Bicycle Mechanics - Bianchi Question

Hi all,
New to the Forum, and have a quick (hopefully) question.

I have an old Bianchi (about 1993) that was retrofitted with Shimano 105 gears, two up front, seven in back. When I got the bike a few years ago the components had been on for a long time and they are now costing more to maintain that I care to cough up. So, the rear derailleur has broken and gone skidding across the street one too many times and I'm turning it into a single speed. Derailleurs are now off, and I want to use the existing gears instead of buying a new cog. When i place the chain on the smaller ring on front and a middle gear in back, remove some links so it's tight and try to ride it, the chain jumps down to the smallest gear in the back, then right off the bike. I've tried all sorts of combinations of chain tensions and gear combos, and the only way the chain stays put is it it's on the large gear in front, and small gear in back. So now i can't ride up anything that resembles a hill. Is there anyway I can get the chain to stay in whatever gear combination I want it to reamin in? Why does it automatically jump gears? OK, enough for now.

Any help would be greatly appreciated, I'm now walking to and from class everyday, but would much rather be riding.......


Tighe

stay in whatever gear combination I want it to reamin in? Why does it automatically jump gears?

You have to have a straight chain line. Normally for this type of conversion, you remove all but one of the front chainrings and replace the rear freewheel with a single speed BMX freewheel (under $10 at biketoolsetc.com). You then may need to redish your rear wheel to make up for the narrower freewheel. If you have a cassette instead of a freewheel, it's a little different. See http://sheldonbrown.com/singlespeed.html.

Yeah your chainline isn't straight. It really has to be PERFECT otherwise it will never work right. I've had pretty good success using a high quality compact level to gauge mine on my singlespeeds. Also, the gears on your bike now are "ramped"... thats what assists the derailleur in changing gears are the ramped gears which are beveled in a manner that pushes the chain off a gear into another one.

thanks guys, i appreciate it a lot, i never planned for this to happen, so i never looked into it until now. i checked out the link above, a lot of good stuff. I will be going to the local bike shop to pick up a bmx freewheel to see what i can do with that.


thanks for your help!


tighe

thanks guys, i appreciate it a lot, i never planned for this to happen, so i never looked into it until now. i checked out the link above, a lot of good stuff. I will be going to the local bike shop to pick up a bmx freewheel to see what i can do with that.


thanks for your help!


tighe

I recently completed a similar conversion: http://bikeforums.net/showthread.php?p=449983#post449983

If you have any other questions, don't hesitate to ask.

I recently completed a similar conversion: http://bikeforums.net/showthread.php?p=449983#post449983

If you have any other questions, don't hesitate to ask.


I completed the conversion the other day, i realized i had vertical dropouts and needed horizontal, but, instead i just broke the Dremel out and grinded about .125- .25 inches horizontally. so now the dropouts look like upside down L's. the chain is on the highest gear in the back, and liwest up front (took off larger cog in front and put smaller one where it used to be) so the chainline is straight. hopefully no damage was caused to the frame by grinding, there is a lot of material left so im not too worried about it. thanks for the help!

tighe