meddyliol

I need a cheapo rear derailleur and have seen several on ebay, Amazon etc. My problem is I don't quite understand whether I need a long or short cage one. I have 6 speed bike if that is any help.

Thanks

Brian

maddog34

how many sprockets on the crank? how many teeth on the rear gear's largest gear?

meddyliol

Thanks for your prompt reply. There are three sprockets on the front and the largest cog on the back has 28 teeth.
As a matter of interest, what is a 34t cassette?

Brian

Retro Grouch 

Three sprockets on the front generally equates to a long cage rear derailleur. The longer arm is needed to take up the chain slack created by shifting from the biggest front sprocket to the smallest.

A "34t cassette" refers to a 34t largest rear cog. That will give you an easier climbing gear but at the expense of larger gaps between gears. Many rear derailleurs aren't rated to handle a 34t cog so you are best off choosing a mountain group derailleur.

fietsbob

the difference between tooth counts (big+big) - (small + small ) = the amount of chain slack you have to take up

greater the difference, longer the cage , distance between pulleys.

meddyliol

Thanks very much for that, am thinking about getting a Shimano RDTY21.

Cheers

Brian

Andrew R Stewart 

A longer cage will fit a lesser ranges cassette but a short cage might not work with a wide range gearing set up. So the safe answer is a long cage. Worst result is a bit less crisp shifting. But with the low cost TY series crisp shifting is not going to happen any way... Andy

nfmisso

Brian;

Do yourself a favor and get a Shimano RD-TX35 SGS; they are inexpensive, shift smoothly, and ruggedly built. They maybe the heaviest RD Shimano currently offers.

meddyliol

That is exactly the other model I am looking at. Any idea what 'SGS' means? Just ordered one from ebay but no mention of 'SGS'.

Cheers

Brian

meddyliol

Thanks to everyone who answered my question

Brian

maddog34

spend a bit more and get an Altus or Alivio Derailleur... you will be much happier with either of them than with a Tourney.

and definitely you will want the long cage... a mid cage length would probably work, but the long cage option will allow installing a wider range freewheel or cassette in the future...

maddog34

spend a bit more and get an Altus, Alivio, Claris, or Deore Derailleur... you will be much happier with any of them, than with a Tourney.

and definitely you will want the long cage... a mid cage length would probably work, but the long cage option will allow installing a wider range freewheel or cassette in the future...

Retro Grouch 

Oops, now you've got another thing to check. That particular derailleur comes in 2 versions.

Look at where the derailleur attaches to your bicycle. Do you see a finger hanging down with a threaded hole at the bottom?

Yes = direct mount
No = claw hanger required

jj1091

SS is short cage. GS is medium cage. SGS is long cage. No real idea for the acronym, maybe Gran Sport and Super Gran Sport?

AlexCyclistRoch

Do yourself a favor; take this bike to a mechanic. You don't know what you are doing.

Retro Grouch 

He'll never learn that way.

Everybody has to do everything for the first time once. I don't call blunders that I make along the way "mistakes," I call them "tuition."

HerrKaLeun

how to chose derailleur length
hope this helps.

meddyliol

Thanks for your very helpful advice, I do know what I am doing, just with loads of different modern derailleurs out there it's a minefield. I was probably putting bikes together before you were born.

Have a nice day

meddyliol

I looked at the other types but the one I ordered was the only one I could afford. I am a pensioner not rich. I did check the mounting and mine is a direct mount.

Thanks again for all the (mostly) helpful advice. I will probably be banned now for my comments.