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big noise from rear hub area when pushing bike backward

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big noise from rear hub area when pushing bike backward

Old 01-25-17, 04:04 PM
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mozad655
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big noise from rear hub area when pushing bike backward

I'm working on a bike that has internal 7 speed hub, shimano nexus I think. When I push the bike slowly backwards with one foot while keeping my other foot on the pedal, the cranks eventually give in to the pressure from my second foot and the chain sounds like it jumps forward on the rear single sprocket creating a a big bang. How do I fix this? I recently took off the backwheel and also had to change the gear cable going in to the hub. I suspect it might have something to do with chain tension but not sure.
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Old 01-25-17, 04:16 PM
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When you push a bike backwards, the crank will turn back also. This is unavoidable since turning the wheel back, or the pedals forward are essentially the same thing.

As to the exact problem, I suspect that you my have left the chain slack, so start there. Lean the bike on the wall and pull the lower chain loop down in the center, then up. It should have some free play, but the difference top to bottom should be less than 1/2".

You might also have misadjusted the gear cable. This can be dangerous because if the hub isn't properly engaged in a gear, it can slip under load which is a nasty surprise that may affect your control.
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Old 01-28-17, 08:25 PM
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I tightened the chain to the very extreme when there is only a couple mm movement and the bike is almost unrideable just to see if it removed the chain slipping. Put right foot on pedal and pushed the bike backwards with my left. Chain is still slipping loudly over the rear single cog. Some times when I reach an intersection and I stop a bit too late I push my bike back using above method. This problem stops me from being able to do that. Is this normal with bikes that have internal hubs?

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Old 01-28-17, 08:42 PM
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Is it possible that the chain is too tight?

Chainrings and sprockets are always slightly eccentric. If you don't allow enough slack, the chain can be too tight when the high spots line up opposite. Check by backpedaling a few revolutions while observing the lower loop or monitoring it with a finger check slack in the middle. If it ever achieves zero slack, then you need to move the wheel forward until the chain NEVER become taut.
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Old 01-28-17, 08:56 PM
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No I think there's plenty of slack right now. I just tightened it to the extreme to test if it would solve the issue and it didn't. After that I installed the wheel again normally and with slack.

Backpedaling is not possible. Its an internal hub with coaster brake. What I'm referring to is keeping one foot on pedal almost locking it in place while pushing bike backward with other foot. Instead of the pedals resisting and bike ending up going nowhere as one would naturally expect, the chain slips loudly on rear cog and pedals give in. Does my explaination make sence?
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Old 01-28-17, 09:03 PM
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PK, so now just try to back pedal while the bike is on the floor and see if you feel any resistance or if the chain seems to rise and fall on the sprockets as it engages or disengages. It's possible that your chain is a bit narrow compared to the sproclet teeth, especially the rear.

Also, have a friend hold up the bike and back pedal. The rear wheel shouldn't move, or accelerate very slowly owing to slight friction in the ratchet mechanism. If the wheel acts like the pedals are driving it, with little slippage, you have excess friction in the ratchet.

You can also remove the wheel and feel for drag in the ratchet.


This is pure and simple detective work, a process of isolating the problem area through various tests. I can't see, feel or hear the bike, so I'm limited, but you're not, and will have to work through possibilities, keeping your eyes and ears open.
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Old 01-28-17, 10:10 PM
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Originally Posted by mozad655 View Post
I tightened the chain to the very extreme when there is only a couple mm movement and the bike is almost unrideable just to see if it removed the chain slipping. Put right foot on pedal and pushed the bike backwards with my left. Chain is still slipping loudly over the rear single cog.
The chain isn't slipping over the cog with anything close to maximum chain tension; it can't. What you're feeling is the internals slipping.

If the hub has been ridden for long with poor adjustment, expensive things happen inside it. The only way to solve it is to make sure the cable adjustment is correct, and if that doesn't sort it, you have to drop $$$ on new guts for the hub.

IIRC, this hub should be in fourth gear when you line up the marks (google it to be sure). Don't trust the gear indicator, count the clicks from first.
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Old 01-28-17, 11:06 PM
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When your gear shift handle is in fourth gear, adjust the cable tension so that the two yellow marks on the Nexus mechanism line up with each other. Go to YouTube and look at the Nexus adjustment videos.
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Old 01-29-17, 10:41 AM
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Your right. Upon further inspection its not the chain slipping but the cog. Its as if the cog is not completely bound to the rear hub. When wheel is slowly pushed backwards while forward pressure is applied on pedals, the cog just gives in and disengages from the hub. Is there a way to tighten it? What mechanism binds it to rest of hub?

I don't have a problem when pedaling forward. The wheel is responsive and gears work fine and are adjusted correctly as per above instructions.
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Old 01-29-17, 01:36 PM
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Originally Posted by mozad655 View Post
Your right. Upon further inspection its not the chain slipping but the cog. Its as if the cog is not completely bound to the rear hub. When wheel is slowly pushed backwards while forward pressure is applied on pedals, the cog just gives in and disengages from the hub. Is there a way to tighten it? What mechanism binds it to rest of hub?

I don't have a problem when pedaling forward. The wheel is responsive and gears work fine and are adjusted correctly as per above instructions.
This came up in another thread; it's likely the mounting lugs for the sprocket are worn/otherwise damaged, and it should be replaced.

A snap ring/circlip holds it to the hub's driver (which is often a part of the bearing assembly, which means it is hardened, and less likely to be damaged). As long as it is not the square type, it is easily pried off with a small flat-tip screwdriver, after which the sprocket will then slide off. If it is a square profile, retaining ring pliers (of the type sold for automotive work, with jaws rather than pins) are invaluable.

Replacement is straight-forward, if you manage to remove the old one. Make sure that the ring is seated securely, is all. A light tap at 3-4 points around it with a small punch will do.
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Old 01-29-17, 06:27 PM
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For reference, this picture is from Sheldon Brown's website:



The silver bit sitting immediately atop the sprocket (closer to the viewer) is the snap ring/circlip. As mentioned above, they come in two varieties:

Round:


and Square:


The square ones are absolutely evil to remove/install without retaining ring pliers:

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Old 01-29-17, 06:36 PM
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So that tiny ring is everything that keeps the sprocket locked to the hub? Is there no attachment mechanism behind the sprocket? How does that work?

And what could be broken on it? It seems like a very simple part, almost like a spacer.
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