Double single speed

[suburbanreuben]

You could use two SS cogs on a normal 8 spd freewheel, spaced to match the front cogs. However, without doing the mathS (it's too early) my gut feeling is that although the cogs will have the same no of teeth, 36/18 will need a different chain length than 42/12, for example.
This helps explain... http://eehouse.org/fmu/

 

[IDB1]

Wouldn't it be ok to run a slightly longer chain and let the tensioner take up the slack?

 

[ajantom]

I'm pretty sure that as long as the two setups have the same no. of teeth then the chain will be the same length. Otherwise the White Industries double-fixed sprocket would be no use at all.

You could easily set up the back sprockets with either On-One single speed cogs or Surly ones, as they are designed with a wider flange at the base to give sprocket size spacing for setting them up for multiple gears if needed.

*edit*

http://surlybikes.com/parts/dingle_cog/

This is the Surly Dingle fixed cog - but they say that as long as the teeth add up to the same amount the effective chainstay length is kept the same, so no chain growth/loss.

 

[suburbanreuben]

I'm pretty sure that as long as the two setups have the same no. of teeth then the chain will be the same length. Otherwise the White Industries double-fixed sprocket would be no use at all.

You could easily set up the back sprockets with either On-One single speed cogs or Surly ones, as they are designed with a wider flange at the base to give sprocket size spacing for setting them up for multiple gears if needed.

No, they would need different optimum chain lengths, though there's only a few mm in it. Whether this would cause skipping etc , I don't know...

 

[ajantom]

Are you sure - teeth are the same size front and back, so adding or subtracting teeth from either would add/subtract the same length.

 

[PIGEON]

I think it's important how many teeth are engaged. When you couple a 50t front with a 50t rear, typically 50 teeth in total are going to be engaged. A 90t front coupled with a 10t rear might engage more than 50 teeth, but only marginal and this example is extremely exaggerated so I think the difference (if there is any) will be negligible even without a chain tensioner, let alone with one.

Edit: actually I have no idea if anything I just said holds any truth, I think it will work but I'm not a mathematical genius so I don't know really..

 

[suburbanreuben]

According to the magic gear calculator I posted , the chain length difference between 36/18 and 42/12 would be 0.2" or 5mm.
So yes, pretty negligible, but anyone who's played around with gearing for singlespeeds will tell you what a difference one link, 12.5mm, makes.
If you'll be using a tensioner, it'll make no difference at all, of course.

 

[weeman_mtb]

Why not use a double chainwheel up front with or without a front mech and something like a Shimano Alfine (google it) tensioner at the rear with a single cog.

I'm toying with this idea on one of mine when I get around to it, Tazio gave me the idea in a thread a while back.

 

[uktyler]

A really complex (and heavy) engineering method of two gears:

http://www.youtube.com/watch?v=NImjxbWT ... re=related

 

[xerxes]

http://www.youtube.com/watch?v=NImjxbWTZIU&feature=related