Here are some tips for fixing old, found, abandoned bikes etc. [not robbed]:
[bike part drawings by Thomas McC and from various bike books]
Punctures: ok so this is probably the simplest problem to deal with on a bike, nonetheless theres many a bike thats been abandoned or left unused because of a puncture. Check to see if the wheel is punctured (pump it up & give it time to go back down or not...) and if so take a deep breath and follow the simple steps to mending it (you'll find them on the inside of a puncture repair kit). It never ceases to amaze me the amount of people who can't or rather haven't fixed a puncture, once you've done it once you'll be able to do it in 10-15 minutes and won't have to pay 5euro each time and the half day/day lost in a bikeshop.
Frames: The most important reason why a bike may be dumped is because the frame is cracked or bent. A bike can often look perfect but may contain a tiny hair-line crack that only the guy who dumped it knows about. The most common spots for a frame to crack/ break are along its welded joins (see illustration). Examine the frame and in particular these points, try to move them, check if weld is broken. Look along the frame for bent or dented areas.
Also check the forks for breaks/ cracks/ dents. If your found bike comes up positive for any of these, its best to leave it. It is possible to weld some breaks/ cracks but in general is a bad idea. Forks can be replaced if the rest of the frame is good. The wrecked frame may have some good parts though, evaluate if its worth stripping them off.
Chain & back gear block: If the chain is rusty, check how badly so (often a chain and gear block can be oiled/ scrubbed clean), if its is rusted through, you'll have to get a new or 2nd hand set. Yes these two come in a pair so if either one is damaged you have to replace both. A new set can be expensive (it can be possible to find a 2nd hand pair) so again examine bikes' potential.
Wheel: There are a few different problems you may find with a wheel the aforementioned puncture; replacement tyre/tube, buckled wheel/ broken spokes, wheel axle/ axle bearings. A new tyre/ tube is not a real problem, just get one! Depending on how badly, a buckled wheel could mean a replacement is needed (again straight forward enough) or that replacement spokes are needed (this will more than likely be the case anyway) and the wheel to be straightened. Unless you can DIY this is a job for you local bikeshop, but shouldn't be too expensive.
The Axle is in the middle of the wheel
it may need bearings and to be re-greased (if so it will shake at axle,
click, be loose) or the axle may be bent and need to be replaced. Again
buy the new parts and DIY with a bike manual or the bikeshop again (fairly
I once came across the rim of a wheel (where the wheel meets the tyre) that had been badly worn down and was quite rough, - the original owner had obviously cycled a fair distance with a flat tyre. It was still a good wheel though so I sanded down the rims 'till they were smooth again.
Bottom bracket: (this is the part joining the pedals/ cranks/ chainset to the frame). The possible problems are it needs new bearings, to be re-greased, or replaced altogether - you will know this if the cranks are wobbly or theres a recurring clicking sound. There are two types of bottom bracket, sealed (most newer bikes) and not-sealed, - those not-sealed can be re-greased and the bearings replaced. In general working on the bottom bracket can be very tricky, so be careful - eg: if the bottom bracket is screwed badly into the bike frame it can strip the threads on the frame - writing it off.
Cranks, Pedals: This is a fairly straight job if theres something wrong eg: clicking [bearings replaced/ complete replacement]; creaking [cotter pin (on older bikes this is the pin that holds the crank to the axle) needs tightening/ replacing].
Brakes: No matter what the state of this found bike is - the brakes need a job of some sort - in general new cable, new brake blocks, and re-adjusting. Depending on type of brakes this will need some guidance from a manual or bikeshop to get sharp.
Gears: Again this is a tricky area, can be broken into cable (needs replacing), levers, derailleurs. Consult manual or bikeshop.
Basic tools: brillo pad/ good scrubber, hot water for rust; adjustable spanner; spanners: 8mil, 9mil, 10mil, 13mil, 15mil etc.; allen keys; flathead and philips scredriver; plyers.
These are some basic tips - for more detail & more perfection consult a bike book. Anybody can own and cycle a bike, in todays society where economic discrimination is increasingly a factor in everything, owning a bicycle is not subject to this. Whether acquired from a skip, an old garage/ shed, the ditch, or bought 2nd hand - it can be built and maintained without much expense while being completely sustainable and good for the soul!