ASVintage

This topic has likely been addressed on this Forum previously. If so, I apologize in advance for the repetition. I could not find it in a search.

I have a 40's track bicycle and am having problems getting a balanced tension on the chain. Half the rotation has too much slack, and the other half the chain has too much tension. I took Sheldon Brown's advice for centering a chainwheel (Fixed Gear Bicycles for the Road) and it did not work. My crank is a three pronged Schwinn Paramount spider crank with a 1" pitch chain. I have two sets of the crank arms and rings, switched them out to see if one crank was bent and have the same problem with both. I can't seem to balance the tension. Suggestions?

Homebrew01

Could you try rotating the chainring around the spider ? Maybe there's a combination that will be a bit rounder due to slight irregularities in manufacture ?

hueyhoolihan

putting it gently... it's possible that expectations are exceeding reality.

ASVintage

I fear that. I just took it out for a spin and the grinding on each downward rotation (both sides) was too loud to be normal. I am assuming it is the chain tensioning.

FBinNY 

The correct tension is no tension, or so that there's vestigial slack at the tightest point. That's true, no matter how much more slack there may be at the loosest point.

Part of your problem is that steel chainrings of this vintage aren't that concentric. If nothing else, the action of the cotter will pull the arm slightly off center on the spindle.

Don''t fret over so-called "excess" slack. Slack in the lower loop doesn't matter in the least unless it's enough for the chain to fall off, which is probably much more than what you'll ever see on this bike.

So spin the crank and feel the chain tension until you find the tightest spot, then pull the wheel back until you've left only some vestigial slack. There are various tests for this, mine is to hold the rear wheel and jiggle the crank forward and back and see if I can visibly transfer the slack or sag from the upper to lower loop.

ASVintage

Thank you! I will try that. Both upper and lower are in significant tension at the same time, then I rotate the crank 180 degrees and both upper and lower are slack. What you are saying will likely balance the imperfections in the chainring. Hopefully this will eliminate the very bad grinding sound. But I won't be able to field test it till spring, since we just received a few inches of snow today.

hueyhoolihan

i have under some circumstances, and after getting a rough idea of a workable tension, adjust by noise level under riding conditions. i just, as mentioned, keep tabs on a too loose to come off state. i can usually find a happy medium between grinding under power and clanking on the cog/chainring when traversing a moderate bump.

the OP stated, "... I have two sets of the crank arms and rings, switched them out ...".

if i hadn't swapped rings on the crank arms (it's a little unclear to me how extensive "switched them out" is ), i might try that if possible. if it were me, i may not be too hopeful of it being a remedy, but i might do it just to exhaust possibilities. good luck.

hueyhoolihan

as a last ditch effort it might be illuminating to test the crank/spinder/chainring's eccentricity WRT to BB shell/spindle/frame, as mentioned in another post, it may be off a bit. i might remove the chain, tape a square across the downtube, as close to the chainring as possible, and slowly turn the crank arm. that should show you something. i'm not going to venture a guess as to what though as i haven't thought it through. ...and i'm not going to.

Bezalel

Is the spider centered properly in relation to the BB spindle?

(And we need pics of the rest of the bike.)

FBinNY 

It;s not a matter of balancing anything, nor is it at all complicated. 6 year old boys know how to do it on their 20" coaster brake bikes, and it's no different on vintage or modern bikes.

Chainrings are never perfectly concentric. It's sometimes possible to get the worst a bit better by loosening the bolts, and tapping them to center but perfection isn't possible or necessary.

So tension will vary as you turn the cranks, and the object is to have that variation be from slightly lack to slacker, rather than from taut to slack.

ASVintage

I will try the testing as suggested above and it will likely show me the chainring is out of balance. Your comment about the cotter is an astute observation that got me thinking more. You would think production of the day figured out forging to maintain consistency. The Schwinn literature of the time, while being good marketers, boasted precision Paramount racing components. So you would think quality would be dialed in pretty good, unlike their boys bikes which were mass produced. However, the wedge pins are not designed to the same exacting standards. Just for grins, I pulled out a bunch of my 9.5mm wedge pins from my bin, and noticed all the tapers are different. I wonder if the taper could affect eccentricity? I will try some different cotters today and see what happens.

ASVintage

Here are the pictures. It is a late 40's Wastyn Paramount. You can view higher resolution photo's of the restoration here https://www.flickr.com/photos/316405...7645736415391/

ASVintage

I will definitely do this. Thanks.

Trakhak

Slightly slack at the tightest is indeed what you're looking for. That grinding sound the drivetrain makes when the chain is too tight is probably an indication that you're performing a hardness test on the bearing surfaces in the bottom bracket and rear hub---not a good thing.

ASVintage

What is a hardness test? All the bearings are new and BB is re-greased.

FBinNY 

Just his facetious way of saying that the bearings will be seriously overloaded, as the chain tries to pull the sprockets closer together.

In reality, the rear axle takes the brunt of this by flexing forward to accommodate the too short chain.

BTW- take a look at some of these photos of chain drive trucks and machinery. You'll noice that 100% of them have sagging slack chains. This equipment is more expensive and carriers vastly higher loads, but professionals KNOW that the right tension is no tension.

ASVintage

That indeed feels like what happens. I may just check the BB bearings to make sure they are seated correctly. I do not pack many old school bottom brackets and I could have a ball or two out of position. Although they run smooth as silk under no load.

I did check the the chainring using the tape method and it indeed was not turning in a perfectly concentric manner. It was off by one mm. I checked the wedge pins and installed a different set. It did not remove the deviation. I replaced the chainring with another 25T Paramount sprocket. Same one mm deviation.

Slacking the chain will likely be the answer. Thanks again!

Homebrew01

Eccentricity comes from the precision, or lack of, in the fit between the BB axle and the crank, and the spider and chainring. The cotter pin just holds the crankarm on. A poorly fit cotter may come loose, but is not the cause or cure of your problem.

FBinNY 

I suspect that the OP was a victim of semantics. The literature speaks of "tensioning" the chain, and there are a number of devices being marketed as "chain tensioners". The repeated use of the word "tension" in this context implies that there's tension involved and can create misconceptions in people's mind.

If, instead of speaking of "adjusting chain tension", we instead spoke of optimizing or minimizing chain slack, then people might find it easier to accept that slack is the normal and correct condition.

79pmooney

The word from pro fix gear mechanics (velodrome, etc.) is to be able to pull the chain up and down about 1/2 an inch, but to never have it go tight. When I have a drive train with rings that are not round, I find the tightest spot and loosen until it is almost, but not quite tight. Then I rotate to the loosest point and check that I have less than in inch of play. If the second isn't possible, I consider that bike not fit to ride fast (downhill or sprint).

The tight chains so popular in the hipster world you DO NOT SEE at the velodrome!

Ben

LesterOfPuppets

Yep, you can run chain pretty slack before dropping becomes a problem, especially on a track. If riding road, avoid the rough spots if running "track slack".

SquidPuppet

I have used a zip tie fastened around the seat tube, with the excess left attached and positioned as a depth gauge to the edge of the chain ring. It told me that my chainring and spider were 99.9% perfect, yet I still had tight and loose spots in the chain. For many years now I have just chalked it up to imperfect chains, because even when new, I experience this on all single speed bikes. I just adjust the tight spot and forget it. Slack is your friend, binding sucks.

ASVintage

Balancing the chain tensioning will be critical. And the only hard riding will be around a track every now and then. Most of the riding will be causal rides on the flat, smooth bike path.

I have made all the adjustments as suggested above, checked the BB and all appears to be in order. But the grinding persists on the downward pull on both sides. I have the bike on the rack and have isolated the noise to be coming from the back hub/cog. The chain appears to catch slightly on the way around toward the front chainring. I am wondering if I am getting some hooking from the old, slightly worn cog against the NOS chain and that is causing the noise? I might need to find a new cog.

SquidPuppet

Time for a cog picture.

FBinNY 

I suspect that many (most people) have no sense of how a tiny amount of eccentricity can be the difference between tight and slightly slack. An eccentricity of as small as 0.01" will cause a chain to move from taut to a sag of about 1/10". So for all practical purposes we must discard the notion of perfectly round (non-eccentric) chainrings and provide for them by leaving adequate slack.