FBinNY 

The guideline is 1/4" sag, or 1/2" total up and down (1/4" in each direction). But you can go tighter than that if you want to reduce the backlash. Slack is slack so as long as there's some slack at the tightest place, found by rotating the cranks a number of times.

Note, that both the front and rear sprockets may be eccentric, so you need to go a few times to ensure that you've confirmed slack at the tightest place which will be when both high spots are opposite each other.

My track bike has very round, non-eccentric sprockets, so I've minimized the backlash by having only about 1/16" sag at the tightest place, but every drive train is different so be cautious as you try to minimize the slack.

You can also use your ears to check if the bike is in a stand. If you don't have enough slack and the chain goes taut at any point, the sound will change and you'll both hear and feel the vibration change.

Scott Bontz

FB, you write 'Slack is slack so as long as there's some slack at the tightest place.' This has been my thinking. If there is always some slack, however small, there is never really any tension. So why would it matter to add lots of slack rather than to just allow the minimum necessary? That's a mostly rhetorical question, at least for you. But in this thread I have been getting, or interpreting, conflicting messages.

1/16 of an inch of sag is very impressive. I don't try to go near that, though I do use high-end, NJS-certified cranks and rings and bottom brackets in an effort to keep things as concentric as possible.

At risk of taking this too long, I'll add one more thing from my fixed-gear chain experience. After being brought to just the right adjustment of slack, a new chain on the first ride away from home and over gravel sometimes actually begins to feel more taut. But the forward pull on the cog should be able to bring only the opposite. I wonder if particles getting between the chain parts take up the slack. Whatever the cause, I stop, get off and move the wheel forward a touch.

There actually seems much ado in the supposedly simple world of the fixed gear. But I love it, and want to get it right.

Thanks again.

FBinNY 

As you correctly gather there's no reason to add more slack than necessary. The standard sag guideline is only that, and based on typical eccentricity in the sprockets. But even industrial applications where there's good ability to control conditions leave generous slack.

On single speed freewheel applications, there's no reason to try to minimize backlash because it's not relevant, so it pays to err high.

But fixed wheel riders like to minimize the backlash because it feels better that way (the bike doesn't care). But dirt on the chains rollers can effectively make sprockets bigger and pull out some slack, so if that's your experience, allow for it.

BITD, this was never a big deal and nobody fretted over it, we simply pulled out the slack, leaving some in accordance with the guideline. However, when fixies became trendy some years back, newbs with no sense of history or old timers to offer guidance, misinterpreted the instruction to properly adjust chain tension to imply that was supposed to be tension.

If, some 5 years back we'd said,"be sure to properly set the chain slack", (as I do now) there probably wouldn't be any confusion, controversy or discussion.

SquidPuppet

Just to further the discussion and add more confusion, I'll expand on my experiences.

I don't know how FB gets away with such little slack.

I have some bikes where I was fortunate enough to be able to achieve perfect (measured) chain line. I installed a brand new KMC Z410. If I adjust the slack to 1/4 inch up from straight and 1/4 inch down from straight, totaling 1/2 inch of slack, it feels like hell and I can hear it making it's way around the sprockets. I can feel it in my hands in the garage and my feet when I ride it. My experience is that just because I can move the chain a half inch, that doesn't mean the chain is free to naturally relax and exploit that range on it's own. If I then adjust the chain to the point where there is visible droop, all sounds vanish (dead silent) and I can't feel anything while rotating the cranks. Huge real world difference. I've used the round shank of a screwdriver while rotating the cranks to attempt to ride the chain up and off of the front and rear sprocket and I can't, even when really trying hard to coax it off.

I'm not saying that FB (or anyone else) is wrong, I'm just saying that ALL my chains exhibit an audible and feelable state of mechanical happiness when they are allowed to be very relaxed.

I'll add this. Most chain adjustment advice runs along the lines of, "As little slack as possible, without creating tension." My approach (came from Dad) is from the opposite school, "Plenty of slack, without the possibility of derailment."

FBinNY 

Excellent (non) eccentricity and good lubrication. It might also be that I never remove and clean that chain, so if dirt build up reduces the slack, I've already adjusted for that. Lastly it might be that my system is well broken in so it's more forgiving than brand new stuff may be.

Keep in mind that the upper loop is always running under tension and that's the same regardless of the amount of lower loop slack.

In any case, reducing slack to minimize backlash isn't necessary, it's just a nice thing. So my advice is to let your ears and feet guide you and go with the minimum slack needed for smooth running.

ThermionicScott 

I gave up on adjusting my chain slack to any specific fraction of an inch. Now I just put my bike in the stand, gently spin the cranks, and adjust so that the chain still has a tiny bit of droop at its tightest point.

FBinNY 

Likewise, I just use touch. I mount the wheel, push it back and slight closer to the left chainstay pulling the chain tight and tighten the right bolt, then when I center the wheel in the stay, it adds the chain slack and I'm good to go.

After mounting the wheel a few times I know just how much left chain stay bias will turn out right after centering, so I can do this in my sleep.

If that method doesn't give you enough slack, you can reverse it, pulling the wheel back and toward the right stay, and tightening the left side first, then straightening it will move the sprocket forward giving you slack.

Either way takes some trial and error at first until you learn how far off center to start with to end up rightmwhen centered. Once learned it's easy peasy from there on.