due to galvanic corrosion problems.
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Old 05-16-11, 09:13 PM   #6
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I would not suggest Magnesium. I am a Mechanical Design Engineer, with a strong back ground in materials.

Mg = Magnesium; Al = Aluminum; St = steel; including Cro-Mo; Ti = Titanium

density (mass per unit volume): Mg 1.8; Al 2.7; Ti 4.5; St 7.8
stiffness (force for a given deflection): Mg 44 GPa; Al 69 GPa; Ti 100 GPa; St 205 GPa
yield strength: Mg 70 MPa; Al 6061 T6 276 MPa; Ti-3Al-2.5V 500 MPa; St 4130 435 MPa

reference: http://www.matweb.com

In practical terms; you can easily permanently bend magnesium with your fingers; it needs to be protected from high pressure loading. For example, if you made handle bars the same size and thickness has standard aluminum handle bars; you could easily leave permanent and deep impressions just by squeezing it with your hands; and the first heavy braking would result in severely bent if not broken bars. As noted, magnesium has some corrosion issues; but they are not significantly different from aluminum.

Magnesium is used for some notebook computers, where its ability to be cast into very intricate shapes provides enough of an advantage to make sense.

For fabricate a traditional style bicycle (joined tubing); magnesium does not make sense. If were to cast or mold the frame in one piece, magnesium MIGHT be worth another look.

Last edited by nfmisso; 05-16-11 at 09:14 PM. Reason: reference
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Old 05-16-11, 10:02 PM   #7
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Here is a link to a mg frame builder:

http://www.paketabikes.com/index.cfm?page=paketa_scud

The mg tubing is proprietary. They claim there is no more corrosion then Al or Steel.
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Old 05-16-11, 10:08 PM   #8
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There was a cast magnesium frame available a few decades back. Brand name of Kirk, IIRC. It was a couple of pounds heavier than the steel frames of the day and apparently didn't offer any advantages to offset the weight. The only one I ever saw in person was hanging on a bike shop wall alongside an Exxon Graftek and a broken Teledyne Titan. The shop owner called it his "shrine of bad ideas".
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Old 05-17-11, 03:59 AM   #9
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My mother rides two custom magnesium frames, both by Segal: One road, one tri bike. Both weigh pretty much the same as a carbon bike - after her Madone was stolen, she had a choice for a fresh start and went with the Mg frames. Several of her friends are now switching, including her trainer.

She competes regularly in triathlons at the top of her age-group and commutes 70km a day by bike and the bikes have been rock-solid and reliable. Sure, the welds don't look as elegant as lugged steel (they're uglier than my Trek 1.2's TIG welds), but if it works, it works.
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Old 05-17-11, 05:41 AM   #10
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hm paketa quotes 1400-1600 g frame weight for their Scud racing model. Not worth it then IMHO, my aluminium bike which is by no means a high end machine comes in at 1300 g. Heck, even my old ALAN is 1600 g frame weight! on top of that people have been building steel bikes in 953 or xCr in the 1500 g ballpark, so even steel can top these units.
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Old 05-17-11, 02:02 PM   #11
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Quote:
Originally Posted by Lawrence08648 View Post
I want to order a custom titanium frame. After riding steel, alum, ti, and carbon, I like the ride titanium has. It has better road absorption than aluminum and in my opinion, rides better than carbon. I don't like the feel of a carbon bike. Plus I like the durability of Ti.

I understand you get get a custom frame in magnesium. The one site I visited said magnesium frames are 1/2 the weight of Ti. and better road absorption of vibrations than, alum, Ti, and carbon, though it did say it was not as durable as Ti. The mix in a magnesium frame is Mg, Alum, and zinc.

Anyone have any more info on a magnesium frame, or mag compared to Ti?
If you want durable Magnesium is not it stick with Ti.
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Old 05-18-11, 08:47 PM   #12
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If you want durable Magnesium is not it stick with Ti.
True. But this could also be a niche market for independent framebuilders.

In terms of weight Al is pretty competitive. I think people willing to spend the money would likely favour Ti or Mg for ride quality and not weight.
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Old 05-19-11, 01:32 AM   #13
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True. But this could also be a niche market for independent framebuilders.
From what I've read and what others siad above it sounds like something UNsuitable for indepent builders, since it's hard to machine right and even tougher to weld. The equipment necessary also sounds like something special and hard to find, hence expensive. So please everyone, get back to your steel, carbon and alu already. Or find some alloys that are tough and easy to weld
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Old 05-19-11, 04:14 AM   #14
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Originally Posted by nfmisso View Post
I would not suggest Magnesium. I am a Mechanical Design Engineer, with a strong back ground in materials.

Mg = Magnesium; Al = Aluminum; St = steel; including Cro-Mo; Ti = Titanium

density (mass per unit volume): Mg 1.8; Al 2.7; Ti 4.5; St 7.8
stiffness (force for a given deflection): Mg 44 GPa; Al 69 GPa; Ti 100 GPa; St 205 GPa
yield strength: Mg 70 MPa; Al 6061 T6 276 MPa; Ti-3Al-2.5V 500 MPa; St 4130 435 MPa

reference: http://www.matweb.com

In practical terms; you can easily permanently bend magnesium with your fingers; it needs to be protected from high pressure loading. For example, if you made handle bars the same size and thickness has standard aluminum handle bars; you could easily leave permanent and deep impressions just by squeezing it with your hands; and the first heavy braking would result in severely bent if not broken bars. As noted, magnesium has some corrosion issues; but they are not significantly different from aluminum.

Magnesium is used for some notebook computers, where its ability to be cast into very intricate shapes provides enough of an advantage to make sense.

For fabricate a traditional style bicycle (joined tubing); magnesium does not make sense. If were to cast or mold the frame in one piece, magnesium MIGHT be worth another look.
I'm sorry, but I am materials engineer and a large volume of that is nonsense.

If you can manufacture a frame and chassis for a notebook computer in 'magnesium' - which, BTW is as thin as 1mm in some places, and regular handling and impacts do not damage either the chassis or its contents, then how would you be able to bend a large-diameter tube with walls much thicker than that by heavy handling?

No-one uses pure magnesium for structural engineering, in much the same way no-on euses pure aluminium. The lowest strength aluminium alloys used in bicycle manufacture, the AA 6xxx series are of the order of 5 times stronger than pure aluminum. In much the same way, the lowest strength magnesium alloys used in structural engineering are of the order of 7 times stronger than pure magnesium. To put this into perspective, AISI 4130 is at least 3 times stronger than pure iron, so taking the mechanical properties of a pure metal as your starting point is pretty broken. You could have picked one of many Mg- rare-earth or Mg-Mn- rare-earth alloy systems. You'd have found they're very similar in strength to the intermediate aluminium alloys.

For reference, magnesium is not actually very easy to cast at all. Its surface tension is awful and it explosively reacts with water vapour when molten. Old foundry casting techniques for magnesium alloys required someone to dust the stream of metal with sulphur powder to reduce the surrounding oxygen levels, preventing too much burning of the metal. Magnesium sulphide readily disoociates in the presence of air anyways, so there is no risk of it being entrainied in the casting.

I have never felt comfortable with magnesium for bicycle manufacture, that's no secret, but not for any of the reasosn you've mentioned.

It's a film-forming alloy, very susceptible ot reaction with oxygen and forms stable, thin, ceramic oxide films on it surface, all the more so when liquid. As with aluminium, titanium and stainless steels, as a result, no welds in it are sound and no castings with a feed rate above the critical velocity are integral. It is a hexagonal metal, with the concomitant limited slip systems for plastic deformation and it demonstrates low fracture toughness in all three modes.

Titanium I can forgive for its issue with oxide films, because above 600 degrees most titanium alloys dissolve their own oxide and it is the only commercially available metal that actually has an alloy system with oxygen. So while it forms films, its welds are, by definition, oxygen alloyed and safe. Stainless steels have enormous fracture toughnesses no matter their crystallography because they're iron alloys and in almost all cases are blessed by iron's huge ability to absorb plastic work, so a 'defective' weld is still extremely forgiving.

Neither aluminium or magnesium have either of these strengths. At least aluminium is cubic.

Galvanics now, that's whole other argument.
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Old 05-19-11, 02:28 PM   #15
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The Kirk, a cast magnesium frame, comes to mind ,
I think it was a British Mountain bike ..

I beams rather than tubes as I recall, was the structural theme.
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Old 05-20-11, 03:26 AM   #16
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The Kirk, by Dawes was a disasterpiece.

One of my old lecturers told them their product was essentially a deathtrap, what with the horrific quantity of inclusions and voids scattered willy-nilly throughout the structure, and he wasn't just a materials expert, he was a regular and proficient cyclist, too.

Again, it wasn't pure magnesium used, but an alloy. I don't recall the designation.

Actually, I think casting in magnesium with modern casting technologies is probably a very good way to go, if there is a way to ensure big, fat radii on the hollow interior. It eliminates weld defects and as long as the mould were filled quiescently, oxide fold defts, too. The downside to quiescent, smooth-front filling is it would take about seventy years to fill the mould cavity. Ho-hum.
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Old 05-20-11, 07:21 AM   #17
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Nice read Falanx!
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