Chains will stretch over time and can accelerate chain ring and sprocket wear. It is also recommended that you replace chains at the same time as you replace the chain ring and sprockets. The type of chain you can use is depending on the thickness of your sprocket. 2/6-speed Bromptons are factory equipped with 8 speed chain, while 3 speed Brompton uses single speed chain. For 2/6 speed Bromptons with 2mm sprockets (i.e., non-12T sprockets), you can safely use higher speed chains to decrease weight as well as rubbing noises.