This article was published in 2001, in Newsletter 36.
If someone offers you a second-hand bike or you are looking for one at a Police auction, how do you distinguish a bargain from a ‘booby prize’? Assuming you have a machine of the correct size in mind, here are the things to check before accepting it.
Frame and forks
|[A] and [B] These respective tubes should look parallel|
|[C] Wheels should be perfectly aligned, otherwise handling will suffer||[D] Derailleur should hang vertically|
The frame and forks are the most important part. If they are bent or distorted following a crash, the wheels won’t be in alignment and the handling will be noticeably unusual (or even dangerous). Frame damage can’t easily be fixed, so second-hand bikes with suspected frame or fork damage are best avoided. Major damage will be easy to spot by eye. Stand beside the bike, looking straight down onto the top tube, and check that it looks ‘parallel’ to the down tube [A]. Crouch down at the front and back of the bike, sight along the frame and check that the seat tube looks parallel to the head tube [B]. If you have a helper to hold the bike up for you, stand a few metres behind the bike and check that the wheels look parallel [C]. Any displacement or twist in any of these tests should sound alarm bells!
While you’re behind the bike, check to see if the rear derailleur mechanism hangs vertically [D]. If it leans inwards towards the wheel, the mount (‘hanger’ [E]) at the rear dropout is probably bent which will prevent you changing into all the gears. This can be fixed quite easily (DIY or bike shop) but it’s not a good sign.
|[E] Derailleur hanger can be straightened if it is bent||[F] Tiny wrinkles here are a bad sign||[G] Forks should not be pushed back like these|
Now try running your finger tips along the frame and fork tubes where they meet the head tube [F]. Crash damage often bends these tubes, leaving tiny ripples in the paint that are easier to spot by touch than by eye. Lastly, sight along the forks from above. They shouldn’t curve backwards at all, and both arms should be symmetrical [G].
|[H] Broken spokes will cause rim to move sideways|
Wheels are the next most critical component, and badly buckled wheels may make you think twice about buying a bike. Lift the bike up and spin each wheel slowly, looking for any wobbles. Minor wobbles (less than a few millimetres) can usually be trued or accepted. Localised lateral wobbles in the back wheel often result from a broken spoke, also easy to replace [H]. Beware of wobbles in heavily rusted wheels, which may be impossible to true (the spokes break rather than the nipples turn) and you may end up buying a completely new wheel!
Theoretically, bicycle chains ought to be replaced before they are 1% worn, otherwise you have to replace the cogs at the same time! (Twelve complete links of any new bike chain = 12 inches exactly.) However, when assessing a used bike, you may be content to put up with a chain (and the chain rings and cogs) which is slightly ‘worn’ as long as the bike is running smoothly and the drive is not causing problems such as very bad gear shifting-even a worn chain will last for years if it is well-lubricated. So, if possible, test-ride the bike to assess smoothness and feel. Be alert for any sensation of the chain ‘skipping’ or very hesitant gear shifting. A new chain which is skipping is running on worn cogs which you will need to replace. If you can’t do a test ride, look for signs of excessively worn chain. New chains cannot be lifted away from the chain ring, but worn ones can![J] A dry or rusty chain is a bad sign. Slip the chain off the chain rings (by disengaging it and turning the cranks backwards) and look closely for excessive ‘wave-shaped’ wear on the teeth. [K]
|[J] Very worn chain lifts easily||[K] Very worn chainring teeth|
|[L] Try moving cranks like this|
While the chain is still off the chain ring check the bottom bracket. Grab each crank in turn and see if you can feel any ‘play’ when you push it in towards the frame.[L] If you feel the same movement in both cranks, the bearings are loose or worn. (If only one crank moves, that crank bolt is just loose!) Also, try turning the cranks without the chain. A smooth easy action is good, but any ‘grinding’ sensation you feel indicates worn bearings. Cartridge bottom brackets are non-adjustable, so problems mean that the whole unit needs replacement. Traditional ‘cup and ball’ systems usually respond to careful disassembly and regreasing.
Worn or damaged tyres will need replacing. Slowly spin each wheel. Any bulges, ‘frayed’ casing or cuts in the tyre sidewalls require urgent tyre replacement (but a ‘rash’ of micro-cracks is OK). Minor cuts in the tread are OK, but a completely worn out tread pattern will be prone to punctures and will need replacing soon. Localised ‘bald’ areas (from skidding!) say a lot about how well the bike has been cared for. Tread cuts with inner tube visible will require tyre replacement.
Seatpost and stem
If possible, partially loosen the seatpost pinch bolt and try rotating the saddle. If the seatpost turns, it’s good news because it is not corroded solid inside the frame. This means you will be able to adjust the saddle height to fit you. Try the same test with the stem (unless it’s a modern threadless headset) to check that you’ll be able to adjust handlebar height. If the bike fails either of these tests you might be in for a difficult job freeing them.
Some components on a bike are relatively trivial to correct and are less critical than the items already discussed. Don’t worry about problems with the following:
- Most gear problems boil down to poor adjustment or are fixed with a new gear-cable. Hub gears are generally robust. With derailleurs, providing the rear mechanism hangs vertically (see frame section above), a bit of standard bike maintenance will rectify most problems.
- Wheel bearings that refuse to spin easily or which have sideways play are just in need of some adjustment (to the cones), a 10-minute job.
- Do not let brake performance put you off an otherwise sound bike. New brake blocks and a little DIY servicing will transform them.
- Rusty or frayed cables look terrible, but cost little to replace. Ignore them in your assessment.
Finally, extras like mudguards, racks and dynamo lights are helpful if you intend to use them. This saves you (a lot of!) money and proves that the frame can accommodate such things: some frames simply don’t have room for these extras.
For information about Bicycle Maintenance classes, email firstname.lastname@example.org