Bicycle Mechanics - Chainring or chain problem?
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08-01-10, 07:27 PM
I recently returned to riding and have bought a second hand Giant Cypress. I have a problem which I haven't really found an answer to by using the search here. So my apologies if this has been dealt with before.
Do I have a chainring or chain problem?
Under stress, that is while setting off quickly or climbing a steep hill slowly in a low gear, I have what feels like the chain jumping in sudden jerks. I first thought that I had some bad (low) teeth on the chainring. Reading here on the forum I discovered that those pairs of low teeth were deliberate. I don't see any bad "hooking" of the chainring teeth (I'll get a photo if it will help). So might this be a chain stretching issue? Once under way, this jerking never happens. Also doesn't happen in higher gears, but then I don't set off or climb slowly in those gears. I ride the middle chainring almost all the time so this refers to that situation.
Maybe this is still too vague, but I'll appreciate your help. I'd like to fix this myself if possible rather than just take it into the local bike shop.
08-01-10, 07:30 PM
Is the skipping occuring at the chainring or at the cassette end? I expect it's a cassette issue and maybe be as simple as a minor rear shift cable tension adjustment.
08-01-10, 07:46 PM
I honestly don't know where it is occuring, all I feel is the jolt on the pedals. Any thoughts on how I can tell where the chain is skipping? I wondered if it was simply lifting on a tooth as it entered the chainring and then dropping in and causing the jolt?
08-02-10, 08:11 AM
Skipping is unusual on chainrings unless they are very badly worn or damaged. The chain is "applied" to the chainrings under tension so it tends to seat properly even if conditions aren't ideal. Cogs, on the other hand, get the chain from the low tension side and are smaller so the load is distributed over fewer teeth. Therefore worn cogs and/or a worn chain are very prone to skipping.
About the only way to tell where this is occuring is to look down while doing what causes the problem. Be careful where you do this as you will be distracted so try to do the diagnosis in a safe area.
As Hillrider says, you need to identify the exact cause before spending dough.
First measure your chain for wear (stretch). Lean the bike against the wall in a corner with the front wheel against the other wall so it can't roll. Find a 12" ruler (any will do). Chains have 1/2" pitch so a new chains pins will all line up with the same 1/2 reference on the ruler. With wear, the links separate a bit making it seem as if the chain stretched. Measure as follows: tension the upper loop enough to take up all the slack, by pressing on the pedal, and measure 11" of chain from the 1/2" mark to the 11-1/2" mark. (don't use the ends of the ruler because these are often worn making an accurate measurement difficult).
With one pin at the 1/2" mark you're interested in the distance the far pin is beyond it's mark. Less than 1/16", the chain is fine, 1/16"-1/8" you should replace the chain, but it probably isn't the cause of the skipping, more than 1/8" the chain is highly suspect, and when you replace it there's a good chance that the cassette will also need replacing.
While the bike is in the corner, try to duplicate the skipping by having a friend press on the pedal, increasing the load up to his full weight. You watch the chain and sprockets. You see some movement between the chain and sprocket with the chain climbing outward on the sprocket before it skips. If the chain is OK (not stetched) then it's the sprocket.
BTW- Chain skip is usually limited to a single or two cassette sprockets or chainrings, not all of them. If it only skips on a single sprocket it's the sprocket, not the chain, if it skips on multiples it might be the chain, but most likely is a trim issue in the RD, or a bent hanger causing the chain not to be fed onto the center of the sprocket, and allowing it to hang on the tops of the teeth.
08-02-10, 08:34 AM
Thank you. I'll try that next and also take a photo of the cogs.
08-02-10, 06:12 PM
Usually the problem is a new chain on old worn cogs (cassette sprockets). If you never change the chain everything will wear out. Changing out a chain that shows 1/2% wear (1/16" in a foot) makes the rest of the drivetrain last longer.
08-04-10, 06:04 PM
I have measured the chain. Under hand on the pedal tension the chain (as in the photo) measures as close as I can tell exactly on the 11 inch mark. With heavy foot pressure there is almost 1/32" stretch but definitely no more. I have included phots of the cassette cogs too. I was riding today and had the problem whenever setting off in 2nd or 3rd. Setting off in 4th seemed better but the load was probably less.
I haven't been in a position yet to look down and see what is happening.
Thanks for the help and thoughts so far.
Before you do anything else, lubricate that chain. Your problem might be as simple an rust stiff links now winding onto the sprockets smoothly and climbing out on the teeth.
You might also try back pedaling while pushing the lower pulley forward to remove tension in the lower loop and see if the chain runs with a smooth unifirm sag, or if any stiff sections don't drop into the uniform curve.
08-06-10, 11:53 AM
Ok I checked the chain as FBinNY suggested and all of the links are moving ok, no stiff ones at all. I freshly oiled it too. So the chain looks good and is only showing very slight stretch. The problem still persists. I am not able to get the chain to skip in a static test with the bike in a corner as suggested. Also watching the chain it only rises a fraction of an inch in the sprocket, maybe 1/32" when full weight is applied to the pedal.
When it does skip I would say it feels like the pedal only jumps down about the amount that would be caused by one link jumping forward, or in. But it will do that two or three times in rapid sucession until the bike is moving more and the load reduces. It never skips while accelerating normally or cruising. Only while setting off firmly or on a short steep climb. Only when the load is high.
Quote.... if it skips on multiples it might be the chain, but most likely is a trim issue in the RD, or a bent hanger causing the chain not to be fed onto the center of the sprocket, and allowing it to hang on the tops of the teeth.
Could you enlarge on the above FBinNY or explain what I should look for there?
Thank you again,
08-06-10, 12:07 PM
What he is saying is that the rear derailleur may need adjustment or the hanger holding the derailleur to the frame may be bent. It could also be the derailleur or pulleys are not aligned properly. Lots of things happen under drive train stress, which makes diagnosis hard when we don't have our hands and eyes on the bike. I would suggest taking it to a shop. Willing to be proven wrong, but I doubt you will be able to fix this yourself.
08-06-10, 12:13 PM
Stop looking for mysterious problems! Have your RD hanger alignment checked! If you are convinced that all your adjustments are correct, the drive train is clean, and you do not have a bottom bracket issue, start by installing a new chain. Then move on to the cassette. It's always a good idea to replace both at the same time if you unsure of the mileage.
Could you enlarge on the above FBinNY or explain what I should look for there?
It rare, and almost unheard of for all the sprockets to be worn so a new or decent chain skips. So if, for example, it skips only on the inner chainring, or only on one or two cassette sprockets, you might suspect a sprocket issue, but if is skips with various sprockets you need to look elsewhere.
I suspect that the chain isn't disengaging from the torque but not engaging on the bottom and riding the tops of the sprockets before dropping in. This is usually a derailleur alignment or adjustment issue. Also it's possible that on your wide cassette the chain can ride the top of a sprocket while leaning on the side of the next larger sprocket. Test for this by shifting to the 2nd largest sprocket and backpedaling while pushing the chain against the side of the largest, if you can cause it to climb up and ride the tops of the teeth, that may be your problem.
The problem happens most often when using a narrower chain then the cassette was made for, is a 9s chain on an older 7s or 6s system. Accurate derailleur adjustments helps but in many cases a wider chain is the best solution.
As others have said, it's hard to diagnose without hands on, and you might have to visit a shop, and let a hands on mechanic deal with it. I know a few decent shops in Wisconsin if you want a reference.
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