Changing inner chainring on vintage road bikes from 42t to 39 or 38t.

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There's a video on YouTube from a guy with a vintage Cannondale with an RX100 52/42 chainring who was having trouble on the hills in the Pittsburgh area. I can definitely relate. After a week or so on my new-to-me 3.0 SR400 I knew I needed to lower my lowest ratios. The good news is he swapped the 42 to a 38 and had zero problems. There is no need to shorten the chain or even adjust the rear derailleur. The entire job takes a few minutes and the only tool required is a 5mm hex key. You don't have to pull the crank or even buy the little tool for the retaining nuts on the back of the chainring; they just come right off. He used a black SRAM from Amazon for about $22 but I wanted to keep my bike looking vintage so I went with the U.S. made Vuelta for a few bucks more. The swap will lower your lowest gear 10 per cent to 1.65. My new chainring arrives Sunday and I'll post a few pics and my results then. In the mean time, here's the video.

 

[black lightning 1987]

Sometimes you will need the tool for the nuts. If they turn with the bolt and don't loosen up, it's probably easier done with a helper on the other side of the bike, or with the crank arm off the bike.

If you find that the 38 doesn't give a low enough gear you will want to look into changing the freeewheel. Max cog size is probably 26 on the RX100 but I didn't look it up. A long cage derailleur such as a 7/8 speed era XT or LX will allow you to run quite a bit bigger cogs.

 

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I didn't need the tool or a friend, those little 5mm bolts just popped right off. Weight weenies will be happy to learn that the Vuelta 38t comes in at 36 grams, a full 8 grams lighter than the stock 42t Shimano RX100. The difference is very noticeable (four fewer teeth, not the eight gram weight reduction) and shifting is as smooth and fast as before. Just like in the video, I didn't need to adjust anything. This has to be the easiest drive train mod ever.

 

[black lightning 1987]

Glad to hear that the swap went smoothly.