's Forum Archives - Fixed Gear

Archive Home >> Fixed Gear

Chain Tension: How much is too much?(12 posts)

Chain Tension: How much is too much?p chop
Mar 30, 2003 3:18 PM
No droop, can squeeze about an eighth-inch or less top and bottom at the halfway point; nice and fit or asking for premature wear?

Can send the cranks about 3 times around with a hand push.

Thanks all for your thoughts.
Chain Tension addendump chop
Mar 30, 2003 3:27 PM
Whoops, said that wrong, thinking of my SS, when i said the part about sending the cranks 3 times around. As you would expect, once I send the cranks around with a hand push, the wheel keeps it all going for a while.

The tension I settled on seems to err on the side of tightness, leaving a little noise on the stand but not on the road. This is for fear of throwing the chain, which as a SS this bike did until I learned to get the hub clamped in there tight enough. An experience I'd like not to have in the fixed version. I'm running no ramps, old Sakae 45t chainring and a cheap steel track hub with nice tall teeth, but still I wonder if my paranoia has me running the whole thing a little too tight.

Right, now, thanks again.
Sounds about right if the tension is constant ...Humma Hah
Mar 31, 2003 8:35 AM
... thru an entire revolution of the cranks.

A new chain will need adjustment again in a few miles. I would expect, with that closely-adjusted a chain, that more droop will be apparent by the end of a good, long ride.

The one caution: a steel chain on an aluminum bike, starting out on a cold day. Aluminum has a greater coefficient of thermal expansion than steel, and might actually tighten up the chain as the bike warms up.
Uh...Humma?Dad Man Walking
Apr 1, 2003 10:55 PM
Your counsel is usually sound but I've got to call you on this one. Chainstay growth due to thermal expansion?

I checked out the thermal expansion coefficient of aluminum. An bike with 16" aluminum chainstays would grow less than 1/100th of an inch between the BB spindle and the axle over a 40 degree rise in temperature. Figure that steel grows somewhere around 1/2 as much, give or take; so keeping it simple the chainstays would grow less than .005" more than the chain would lengthen in response to the same temperature rise.

So if I did my figurin' right, I don't believe that we would be able to observe any meaningful effect on chain tension on a real-world rig (where we are loading the chain by hand in the first place while fiddling with the QR or axle nuts).
Yeah, but he's got the chain almost tight ...Humma Hah
Apr 2, 2003 9:39 AM
... the amount of slack change (possible vertical movement) in the chain is many times greater than the change in axel spacing that causes it (I haven't worked this out, but all fixte and singlespeeders notice how touchy slack is to axel position).

This rider is specifying very little slack, and I'm merely pointing out that it bears some watching on an aluminum bike when the chain is almost tight and slack will decrease maybe 10x as fast as the expansion difference. If that 0.005 becomes 0.050 change in slack, that's around 1 mm, and you could probably feel it if the chain started almost tight.

I haven't worked up the numbers, and ought to if I can get a chance. I don't think the difference in coefficient of expansion from aluminum to steel is 2:1, more like 13:10 if memory serves.
Yee GODS! Its worse than I thought!Humma Hah
Apr 3, 2003 2:07 PM
So I take this hypothetical idealized "chain" of 20 inches length. I pull it taut. Length is 20".

Then I let it slack by a small amount. The chain length is still 20", but the spacing is less, so it can be pulled down to form a triangle. Where half of the original chain length is the hypotenuse, and half the axel spacing is x, you can calculate vertical slack, y, fairly simply. A little application of the Pythagorean Theorem yields the following:

The first 0.001" of axel movement produces a full 0.100 inches of slack!!!

The first 0.005" of axel movement produces 0.224 inches of slack!

So even a small difference in expansion coefficient CAN make a significant difference in chain tension if the chain is almost taut. However, if the chain is already a bit slack, the change in slack is not nearly as noticable.
Could I 'expand' on that?p chop
Apr 2, 2003 3:12 PM
Actually, the bike's steel, though undoubtedly of a different type than the cog -- but the chainring's aluminum, right? So here's what I pondered while charging around the local roads:

When metal expands with heat, it does so in all directions. In the case of the chainstays, the only direction of expansion that matters is in the line from the bb to the rear axle. But in the chainring, all directions of expansion should contribute to its pull on the chain.

Maybe the difference is so slight as to be nominal, but perhaps not since, as Hummah pointed out, I'm already running the chain pretty tight. I guess if this is true, though, everybody with a tight chain would start hearing some extra chain noise when it gets warmer outside (especially if they set up their bikes in the winter).
Hence we'd all know about this phenomenon already.

Couldn't help but ponder, anyway.
Don't worry too much about it ...Humma Hah
Apr 2, 2003 3:36 PM
... the slight differential expansion difference between chainring and chain happens on all your bikes (but not on the cruiser, which sports a nice steel chainwheel). Its a daily fact of life ... to some small degree the pitch of the chain won't match the pitch of the chainring ...

... for a very short time ...

... because the mismatch causes both chain and chainring wear, also a daily fact of life, because those two components are always working to wear themselves in to each other's pitch.

I suspect the normal daily wear will usually tend to loosen your chain before you get into trouble, except maybe in rare occasions where you already start too tight and then encounter a huge temperature increase.

Just check it occasionally. Getting it too loose is a much bigger problem, as a derailled chain on a fixie or SS tosses your butt on the ground very hard.
frame torque?DougSloan
Apr 3, 2003 4:04 PM
I imagine twisting of the fame just a bit while pedaling hard might overshadow any thermal expansion of the parts involved.

If a chain is so tight that thermal expansion is an issue, then it's waaaay too tight.

See Yee GODS above ...Humma Hah
Apr 3, 2003 4:22 PM
... maybe not. 0.001 inches of thermal expansion difference could use up 0.100 inches of slack. 0.005 inches could use up about a quarter inch of slack.
Apr 5, 2003 6:07 AM
The part that's signficant is the difference in expansion rates of the frame/cogs system and the chain, right? How much could that difference be?

Of course, I'll defer to the rocket scientist! (normally, that would sound facetious...)

Why I switched to boxers one summerp chop
Apr 4, 2003 11:46 PM
"If... thermal expansion is an issue, then it's waaaay too tight."

Really, though, I don't think we're talking about my actual chain anymore... we're just, you know, CHEWING it, like my tight chain is doing to my chainring.

-- although, having filed my vertical dropouts by hand to the present effective chainstay length (filed forwardly, meaning I don't have to worry about hammering the hub forward, simple quick release'll do, but also why I'm loathe to file further if I don't have to), I think that Humma's hypothetical 0.2 inches is a significant margin between a little tight and a little too tight (as opposed to waaaay too tight), having experimented with much smaller increments in filing to get the chain where it is now.

Then again, as I said before, I guess if thermal expansion were all that, then all of our fixies would bind up when it hits 90 outside.

Maybe the thermal expansion IS all in my shorts.