[*BCM*] bike questions

[Tom Revay]

>> - Why do some bikes come with a chainring guard and some don't? Could
>> something as simple as that really affect performance or weight?

Do you mean a chain-RING guard, or chain guard?

A chainring guard is a round piece of metal that is on the outside of the chainring.  It keeps the chain from falling off if it's overshifted to the outside (a situation that can be remedied by a simple adjustment to the front derailer).  Mountain bikes have also had these devices attached to keep the chainring from being damaged by a log or a rock ridden over and scraped.  The Specialized Rock Ring is an example of one of these products.

Here's a picture of a chainring with a guard:
http://www.rivbike.com/images/QB3.jpg

That bike, by the way, is the Rivendell Quickbeam, which has a two cogs on the rear hub, one on each side in a flip-flop arrangement, and two chainrings in front.  The bike has no gear-changing devices, though.  If you want to change gears, you can either remove the rear wheel and flip it, or you can use your fingers to pull the chain onto a different front chainring.  That process is illustrated at
http://www.rivbike.com/html/101_quickbeam.html

Chainguards are something different.  These wrap around the chain along it's upward (forward) path, and partly or fully around the chainrings.  This bike has one:

http://www.kogausa.com/StreetLiner(M)specs.htm

If your bike has a front derailer, it'd be difficult, although not impossible to put a changuard on it.  That Koga-Miyata, in particular, is spec'd at 24 speeds, which ordinarily implies that it has three chainrings in front, and an 8-cog cassette on the rear hub.  That they managed to put a full chainguard on such a bike is nifty, IMO.

Chainguards keep your trousers from contacting the chain.  They also help keep the chain a little cleaner, though I think a full-sized front fender would do nearly as well.

...................................Tom