Have I bought the wrong rear derailleur?

Question

I cycled through a plastic bag and it bust my rear derailleur apart. Photo attached. My bike is a Raleigh Equipe road bike, which looks like this: https://www.ecosia.org/images?q=raleigh+equipe+road+bike#id=EFEC15345FC6211574940A99C94FF6014623E79F

I have just bought this replacement rear derailleur: https://www.amazon.co.uk/gp/product/B0044DGH2E/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1 and having received it, I think that it might be a bit too big and chunky. I also just noticed that amazon said it is for mountain bikes.

I think that rear derailleur capacity is around 24 (it's difficult to properly count).

Have I bought the wrong rear derailleur? If so, how do you find a good one for my bike. I wouldn't want to spend too far north of £35.

Thanks bike people. You're all very kind.

Answer 1

The derailleur you bought has a capacity of 43 teeth, so if you need only 24 teeth, you're fine on that count. The fact that it is marketed as being for mountain bikes is not relevant in your case--it only comes into play on bikes that have indexed shifting, which yours does not.

Mount it up, set the high/low limit screws so you can't shift past the inside/outside of your freewheel, set the B screw in back so that the jockey pulley rides a few mm below the gears, and you should be ready to ride.

I'll mention as an aside that if your other drivetrain parts are of similar vintage to your late derailleur that something else could fail at any point, and you may need to budget for a new chain and freewheel (at this age, replace both together to ensure they mesh correctly), and possibly chainrings (for the same reason).

Answer 2

FYI: The capacity is calculated by adding the difference (largest cog - smallest cog) to the difference (largest front ring - smallest front ring). The number 24 you quote isn't the capacity but just the difference between largest and smallest rear cogs. The maximum rear tooth count will be 34. If your freewheel/cassette doesn't exceed 34 it will be fine. (You can easily count the teeth by sticking a bit of tape or some small magnet to the one where you begin.)

The website says that the derailleur is intended for 7 and 8 speed shifters. Your freewheel has 7 cogs, so no problem there. The derailleur will work as an indexed unit but also with a friction shifter.