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Old 01-23-2008, 08:28 PM   #28 (permalink)
trebuchet03
MechE
 
Join Date: Dec 2007
Location: Bay Area
Posts: 1,151

The Miata - '01 Mazda MX-5 Miata
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Quote:
Originally Posted by GenKreton View Post
May I ask why Ti is good? Why is Aluminum a poorer choice?
I know from my extensive theoretical work with metallurgy that aluminum has a cyclical limit. I would assume the bicycle manufacturers take this into consideration but knowing the lack of integrity in many industries it wouldn't surprise me if aluminum was an excuse for specific sections or components to fail.
It's very simple... Stiffness (the material property) et. al.

Stiffness
Steel: 30 million psi
Titanium: 15-17.5 million psi
Aluminum: 10 million psi

Density
Steel: 7.8 g/cm^3
Ti: 4.506 g/cm^3
Al: 2.74 g/cm^3

Hardness
Steel: 120HB
Ti: 330HB
Al:15HB

Ductility
Steel: 10-15%
Ti: 20-30%
Al: 6-12%

Aluminum is 1/3 as stiff as Steel - very soft and not very ductile by comparison. Titanium, while a little more than half as strong as steel is also nearly half as dense - but incredibly hard and ductile. Aluminum has no fatigue threshold whereas Titanium and Steel do - that means, as long as you stay below that threshold for steel and Ti, it will not fatigue (sounds like you know this already - just posting for thoroughness).

Al has it's place though - my bike is Al. To compensate for the low stiffness and fatigue, my frame's backbone is has a large diameter (if you can't win on materials - you can make up for it with geometry). But, both of my forks are steel (chro-mo). My roommates hybrid like mountain bike is Al (with steel fork) - and I can see what Trek did to keep stress concentrations from accumulating. He beats the living **** out of his frame - it's even crashed into a river once (hit him in the back on the way down) - so far, so good

So why not Ti considering it's awesomeness? It's freaking expensive to extract, process etc. And, it's finicky to manufacture with...

Quote:
And fenders are just more money to keep me a bit cleaner/dryer...
I have a rear fender on my 'bent... And thus far - I've loved it I'm in Florida, so it rains more often. I'm also riding a 'bent - so my back gets plenty wet (but water from the front wheel gets deflected by the frame and seat ). Fenders are a local decision - if you'll be riding in the rain frequently, it may be worthwhile

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I second the comment about no integrated headsets... F'ing retarded.

Panniers are great though When I ride my road bike - I use them all the time
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