1 x???

Yes you can run a 1x9. You do not need to run 1x10 or 11.

You could just use your middle ring and remove the other two, or even change the middle ring to a smaller or larger ring to better suit where you ride. Once you are OK with that, then remove the front mech and the shifter.

BUT! What you will be giving up is gear range.
A typical 9 speed set up is an 11-34 cassette and a 22/32/44 set of front rings. That will give you a 22/34 climbing gear of 0.647. The speed gear at the other end will be 44/11 = 4.0. Your total range will be 4.0 divided by 0.647 = 6.18 That is an impressive range and will suit just about any circumstances you are likely to face.

What you will be left with if you only have one ring is the range offered solely by the range of the cassette. ie 34/11 = 3.09. In other words half of what the 3x9 offers. You the have to decide whether you want that range to be more towards the climbing end or more towards the speed by end by choosing what size of front ring you want. A bigger ring than 32 will help you go faster, a smaller one will help with the climbing.

If you can find a bigger range for a 9 speed then that will help, but to be honest I doubt it (but I haven't looked). Most cassettes have the gear steps at 15% intervals. This is not just on bikes, it is a widely accepted rule of thumb that 15% is a good %step in all sorts of power transmission. We are talking averages here across the cluster, individual steps can be difficult to achieve at exactly 15% because gears have to have a round number. So the way to achieve more range on the back and still have an average 15% step is to have more gears! Hence 1x10, 1x11 and 1x12.

A 10-speed cassette will fit onto your hub and I believe that you can go from 11-42. Some aftermarket cassette producers may even take you higher, but the steps can be a bit odd. With a 10 speed cassette comes the need for a 10-speed shifter and new cables. The larger gears on the back also will need a new rear mech, because your old one will not open wide enough to shift the chain onto the biggest gear. But again the aftermarket providers have a solution for that in some instances, it depends.

As you can see the cost soon starts to mount up if you are unhappy with the simplicity that going 1x9 offers.

Before you remove any of the front rings, pick one and stick with it for the entirety of one ride of the sort you do a lot. Personally, I'd start with the middle ring. That will give you some idea. If you find that climbing is impossible in some areas and you don't mind spinning out in others, then go for a chain ring one step smaller (usually 2T less). If you can clean all the climbs but hate spinning out on the flats, then fit a chain ring that is one step larger.

When you go 1x on a 9 speed you will have to compromise because of the short range. I went from 3x9 to 2x9 with two different front rings. I went from that to 2x10, then 1x11, then 1x12 (cassette of 10-50, ie a range of 5:1)
When I went to an emtb, the bike I bought happened to have 1x11, but I would have been OK with a 1x10. When the 11-speed kit wears out I will stick with it though. I believe that 1x12 on an emtb is one step too far.

Why do you want a 1*9 ?

I don't see the benefit.

As has been said easier to leave it in one chainring and see how you fare.

fenix said:

Why do you want a 1*9 ?

I don't see the benefit.

As has been said easier to leave it in one chainring and see how you fare.

Simplicity and between 1/3 and 1/2 a kilo off your bike.