This is a replacement chainring that is larger in size than the standard version that usually comes on a Brompton. It increases the gearing by 8% allowing you to go faster, but your low gears will be raised by the same amount, reducing your ability to climb steep hills. Get this option if you live in a flat area, or are a particularly strong rider. When purchased at the same time as a new Brompton, we will install and alter the chain length to the proper length. 

54T black chainring & guard assembly for Spider crank. Brompton part QCHRINGA-SPI-54-BK