Strange clanking sound when going over bumpy surfaces

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Apr 3, 2013
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Hello everyone,

Yesterday I swapped out the 18t cog for a 20t, to make it a bit easier to pedal. I also added some grease to the internals of the hub, and as far as I know, got all the bearings back in the right place, and the brake still works. After taking out for a spin, now it seems like everytime i hit bumpy terrain it sounds like something is clanking around in the frame. The chain is tight and I am unsure at this point what could be causing this. Any thoughts on what it could be?
No Kickstand, Chain Tight, and the headset is tight. It sounds like a loose bearing or something rattling in the frame? Pulled apart the rear hub again, thinking maybe a bearing got loose, but all was well. I am out of ideas at this point.
My first thought was chainguard too.
if by bumpy surface you mean grass or gravel it's sand & grit kicking back from the tires.
Chain guard has been removed. I will try and flip the cog. It's not gravel. Sounds like a rattling bearing. Double checked the chain and it is tight enough that it is not slapping.
Someone else was missing a bearing. Maybe there bearing was place inside of your frame? :p
Muffler bearings ? :lol:

Maybe it's just debris in the frame rattling around.... Shavings from the manufacturing maybe ???

If its got loose ball bearings sometimes its possible for the bearings to get past the cups on the hub and into the "axle chamber". Will it make the sound just spinning the front wheel ?
Check your wheel bearings (front and back) while they are mounted to the bike by pushing the tire firmly side to side. If you have lateral play, loosen the mounting nut on one side and tighten the bearing slightly, tighten the mounting nut, check again for play, repeat until no shifting is detected.At least 1 (2 is better) 13mm cone wrench is a must to get them adjusted and tight. They will have a little resistance to turning when dismounted, it's the nature of the beast.
I had a GT bmx that came from the factory with a piece of weld in the down tube, I know because when the frame broke I dug the offending piece out.
"tink, tinkly, tink, tink," over every bump.......
I am betting it's not this, you have a bad bearing, loose race, something in the BB you just worked on.
The likely problem is the replacement cog is thinner than the stock cog.

Old school cogs were all for 1/8" wide chains.

A few years ago the boxmarts started using 3/32 drive lines for some coaster brake bikes (old 10 speed width).

Some 3/32 cogs have little stampings adjacent to the hole to take up some space on the driver.

But many 3/32 cogs don't have those offsets so they fit loose on drivers intended for fatter cogs and will rattle around. You can probably feel it move around with your fingers while stationary or if you just bounce the rear end up and down.

You can add a spacer. Some try cutting one out from a cottage cheese container lid, that's about the correct thickness (1/8 - 3/32 = 1/32" thick). Sturmey Archer 3 speed hub spacers are a bit too thick.

Another issue with cog swapping is the diameter of the hole. The Sturmey Archer size is different from Shimano size, the two most common sizes. If you use a larger diameter cog on a smaller diameter driver, the 3 little ears won't engage the driver by much so they are prone to slipping (and rattling).

You can run a 1/8" chain on 3/32" cogs and sprockets (chainrings) okay but you can't run a 3/32 chain on a 1/8" cog or sprocket.