Bicycle Mechanics - Minor Ulpgrade Question

The low end mtn. bike I bought a while back has the Shimano Tourney stuff. It works okay for my needs, but frequently the rear derailleur, with its big ol red pulley, grabs a piece of vine or something, and binds itself up. Would a derailleur with a smaller pulley, and generally less gangly arm, be less likely to grab foliage? Or should I just try to avoid weeds? Why the big ol pulley on this cheap RD anyway? For that matter, why red? I'm thinking of getting maybe a Deore or something (This is a 7 speed).

I don't there'd be much of a difference when it comes to picking up junk... The Deore might shift slightly better.

Red goes faster!

For rear deraillers, don't pay any attention to the "speed" designation on it. Rear deraillers have no internal indexing (for that matter, neither do front deraillers but the geometry is different between double and triple deraillers, as well as for different chainring sizes, but I digress). Indexing is done by the shifters so as long as the rear derailler can move along the full width of the cassette, it'll work.

Basically, the only things you need to pay attention to are making sure the derailler pulls from the right direction (look for "top" or "low" normal depending on your setup, check Shimano's website for more info), that it will work with your largest cog (most MTB deraillers will handle up to a 34 tooth cog but many road deraillers are only spec'd up to a 27), and that it has enough chain wrap capacity (if you have a triple crankset, you'll need the long cage version of whatever derailler you go with).

Someone else will let you know if I missed something or made a mistake.

This sounds like a jerry-rig-a-cage-out-of-wire kind of job.

Thanks for your iinputs. Looks like I need to do some research on the Shimano website.

The real issue is that any "mountain bike" with Tourney gear really isn't made for hard off-road duty, it's a beach cruiser with gears.

The Tourney rear derailleur is a huge step down from even an Altus, has much more flex due to looser manufacturing tolerances and more of a tendency to swing into the spokes when used hard. Moving up the ladder, Altus and Acera are both listed as "MTB-like" groups by Shimano. Alivio is their bottom rung "recreational mountain bike" group.