Broken Derailer.

So I have an entry level Trek. I think the model number is 820 and I believe it was purchased in 07.

Anyhow I was pulling a slight incline at a moderate pace and my derailleur exploded.

The derailleur itself is a Shimano Acera, model number rb-m340. I would like to upgrade to better derailleur but I'm not looking to spend a lot of money. Any recommendations?

I'm pretty mechanically inclined, just haven't really tore too deeply into bicycles until now. So all information is appreciated.

I also have a question concerning my crankset, but let us conquer the completely berkeleyed up part before we move on.

A step up from Acera is Shimano Alivio, but the differences are so small that I doubt you would notice much. Deore is a step above that and should still be reasonably affordable, but Deore is designed for use on 9-speed bikes whereas Acera and Alivio are designed for 8-speeds. I have had good luck using 9-speed Shimano derailleurs as replacements on 8-speed Shimano bikes, and generally think that being optimized for a slightly narrower chain is a minimal problem and that being better made with better materials is a fairly large bonus, but YMMV.

Derailleur failure can be a problem with the derailleur, but in many cases starts with the chain failing and then the broken chain links or plates ripping the derailleur apart as the broken chain comes around. I would replace the chain while you are at it, and make sure it is properly installed, since improper installation is one of the biggest causes of chain failure. Shimano chains work well and come with a special pin for assembly. They come in a long length and often need to be shortened a bit on most bikes. SRAM chains also work well, also often need to be shortened a bit, and come with a special link for assembly. The benefit of the SRAM is that the link can be used for disassembly and reassembly down the road for repair or cleaning. IMO it comes down to personal preference. For either brand, be sure to match the chain to the number of speeds on your bike.

Once the new rear derailleur is installed, it is a good idea to periodically check the bolts which hold the rear derailleur pulleys (AKA jockey wheels) in the derailleur cage. Depending on mileage and use, every few months (or a few times a year) is probably enough. They are usually a 3mm Allen, but sometimes use a 8mm hex. If you ever take them off, reassemble with blue Loctite or the derailleur will vibrate apart eventually.

If you want particular tips on chain length, derailleur setup or adjustment, feel free to ask.

The thing is I've never really adjusted a derailleur before. Is adjusting a 9 speed derailleur on a 7 or 8 speed wheel going to be harder for me to adjust?

I definitely want to upgrade the thing from where I was but I'm not willing to pay double for something that either won't be more reliable or more durable.

What sort of special tools on top of parts am I going to need for shortening the chain or whatever? I have plenty of mechanics tools, just nothing bicycle specific.

I'm not afraid to tear it apart and learn. I've built engines and transmissions before so I'd rather bite the bullet and learn the ropes rather than pay someone to do it for me if at all possible.

I plan on picking the brains of the folks at the local shops for tips and parts. Do you have any online resources where I can get an idea on prices before I walk blindly into the local shops?

So I need a derailleur and possibly a chain. Anyway to determine whether the chain is toast for sure? How essential is the plastic ring between the spokes and largest gear on the wheel, I assume its existence is to prevent the chain from getting caught down in the spokes.

My rear wheel has 7 speeds, but the largest cog is much bigger than the second largest. 'Mega-drive' if that makes any difference.

Thanks again.

obligatory carnage photo

A rear derailleur designed for 9-speed is no different to adjust than one designed for 8-speed or 7-speed. Sort of like installing a 4x100mm Honda factory alloy on a VW...the process is exactly the same no matter what car the wheel was designed to go on originally.

Shortening a chain requires a bicycle chain tool. Shimano, Park Tool, Pedro's all make good ones, but there are generic ones available also. Nicer ones usually have better handles, fewer sharp edges, more leverage, and are built to a slightly higher tolerance. They are basically a T-handle that threads into a tool which holds the chain. A pin on the end of the T-handle (replaceable pin on nicer tools) drives a single chain pin out of the chain, and is also used to drive the pin back in when re-assembling. If the chain doesn't come with a replacement pin, don't drive the old one all the way out, just most of the way out, otherwise you won't get it back in. Proper reassembly means pushing the pin in until it is centered (and doing so without bending the side plates or links by being ham-fisted.) If the link is too tight, you may need to work the pin back and forth a little bit to loosen it up so that the link doesn't bind, AKA "a tight link."

As far as other parts go, you may want to pick up a replacement derailleur cable in case the old one is crimped or frayed...they should be $3-5. You will also need a 5mm Allen wrench.

As a longtime bike industry guy, I don't spend much time surfing the web for prices, but one of the best resources online for general info and repair guides to bicycle repair and DIY is Sheldon Brown. I think his site is www.sheldonbrown.com but if not, google his name and Harris Cyclery to find the site. He also links to a lot of other sites, so be prepared to spend a few hours surfing! Park Tool also has some good online repair guides.

http://www.parktool.com/repair/

Nashbar, Performance, and REI are all online retailers of bicycle stuff, but the local shop will always give you more information, usually give you better information, and can help make specific recommendations for your bike that you can't get online.

If the chain is broken into mulitple bits, has visibly twisted links, is very rusty, or has links which are bound up and won't move, best to get a new one. A name brand replacement chain should be $18-22 or so. If the chain is intact and each link looks uniform and moves freely, it is most likely fine. If one link is broken and the bike has less than 1500 miles on it since the chain was new, you can most likely put it back together, or put it back together with a new link (about $2) added.

The plastic ring (AKA spoke protector, pie plate, or dork dish) isn't necessary if the limit screw (stop adjustment) on the derailleur is adjusted to keep the chain from shifting into the rear wheel. If you take it off, just be aware that if you crash or the bike falls or gets leaned on the drive side, the derailleur could get bent enough to shift into the spokes, so double-check and re-adjust as needed before you hop on and ride off into wheel damage.

Does the world no longer use master links?

SRAM has got the PowerLink

...which I'm going to try to take with me in my kit instead of a chain tool. I've found that whenever I break my chain it weakens the links where I use the tool, so rather than using the tool anymore I'll just put the PowerLink in.

I've broken oh....3 chains on 2 bikes in less than a year.

DILYSI Dave wrote: Does the world no longer use master links?

Sure, on single speeds, BMX bikes, and beach cruisers. They don't really work for multi-geared bikes because both the cogs and the chainrings are designed to engage and lift up or drop down the outside of the outer plates when shifting (wild stuff!!) so every link has to have identical external dimensions.

The SRAM power link is the main reason I generally prefer SRAM chains to Shimano chains, even on Shimano-equipped bikes.