I realise that there are other instructables on this subject, but hope this adds something to anyone tackling this project.
The primary goal of this 'ible is to provide an answer to the question "Is it possible to convert a regular kids' bike to a balance bike and back again?" I certainly couldn't find an answer on the internet when our daughter was learning to ride.
The answer is YES!
It seems crazy that a bike with no pedals will routinely cost more than a bike with them and the former will only be used for a few months max.
I can also confirm that when learning to ride a bike - a balance bike is THE way to go. I learned the painful way over many weekends with my dad running behind holding the saddle and many falls and bruises.
By contrast my daughter spent a couple of months playing on the converted balance bike to get confidence. When I put the pedals back on, she was racing round the yard in less than 15 minutes. No falls. No bruises.
This instructable came about partially because I wanted to photograph how the bike went together for my own records. It is likely to be a few months between removing the pedals and putting them back. You may also want to photograph each stage so that you are confident about putting everything back in the right place.
Step 1: Obtain a Suitable Bike
First you need to find a suitable bike.
Almost any bike should be convertible but bear in mind that the saddle needs to be low enough so that both feet can sit flat on the ground. Look for one on the small side, obviously, but I bought this one on the internet without being able to check it too closely all went well.
Step 2: Remove Pedals
The pedals are attached to the cranks with a threaded shaft. The bike I bought came with a thin spanner for attaching these, otherwise a flat multi-purpose bike spanner will do the trick.
Note: The left hand pedal has a left hand thread i.e. it will unscrew by turning it clockwise. The right hand pedal will unscrew normally.
When you re-attach you should find that the pedals are stamped L and R on the shaft to help you. Otherwise a bit of trial and error will get them fixed again.
Step 3: Remove Chain
The chain is attached using a master link. This is identifiable by the retaining clip that sits on the outside surface.
To remove the chain, (carefully) push the retaining clip out with a flat screwdriver. Don't worry, it shouldn't ping off when you push it forward. You can then remove the master link and keep safe for when you come to reinstall.
When reinstalling the closed end of the clip should really face the direction of travel of the chain (so that it can't catch on something and unclip).
Step 4: Remove Cranks
There are many different types of cranks available.
On a modern kids' bikes like this you are most likely to encounter a simple one-piece crank as shown.
To remove, you first undo the retaining nut as shown. Underneath this you will find the outer ball race which needs to be unscrewed. In the picture you can see this is the part with slots in. These can be used to give purchase with a screwdriver if necessary. If applying force with a screwdriver be careful and keep your hands out of the way.
Once this is out, you will see the basket which holds the ball bearings. The ball-bearings in this case were captive i.e. they are held in a cage and didn't fall out all over the floor. Yours is likely to be the same. If ball bearings do come loose, don't panic - collect them up and put them back in when you reinstall.
Step 5: Remove Chain Wheel
Here you can see the whole crank including chainwheel being removed. The race (where the ball bearings run) is part of the chain guard assembly). You can also see the right hand ball bearings in their cage.
Pull it all out and put it somewhere safe.
Step 6: Remove Chain Guard
The chain guard on this bike was in two parts. The front part pushed into the bottom bracket (where the crank goes), and the rear part was retained by a couple of small screws.
Step 7: Lower Seat
You may or may not have to perform this step.
As mentioned, the rider must be low enough on the bike to allow both feet to sit flat on the ground. If this is not possible with the original configuration, you may need to alter the seat tube.
Here you can see that the seat clamp has been removed (it just pulled off) and about 10mm removed from the top of the tube. Stick the seat clamp back on and reinsert the saddle.
Step 8: Balance
You should be ready to go. Check all the nuts and bolts are tight.
I found that our daughter didn't need any tuition or guidance at this stage - the combination of balancing and pushing along seems to come naturally.
When the rider is confident about pushing themselves along, 'gliding' and steering round corners, it is time to revisit the instructable in reverse. You should find that they are cycling with confidence in no time.