The frame is the part of a bike that determines how useful the bike will be, more than any other component. This page explains the most important ways in which frames vary.
The first consideration when choosing a frame is that it fits the user. This is crucial and is not worth compromising on. There is information on how to determine a rider’s size on the page about frame size.
When it comes to utility cycling in a rainy climate, such as the UK, the next consideration is compatibility with mudguards: the frame needs space between the frame and the tyres otherwise mudguards won’t fit. That space is known as ‘clearance’ and it varies between types of bike and between models. For instance, modern road bikes have very little clearance. The frame also needs places to secure the mudguards to. These usually include eyelets and bolt holes or bosses.
In order to be suitable for cycle luggage, a frame needs to be designed so that a rack can be fitted. This means it needs to have a relatively long wheelbase (with chainstays chainstays of at least 44cm, to prevent heel strike on panniers) and, ideally, a horizontal top tube, or one that is near-horizontal. The most useful frames are those with separate eyelets at the dropouts for mudguards and racks, and braze-ons at the seat stays for the rack.
Frames constructed from different materials tend to have different ride characteristics, some being more compliant and therefore more comfortable to ride. So steel frames tends to ride different than aluminium, carbon fibre composite or titanium. This is worth considering if you plan to ride long distances or to use the bike everyday. More about bike frame material.
A bike for day-to-day transport does not need suspension.
Paintwork can get chipped or scratched over time, which can lead to corrosion. I’ve written about my experience of repainting and retouching steel frames, including which brands of paint have been suitable and which haven’t.