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Problem after swapping chain rings
#1
Hi everyone

I'm new here and I'm a novice bike mechanic so be gentle with me ;-)

Basically I have two bikes, ad old rigid frame MTB (must be a late 90's model) and my entry level road bike. I still want to use the MTB for family bike rides and pulling the little kids in the trailer I have. But, the thread in the drive side crank arm sheared and now I can't install a peddle in it. The chain rings and crank arm are fixed together so it's not a simple case of fitting a new arm so knew I'd need a new crank set. It's a 28/38/48 set and I'd bee watching quite a few on eBay when a chance conversation with my neighbour revealed that he had a scrapper of a MTB he said I could canablise. This was a more modern bike than mine, full suspension and all that but in a real state. I suppose a better man than me would have opted to repair that using my old bike as a donor. Except I like the rigid frame, not bothered about suspension and to be honest I can't fit a child seat to the back of a full suspension bike anyway.

So, the chain ring set on this donor bike was also 28/38/48 so that'sa start. I removed the rings of both bikes (thoroughly cleaning the donor) and compared them. Same size, same gap between the rings and both square taper. The donor part looks spot on, without any damage so I went ahead and fitted it.

The donor part went on fine and after a little adjustment of the dérailleur the gears were shifting ok. SO then a test ride. It seems ok but then I became aware of some grating noises in certain gear combinations and then I looks down and noticed as the rings we spinning as I pedalled it look to be wobbling, or going in and out. I stopped and check but it was firmly attached. So I guessed that the rings set must have been bent. On returning home I remove it and attached it back to the donor bike and spun it - no movement. I put the old set back on my bike and spun, no movement. I only get this movement when the donor crank set it on my bike.

I also noticed then that the new set were flat rings, but the old one at a recess that made the rings closer to the frame that the new ones were - thus this being the likely cause of the noise in certain gear combos.

So what do I do? Why would it seems to move in and out only on my bike? Should I also swap the bottom bracket from the donor bike to mine (assuming that it's the correct size)?

I've looked around on-line and can't see any sets that have this recess type setup either.

Any help would be greatly appreciated.

Many thanks

David
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#2
How much closer were the rings, but I don't think that would do it, did you recheck the BB? You can have the old crank drilled out and a Heli-Coil and some loctite on it so it will stay and then put the peddle back on.
Have fun and just enjoy the ride
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#3
(02-18-2013, 04:10 PM)FredS Wrote:  How much closer were the rings, but I don't think that would do it, did you recheck the BB? You can have the old crank drilled out and a Heli-Coil and some loctite on it so it will stay and then put the peddle back on.

Just Google Heli-Coil - interesting! I'm going to have the cranks off again and look more closely at the BB though - I refuse to be beaten! I reckon the inner ring on the original set is at least 5mm close to the frame than the donor.
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#4
First thing is to figure out what is "grinding". Is it the chain rubbing on the front derailleur? If so, you may just need a little more shifter adjustment. If the chain is rubbing on the other chainrings, then it might do better with the BB from the donor bike.

On the chainring wobble. It could be that you didn't get the crank seated properly and tight enough on your bike. If it stills wobbles when you put it back, you can also try removing and remounting at 90 degrees change. It could be your BB is bent. A little wobble is not too bad, but if it's enough that it's causing the chain to rub on your front der, then it's probably a problem.
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#5
Well I didn't figure out the "wobble" so ended up installing a triple set from another donor bike. The sizes are a bit smaller but as this is the family day out and trailer pulling bike I'm not too fussed.

Thanks for the help. I think I'll be a regular here'

David
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#6
Could it be that the crank set that wobbled was from an incompatible square taper bottom bracket, like JIS versus ISO or JIS low profile?
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