Bicycle Mechanics - What is a 3 piece crank?

Stupid question i know- and one i should know too :eek:

What is a three piece crank? What is a one piece crank? What is the difference!

I feel ashamed to be asking this. :o

A three-piece crank has three pieces: a spindle and a pair of arms. The arms bolt onto the spindle.

A one-piece crank has a single, forged piece that serves as the arms and the spindle.

This is a 3 piece crank. It has a left crank arm, a right crank arm with a "spider" that holds the chainrings, and the bottom bracket. There is no bottom bracket in this picture.
http://www.os-bikes.de/TRUVATIV-Parts/600FiveD.jpg

This is a one piece crank. I dont really know how it attaches to the bottom bracket though.
http://www.chainganglowrider.com/31885%20Crank%20Twist%20Ch.jpg

One piece is generally a BMX crank. Or single speed budget bike. It uses a larger BB housing, big bearings, and basically a big nut on each end to hold it all together and preload the bearings. And they're pretty much a cheap casting. 3 piece can be cast, forged, or machined. Much better strength to weight ratio.

One piece is generally a BMX crank.

Only on the kiddie bikes. Those things might have cut it for us in the '80s, but no one runs them anymore.

Uh huh, just as i thought. Thanks for your help guys.

Only on the kiddie bikes. Those things might have cut it for us in the '80s, but no one runs them anymore.

Maybe on your planet, but over here you can still get a BMX with a one piece. Yes, scary.

On a complete bike.

Like I said, on the kiddie bikes.

I've seen plenty on adult bikes, not just kids BMX, and not just at Big W. Cruisers, choppers, and single speed grocery getters. The simple and cheap one piece crank is alive and well down under.

If you're referring to something like a Specialized, Fit, or Haro complete with a one-piece crank, that's exactly the kind of bike that you're going to find a kid riding. Go to your local bike shop and ask how many adults (or older teens) are buying complete BMX bikes. The number will probably be pretty low.

Anyway, to get back on topic:
if 3p=[3 piece cranks], and 1p=[1 piece cranks], then 3p>1p when 3p is not forged

oh but not when x-4y > 3z-4/d !

Back in the olden days when a square BB spindle* was the only option with 3 piece cranks - we larger fellows often needed to go 1 piece for the strength. After stripping 2 sets of alloy cranks and unable to afford the Profiles, I threw in set of Tange Cro-Mo 188mm cranks. Those boat-anchors lasted through 3 seasons of BMX racing and a fair amount of jumping. They did weigh a ton, though.

*in grammar school I used to sketch ideas for bike parts. Frustrated with replacing cranks I sketched a notched BB/Crank system that looked almost exactly like the future Shimano Octalink. I should sue... ;-)

Before the square BB spindle came along the 3 piece cranks were held onto the spindle with tapered cotter pins - I hated them.

the primo cranks use a square spindle.

There are 2 types of square spindle, the square taper and the Primo style which uses pinchbolt arms.
And some pros still use 1pc cranks, specifically flatland bmxers.