Bicycle Mechanics - Single Chainring Six Speed?

ok, let me preface this question by saying that i have never built a multispeed bike before though i have built a couple of fixed gear bikes (the reason I got into working on my own bikes), so i'm sorry if this is a dumb...
i'm throwing together a bike from some parts i had lying around. right now its got a single 40 tooth chainring on the front and i also have a wheel with a six speed cassette that i'd like to put on. what i'm wondering is if i throw a rear derailleur on will it be able to shift without the chain falling off the front chainring? if not will putting a front derailleur on serve as an effective way to hold it in place? i'm just wondering if this is possible, i haven't really seen any bikes set up this way.
thanks

I have a folding bike with that set up. I use a fixed front derailer as a chain guide.

http://home.comcast.net/~cheg01/20_light_1.jpg

six speed cassette

its not a casette, but a freewheel.

the front der will help keep it in place, but if your chainline is good, then you prolly wont need it. or, you could always add a granny and a big ring.

If your chainline is dead center it should be OK. If you are putting on a front derailer, why not consider multiple chainrings? Your gear range will greatly increase.

My sister has an Eddie Bauer MTB that is set up like this, (although it's a 7 sp). The bike came with a plastic "bash guard" on either side of the chainring to keep the chain from hopping off. I'm sure there's another name for the part, but I don't know what it is.

Usualy a single ring setup for offroad use either a chainring guard/ bashguard in the big ring position with a chain watcher (Third Eye, Dog Tooth, Jump Stop) on the inside attached to the seat post

OR

A chainring guard/ bash guard on either side.

But since you won't be going off road and you have suspension you can probably get away with just the single ring assuming that you have a good chain line and the derailleur set tight. I haven't tried this yet but I plan to in the spring. Good Luck.

you'll be just fine. I've run similar setups on 3 different recycling center/thrift/junk 10-speeds in the last year, effectively shedding a pound or so and giving me a reliable 5-speed. I usually just remove the small chainring (sometimes the large one, in the case of my winter ride) and leave the rear derailer as is. You might want to play around with some spacers to get the chainline as good as it can get, otherwise there may be a little binding in either the low or high gear. But i've never left the front derailer on as a chainguide or really done anything else to them and i've never had a chain jump off on me while shifting or otherwise. Oh, and i've got a couple of friends doing similar with no problems either.

The only jumps will probably be when you are in high gear with the relxed chain tension and jumping kerbs.
You can remove a few links if possible to keep a tighter chain.
Sandwiching the ring is the best option, the chain grabbers do not stop the chain from jumping, just from getting jammed between the bb and spyder.

Chain tensioners will help stop chain jump as well, a bit more expensive fix.

I am currently running a single chainring up front with an 8spd cassette in the rear, and no front derailuer or chain guide. However, the chain does fall off every now and then - primarily when I am quickly droping gears and go over a bump/pothole. I would recommend using a front der. to prevent this from happening (I'm just too lazy to grab one from the parts bin and put it on...). One tip to minimize the chain from falling off is to use the the shortest chain you can (and still shift gears) so that you can maintain chain tension.

Here's one that uses the chainguard method. (http://www.rei.com/online/store/ProductDisplay?storeId=8000&catalogId=40000008000&productId=47843133&parent_category_rn=4500943&vcat=REI_SSHP_CYCLING_TOC)

thanks for all the tips, the chainline is damn near perfect but i'm running a front derailleur anyway just to be safe. the bike is running great.

its not a casette, but a freewheel.

the front der will help keep it in place, but if your chainline is good, then you prolly wont need it. or, you could always add a granny and a big ring.

It is more likely to be a freewheel, but it could well be a cassette. It doesn't pay to assume about these things!

Anyway, with a rear derailer, chainline will change as the gears are shifted. Still, if the deflection is small enough, it might stay on the chainring by itself. You could try riding with the front derailer for a while, and if chain derailment turns out to be a problem, put the derailer back on.

thanks for all the tips, the chainline is damn near perfect but i'm running a front derailleur anyway just to be safe. the bike is running great.

Try it w/o the FD for awhile; if it works it's much more sano...

Also, depending on the size of the wheels and the sort of tires you are using 40 teeth might be a bit slow. You might want to invest in a bigger chain ring.

JamesBlah

I agree with dbg, a perfect chain line

http://www.sheldonbrown.com/gloss_ch.html

is enough. You can remove the front derailer, the chain will not jump. Did you set your cainline between the 3rd and 4th cog?

its not a casette, but a freewheel.

You're probably right, but I had an old wheel with a 6 speed cassette. Nashbar sells 6 speed cassettes, and I was redoing the drivetrain and found out that there are actually at least two different ways of securing 6 speed cassettes. Unfortunately, I wound up with a cassette and hub that did not agree.