First, let me note that there is actually nothing special about fixing a folding bike chain. It’s the same as fixing a chain on any bike. As far as I know, there are few if any folding bicycles with special chains.
Folding bike chain philosophy
My philosophy is that you should replace your chain once per year. If you do that, it is highly unlikely you’ll ever wear one out, or be left stranded on the side of the road with a broken chain. I like to change mine at the transition to daylight savings time in the Spring each year. A chain costs 10 – 30 dollars. It’s worth spending that much once a year to be sure you won’t have chain problems.
Having said that, sometimes you need to deal with a pesky chain issue while on a ride, and can’t replace the chain at that moment. Most common is that a link gets sticky; meaning that the joint associated with two adjacent links doesn’t pivot very well. That causes the chain to make a funny skipping sound as it goes around the rear cog and through the derailleur pulleys. You can try carefully bending the chain laterally (sideways against the natural direction of pivoting of the chain) back and forth a little bit in an attempt to loosen up the offending joint. That usually works. Still, if you’re having that kind of issue and you’ve been lubricating your chain, I’d be inclined to replace it when you get back home.
Keep the folding bike chain clean
And as a second aside, you really need to make sure your chain is clean and well lubricated. Many of the guys I ride with believe in all kinds of mystical properties of chain lubricants. They can spend dozens of dollars on a tiny tube of exotic aerospace lubricant. As far as I can tell, there are really just two kinds of lubricant. There are “oils” which stay oily on your chain and there are “dry” lubricants which use silicone, graphite, and sometimes waxes to give you some lubrication without any liquid-y qualities. The argument for the dry lubes is that they don’t attract and bind with dust and dirt. Yeah, maybe. I’m not really convinced. Personally, I favor wiping down the chain before every ride and applying motor oil to the chain via a little squeeze bottle. Then, you wipe it down again before you head out on the ride. I put my bike up on a stand, then I rest my left hand on the derailleur cage and place the spout of the squeeze bottle on the inside edge of the chain. Then I use my right hand to crank the pedals backwards. I squeeze lube onto the inside edge of the chain until everything is nice and oily and I keep cranking for a dozen or so cycles to work the oil into the joints. I then hold a dry cloth on the chain with my left hand as I crank backwards with my right hand, to remove excess lubricant. I’d rather spend my money on a new chain every season than on some exotic lubricant.
Replacing a folding bike chain is not that hard. We give instructions here.
The main thing you have to worry about is getting the right chain for your rear cluster and derailleur. In simple terms, chains come in different widths for different numbers of cogs in the rear cassette. The widest chains are for single-speed bikes (BMX and Fixies). Next widest is for 5-6-7 speed cassettes. Then 8-speed cassettes. Then 9-speed cassettes. Finally, 10-speed cassettes take the narrowest chains. Having said that, I think it makes sense that if you have a 9 or 10-speed cassette that you match the manufacturer of the cassette and derailleur with the chains. In most cases that means using a SRAM chain with a SRAM cassette or a Shimano chain with a Shimano cassette That may be superstition, but the geometry of the teeth on a Shimano cassette is different enough from that of a SRAM cassette that I think it makes sense to match up brands.
The Xootr Swift uses an utterly standard 8-speed SRAM cassette and derailleur. You can use pretty near any 8-speed chain on it. We like the KMC Z72 because it’s inexpensive and bullet proof.