I have to change the crankset on the quite old bicycle. I need to order it online so what parameters should I consider when choosing the right chainset? It's used 7 gear bike and I don't know what type of crankset it has.
In general, the basic considerations are:
- Number of chainrings and chainring bolt center diameter (BCD) which dictates the chainring sizes that can be fitted.
- Bottom bracket type: cranks are generally advertised as being compatible with a particular type, of which there are many.
- Crank arm length: measured from center of crank spindle to center of pedal spindle, typically 170, 172.5 or 175mm.
- Chainline: how far away from the center line of the bike the chainrings are, this needs to be compatible with the width of the rear hub and position of the cassette.
- Rear cassette speeds: 9, 10 and 11 speed drivetrains use progressively narrower chains which means the chainrings can be closer together.
In reality it's not quite as complicated because a crankset intended for a particular style of bike will have the appropriate combination of BCD, choice of bottom bracket types and chainline.
Older bikes used a threaded bottom bracket shell with either a loose or cassette type bearing and spindle. There are a couple of different types of interfaces between the spindle and crank arms. The bottom bracket bearings on an older bike may be worn out so replacing the bearings should be considered - this also gives you some more crank choices as the there are a few types that work with threaded shells.
If you add photos of the existing crank and bottom bracket from both sides we can give you more specific guidance.
Not an answer to original question, but cranks' properties, and what it affects:
cranks have different attach holes: square, octalink, hollow. on old bikes it's probably square. Square have 2 positions. If you replace both, you don't have to mind of this, but if replacing only one side, you should get the new one like the one you have.
cranks can have different amount of teeth. That affects the position of front derailleur. The derailleur support some range of teeth, so if the difference is large enough, it may require to change the derailleur.
crank's amount of teeth also affects the chain length. as if the crank will be pretty much smaller, your chain will be to long and should be shortened. And vise versa, lengthen the chain if crank is much larger.
cranks also have different shape of the chainrings on it: some of them are flat straight, while others have depression, therefor the teeth ring itself will be much closer to the frame. On some frames with some cranks derailleur will become unadjustable because the inner ring is too close to the frame, or because the outer ring is too far from it.