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I want to convert my 27 speed 3x9 to a 1x9. What do you recommend for chainring and cassette combinations? What are the cautions?

Argenti Apparatus
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Russ_L
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    Voting to close as its too broad. 1x setups can be made simply by removing the unused chain rings though to new everything. Without knowing budget, rider strength and terrain being ridden, no recommendation can be made. IMHO the best 1x9 setup on the market today (Microshift Advent) is still too much of a compromise for off road use. – mattnz Jul 24 '19 at 08:35
  • We can't tell you what gear ratios to use because we don't know how strong you are, or where or how far you'll be riding. Can you re-word your question to something like 'how do I figure out what chainring and cassette sizes i need if I convert to 1x9?' – Argenti Apparatus Jul 24 '19 at 11:45
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    change to your existing middle front ring, and ride around for a while without altering the front gear. See if you run out of gears at any point. That will tell you if this is a good idea or not, and its no-cost. – Criggie Jul 25 '19 at 21:01

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To figure out what cassette and ring size to get, the best approach is to use a gearing calculator to play with the options in comparison to what you need or would like based on your experience with bike's current gearing and how it's working for you where you ride. With 1x conversions the usual question is how much high end you can accept losing to get the low end you need or want.

The cheap way is to figure out what ring to go with based on the above and then get a narrow-wide ring of the right BCD to achieve it. Do all chainline measurements up front and use chainring spacers, a different BB spindle, and/or an offset chainring as applicable to achieve perfect chainline.

Nathan Knutson
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As I understand from your situation, you need a cheap option just to try. Otherwise, you can just buy a drivetrain set like 1x11 for $130+.

I did this trick a month ago on my mountain bike. My option was 38t in front with a cassette 12-36. My experience was positive, but I recommend to use 32/34/36t in front, depending on your riding style and your power. Fewer teeth if you ride out of roads. Much better to use narrow-wide chainring in the front, to keep the chain on the chainring. If the price is a big question for you, check aliexpress for it.

I think this setup is good to ride 30-32 km/h on asphalt and for normal uphills, maybe even for not difficult mountains.

Advantages: fewer details, less weight ~400g I think, comfortable to use just one shifter instead of two. Also, you will not get a big piece of dirt in your front derailleur as it happens often.

Disadvantages: After 100-120 km I started to feel that 9 speeds can be not enough for long rides when you started to be tired and you will get more load on your knees.

Summary: I like 1x9, it's very cheap (cassettes, chain), 9 speeds enough to ride on roads, out of roads and hills. But will be not enough for long rides, 100+ km in my case. But I will use it for winter rides, in my region, regular temp is -15C and my usual winter ride is 25-50km.

Eugene
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    one more advantage: you can easily install full sized rear fender - otherwise the front mech can be a problem – k102 Jul 24 '19 at 08:00
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One thing to keep in mind is that you can use 40t 10 speed cogs to extend a traditional 9spd cassette to an 11-40. The tricky part is finding a 9spd derailluer setup that can shift to 40t. I've used a 9 spd XT rear long cage with a Wolftooth roadlink derailluer cage extender to successfully shift a 9 spd 11-40 cluster.

This page is a good summary of do it yourself options Building wide range 9 speed cassettes

SunRace and some other manufacturers now also make 11-40 and similar wide range 9 speed cassettes.

Fred the Magic Wonder Dog
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