Spare tube:  Sometimes, a tube is just too damaged to be patched, or has had a failure such as the valve stem breaking off which isn’t something that can be patched.   To save weight and space, I usually keep a lightweight version of the right size tube for my bike.   Since one of the first things I do with most bikes that I own is switch them over to heavy-duty thorn resistant tubes, I just take one of the originals and put that in the toolkit.
In spite of seat bags’ diminutive size, a lot of people really, really want them to accommodate more stuff. If that’s you, the medium and large seat bag (but not the small) have an extendable gill at the bottom, which you can see in the image up top. It zips open to create more room. With it unzipped, we could cram a wallet, keys, and phone inside the medium as well.
Modern bottom brackets use a sealed bearing type construction that keeps water and debris locked out, so they seldom go wrong. Maintenance is impossible so they need replacing when they do wear out. Older style cup bottom brackets need to be re-greased every now and again, and occasionally will need to have their bearings replaced. There are a lot more teeth on the chainring cogs than those on the rear cassette, so need changing less regularly. Cranks usually a solid one piece construction so there is not much to go wrong.
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