Why does the rear hinge need to be serviced?
The rear hinge - the join that allows the rear triangle and wheel to swing underneath the main frame to 'park' the bike - consists of a steel spindle within copper and nylon bushings, which are housed in in the main frame.
Those bushings are a wearable part; the nylon lubricates the spindle and allows it to rotate smoothly. Without them, all the contact would be steel-on-steel - which would quickly wear out the frame, not to mention being incredibly noisy. As the bike is used, the nylon wears away which eventually will necessitate their replacement. This can be done at your local Brompton dealer, and is best booked in advance.
How often will this need doing?
The time in which these bushings wear out varies enormously dependent on the amount of use, style of riding, rider weight/power output, number of folds, general cleanliness of the bike, etc.
Our resident test-to-destruction chap (a pretty nimble road racer when he’s not at work) rides about 30 miles a day, 3 days a week on his Brompton and he requires a rear hinge replacement service around every 4 months. For most daily riders though, we expect this to be done as part of an annual main overhaul service. For sporadic users it could be years before this work is required.
Keep an eye on the movement of the rear frame - if play develops, you want to think about booking in for service. Riding on worn bushings puts increased stress on the rear frame and retaining bolts.
Why can't I do it myself?
The task is reasonably complex, but more importantly it is very precise and safety-critical. Specific (and costly!) tooling is required, and if the job isn't done correctly there is a risk that the main frame will be written off. The responsibility for this one is really best left to the Brompton dealer network.