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.
More than any other tool in the kit, a reliable lever makes changing a tire easier, especially if you have road bike tires, which are difficult to remove. However, throughout testing, levers seemed to be the one item more prone to failure and poor design than anything else. For example, I found a random orange lever floating around my basement and I threw it in the test pool for fun—it seemed solid enough … until I began prying at the rim of a tire. It bent directly in half, slowly and smoothly, like taffy, and then was boomerang shaped forevermore.
Otherwise, this tool should serve the average commuter well. Specifically, we think you’ll find the size 4, 5, and 6 hex keys, extremely common sizes in bicycles, very helpful. They’ll adjust seat-post heights or let you remove the saddle entirely, or tighten a loose stem that’s always rattling apart. The Phillips head will tighten loose bolts on shoe cleats or the rear derailleur. The most common use for the torx would be adjusting disc brakes if you have ’em.
If you are planning on going on a multi-day ride or just riding a long distance in a day, you should have something for first aid, just in case. Unfortunately, almost everyone who cycles frequently will go down at least once in their life. This kit can be really small and compact and can include bandages, some gauze, and a small bottle of a disinfectant.
Brakes come in a variety of designs depending on the age and style of your bike. Rim brakes offer more modulation but have less stopping power, so are more suited to riding on the roads. Disc brakes can provide huge amounts of stopping power so are best suited to off road riding, although ‘V’ rim brakes are also well suited to mountain biking. All brakes use a friction system to stop the movement of the wheel, so regular checks of the contact pads should form part of your regular cycle maintenance.