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I have just changed from a 34T inner chainring to a 38T, 34 is too small for the riding I do. When changing down the chain is not engaging with the chainring, it sits on top of the teeth and im just spinning. The bike is a new (6 weeks old) giant fastroad, the new chainring is from stronglight.

Can anyone help please?

Chainrings that were purchased:

enter image description here

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    Welcome to Bicycles.SE! It might help to include details of the number of speed your dérailleur system is (ie: The number of cogs on the rear cassette), as well as the model of stronglight chainring you went for. A photo would be good too. – James Bradbury Oct 10 '16 at 8:40
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    You may have flipped the inner chain-ring, in that case the space between the rings is too narrow and the chain will rub against the outer ring. – Carel Oct 10 '16 at 9:31
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    The text should be facing the outer ring. – Will Vousden Oct 10 '16 at 10:27
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    Most likely you need to adjust the derailer. – Daniel R Hicks Oct 10 '16 at 12:02
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    Look at the holes for the bolts. They have a step where the head of the bolt has to fit in. The big ring has the wider part of the hole facing the right pedal and the small ring has the wider part of the facing the bottom bracket. – Carel Oct 10 '16 at 16:18
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As the bike is fairly new the problem may have always there but you'd not noticed it, or it occurred at the same time as you changed the chainring.

I'll assume though, that everything was working before the change and all that you changed was the chainring, and made no other adjustment. One of the following could be the issue. Start at 1 as the most likely.

  1. The chainring has been installed incorrectly. Check all bolts and spacers are set correctly and tightly. There is info in the comments on your question. If necessary remove the new chainring and refit the old one, check the spacing between the chainrings and that the shifting works correctly. Then refit the new one and compare the spacing.

  2. The front derailleur needs adjustment. You said in the comments that the front derailleur changes from small chainring to large ok. It may need some adjustment to the lower limit screw to make it move down / inwards a little more to give a better change from large chainring to small. This page gives info on adjustment: http://www.parktool.com/blog/repair-help/front-derailleur-adjustment#article-section-5

The less likely solutions:

  1. The chainring is not compatible with your crank or chain, compare it with the chainring supplied with the bike.

  2. Chain length. When you change from large chainring to small the amount of chain used will reduce. This slack is taken up by the rear derailleur. As you've put a larger small chainring on, the rear derailleur is taking up less slack, there's a very slim possibility this is affecting the change.

  3. If you removed the crank arm to change the chainring, it's not attached as tightly as before.

Try 1 and 2 and see if they solve the problem. Steps 3, 4 and 5 are included for completeness.

  • Thanks for the help, I think the problem lies in the fact that the new chainring is described as 9/10 speed. There is hence a slightly bigger spacing between the two chainrings wide enough for the chain to sit between them but not drop fully into the gap. I have not removed the crank to change the chainring. Adjusting the FD has helped a little, but the chain is still getting caught between the two chainrings. I will be investigating further at the weekend. – KevMur Oct 13 '16 at 9:49
  • @KevMur How has it gone in the last month? Did you find anything? Did you try flipping the smaller chainring ? – Criggie Nov 12 '16 at 10:37
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    @Criggie - everything is fitted as it should be, I was going to grind off part of the chainring to bring it slightly closer to the larger chainring to decrease the gap but have not got round to it yet. As long as I only change down when on certain sprockets it works fine, for now. – KevMur Nov 14 '16 at 15:19
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Have changed back to the original 34T chainring due to changing not being reliable with the 38T chainring. working fine now with original setup.

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