1990 Bridgestone CB-Ø Single Speed

On a recent weekend I did a frame-up build of a 1990 Bridgestone CB-Ø. With its Cunningham style U-crown fork and longish chainstays, at first glance the CB-Zip (also known as a CB-0, Zero or City Limit) seems to be just another '80s mountain bike. Look closer and you'll see noticeably smaller gauge frame tubing, a lower bottom bracket, and some indefinable combination of design and geometry that puts it into the "cool bike" class.  

Since the letters "CB" might stand for "City Bike", and it's been said that the CB-Zip has very similar frame geometry to the famed B-Stone XO-1, it seemed appropriate to assemble it as a single speed town bike version of the XO, which was designed for use both as a touring bicycle and also an all around rough terrain racer.
 
Doing a single speed or fixed gear build of any bicycle with vertical rear dropouts (as does the CB-Zip) is a challenge; luckily there's some great resources out there. The FixMeUp! page at Fixed Innovations has a handy calculator that helps find just the right combination of chainring and rear cog so you don't have to resort to using a spring-loaded chain tension device. Those gizmos are easy to install and get the job done, but they tend to have as much chain sag (and related energy loss) as any derailleur, and can be unruly on a single speed, and possibly dangerous on a fixie.

I plugged the Zip's chainstay length and wheel size into the FixMeUp calculator, and okay! I got lucky - one of the possible combinations was a 40 tooth chainring with a 17 tooth cog, both of which I happened to have in the bike garage. The resultant 61.2 inch gear is somewhat lower than the usual "ideal" single/fixed ratio in the low 70s, but it turns out to be a great all around town and country gear. It's not too high for dirt, gravel and hills, and not too low for fun fast zipping through city streets.  

In this particular case, the chain was a bit tight, but filing about 1/8" from the front faces of the dropouts cured that. Trying out different chains, if you have them, can also help if the assembled chain is too tight or too loose - they all seem to have slightly different sized links.
 
Another thing to watch out for is setting the chain line - that is, aligning the chainwheel and cog as closely as possible. Removing the cogs from an old freewheel and reassembling with just one cog, along with a few spacer shims, works quite well, and it's a lot cheaper than buying a new freewheel, bottom bracket and possibly replacing the crank, as well as the hassle of having to re-dish the rear wheel.

After getting the gears sorted out, the rest of the build was fairly routine and a lot of fun, especially finding a nice old thin-wall steel French moustache-style handlebar and wrapping it in a light grey cotton tape. Except for the 'bars, most of the components I added are older, lightweight, quality made in Japan or Taiwan parts (including NOS Panaracer skin wall tires!), and they all add to the feeling that this Bridgestone, although now 24 years old and a bit scratched up, is nevertheless truly a classic bike for the ages.

To see another post featuring this Bridgestone CB-Ø, go here.

Click or tap on any photo in this post to see a higher resolution (larger) image. All photos taken with a Lumix ZS-25.