I want to modify a 1980' Bianchi to make it a 2 spees fixie. I have ride fixie for over 3 years but I wanned to do something different and modify this one. I will change the front derraliur between the two gears and keep the derraliur in the back to keep tension on the chain. I preffer fix a lot more, however for the landscape in which I ride two gears would help a lot. Also all te accesories of the bike are campagnolo( caliper breaks, crank, seatpost, wheels) and except for the derraliur in the back which is chimano that way I would keep all of them on the bike. What suggestions do you have?

Won't work fixed...

You might consider this three speed Sturmey Archer fixed gear hub.

Sturmey-Archer S3X 3-speed fixie internally geared hub kit rear 36h 130mm Black, Hubs

Sheldon Brown describes how to convert a standard Sturmey-Archer AW 3-speed hub into a two speed fixed gear hub:

Converting a Sturmey-Archer AW Hub Into a Two-speed Fixed Gear

or a schlumpf 2 speed crank

The Schlumpf Mountain Drive is probably the best (albeit costly) way to get two widely spaced gears (e.g. one for flats, one for hills), which is what I assume you want to do, mondonson.

Other internal hubs usually aren't spaced wide enough to actually give a gear that would help you up those "Damnit!" hills, but then again it depends how big the hills are where you live and ride.

They do make multi-speed dual cogs that you can switch between like this one at Wabi's website:


Wabi Cycles fixed gear cogs

But I get the feeling that you want a bike you can shift on the fly. In which case, I would direct your attention to a new thing they have called a "derailleur."

true dat, dingle cog + double crank = 2 widely spaced gears, but you still have to stop and move the chain

I've seen it done but it had to be shifted by hand while stopped.

I have a Surly dingle cog on both my bikes. 46/43 x 17/20 on my commuter/trail bike. 50/46 x 17/21 on my distance/climbing bike. It's not a huge hassle to stop and move the chain over, but if you wanted to change gears often I'd look at one of the 2 speed hubs or cranks.

I am not sure you understand that a fix gear cannot have anything serving as a chain tensioner because as soon as you backpedal (and you will), the force of backpedaling will trash anything ever made. That means no derailleur will ever work. On top of that, it is the experience of many of us that a fix gear must be simple and rugged. Now there are fix gear internal hubs that allow the rock simple chain setup you must have to have a safe and feasible fix gear as posters above have discussed.

Now, perhaps when you say "fixie" you mean single speed which is a one speed bike that can coast. In that case,you proposal can work. But say single speed or SS from now on if that is what you mean. A fix gear is a completely different animal. Fix gears have a single cog in back that is screwed directly onto the hub then locked solidly in place. A fix gear can never coast

Ben

That would be awesome: here, look at my 2-speed, freewheel, "fixie", I R so hip!

I wonder if you could somehow spot weld a freewheel cassette so it was fixed or something like that?

Sure. But since you couldn't use a derailleur to change gears and take up chain slack, you'd need to stop, move the chain manually, and move the wheel in the dropout slot to take up the chain slack. And unless you have super-long dropout slots, you could only span a few teeth difference in cog sizes. May as well just use a flip-flop hub in that case.

Yop

The post is a little old, but i guess it could help:
VeloSolo Twin Cogs - Dingle?
double fixed cog on a disc hub, if you're good with a press drill you can do it yourself for ~12€ (without the price of all cobalt bits you will burn if you don't lubricate). It's safe, cheap, quick to change for an other cog. And to keep your chain tension, i think the better is to keep a double crankset, and make 2 ratio (like 39-20 and 42-17). You haven't to change your BB, cool no ?

as mentioned Sturmey-Archer S3X 3-speed fixie internally geared hub kit rear 36h 130mm Black, Hubs if you want to change gears without stopping. Keep in mind you shouldn't back pedal with this hub and should only be stopping with brakes, that means no skidding.

Otherwise a double fixed rear hub with a quick release would be your best bet. You will have to take the wheel off and flip it to change gear ratio's.