Iv got a old mtb and i am thinking of putting drop bars slicks
and using a fixed wheel system.
Is it possible to convert the back wheel which is already on the bike to a fixed wheel.Or will i need a new rear wheel.
Ps i have never tried a fixed,but theres that many that say it is
a great way to ride i thought i give it a try.

Surly makes a device called the Fixxer that converts a cassette hub to a fixed gear. It will cost you $70 US. Unless you have horizontal drop outs, it could create chain tension problems, though. Most singlespeed conversion chain tensioners cannot be used with a fixed gear, because they don't like backpedalling.

If you can find a workable gear combination for your bike, getting a new wheel built up may be your most economical move. RiderX posted this link in another thread. There is a calculator there that, based on the length of your chainstays will tell you what gear combinations will work without the need for a tensioner.

I'm not real sure about putting drop bars on your mountain frame. Depending on your frame's geometry, the drops may not be very useful.

Good luck

Is this device by Surely obtainable in England ?
Drops are used on a lot of mtb converted for the road in the uk.

you may be better off buying a new wheel. you can purchase a wheel built up with a Surly singlespeed hub and Sun Rhino lite rim for around $100. This is what I use. The hub is made for freewheels but you can put a track cog on instead. You can then take a lockring from an old style bottom bracket and thread that on to keep the cog from backing off the hub. This is what I run on my bike and I haven't had any problems with the track cog loosening up.

here's a site that sells the wheel I was talking about. This is a standard wheel built by Quality Bike Products.

https://aebike.com/site/page.cfm?PageID=30&Category=1973

Surly is also supposed to be coming out with a mtb hub design for a freewheel on one side and a track cog on the other. That may be worth waiting for.

Thanks,but it looks like my biggest problem is that the frame in question has vertical drop outs.Is There any way around this with out changing the drop outs?

this website https://www.peak.org/~fixin/ has a program that will tell you what combinations of gears you can use on your frame and have good chain tension. I've never tried this but it seems like it would work at least until your drivetrain gets worn.

If your interested in turning the bike into a freewheeling singlespeed you could use your cassette wheel and put a BMX cog and spacers on it. You could the program listed above to figure the right gears to use on your frame or you could use a chain tensioning device to take up the slack. A chain tensioner will only work on a freewheeling bike. You can't use one with a fixed gear.

Take a look through Sheldon's site. https://www.sheldonbrown.com/fixed-conversion.html