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a few months back I installed a 34 tooth narrow-wide chainring in my Santa Cruz Bronson. I decided to install the 34 tooth chainring, basically, because I couldn't fit any other smaller chainring using my current crankset.

You can see my current build in the following picture:Crankset

I use a "standard 73mm threaded BB" GXP (or that's what Santa Cruz website says).

How could I change it to a 30t chainring?

Do I need to buy a new crankset? If so, do you recommend direct mount cranksets (in case I need to switch it to another side of chainring in a future), and, does any GPX-adapted crankset work?

UPDATE: my crankset BCD is 104

  • And, how can I find the BCD of this crankset? – Bernat Nov 15 '15 at 18:56
  • You're heading for the slippery-slope of spending money here. Yes it would be possible to fit a smaller chainring, but the BCD of your existing spider looks to be too big. So you need a smaller BCD which means a new crank on the right side. This probably means a new left crank too. I suggest you explore a new cassette at the rear, with a more suitable range of teeth. This should be a cheaper solution too. – Criggie Nov 15 '15 at 18:58
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    @PeteH for example, I think this one (chainreactioncycles.com/es/es/…) 30t/104BCD should fit, right? – Bernat Nov 15 '15 at 19:23
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    @PeteH Narrow wide is common on X1. SRAM site has a good discussion. – paparazzo Nov 15 '15 at 19:40
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    So 32 on the front chainring and 42 is the biggest on the back? What are you doing, riding straight up Mt Everest? On 26" wheels that's 1.5 metres per pedal stroke, and 1.6 metres if you're on 29" wheels. What grades need these crawling ratios? – Criggie Nov 15 '15 at 23:29
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The key measurement for chainrings is the Bolt Circle Diameter (BCD). If you imagine those four bolts being points on the circumference of a circle, then the BCD is the circle's diameter. As always, Sheldon Brown's site is a good reference here (and his crib sheet will help you work out your bike's BCD).

So having found out your BCD, any shainring with that BCD, and with the same number of bolts, should fit onto your existing crankset.

Now, you also need to consider how wide the chainring is. If you imagine the chain going around the chainring during cycling, what you're looking for is to have a perfect fit as the chain feeds into the teeth.

The problem is that there are two common chain widths out there: 1/8" (generally used by track bikes, some fixies, bmx, probably others) and 3/32" (generally, any bike with a derailleur, including road bikes and mtbs). So when you're looking to buy a chainring, it will generally be spec'd either 1/8 or 3/32 to denote the chain that it is paired with. Without getting too bogged down in detail, the best combination is where both chain and chainring are matched, although you'll get away with a mismatch, as long as the chain is wider than the ring.

Now, you say that your BCD is 104, and point to a chainring that might be a good fit. Certainly the BCD is a match. Also, while that url doesn't say explicitly 1/8 or 3/32, it does say that the ring is compatible with 9/10/11 speed drivetrains. In other words, with a geared bike. So you can deduce that it is a 3/32 ring, and should therefore fit with your current chain.

Lastly, note that if you are going to fewer T, you may need to shorten the chain.

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    Having trouble sleeping? – andy256 Nov 16 '15 at 4:24
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That sure looks like a 112 BCD to me. Just looking at you you can see there is not much if any room to get smaller. There are some around that squeeze in a 33T.

If it is 104 BCD then according to Sheldon you can go down to 32T but I would look around as the just does not seem right to me. To me it looks like 32T is the smallest for 104. Again just looking at the set up it is clear there is very little room to go smaller.

A new crank for a few teeth is a bit of money.

Yes you can fit another crank with a smaller BCD or look at like RaceFace with spline based chainring. Also called direct mount. Looks like Santa Cruz is using that on the some of the latest models of that bike.

  • it's a 104 BCD! – Bernat Nov 15 '15 at 19:09
  • Various companies such as Hope do manufacture 30 tooth 104 BCD chain rings. hopetech.com/product/retainer-ring – Jackson Nov 16 '15 at 14:07
  • @Jackson OP already has a link to a hope in his comment. – paparazzo Nov 16 '15 at 14:16
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I'm sure it is 104 mm BCD there are a lot of 30T chainrings for it. I recommend you find a Race Face Narrow Wide 30T chainring.

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    We don't do product recommendations here. Better to say something like "I suggest you look for a chainring that has larger this and fuzzy that. These features are good because a larger this means more theother and fuzziness means greater blah. A Race Face Narrow Wide 30T would fit these requirements." See how that de-emphasises the product while focussing on the features? – Criggie Nov 18 '15 at 19:12

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