Why would you convert a perfectly good 2×10 or 3×9 crankset to 1×9/10? Well there are two reasons to do it, save weight (500 grams) and simplify your bike. If you do the math and look at a table of gears the 11-36 cassette from Shimano or Sram give you almost all the gears with a 2×9 setup. So strictly from a gear standpoint you don’t even lose much. For this reason converting my cranks to a 1x configuration is my favorite mod. The added bonus on the trail is you have less things to worry about, just one shifter on the right. Going uphill, shift down, going downhill, shift up. No worrying about if you are in the correct big ring or not. Having the chain slip from one cog to the next and losing momentum is no longer a worry. So how do you do it? Let me tell you.
1. Decide if you are going to use your current crank or buy a new one. Your crank has to be one where the rings can be removed. Some of the lower end cranks have all the ring riveted on. My personal favorite crank sets are the Shimano SLX/LX/Deore line. They can be found cheap on sale and when stripped down to one ring weigh about the same as the XT crank. Also the Race Face Evolve line can be found reasonable and the latest ones are pretty damn good.
2. (Optional) You can use your current center ring if you want to, but it was designed for shifting so it has pins, ramps and is probably made out of a heavy steel. It will work fine for a while, but a dedicated single speed center ring should be purchased at some point. So you will need to know what your Bolt Center Diameter is on your current crankset. Most new cranks use a 4x104mm BCD configuration. Google and the model number stamped on your crank will help you figure it out quickly.
After you figure out your BCD order a new single speed middle ring. They will run from $15 up to $100. I have been going back and forth between 32 and 34 teeth and both will work fine. If you need a bit more top end you can also find 36t rings, but know you will be giving up some climbing gear on the bottom end. I have found 34t to be the best compromise. Last is to look at the material the ring is made out of. Aluminum will be what you find the most of, but it will be in two materials 6061 and 7075. The 6061 will work fine, but wear pretty quickly. I have had them only last a season. The 7075 will cost a bit more, but wear better. Weight is about the same between the two materials.
3. Selecting a chain guide is your next step. You have 3 choices here, bash ring with an inside guide, no bash ring and guide, or 2 bash ring sandwich. I will start by saying I don’t like the 2 bash ring sandwich. Pain in the butt to clean and can actually allow the chain to still jump out. Also the weight of the second bash ring almost negates everything you are trying to do. My personal favorite is the MRP chain guide. You take off your bash ring and the guide mounts via your bottom bracket. I have yet to have it fail and let the chain jump, they are fairly cheap for $50 and simple to install. This would be my first choice. If you want to keep your bash ring the N-Stop guide works well and is really cheap at $10. It clamps to your frame so you will need to measure your frames seat tube diameter and order the correct size. There are other similar products but these two I know work well.
4. (Optional) For me the entire point of doing this conversion is to have a simple setup, but not to sacrifice any gears. This means swapping out your rear cassette for a better spread. For 9 speed setups you can find 12-34/6 cassettes pretty easy. On 10 speed systems the 11-36 spread is very nice. However, this is not necessary and if you can live with a few less teeth in one direction or another. Then just use your current rear cassette. You may be surprised to find out you already have the cassette you want so make sure you check your bike before just ordering something.
5. Put it all together. So if you are using your current crank pull it off and disassemble it. Put on the new center ring and then your chain guide. When putting on your center ring and taking off your bash ring you may find the stock ring bolts are a little too long. The MRP chain ring comes with spacers for this or you can go to your local hardware store and get some washers for spacers. In the event you are not comfortable with spacers you can order shorter ring bolts online for the single ring. I have had no problems with spacers or washers over a few seasons of riding. If you ordered a new cassette and don’t have a cassette tool your LBS can swap it out in 5 minutes and usually for only a couple of bucks.
6. The trick to the setup comes in the fine tuning. You want your guide to only have a couple of mm of clearance. If you have more than 3mm of clearance in high or low gear to your guide then you will want to tighten up the clearance a bit. On the MRP guide it may mean gentle bending the bracket back and forth. With the Nstop guide you slide the guide in and out from the frame. Once your clearances are set shift through all your gears on the stand and make sure everything is smooth. If you replaced your cassette you may need to adjust your rear derailleur a bit. Just make sure you can pedal forward and backward in every gear without the chain walking. Any problems may indicate you need either a new chain or something is still a bit out of adjustment. Don’t be discouraged as this may take a bit of tinkering to get it perfect.
7. If you have a lot of slack in the chain you can now shorten it. Since you don’t need to shift cogs in the front you may not need as long a chain. Shift into the big ring in the back and shorten your chain according to this gear. I would say this step is optional, but with a lot of slack in the chain it can allow for some sloppy shifting and more slap than necessary.
That’s it you are done. Go for a ride and enjoy your lighter simpler setup. If after your first ride you need to make an adjustment or two that is normal. However the most important step is to return home and crack open a cold one and think about how great your new bike mod is.