What is the carbon footprint of a new bike? When you look at the complete journey from bauxite to bicycle via China, a big container ship and the bicycle showroom, there probably is quite a lot of CO2 emissions involved in making a new bicycle.

Are their any easily quotable figures for this?

Are there any handy equivalent metrics, e.g. equivalent to x miles in a family saloon or equivalent to x hundred packets of Walkers Crisps (at 80g per pack)?

I would like to know a generally agreed figure as I have found myself being stumped in conversation with motorists that (after conversation has moved onto their planet trashing ways) have the cheek and audacity to criticise cyclists.

What happens is that they go ad-hominem, 'pointing out' that buying a new bicycle is 'bad for the environment', particularly one made on the other side of the planet. Rather than have no defence I would like to be able to say 'yeah but that is only equivalent to x hundred of your gas-guzzling miles/hours 'sat' in traffic.'

Has anyone done a carbon audit for a bicycle yet? If so, are there any particularly low-carbon bikes out there?

Note:

I mention the Walkers Crisps in passing - no I don't want to know that cycling has a huge footprint if the fuel (food) is caviar/fancy-mini-corn from Zambia/lamb from New Zealand/fish from the Arctic (or even a nice pack of Prawn Cocktail crisps prepared with sunflower oil grown in Eastern Europe). Just the carbon footprint from whatever bike brands have bothered to measure it, not the on-going fuel costs (food).
I don't have numbers on hand. But if you're after a cogent argument, I would point out that the car they drive has a far larger manufacturing footprint, even assuming it is manufactured in the local economy, which is rare. If it is, then was the raw steel manufactured there or shipped in? 

For that argument of theirs to be effective, they must start with the idea that their car popped out of thin air. Even assuming that the manufacturing footprint is identical, the car continues to build that carbon debt, while the bicycle mostly ceases to charge that card after the manufacturing is done. 

Of course there will be minor maintenance items, and consumables like tires and tubes, but the car has those, too. No matter how you look at it, a bicycle doesn't burn fuel to function and a car does.

So the car will use far more resources over the course of it's useful life.

Also, whether you believe that global warming is real and important, or not, it will affect your finances far more to drive a car than to ride a bike.