And ease of fabrication don’t enter into it. Not on high end bikes. Do you think Titanium is easy to fabricate?
Last thing. I promise. I think the OP has a mistake idea about the thickness of a lightweight bike frame. It is not like a lead pipe, or even a copper pipe from the plumbing section. If you sawed through the middle of a double butted bike frame you would see a very thin wall, less than a mm. Imagine making an I beam of that thickness. You could bend it with your bare hands.
Why are axles and halfshafts and made of solid tubes?
For very high performance applications, axles are gun drilled. Here are some that would work fine in your NASCAR Nextel cup car.
Or worse, the all plastic Itera. :dubious:
What about cost?
Cost is absolutely no object in high performance bikes. Joke:
How do you tell a guy from Colorado?
He’s got a $10,000 mountain bike on top of his $2,000 car (usually beater Subaru).
Do you know how much it takes to field a compititor in the Tour De France? Cost? Pshaw. I can show you a bike that costs more than a new Benz.
Because you aren’t trying to shave every gram off you Ford Fairmont. Yeah, lets make bike frames out of solid steel shafts. Why didn’t I think of that! :smack:
I agree that you are probably right when applying the OP to high performance bicycles. However, I hadn’t assumed Key Lime Guy specified that he meant only high performance bikes. You’re probably right though.
Now, despite that, what kind of cost would we be talking about? Not everyone can afford a several thousand dollar bike. When I used to ride, my bike cost me less than a thousand bucks. I used to ride on average between 50- 60 miles a day.
A hollow tube is stronger for the same mass. Meaning if you take the material from a solid tube and reshape it into a wider hollow tube, it will be stronger.
But for a given diameter, a solid tube is going to be stronger – drilling out the center of a tube only makes it weaker.
If there’s limited diameter space for an axle, it is stronger (and cheaper) to just make it solid.
BTW the axles in bicycle wheels are not solid – they’re fairly wide tubes. So is the ‘axle’ that the handlebars & front fork rotate around. The ‘axle’ that connects the two pedal arms through the bottom bracket is solid, though.
Yah, that is good for cars. Not bicycles.
You are thinking of the HUBS, through which the axles must pass. The axles are also hollow in some cases to accomidate the quick-release mechanism, but this is more for the mechanism than for reducing the mass of the axle.
That is because there is more stess on the bottom bracket than any other part.
Lissen folks, go make your bicycle out of I-Beams and enter it into the Tour De France. I am sure you will be the weener.
Mods? Has this been asked and answered?
Actually, this thread has made me think of getting a nice road bike again. I expect to pay maybe $600. I have one of Raleigh’s mountain bikes and have owned a Record, a Super Course and a Gran Sport in the past. My only non Raleigh bike was a Dawes, and I liked it a lot, but am going back. That being said, I am amazed at the tech you get in a moderately priced bike today. 
A good road bike shouldn’t cost a thousand dollars, even today.
No, the reason is limited room. The diameter of the bottom bracket shell (the “hole” in the frame where the bottom bracket fits) is standardized to 1-3/8". You can’t fit a ball bearing and a sufficiently strong hollow shaft in there; you have to use a solid shaft to get sufficient strength.
But if you weren’t so constrained, you could save weight and improve strength by using a larger diameter hollow shaft. Truvativ actually makes such a system; it still fits in the same shell, but the bearing sits outside the shell, allowing a larger diameter shaft.
But of course, a hollow shaft is also more expensive to manufacture, and it’s more tricky to attach the cranks. Which is why bikes traditionally used solid shafts (and still do, mostly), and the bottom bracket shell size was standardized to such a small size.
Stiffness and weight are only a couple of the design constraints facing a bicycle manufacturer - others are materials availability & cost, ease of manufacture
Umm. nope. Cost and ease of manufacture don’t play into this. With bikes costing upwards of $30,000, any edge that can be had will be had for almost any price. Like I posted before, Titanium isn’t exactly easy to work. Or carbon fiber. Pound for pound, a high end bike is one of the more expensive manufactured objects you can name, running about $2000 per pound. Oherwise your post was right on.
Despite** Happy Wanderer’s ** musing on the subject, new bottom brackets do use a hollow spindle. Shimano has been doing it for a few years with the Octalink and the more recent Hollowtech II bottom brackets. Campagnolo is introducing it’s new Ultra-Torque crank soon. Look here.
Wouldn’t that measurment depend on the frame’s origin? Last time I was cycling seriously, there were English, Italian and French sizes. In any case, a bottom bracket that accomodated a thicker hollow shaft would also wind up giving you a weight penalty with bigger bearings, mounting plates, size of the bottom bracket tube, etc. Just make it out of Titanium and drill it out 
I don’t believe I ever said that this was true. I just responded to another post who said why is the bottom bracket solid. I was unaware of the new designs. I would be interested to see the nature of these designs, and how they hold up. A solid axle makes sense in the bottom bracket not only due to limited space, but also to simplify the mounting of the crank arms though either a cottered or cotterless system. I suppose a hollowed out bottom bracket axle could be machined out of titanium or something. The lengths they will go to to save a gram or two!
Hmm, I didn’t realize that. I guess the Truvativ external-bearing design is not really about allowing a hollow shaft, and more about moving the bearings outwards for improved strength and/or stiffness, or allowing larger, more durable bearings.
Almost all modern bikes use the ISO size. Though BMX bikes actuallly have larger size shell, I believe.
Cite?
The Cervélo Soloist Carbon goes for $5,200, built with Dura-Ace. Similar top of the line bikes from Trek, Specialized, BMC, etc. are about the same price. Where are these ultra expensive bikes you keep talking about?
Since when did this become a thread about high-end bikes? The $90 Walmart bikes are made of hollow tubes too, and it’s reasonable to wonder why it’s better than I-beams.
And I haven’t come accross $30,000 bikes either…