front derailleur guard thingy

[ShanDog]

Hey guys,

My brother somehow snapped off that circular plastic guard thing that attaches next to the third gear chain ring on the front. I don't know what its called exactly but my guess as to what it does is stop the chain from going past the third gear and coming off. I know this because my brother can't use third anymore without the chain coming off. So anyway, I was wondering firstly what this plastic thing is called, are they easy to get and is there anything I should know about sizing?

I have now turned into my brothers bike mechanic after completely overhauling his cantilever brakes that weren't really braking at all. I also replaced his buckled rear rim that he was riding with but he didn't seem to care that his bike wouldn't ride in a straight line. I don't know how he rides that thing but it's starting to become a bit more roadworthy now

Thanks guys!

Shannon

[LuckyPierre]

Shannon - I assume that you mean the guard that goes outside the large chain ring?
If you can't get one, then the way to stop the chain coming off is by adjusting the top and bottom stop screws on the derailleur. This link http://www.shimano.com.au/media/cycling/techdocs/en/bikecomponents/FrontDriveSystem/SI-6KBFB_v1_m56577569830563520.pdf shows how to do it (for a Shimano Deore derailleur, although instructions for all their other groupsets are also on the website).
That said, you should be able to get them - and most cranks have the same 'bolt circle diameter' which is the measurement that you need to get the right size - it's different for road vs mtb's. A bike shop should be able to tell you what you need if you can tell them the model of the cranks.
Hope this helps - I'm in bike mechanic mode for the next couple of days. I bought the girlie bike some new bar tape and Tojo some new pedals for Christmas!

[europa]

You can get them for the small chainring too. You're right in assuming that they are to catch the chain if it comes off, but if adjusted correctly, it shouldn't come off. What's happening Shannon, is the derailleur is stepping across a little bit too far and taking the chain off the chainring and, like Peter says, all you have to do is adjust the stop screws. It's an easy and satisfying adjustment, made easier if the bike is suspended so that you can spin the pedals and whip through the gears. You'll probably find you need to adjust the cable tension as well, but work through the Shimano links Peter gave you and then come back with any questions.

You're lucky having a brother who rides a clunker - you're learning all this good stuff without messing up your own bike

Richard