possibly a dumb question... maybe?
 

simple question. why aren't bike frames ever cast? especially downhill/freeride frames? they'd be rock solid, and all one piece.... though probably ridiculously heavy?

I bet they'd be really expensive.

I'm guessing to quickly change designs/sizes. If they made one mold for every model, it would only be one size and everything. It is easier to cut up pieces of metal then weld it together to different sizes and so they could modify the design instead of just throwing out old molds...

Yeah, have you picked up a cast iron skillet? Now that's iron obviously but it's still heavy as a brick. I have a feeling new welding techniques make it close to the same strength.

You answered your own question. They would be ridiculously heavy. They would be too heavy even for the dowhillers.

Even those cheapo 50# Steel Toys 'R' Us bikes use tubing. Imagine how heavy they would be if they used solid metal???

Now a forged frame, that would be cool.

Welded joints are actually stronger that one piece angled curves....

As for castings, there are too many imperfections with the process, and plus, the molds are very expensive. The ones used for turbo (turbine wheel's) are over $50,000 each.

Cast iron by loose definition is 1.7% to 6.67 % carbon in steel and not really a process.
They do cast this liquid metal because the stuff is unworkable after. It is also very hard and brittle. But that is cast steel.
They do have cast aluminum such as die cast aluminum and others.
If a person were to cast 6061 aluminum what would happen???

Good question.
That is certainally not a dumb question
That is a question that had to be asked.
I will see my metrology teacher on Thursday and ask him.

thanks :D

Its a strength issue. Without going into the science of metal micro-structure and the difference in crystaline structure between a cast tube and a rolled&welded or extruded tube, the cast item is always going to be more brittle than a rolled/welded or extruded item of equivilent material subjected to similar heat treatment. Think about the crown of your (suspension) fork, it is without doubt a forging. Were it a casting it would need to be in the order of 50% larger for a similar strength. rolling and welding and extruding also have more in common with forging techniques than they do with casting. Look it up, a good book to start with is " Materials for the Engineering Technician" by R.A. Higgins. It goes deep enough without being immposible to understand for non technical types.

Well, for the metallurgists here: (slightly off topic, sorry)

Where can I find a good online explanation of the difference between various aluminum alloys--for example, between U6 and 6061?

Any supplier or manufacturer. Google is your friend.

This article is cool
"Metallurgy for Cyclists The Basics"
http://www2.sjsu.edu/orgs/asmtms/artcle/articl.htm
Yes. What Haro V3 said
The casting of say 6061 would have a crystaline structure and be too brittle so it would have to be alot thicker
But who knows. Maybe someday