it has 18 gears.i snapped the rear derailleur some how and l need a new one.i've been lookin on ebay and it says some thing like ''8 speed system''????how do l tell??plz help

18 is either 9 in the rear times 2 in the front or 6 in the rear times 3 in the front. Count the number of cogs u have on the rear wheel. An 8 speed system refers to 8 cogs in the rear.

That said, rear derailleurs r indifferent to how many cogs u have in the rear, & one designed for 8 will work fine with 6 or 8 (or even 9 or 10 for that matter). It is simply a function of setting the upper & lower gear limit stops correctly. The number of gears in between does not matter.

What u do need to make sure of is whether ur rear dropout (where the derailleur attaches to the frame) is threaded for a direct derailleur attachment or if u require (which is common on cheaper bikes) a derailleur that has a dropout ''ear'' for attachment. See pics below -- first has ear & second does not.

Now, it all depends on the number of chain wheels u have on the chain set.

If u have three chain wheels then u have 6 cogs at the back. (3*6=18).

If u have two chain wheels then u have 9 cogs on the rear cassette.
(2*9=18)

The result is that on a 3*6 system virtually any rear derailleur will fit. If the gears r indexed, the changer will move the rear derailleur the correct amount.

If u have a 2*9 system u will have to have a 9 speed derailleur system so that the index system will operate the rear derailleur the correct amount in order to engage the rear gears.

Up to 7 speeds the spaces between the cogs at the back is the same but the freewheel gets wider. Once u get above 7 speed there r more gears but they r spaced narrower resulting in the need for a narrower chain & a different index system.

If u r running a non indexed system such as down tube changers, u only have to worry that there is enough 'swing' on the rear derailleur in order to be able to move from the largest to the smallest sprocket.

''''If you have a 2*9 system you will have to have a 9 speed derailleur system so that the index system will operate the rear derailleur the correct amount in order to engage the rear gears.''

not true
the indexing is done in the shifters
not the derailleurs

wle.

Just count the number of gears on the back and that is the derailleur you need. If you have 6 gears a 7 speed derailleur will work. The important thing is to get the correct type of mount. There is a direct mount and a hanger mount. If you have a inexpensive bike the Tourney shifters will be fine.

http://amazon.com/b/ref=sc_bm_br_3403201_15_mo_4?ie=UTF8&node=3494361&no=3403201&me=ATVPDKIKX0DER