Old 01-21-17, 07:26 PM
Andrew R Stewart 
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Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Raleigh Pro, Trek Cycle Cross, Mongoose tomac ATB, GT Bravado ATB, JCP Folder

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If this bike has only a coaster brake then the possibility of the cog coming loose from the hub's driver is A VERY BIG DEAL As in not being able to stop.

But I'm not entirely sure what the problem is exactly yet. I suspect the term rivits that you use is actually the driver's splines and the cog's tabs. Be at ease that 3 tabs/splines have served millions of riders for, maybe, a hundred years if the cogs is properly attached.

The cog is usually held in place by a clip in the shape of a ring with a split. This allows the clip to be opened slightly while it is slid over the driver body then the clip settles into a groove and the clip closes back down. In an ideal world the cog has a slight slop on the driver. Since there is no real side to side forces acting on the cog this slight slop is no big deal. But if enough force is applied to the side of the cog (like a bent/twisted chain link or significantly out of line cog/chainring) the clip can be forced open and jump out of the groove. If the clip was worn out (not common but can happen) then this is even easier. If the cog location on the driver is such that the clip won't fully settle into the groove then it's slipping out and off is also easier.

While this design is very universal in it's basic layout there is some difference in cog thicknesses and driver widths. I have seen cogs which have been from other brands/versions of hubs installed on the wrong driver and then the clip won't settle fully and will let the cog to come off the driver.

This needs to be figured out ASAP. The worst case is VERY UNSAFE. If the mechanic can't figure this out you should seek one that can fully deal with it. Of course since we're here and you're there we really can't say/help much but to worn you of the need to get this fixed before you find out what the cost of gambling really is. Andy
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