As of right now I currently have an Apollo 18 speed freewheel bike (3 Front and 6 Rear) I bought from Halfords but I have decided that I want to convert it into a single speed bike. This is because I want something simpler but still my size, and I also want to understand bikes a bit more.
How would I go about doing a conversion such as this? Is it even possible?
Please let me know as I want to begin cycling again soon.
This is my bike BTW
TLDR: Yes, you can convert your bike to a single-speed. But as you have a free-wheel on your bike it's more complicated and expensive than a free-hub conversion.
Your bike currently has a free-wheel as opposed to a free-hub. This means that hub assembly is threaded and not slide on, so instead of a conversion kit as posted by @Criggie you'll need to buy a single speed free-wheel. An example is the Shimano SF-1200 (These can be a bit difficult to buy, RRP is around $30 USD). You'll also need a freewheel tool if you don't already have one.
If you change to a single-speed free-wheel you may also need to change from a 3/32 derailleur chain to a 1/8 single speed chain (some work, some don't). A change of chain could also necessitate a change of the front ring. Free-wheel bikes generally come with a steel crankset with the rings riveted or/ and welded on. If this is the case you will need a new crankset to suit a single-speed front ring.
You will still need to use a chain tensioner to keep the chain tight. If you have changed to a single-speed chain you will need a tensioner designed for a wider chain like the Surly Singlator. If you can't get a good chain tension, and are using a single-speed chain, then you can use a half-link to get it tight.
So really it's up to you. You may get away with a $30 conversion or you may end up paying over $150, at which time it's worth considering buying the bike you want rather than converting the one you have!
You can do anything you want - it is your bike.
And its okay to just stay there if it suits you. So far, this is a no-cost change.
This #2 section presumes a cassette, whereas your
bike has a freewheel. Thus the middle of this
answer does not work for OP's bike, but leaving
for completeness.
Example shows a 13 tooth, but seems to be available in 13/14/15/16/17/18/20 tooth options. There are shims and spacers to fill the spare, and you can put them on either side of the single cog to get a straight chainline.
On the front, you probably have three chainrings. It may be possible to unbolt the chainrings you're not using, or the crankset may be rivetted together. It is possible to drill out rivets, but those cranksets may need the added strength of a steel sandwich, allowing one single chainring to bend if left unsupported. In the first instance, you could just leave them in place.
You can choose to remove the front derailleur, cable, and shifter. Sometimes the front derailleur can be locked-out using the limit screws so its only in one position, to act as a chain guide and prevent the chain jumping off.
2b) Your other option is to replace the entire rear wheel with another using a cassette-based interface, or one designed for a BMX style of single cog.
Your bike probably lacks any way to adjust the chain's tension using the wheel placement in slots, so you'll need to either fit a tensioner like this one:
Or it may be possible to hack up your existing rear derailleur to provide tension on the chain, and use the limit screws to hold it in place. Remember you want as many of the teeth engaged in the rear cog to the chain as possible; ideally half of the teeth or more, and not 3/8 like in a derailleur bike.
Additionally, you still require two working brakes. Single speed offers zero ways to slow down.
I suggest you store all the takeoff parts, in case you want to revert later. Single speed is not all it is cracked up to be in terms of rideability, and offers simplicity with a loss of functionality too.
Generally, derailleur bicycles are made with vertical dropouts. It means there's no way to adjust chain tension apart from eccentric bottom bracket (but such a solution would leave less space for bottom bracket bearings, and I'm not sure if the superior external Hollowtech II bottom brackets are available as eccentric), or a chain tensioner. Usually chain tensioners are considered undesirable on single speed bikes. The reason is that with a chain tensioner, a single speed bike has the same problem that it's very sensitive to rear sprocket wear and thus requires you to change the sprockets once slightly worn, instead of allowing using them for very long like good single speed bikes allow.
Thus, the easiest way to make your bike single speed is to select some gear and never touch the gear shifters again.
Yes, it can be done. You need a BMX freewheel, chain tensioner and a selection of bike tools that will probably cost more than the parts if you can't borrow them. It used to be a common modification for keeping old MTBs running back in the days when strong legs and mechanical ability were something to brag about.
The singlespeed BMX freewheel goes on the same thread as multi-speed freewheel. The tricky part that might require help from a mechanic is removing the old freewheel. The singlespeed freewheel can then be installed finger tight and riding will tighten it. The second thing that might need help from mechanic is adjusting the chain line. The BMX cog is going to be too far to the left of the bike, and the chain will run smoother if you adjust the wheel so that the chain runs straight. To do this you need to remove some spacers from the right side and add them to the left, then readjust the bearings and finally re-center the rim by loosening spokes on the right and tightening them on the left.
After the difficult part, the rest is just removing the derailleurs and putting on the chain tensioner. It's likely that the front rings have been riveted together and there isn't easy way to remove the unused ones.
