c4p

Hi all

I'm somewhat new to bike repairs. I can change a flat tire and a few small things here and there but have been trying to gain more knowledge about bikes in general, seeing as I'm a bike courier, so I can fix issues when they come up instead of going to the bike shop. I put a ton of daily wear and tear on my bike so it's a good investment for me to learn.

Anyway, I have a Mongoose spire. Not the greatest bike but good enough for what I need it to do. The other night on my way home, I was shifting gears and my rear Shimano derailleur fell off after part of the plastic(?) piece that houses the spring chipped off. It's been snowing / raining a lot here and very cold (11-21°) and even though I store the bike indoors, I have been riding several miles (11-20) a day in some pretty crazy whether conditions. I'm guessing that may have something to do with it as I haven't been properly cleaning/lubing the chain, etc. I also suspect the insane amount of rock salt being put down all over the city streets lately could have contributed in some way.

I'm now looking to replace my first derailleur. The bike shop wanted $50-60, possibly more, so I'm wanting to do it myself. I know I'll have to get a bike chain tool but my question is about the derailleur itself. I'm having an issue understanding exactly which kind to get. My Mongoose Spire is an 18 speed. The total noob question here is, do I need to get an 18 speed rear derailleur?

I'm just seeing a lot of places selling 12-10 speed derailleurs. Is there just a basic derailleur I can use or do I have to find one that's specifically made for 18 speed bikes?

Bill Kapaun

A Shimano Tourney RDER is inexpensive and appropriate for your bike. Chances are it'll be a SLIGHT step up in quality.

They come in 2 types-
A claw mount type (hooks over the axle and is retained by a screw) found on many inexpensive bikes and the other, more common for bikes with a RDER hanger as part of the drop outs.

IF you post a pic of where your current RDER mounts, we can tell you which you need.

BTW- Consider the rear as a 6 speed.

fietsbob

Rear derailleurs are not the 'Speeds' part , that is a count of how many rear sprockets there are ..

It just pushes the chain from one cog to the ones next to it, from the cable pulling it.

Andrew R Stewart 

Kind of right but also wrong. The parallelogram movement WRT cable pull amount changes somewhat over the range of movement. So a der designed to have correct cable pull ratio for a six cog set might have the movement range to cover a 9 cog set up but not have the ability to index at the extremes. Also the pulley and cage width needs to be wider for a 6 cog VS that for a 9 cog set up. Lastly is that most 5 or 6 cog set ups have more clearance between the large cog and the spokes. better able to not have the wider cage of said cog count der catch the spokes. If this wide cage der was mounted to a 9 cog system the spokes might snag the cage. Andy.

fietsbob

I kept my Friction shifting skills up inspite of the industry pushing indexing. de skilling Cycling to increase sales to those who want it to be more like driving cars.


there you have the complications the clicks on the handlebar

working the RD at the far end of the cable to go just the right amount to klick from 1 cog to another. ..

I hate my spokes breaking from the RD going in the wheel , so Spoke protectors
were part of my reliable Touring bike Build..

Practical though people who need admiration for their bike builds often leave them off.

To Each , their Own.

dedhed

AS said a Tourney will be fine or for a few bucks more the next steps up are Altus or Alevio. While chains get narrower as the speed count goes up and the RD cages also narrow I've had no problems running 9 speed rear Shimano RD with 6 speed systems and chains. Claw or direct mount will be the next thing you need to know. I'd also replace the inner (stainless) and outer (compressionless) cable while you have it apart.

Just looked your bike up, the Tourney will be a step up from original. Looks like a claw mount. https://www.mongoose.com/usa/spire-19148

bulldog1935

an 18-speed bike is probably a 3x6 - a 6-speed derailleur (and if it's a 2x9, you're still ok).
Any Shimano RD up to 10-speed (except Dura Ace) will have the same cable pull requirement and will work for you.

ps - you also need to make sure you match cage length for chain wrap capacity.

HillRider

Where is "here" and are those temperatures in ºF or ºC? 11-21ºF is brutal for riding, 11-21ºC isn't that bad.

AlexCyclistRoch

BTW, am I the only one who finds it improbable that a BIKE MESSENGER doesn't know squat about fixing his bike?

curbtender

If people would just post their locations they may get access to some local help.

travbikeman

Good video that might be helpful:


Plus, there are lots and lots of other youtube videos' showing how to do this.

Dredpiratedanno

This is a related (sort of) question to the original about the mongoose rear derailleur. I recently bought a new Mongoose. Yes it's heavy, not expensive but it seems that it will work for me. The problem is that while riding it seems the rear derailleur "skips", meaning it stays on the same gear but skips a tooth or two. Kind of a pain and disconcerting. My guess (WAG) is that the springs in the derailleur do not keep the unit in the right position, i.e. it leaves too much slack on the chain allowing the skipping to occur. I've excerised the derailleur by hand and lubricated it to no avail. This is an inexpensive derailleur and I can replace it but is there anything else I can do? Any adjustments that will work?

HillRider

Go to the Park Tools and watch the tutorial on rear derailleur installation and adjustment. It sounds like you need a cable tension adjustment but go through the set up from the beginning.

alcjphil

Reviving an old thread this way is counterproductive. First, your bike does not have a Mongoose derailleur, it has a derailleur made by Shimano, SRAM, Microshifrt, or some other component maker. Your guess about the springs in the derailleur is probably wrong. It might be an adjustment issue, it could be a stiff link in the chain, or some other issue that affects inexpensive bikes. The Mongoose name on your bike is a brand, the company who contracted the manufacturer of the bike does not make any part on it. Your rear derailleur does not "skip", it is the chain that does not engage with the teeth on your freewheel or front chainring that "skips". The most common reason for new, inexpensive bikes to have the sort of problem you describe is poor factory quality control and poor assembly and adjustment at the retailer where the bike was sold

Bill Kapaun

Might be a stiff chain link?
SLOWLY rotate the cranks backwards and observe the RDER.
Look for a major "twitch" which will occur when the stiff link goes through it.

Dredpiratedanno

To Bill. Thank you, I had thought of that but tried to spin the chain forward rather than backward. I'll try it the way you suggest and let you know what I find. If it is a stiff link is there a good method to use for loosening it up?

Dredpiratedanno

To alcjphil. Sorry to disappoint you Phil but I am not a bike expert. Yes, I am well aware that the derailleurs are made by Shimano and the the bike is officially called a Terex Mongoose. It is a low end bike that I bought at Wal Mart simply with the intention of riding it around my neighborhood to get some exercise and visit the neighbors. By "neighborhood" I mean the wide open spaces of Montana where a rough gravel road is the norm and my nearest neighbor is 1/2 mile away. This bike has fat tires to absorb the bumps and I doubt I'll be setting any speed records. It should work out perfectly if I can just get this little problem solved. Doing a google search with key words of "Mongoose, Shimano, and rear derailleur" brought me to this forum and thread. I saw that it was a bit aged but since it was related I thought I would post anyway. Couldn't hurt. I got a nice reply from Bill with a good suggestion. Then from you I got a lecture telling me how wrong I was in every way that I posted. I apparently don't fit into your club. Maybe next time just a brief suggestion as to what may be the problem would be helpful, thank you.

Dredpiratedanno

HillRider. Thanks I'll take a look at it.

Retro Grouch 

The first response was the best one. After that it started getting confusing. If you need a claw derailleur (which I strongly suspect) just get a tourney with a claw and be done with it.

Probably need to get a new shift cable too because the one that you have is probably frayed and hairy on the end and is going to be hard to anchor. My trick is to solder where I intend to cut the cable and cut through the solder to keep it from fraying again. I've heard people tell me they use super glue the same way but I've never tried that myself.