• LAVA Moderator: Shinji Ikari

Bicycle chain question (varying tension based on where cranks are in their rotation)

Jabberwocky

Frumious Bandersnatch
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Nov 3, 1999
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((MODS plz move this to 2nd opinion if you think it's a better spot, i wasn't sure where to leave it))

figured i'd ask this here, maybe someone can tell me what I'm doing wrong.

I ride bmx and fixed gear. BOTH of my bikes have this problem (particularly the bmx), and I am clueless on what to do.

When I put a tire on (or just go to tighten the chain/rear wheel), I find that, depending upon the crank's position, the chain tension fluctuates. I like to ride my chains tight as hell, but, for example, on my bmx the chain goes from loose to tight every goddamned crank rotation!

I'm guessing i'll be told it's un-true sprockets, which may make some sense here. i figured the lil sprocket (25T) on my bmx would exaggerate this. but finding that this happens on my fixed gear too is bothering me. the fixed's sprocket is like 44T and I don't grind with that bike so the sprocket should be pretty damn true!
 
The most simple step it to check again to make sure everything is straight in the rear drop outs. What kind of bottom brackets are you running, if external is everything faced and straight (though hard for me to see this being the cause of your problem without obvious bearing issues)? Are the sprockets/chainrings bent or warped, or reallllly worn down?

Without seeing it it is really hard to imagine what is off. It sounds like you are making the same mistake on both bikes. I've never really heard of such a problem. The way chain rings and sprockets work it is weird that there is any variation in chain tension. Maybe it would be worth it for you to take the bike into a your local bike shop, and have them fix it on one bike and tell you what was wrong, and how to avoid/repair it in the future.
 
it's cuz your chainring aren't centered on the crank. the simple solution is to stop being so anal about chain tension. the more complex and time consuming solution is here.

if it was an untrue (if by which you mean, side to side or up and down) of the chainring itself, which probably isn't perfectly round or straight, it would be exponentially worse with larger, not smaller, chainrings.

but i recognize the preference for tight chains all the time for brakeless riding or for keeping pedals still during tailwhips etc.
 
shimmerfade- everything's fine in the rear drops (hell i even ride with chain tensioners on my bmx). both bikes have horizontal dropouts and american bottoms; both bottom braackets appear unbent/unbroken in any ways and both are cleaned/lubed/tight-ish (as tight as i can get them..i can always get my headset packed/tightened 100.0% i don't know wtf i'm doing wrong on the bottom

i againsti- i guess this is it: "If the chain is too tight, the drive train will bind, perhaps only at one angle of the pedals (chainwheels are not usually perfectly concentric). It should be tight as it can be without binding. If the chain is too loose, it can fall off, usually at the most inconvenient possible time. "
so i guess just deal with it? even though on my bmx my chain goes from perfect-tight to, by any definition, loose, depending upon pedal position..
both bikes are tuned incredibly well *except* this single spot..and ya it matters a lot on fixed but honestly the real problem is the bmx the varying tightness there is real pita
 
i against i - I thought most chainrings had to be centered on the crank or you couldn't even attach them properly with screws? Or are some like disc brakes where there is some play around the screws?

bmxxx...just save up the $10 it would cost to have the LBS look at it real quick, and have them tell you what was wrong so you know in the future. It is hard to know what is going on without being able to fiddle around with the bike.
 
I get this problem on the fixie and dirt jumper .. I reckon its down to a combo of the back hub not lining up parralel in the dropouts and chainwheels being naturally a lil bit warped... I found that the problem was a little bit less prolific with a halflink chain though ... dont know why :?
 
I ride a totally custom full suspension 18 speed 26" set-up for street assault style riding.
Hello, bmxxx. I am a transplant from upstate ny now residing in Clearwater. Tampa bay. GO BUCANEERS!

Anyways, I had a mtb that once had a rounded or oblong front sprocket. The chain, even with the rear derailer spring taking up the slack, kept falling off.

So, I ride the Pinellas Trail. It starts in Dundein and ends up in tTampa. Longest trail in the US. 35 miles long.
 
bumboclaad- that's good to know, will speed up getting one :)

beagle- that's all it can be, a sum of the imperfections between sprocket alignments, sprockets' flatness, etc etc.
oh and where are you finding to put that suspension to good use? that's just slowing you down on the that trail! (check your pm's)
 
i against i - I thought most chainrings had to be centered on the crank or you couldn't even attach them properly with screws? Or are some like disc brakes where there is some play around the screws?

it is the play around the screws. especially on bmx chainrings, as most only have one attaching bolt.
 
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