Why do bikes need Allen keys?

[Bonez]

Though for general bolts around the bike is torque an issue?

Just tighten them up till the threads strip, then back off half a turn.

[Motu]

Have you worked on my bike before? Just wondering.....

[Bonez]

Valuable lesson I learnt working on my old Hondas. I'm sure it's cheese they use in the castings..................

[Motu]

I found cheese too - so it was you!

[Oscar]

What puzzles me is why Allen....what if Desmond invented them....would we then be calling them Desmond keys?

Good thing I didn't invent them..."...could you pass the grumpycahnt key please?"

[HenryDorsetCase]

One Hornet owner I know recently bought a set of gold plated Allen screws for his new pride and joy.
....

oooooOOOOOOoooooo where from??

*slap me for gods sake*

[McJim]

I was just looking at some of the cheap flat packed furniture we bought (like the computer desk I'm sitting at) they all use allan key fittings so that puts modern bikes in the same quality bracket as flat packed furniture.....hmmm....anyone wanna buy a VTR 250? - I'm gonna buy a cage.

[jonbuoy]

Allen keys still look pretty mint even after being used a few times, screws get all chewed up especially when people use a pozi in a philips and vice versa although they say you can they still don't fit right.

[Pixie]

Not entirely. The screws were tightened at the factory using air operated equipment. Often they were very tight (and I believe tended to tighten up in use, too).

Removing them with an ordinary Phillips screwdriver could be very difficult. The design of the Phillips screw is such that , unless a considerable axial force is maintained, the driver , with its angled tips, will tend to "cam" up the slots in the screw.

I beg to differ.
If this was the case the air tool would "cam up" on the head when the screw was fastened.
I never rounded a screw that was pristine and rarely had to resort to an impact driver.
I've also seen many people try to undo a # 3 phillips ( the one commonly used to retain cases on Japanese motors ) with a # 2 driver.

[Ixion]

They didn't because the air driver had a powerful longituditional force applied to it. ie it was firmly held down into the "slot". That was (and is) the secret to undoing them. Lots of pressure along the shaft of the screwdriver. I seldom (I'd like to say never, but I'm not tempting fate) mangled a head. But there have been some too tight to undo without recourse to the impact driver.

[Pixie]

One of the major attractions of capscrews is the thread is cut right up to the head there is no "blank" section of flank.

Whether a fastener has an unthreaded shank section is pure related to whether it is a bolt style fastener or a machine screw.
Capscrews are available as bolt style (partially unthreaded ) as well as machine screws
http://www.boltscience.com/pages/glossary.htm
http://euler9.tripod.com/bolt-database/22.html

[Ixion]

There are also good reasons why an unthreaded shank is often desireable.

[Bonez]

There are also good reasons why an unthreaded shank is often desireable.

I was going to point that out too. Also some are recessed-dia of the threads groove or less, and some are the same dia as the threads outside diamension.

[Pixie]

There are also good reasons why an unthreaded shank is often desireable.

Yes.
The unused threaded section can "saw" away at the hole in some cases.

[Ixion]

It can also act to draw oil past gaskets, along the thread . And in some cases tightening a "fully threaded" bolt retaining a cover or such like , can cause the thread to "pick up" on the retained cover, twisting it (the cover) or subjecting it to a non clamping force. This will "unwind" with time causing the torque on the bolt to also relax - ie it "comes loose", to the vexation of the owner.

If the designer specified an unthreaded shank, you may be sure he had a good reason to do so. A wise man respects such design decisions.