Beware of the "Bicycle Shaped Object" or "BSO"

buying a bike flat-pack in a box means taking total responsibility for ensuring it is assembled to a safe standard. Unfortunately BSO's are almost always particularly hard to assemble to a safe standard, even where part of the assembly has been done for you. The tools supplied - and often no tools are supplied - are cheap and nasty pressed low-grade steel and not sufficient for the job. The two tools commonly supplied are a flat spanner and one or two hexagon (allen) keys. The metals these tools are made of is almost always very low-grade and soft, meaning that they are incapable of fastening some of the nuts/bolts up to the required torque levels as the tool is damaged in trying to do so. If you have your own tools, proper spanners and quality hex keys, then this is less of a problem.
You will not be supplied with a pump, so unless you have your own, you will be unable to inflate the tyres.

The parts to be assembled are almost always of a very poor quality which means they may be hard to fit and calibrate correctly. Instructions are often incomplete or sometimes missing or just so unhelpful as to render them worthless, which means ensuring parts are fitted correctly and safely - such as brakes and pedals - can be particularly difficult. Instruction/owners manuals supplied will not necessarily be specific to the product you are assembling.

Plus of course the owner themselves may have little or no experience of bicycle maintenance or correct use of tools.

For example, many BSO's are spotted with the front forks facing the wrong way because the owner did not know they had to be changed when removed from the box. This will mean the bicycle will not steer correctly and the brakes will not work properly. See our Where should I buy a bike from? page for actual examples.

to keep the price low the parts and components on a BSO will be of particularly low specification and be made from poor quality materials. That means they are really only able to cope with very light use, and will be prone to mechanical problems which are difficult to fix because of the quality. In particular, more steel will be used which rusts very easily and quickly. Many of these bikes are showing signs of rust as they come out of the box! A few weeks down the road and pretty much all unpainted surfaces, and nuts and bolts will have started to develop rust.

Features such as suspension or disc brakes will be of very poor design and quality, meaning they won't be able to do a proper job. Plus of course there's the weight of your BSO, which will almost certainly make getting around much harder work.

Brake levers will almost always be made of plastic with a thin metal rod inside the moulding, which means they flex when any pressure is applied to them.
The brake mechanisms with likely also be plastic, moulded over a metal former which will again flex when the brakes are applied.
Combine flex in the levers with flex in the brakes and often, the brake levers are touching the handlebars and still you can't stop the bike... even when the brakes are adjusted correctly!

Chainsets are another big problem area, with cranks being either solid steel, or thinner steel coated in plastic to make it look stronger. The chainrings are almost always hammer stamped on and it is not uncommon for this friction joint to come loose after a while allowing the crank arms to spin without driving the back wheel!
Even more common with cheap chainsets is the misalignment of the pedal threads such that the pedal feels odd when riding... it's very hard to explain this!

poor quality parts and components will almost certainly not survive regular use which means they will need to be replaced or repaired regularly. In fact on some BSO's they may never work properly in the first place. Ultimately this could mean spending more than you originally paid for the bike itself, just to keep it working, or that you might have to replace it sooner than expected.

We generally recommend spending a MINIMUM of about £200 on most styles of adult bike. In most cases a bike at this price will be of considerably better quality - and therefore value - than a bike that cost £100 - £150. Which means the "bargain" you picked up whilst at the supermarket really won't be such a good deal after all.

It isn't always so black and white and there are some reasonable quality bikes at below £200 but if you intend to buy at this price, do so from a reputable bike specialist.

perhaps one of the main reasons to avoid a BSO is because it will not offer you a particularly enjoyable cycling experience. Mechanical problems or safety issues - and the costs associated with fixing them - ultimately spoil the simple pleasure of riding your bike.

Gears that don't work properly, brakes that give-up at the first sign of a descent, uncomfortable saddles, and constant unseen rattles and shakes; will lead to a very frustrating time on your bike.