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130bcd chainring on 135bcd crank?

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130bcd chainring on 135bcd crank?

Old 05-24-20, 11:23 AM
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rosefarts
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130bcd chainring on 135bcd crank?

I really really want a 38 tooth on my Franken-gravel bike. This would allow me to go with the holy grail of gravel 1X, 38x 10-42.

When you run the numbers, what it offers over 40 x 11-42 is substantial.

So the bike in question has a 135bcd Campy Chorus crank paired up with an IRD BB (pretty much the best square BB still available). This results in a perfect chain line.

Switching to an older Shimano 130 BCD would move the chain line in 4.5mm. My chain stay will clear this, and I could mount the chainring on the outside instead of inside of the spider. This would work ok, but I don't want to mess with it and I don't want to hunt eBay for an appropriately high end square Shimano crank from the 90s.

So this finally brings me to the question of the day. Using a file and elbow grease, can a 130bcf chainring fit on my current Campy crank. I think I could file the holes in the ring pretty oval. I would also need to file the inside edges of the chainring down to fit within the tabs of the spider.

Has this been done? Did it fall apart? Did it spin round?
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Old 05-24-20, 11:51 AM
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What about a 39 tooth chainring? They have a 39 that fits 135 BCD
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Old 05-24-20, 11:54 AM
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Originally Posted by cyclist2000 View Post
What about a 39 tooth chainring? They have a 39 that fits 135 BCD
In narrow-wide? I would compromise with that but I haven't seen it.

I'm looking now at a 130 vs 135 from my parts bin. I don't think it can be done.
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Old 05-24-20, 12:01 PM
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I've never attempted, but it seems...theoretically possible. You'd have better luck with a mill/CNC than trying to do it by hand, if you have access to one. But if you make good marks on the chainring (maybe using a compass with a pointed tip instead of a pencil?) you might be able to do it. But Sheldon Brown says the smallest ring that will fit on a 135 BCD crank is 39T. So maybe a 38T would interfere with the spider?

As an aside, I don't understand the aversion to a 2x setup with a reasonable sized cassette, aside from it being cheaper for OEMs. My road/gravel/etc bike is 2x with a 46/28. The 46T chainline lands in the middle of the cassette and I almost ride as if it were a 1x. But the 28T is there for really steep hills when needed.
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Old 05-24-20, 12:30 PM
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I donít understand why a narrow-wide on a gravel bike. Worst case would be a chain guide. By design narrow wide is only found on even tooth chainrings, alternating teeth, so it is not possible to even make a 39t narrow-wide. Seems like a lot of work that is probably set to fail just for a specific 1x chainring.

Iíd just get a narrow 39t 135bcd and run a chain guide if I were that concerned. It is a gravel bike after all.

John
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Old 05-24-20, 12:40 PM
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Originally Posted by 70sSanO View Post
I donít understand why a narrow-wide on a gravel bike. Worst case would be a chain guide. By design narrow wide is only found on even tooth chainrings, alternating teeth, so it is not possible to even make a 39t narrow-wide. Seems like a lot of work that is probably set to fail just for a specific 1x chainring.

Iíd just get a narrow 39t 135bcd and run a chain guide if I were that concerned. It is a gravel bike after all.

John
Good point about odd numbers. Why didn't I realize that?

For your other point, I don't think you have any idea what terrain I ride. I have a narrow-wide, chainguide, and Shadow plus derailleur. They're all needed.

Fast washboards are so much worse than big hits for throwing the chain. I'm not going casual rides.
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Old 05-24-20, 01:11 PM
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Is there an issue fitting a chain guide with a 42t? Or, have you made up your mind you don’t want to use one?

John
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Old 05-24-20, 01:20 PM
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Originally Posted by rosefarts View Post
Fast washboards are so much worse than big hits for throwing the chain. I'm not going casual rides.
Time for wider tires! Then again, they can only do so much. Washboards are difficult.
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Old 05-24-20, 01:45 PM
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Having watched chains skate over crank spider flanges of very slightly worn 38t chainrings mounted correctly on 130mm BCD cranks, I believe you would have to file these down to the chainring bolt holes for the chain to clear 135mm on a 38. If you need 38 teeth, get a compact crank.
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Old 05-24-20, 02:14 PM
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Originally Posted by 70sSanO View Post
Is there an issue fitting a chain guide with a 42t? Or, have you made up your mind you donít want to use one?

John
I currently have a 40t NW on the bike. It's pretty good and was like $27 on the eBay.

I'm just daydreaming here. The max speed before I have to tuck and coast on this bike is a little less than I'd like on those occasions that I cross a steep paved road when connecting trails and dirt roads.

On the other end, 40x42 will literally get an unloaded bike up anything but there are occasions when I can't quite keep my cadence where I'd like.

Switching from 11-42 over to 10-42 and also dropping to a 38 would give you those precious few gear inches at either end.

I don't think it will work without major compromise or wallet biopsy.

As for 1x vs 2x. Weight and simplicity. If I were racing though, I think 46/34 x 11-36 would probably be best for me.
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