SquidPuppet

Why switch cogs or chain wheels when there is no problem?

The "right amount of slack" covers a big margin

How much does a short chain elongate from it's new state to replacement time?

More importantly, how much does the slack increase? Negligible. Nowhere near, not even close to enough to risk derailment.

I have no opinion on what the OP should do regarding this conversion.

ThermionicScott 

Well, here's my thinking: assuming that you are able to start out with the perfect amount of slack due to right chain length and chainring/cog selection, and you replace your chain when it elongates 1/8" over 12" (arbitrary but I think pretty reasonable/common for FG/SS), that would be 1/2" over 48" of chain, which would be about 1/4" worth of axle adjustment on a bike with horizontal dropouts. Just for ****s and giggles, try moving your rear wheel forward 1/4" and watch how much the chain droops.

That might not be a big deal on a single-speed bike, but it would make me nervous on a fixed-gear. By adding four teeth between the chainring and cog, you'd be able to take up that extra slack, and if it were my bike, I'd be considering adding a tooth or two at the midway point, or replacing my chains fairly early in their lives. But it wouldn't be my bike, because it's way simpler to start with the right frame.

Tradarcher

Well what is a good gear combo? I understand that it's not optimal but I want to learn how to do this and what's a better way to learn it than to use a less than optimal bike. I welcome the challenge!

SquidPuppet

You are only off by 50%. Each tooth moves the axle 1/8 inch. A quarter inch axle movement would need only two teeth for a 100% slack correction.

Regardless, waiting until you reach 1/8 inch elongation is a no-no according to Sheldon, resulting in damaged sprockets. (TRUE too) He says 1/16 inch is the time to replace the chain.

I based my math on that ^, which resulted in a maximum 1/8 inch axle movement. Equal to one tooth. You'd for sure see a little droop. But you wouldn't be able to derail it, even if you manipulated the chain with your hands while pedaling the bike on a stand. I don't recommend that experiment though.

T13

If you "welcome the challenge" then figure it out, instead of getting the nerds all worked up to figure it out for you.

SquidPuppet

Do you know what ratio you'd like to run? If so, there are charts that will tell you all the different combos that could be used to arrive at (close to) that ratio. Then it's just a matter of counting teeth and links and picking the one that will work. Just google "Bicycle gear ratio chart".

I have been defending the notion that magic gearing can be achieved and is functional. While I believe both are true, there are certainly easier and ways to go about things, also resulting in a more versatile bike.

prooftheory

This is only a heuristic.

SquidPuppet

Your point?

Nagrom_

That's one of them book learned words ain't it?

prooftheory

That people shouldn't rely on it for calculating the magic gear. It isn't an argument but simply trying to dissuade people that think they can use it reliably. My experience has been that my one friend who tried to use a magic gear calculator ended up buying several different chainrings before he actually found one that worked and he wasn't happy with the bike for very long. He eventually just bought a track bike. I don't know if it was because the chain stretched or not.

SquidPuppet

I've never used a magic gear calculator. I'd assume that each one is only as good as the formula built into it. Just like a spoke calculator. The Trig formula for spokes is solid, so if it's properly input into a calculator with no bugs, the results should be good. As far as 1/8 inch per tooth though, that would only be one component of many needed to build a good magic gearing calculator. I aint a math wiz so I wouldn't know where to begin. But 1/8 inch per tooth is accurate, over a foot or fifty feet.

And I agree that it's a lot of work for a less than optimum result.

ThermionicScott 

Oh, right. Two teeth. The rest is correct, though.

Sheldon's advice above is for derailleur bikes, of course, where small differences in chain and sprocket wear can easily cause skipping. Our FG/SS bikes *can't* skip unless the chain is extremely loose or the parts are extremely worn. The rule of thumb for chain replacement over the years has generally been given as somewhere between 1/16" and 1/8" (1%) before Jobst and Sheldon got people to think more conservatively about it, and modern cassettes seem more sensitive to wear -- now 1/16" over 12" is gospel. I change all my chains at 1/16", but you can let them go much longer on an FG/SS without issue.

Anyways, I was just trying to get people to think about the magnitude that chain wear could cause annoyance for someone with a magic gear... our intrepid OP will find out all this stuff for himself soon enough.

SquidPuppet

It's kinda funny too. It's older meaning seems to be the opposite of it's modern meaning. Sort of.

TejanoTrackie 

Y'all are so eclectic.

hairnet

It goes hand in hand with being hipsters, gontchya know.

79pmooney

Squid and Therm: reality check. 1/8" in 12" is a LOT of stretch! But fix gears and single speed can go well past the 1/16" point. Yes, new chains will not work. But the existing chain will work just fine on some very pointed and worn chainrings and cogs for years on the tall toothed single speed cogs and rings.

As for hub movement per tooth change: I had a bike built with a dropout where I can move the hub 1 3/4" c-c. I run cogs from 12t to 23t without messing with the chain length. (A 24t would fit, I just haven't seen one to buy yet.) 1 3/4" divided by 11 (23t - 12t) = 0.16" or a little over 1/8" per one tooth change. And I still have a little room at both ends of the dropout so the hub doesn't move that full 1 3/4". (Both ends come to hard stops; there's no fudging.)

Ben

Tradarcher

I'm thinking something like 42/18. But T13 this is pretty unfamiliar territory for me. If I am building it as a single speed and not a fixed gear can I not just run the ratio I would like and add a tensioner if need be?

T13

Yeah man, that's what i'd do. I'd use a tensioner and then you could avoid all the "math" and switching around chainrings etc. Just get a decent tensioner and not a real cheap one. I don't know enough about them to suggest one, but I'm sure one of these guys could give you some good advice about that.

TejanoTrackie 

Actually, you can probably use your existing rear derailleur as a chain tensioner. Just turn the high gear stop screw in until the derailleur jockey pully lines up with the freewheel cog.

wphamilton

Downside is having a pound extra weight more or less, a couple of percentage off your drive train efficiency, and something else to break. It would work - but are you going to run a 1/8" chain through that derailleur?

TejanoTrackie 

Why use a 1/8" chain at all, when a 3/32" chain will work with a 3/32" single freewheel or cassette cog and a 3/32" chainring ? I think you exagerate the drawbacks of using a derailleur as a tensioner.

SquidPuppet

Reality check? I didn't know chains stretched.

wphamilton

Along with a thicker chainring and sprocket, less wear and tear. The 1/8" is also designed to run straight (stiffer laterally). Both are perhaps minor considerations, but I'm not exaggerating that the rear derailleur will weigh 400 grams or more possibly, and may have issues with a wider chain! Just an FYI.

TejanoTrackie 

Hmm, well the heaviest rear derailleur I could find in my parts bin was an old Campy Rally long cage that weighs 252 grams, but if you are so worried about weight, then how much heavier do you think a 1/8" chain would be compared to 3/32" chain ? Anyways, I think all this silly harping on weight when we are talking about modifying a $100 bike that is probably fairly heavy is missing the point. The OP already has a rear derailleur, so why not just use it for the cheapest and simplest solution ?

wphamilton

Sure, why not and leave the brifters on also since that's the easiest and cheapest. The front DR works great as a chain-keeper. At least cut the cables off, then you could call it a single-speed.