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View Full Version : Sanity Check Request: Drivetrain is purring



ChampionTier
11-07-2017, 09:36 AM
So I think I know what the problem is, but before I put money into a new cassette, I'm curious if I'm leaving something out.

Bottom line: some part of my drive train is making a grinding/purring sound when I'm in my first/second/third gear on my cassette, my big ring up front, and applying lots of power (as in climbing a hill). I checked and the chain is not rubbing against any derailleurs.

Here's the details:

Replaced my shimano (116 link) 11-speed chain which had ~1000 miles on it.
I did not replace my cassette which has about 2500 miles on it (please don't judge me).
My new chain is a KMC 116 link 11-speed chain which is where part of my problem might be.

Because I am a first-generation upright walker, I tried to use my bare hands to try and snap the missing link into position rather than read the directions and merely apply force to the pedal to get the job done. In doing so I think I may have bent the chain from its natural alignment somewhat.


Because it only happens when I'm applying a lot of power (i.e. more than I can when it's on the stand), I cannot isolate the origin of the sound (other than to say it comes from "down there") because I'm moving and the wind over my ears muddles the sound.


Like I said, I think it's a new chain on a worn out cassette, but I can't figure out why that would cause a purr. Anyone else have a similar experience?

Thanks,
CT

FFX_Hinterlands
11-07-2017, 11:17 AM
Could it be loose pedals, loose chain ring bolts or play in bottom bracket? Those are all really quick to check. Did you check for stiff links when you put the chain on? I find on my IGH (single rear cog) that I get an odd vibration for the first 50 miles after installing a new chain. Probably due to stiff links or (in my case) too high tension.

Tom

huskerdont
11-07-2017, 12:13 PM
I would sometimes get a grinding/purring when I put a new chain on my fixie but didn't change the cog, but I've never had that happen with a cassette, no. But perhaps those three cogs of your cassette are the ones you used most often so they are the ones more worn.

And thank you for playing Stump the Chump.

ChampionTier
11-07-2017, 12:51 PM
Could it be loose pedals, loose chain ring bolts or play in bottom bracket?.... Did you check for stiff links when you put the chain on?

Good call -- I should probably be checking these more than I currently do. I'll take a look when I get back home.

ChampionTier
11-07-2017, 12:54 PM
I would sometimes get a grinding/purring when I put a new chain on my fixie but didn't change the cog, but I've never had that happen with a cassette, no. But perhaps those three cogs of your cassette are the ones you used most often so they are the ones more worn.

And thank you for playing Stump the Chump. Could be; I was wondering if it may be an issue of breaking in the chain, so to speak, but I can't think of a time when I've had to do that before. If FFX_Hinterlands advice doesn't pan out, I'll just move out on a new cassette...probably a little overdue for one anyway.

Thanks!
Ct

FFX_Hinterlands
11-07-2017, 01:23 PM
I'm not a bike mechanic (there are some on this forum who are), but I don't think you should change your cassette without doing the chain rings (and typically chain) at the same time. So in your chase if this is a new chain and you just replace the cassette the chain might not be meshing right on the (front) chain ring.

General info - When your chain wears it stretches and causes the other gear teeth to wear slightly. If you haven't changed a chain in a long time you may find the new chain does not mesh quite as well as it did with the worn chain. Sometimes the new chain is so unmatched from the worn teeth that it will slip or fall off when you pedal after putting on the new chain. This is typically a problem if you keep changing chains but not the rest of the drivetrain.

Tom

n18
11-07-2017, 01:30 PM
I vote for:

1 - Front derailleur rubbing against chain, despite you saying that it isn't. The frame maybe flexing while your are peddling hard.
2 - Pedals.
3 - Dirt on rear derailleur causing sprockets to make that noise.

Also, for reference, my 11-speed road bike, Fuji Gran Fondo which has 50/34T gears on the front, and 11-28T on the back came with a chain that has 110 links. A chain length calculator that I posted about here (http://bikearlingtonforum.com/showthread.php?12523-chain-stuck&p=167678#post167678) says that I should have 108 links, so it's close. 116 links is probably too long for your bike, but I don't know about your setup.

A new chain on a worn out cassette would perhaps slip(not to be confused with skipping/auto shifting, although it might happen), but it won't make sound like you described.

Finally, you may want to get this master link Park Tool (https://www.amazon.com/Park-Tool-Master-Pliers-MLP-1-2/dp/B00D9NW32I/) for both opening and closing master links with one tool, while KMC makes two tools (https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=kmc+Master+Link+Pliers) to perform the same function. I have injured my fingers while trying to open a master link with long nose pliers.