Wednesday, January 23, 2013

Upgrade an American To European Bottom Bracket

How to convert an old single-speed bike into a multi-speed using an insert. Imagine the possibilities that open up by going to a standard European or English thread bottom bracket.

Why make this kind of upgrade? If your bike has a one-piece crank and you want to go faster, or re-build a bike into a fixie or a multi-speed, add toeclips or clipless pedals, or do any number of changes to customize your bicycle drive train, you'll need to change over to a three-piece crank. And to do that, you first have to change the threading of your bottom bracket. 

BBT adapter assembled and apart
You'll need a TruVativ bottom bracket adapter. It's a complete conversion kit with two aluminum bushings and bolts. 

BikemanforU's video above shows how to get the old crank out, the insert installed, and the bottom bracket ready for a new crankset.

Here's what you're working with: The shell itself is the lowest part of the bicycle and the center of your drive train. The shell holds the bottom bracket, which connects the crank. The crank is a term for the chainring or sprocket and the two arms that connect the crank to the pedals.

One-Piece American crank
One-piece cranks are older technology. They're a heavy and made of steel. The photo at left shows one without pedals. The center is a spindle connected to left and right arms. 

This American, or ashtabula, crank fits into two bearing cups. which make up the bottom bracket set secured inside the shell of a bicycle. 

You'll find these rugged, durable cranks on older U.S.-built bikes. They're still in use on cruisers, BMX, even some mountain bikes, as well as less expensive or department store bicycles. We sell a lot of them for repair and restoration.

In the video below. BikemanforU takes apart and replaces an old American crank, showing how the parts fit together. The video thumbnail shows an empty tourquoise BBT shell, with the chainstays, and downtubes welded to the outside. 


FYI: The world calls this a one-piece crank because it's a single metal forging of two arms and spindle.  

Three-piece cranksets for standard thread BBT
A modern three-piece crank, aka crankset, consists of the chainring or sprocket plus two arms. There is no axle or spindle as with the one-piece crank. 

They can come in colors and are made of aluminum alloy, like the single-speed cranksets at left. (Note, the photo shows only one crank arm.) A multi-speed bike would use a crankset with two or three chainring/sprockets. 

The three-piece cranks use different threads than the one-piece, hence the need for this TruVativ adapter. 

The adapter lets you install any bottom bracket set that you want, providing your shell measures 68mm in width. If you're not sure of the shell width, you can measure it with calipers.

Once you've cleared out the BBT shell, you're ready to install a new, sealed cartridge bottom bracket set. 

The spindle, or axle, since it's not part of a three-piece crankset, is included with the bottom bracket set that secures inside the shell. Spindle length is a consideration. For example, double or triple cranksets (with two or three chainring / sprockets), require a rather long length. The BikemanforU video below explores the different spindle sizes.

I've written about finding and fixing up old bikes abandoned on the side of the road, bikes that have been roadkill or whatever. Old single-speed clunkers. There's got to be millions out there. Send us your ideas. What would you build?

-- Mr. Pump

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