Hi Punjanaman,
I tried to reply to your post yesterday but the IN kicked me out.
This subject has been covered a couple of times in the forum. Go the the start page and pump in 'derailleur' into the search box and see what it comes up with. I'm sure someone has done this but I'm not sure if it's been documented in the forum.
IMO fitting a derailleur will present you with quite a few problems.
1. As Chas replied, there could be a problem with ground clearance. On a normal bike the derailleur gear set is close to the wheel, on the C5 it would be located in the middle of the axle, if one wheel dips or you pass over an obstuction there is a chance of ripping off or bending the jockey unit
2. The original pedal sprocket is for a single speed chain and will not take a muti-speed one.
3. OK you can change the pedal sprocket, BUT the C5 pedal sprocket is heavily baiest towards the frame, if you fit a normal pedal sprocket you will end up fouling the body.
4. If you retain the original C5 sprocket then you will have to grind the teeth down to accept a smaller chain.
5. Mounting a derailleur set to the C5 will be a lot more difficult than mounting a hub.
I see no reason to think about the chain line, ain't no worry with the chain line on a derailleur set, it's changing all the time.
And why should you need a tensioner? The derailleur set is fitted with one, though a chain support
might be an idea.
i found on my very short initial ride before strip down that the c5's gearing was low and reducing it further seems unnessecary.
I suggest you repeat this sentence
very load when you're half way up a steep hill. I found the three speed not to be adequate for pedalling long distances without motor assistance, for this reason I'm going to fit a seven speed unit.
I had a Sachs three speeder on my C5. I removed the original sprocket from the hub and fixed a dished sprocket to the sprocket side of the hub case, then replaced the freewheeler sprocket. The fixed sprocket, being dished, allowed enough space for both chains. The hub was slightly offset to the right so the fixed sprocket was inline with the rear axle sprocket, the offset for the freewheel sprocket was so minimal that there was no worry about the chain line.
If you look closely at Chas's photo you will see very faint markings where the original sprocket was mounted on the axle.
Best regards and av phun. KarlG.