Can you show a picture when it is off ?
When does it happen ?

The chain moves around the front chainring to the bottom of the chainring, then back to the bottom pulley of the derailleur, then up to the upper pulley, then to the cog.
Do you mean that on the way from the bottom of the front chainring to the lower pulley of the rear derailleur, the chain comes off the bottom pulley ?

Looking at the picture, I see a bent derailuer hanger. That will cause many shifting problems. If you have another rear wheel in good shape you can use it to straighten the hanger. Remove the der. and screw the other wheel into the hanger. They should use the same bolt size (10mmX1mm). If the wheels are not parallel use the extra as a lever and bend the hanger until the wheels are parallel. Reinstall the der and adjust it to see if that cured the problem. If not, what bent the hanger damaged the der. and it needs to be replaced.

Too hard to tell from those pics, and an inexperienced person could make things worse.

actually rd is bent slightly.... lower pulley cage is bend towards wheel... can rd be fixed? i dont thinks its too complicated mechanism to be fixed

Reading your posts and looking at your photos, my attention was first drawn to your current derailleur which appears to be an older Shimano Alivio. While it is a good choice for touring with it's extended range (something a 105 doesn't have by the way), your model looks to be an M410 7/8 speed. Further the pulleys look like they may be very worn (hard to tell). Regardless, being an older derailleur the combination of wear and the wider 7/8 speed pulleys, could account for the chain falling off the lower or tension pulley. Before you run out and buy a 105 derailleur, be advised that long or short cage, Shimano road derailleurs are ranked at 28t being the largest rear cog they can accept. This can vary for different frames and some tweaking, but with your limited knowledge, it would be best to go with a sure thing. So if you want a larger cog, say a 30T or 32T you need to go with a mountain bike derailleur like your current Alivio but a newer 9 speed model. The newer Alivio M430 is 9 speed and will work with a 34t cassette.

thanks for ur reply....i think now i have whole picture whats going on with my bike.....i have several question and pls answer...i will really appreciate..... so...
1. is it possible to change cassette and derailleur from 8 to 9 or 10 speed? i mean can my shifters handle it?
2. if i can change could u tell me exact sizes i have to buy
3. if my shifters only handle 8 cogs... what other options do i have? ( i could increase my budget to 250 $)
4. mountain gear will work????
5. how to find out for how much cogs my shifter is suitable?

Okay. lets start from the beginning. What ever shifters you have the rest of the other parts must match. That is, shifters, cassette derailleurs and chain should all match make and speeds. There are a few exceptions but lets complicate this anymore than necessary. We need to know what parts you have, without that knowledge we can not help you because they have to match.
#1 What speed shifters do you have. I see they are Shimano Sora; depending on how new it is, Sora came in 7,8 and 9 speeds. Shift the right shifter/ brake lever as far as it will go and then count the clicks as you push the downshifter tab; if there are 6 it's a 7 speed; if there are 7 it's an 8 speed; if there are 8 clicks it's a 9 speed.
#2 How many cogs are in the cassette and what are their sizes.
#3 What chain do you have and how old is it.

Once we know what you have and then figure what you wnat to do, we will be able to help you get there.

This can probably be fixed without a lot of money but we need to know more about the problem.

ok so i have 8 cogs sizes are 12-25, when i put chain on 25, or biggest cog then i started to shift and after 7 it did not click.......(i mean i still could press button but without voice click) about chain i have changed it because old one had rust.....

i am really thinking to buy more advanced gear...but as i guess most important is shift levelers...and they are expensive....so i guess only thing i can do i can buy other parts suitable to my shifters...anyway if u say that i have sora, to buy new one is very cheap (i mean df and rd)
i am thinking to buy long cage rd and 12-30 cassette......it will help me on hills...anyway feel its cheaper then to change chain rings and crank.


i think rd is also sora

This is what makes it difficult to give you a good answer ... you are not giving good information.

Also, you cannot accurately count the clicks unless you disconnect the cable, because the derailluer limit screws might be stopping you from getting all the positions. If you disconnect the cable from the derailleur, then you can freely shift as much as the shifters will allow. Count the total clicks in one direction (up or down). Then add 1. That is your number of gears or speeds. 7 clicks = 8 speed. 8 clicks = 9 speed.
You need a new cable anyway since yours is breaking so this is a good time to replace it.

You need to just consider buying a nice complete bike where everything functions properly.

i know i sound stupid but actually its 8 cogs...sorry about that... even i don't have reasonable explanation why i miscounted lol........
OK i did as u said and i counted 7 clicks so i have 8 speed shifters

i was thinking about it...but for new bicycle price i can buy shimano full sets for example 5700 black....so u got my point

Perhaps your derailleur is bent, or perhaps the hanger (the part of the frame the derailleur bolts to) is bent, or both are a little bent. If you shift to a larger cog like 21 or 23, then from behind the bike look at the gears, everything should be lined up. You should have a straight line from the cog the chain is on, straight down through the 2 pulleys. This requires a careful eye, and is not always accurate. There is a tool to measure & straighten the hanger if it is a little bent. This would have happened if the bike was crashed on the gear side, bending everything in a bit.

You could try to take a picture from that view & post it, but it is easy to be tricked by angles, lighting, blurring etc...

For the chain to come off the bottom pulley of the deraileur, I would think it would be very bent (or twisted) and easily seen.
Can you take a picture after the chain has come off the way you describe ?

https://www.bikeforums.net/showthread...1#post13875211
here is uploaded photoes.......anyway i need to change my cassete to 12-30 and wich deraileur u suggest?

I think it would be less confusing if you put everything in this thread. Don't keep starting new threads for the same problem.

It would be easier to see if you shifted onto a larger cog as I suggested, such as the 21 or 23, because it makes the pulley cage more vertical.

I cannot tell if it's bent.

ok np just did not find better way to upload pics..tell me how to erase threads

ok i will take new pics

You cannot delete the first post. You have to ask a moderator to delete or lock it - click the "Report post" icon: '/!\'

I'm not familiar with different models. Try Shimano website for specifications ??

ok bro np

Pic 3 & 4 does look like the pulley cage is a bit bent.
New derailleur & cable should be a nice improvement.

This needs to taken to committee!

Okay, your new pics are helpful. It is hard to say of your derailleur is bent; keep in mind that the spokes angle away from the rims, so it always looks like the derailleur cage is canted inward. You have 8 speed Sora shifters with a 7/8 speed Alivio rear mountain bike derailleur (this is obvious from the shape and plastic "spoke'd" pullies). If you want a cassette with a 30t, you will need a mountain bike derailleur. This is not a problem, as Shimano road and mountain bike components work together. The question I have is, besides the chain coming off the bottom pulley, does the rest of your shifting work okay?

There are two possible causes of the chain coming off the lower pulley and it could be both. What chain did you buy? If you thought you had 9 speed shifters and bought a 9 speed chain this could cause your problem. Also the lower pulley looks very worn; the teeth are at points and they should be squared off. https://aebike.com/product/shimano-7-...p7781-qc30.htm

Shimano road derailleurs will not be able to shift onto a cog larger than 28T. But that's okay because Shimano makes a lot of mountain bike derailleurs that will handle 32 or 34 tooth cogs. Your current derailleur is a mountain bike derailleur that would be able to handle a 30-32T cog. If the only thing wrong with the derailleur is the pulleys (if one is worn you should probably buy a set), simply replacing them will fix your problem. But you should check the rear derailleur for wear. To do so first disconnect the cable. Now, while looking at the front of the derailleur, grab the cage with your left hand and twist it up and down. What you are checking for is if there is any play in the pins that hold the face plate on the derailleur body. If there is any play up and down, then the derailleur is worn (in and out play is normal)

man u r amazing....i mean it...u r like sherlock Holmes in bike world.....i read somewhere that derailleur can handle any speed its just a matter if its long short or medium.....so i don't want to buy thing twice......and i am strongly planning to replace all my gear with 105 5700....if i buy 5700 derailleur long and biggest affordable cassette? will it work?i am stricktly decided changing cabels and derailleur....so my problem is just transfered to a question which derailleur ?...again brother i can not express how i am glad and thankful for ur answers......u r cool.....
as i guess any shimano vables will fit to my bike? and does v comes with cables?

Sometimes .... but not always due to the geometry of the derailleur and amount of cable pulled with each click.
There are more complications between brands, so let's stick with Shimano for this discussion.