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Wonky chainwheel and guard

ansbaradigeidfranansbaradigeidfran Posts: 526
edited November 2008 in The workshop
Last week, I overshifted my front gears and got my chain stuck between the largest chainwheel and the guard. It came out again, and I've since adjusted the front derailleur, but...

The guard, being a soft alloy, has bent out quite a bit. I wanted to take it off to hammer it back into shape, but the guard and three chainwheels are riveted together (with the rivet head designed to look like an allen keyed bolt, the sneaky people!). I'm thinking of drilling the five rivets out and replacing them with nuts and bolts. Alternatively I could go with just some nuts, and thread the chainwheel, but that'd require a tap thingy and I don't have one. Any advice?

Also, the three chainwheels, while rigid on the axle, are a bit off. The large chainwheel has 2-3mm of side-to-side waver in a rotation. The middle and small chainwheels are attached to the large one, so fixing one will fix them all. But is it bent (due to the above debarcle?) or poorly mounted?

Posts

  • Pat920Pat920 Posts: 55
    Are you certain the chainrings are riveted? They usually have short allen bolts with threaded barrels as nuts - they have to be like that so they don't snag the chain. The nuts have to be held in place with a flat-bladed screwdriver when the bolts are undone. And it is not usual for all three to be fixed with the same bolts. I'd suggest having another look. If the whole think is quite old, it might be worth replacing it rather than trying to tap some threads.

    As for the chainrings all being out of true - how old is the bike? Could it be that the bottom bracket is loose or worn? It would take quite some force to get the spider out of true.
  • Re: rivets, pretty certain. I coudn't turn it with 3,4, or 5mm allen keys, and tried a friend's torx set as well. I was just stripping the very shallow (1-1.5mm) thread. Took it to my LBS, and the guy there tried it too. Second guy looked closer at it, and came to the conclusion that it was riveted. He said it's common with low-end bikes, and this is very much a cheap bike.

    The three chainwheels and the guard are all attached together with these pseudo-nuts. There is no play in the BB, that's solid. Age isn't an issue. The one possible cause that comes to mind is one derailment off the granny, with the chain stuck between the chainwheel and the frame. I couldn't get it out, and opened a link to get back on the road. That overshift may have whacked the chainwheels out, or perhaps the chainwheels were already crooked: the chain snuck into it's crevice through the widest 'throw', and I didn't know to turn the pedals back to find that loose spot.

    On the issue of bolt, the granny chainwheel recesses about 6-8mm to get to the rivet, so there's room for a bolt to sit on the end of a nut of the correct length. I may yet drill the suspected rivets out...
  • I bit the bullet and got hold of a drill: the rivets are no longer. I straightened the flimsy chainwheel guard by hand, but the chainwheels themselves looked straight to my eye. I put them back together with some 12mm bolts to find that the middle and small chainwheel are still a bit off, but by selectively tightening the five bolts that is improving.

    Drilling the first rivet out
    19112008078.jpg

    Chainwheel guard off, some rivets needed attention from a hammer and punch to get the chainwheels free
    20112008079.jpg

    All cleaned up, with new nuts and bolts. Ready to go back on the bike
    20112008083.jpg
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