vincentc777

I bought a bike for a family member online and am trying to assemble. It is a 20" single speed, but it came with a derailleur. Do I really need this? It doesn't seem to fit very well in the place it's supposed to go, and I'm not sure about threading it, and it's one more thing that could get damaged. When I was a kid I don't think my bikes had this. Can I skip this piece of the assembly? Thanks.

TejanoTrackie 

A rear derailleur is needed for shifting gears on a multi-sprocket freewheel or cassette and to maintain tension on the chain. It is not needed on a singlepeed, unless the rear dropouts are vertical, and don’t allow for chain adjustment. It is probably just a mistake made at the factory when it was boxed.

vincentc777

Thank you, TejanoTrackie. Based on your explanation about the rear dropouts and needing some way to adjust the chain, it seems I might need it. This bike does not have vertical dropouts -- it really doesn't have any dropouts. It's actually a 20" adult tricycle for an older person who can't risk any falls due to weak bones. The 2 axles just sit in tubes. So there really is no way to adjust the chain without the derailleur. It looks like I need it. The assembly instructions are not very good so I'm struggling a bit. Might take it to a bike shop for this part. Thanks again for your detailed reply -- I appreciate it!

veganbikes

You might consider returning that trike, they can be quite dangerous and the cheap ones online are certainly the worse offenders but really any upright trike has the same instability issues. A really narrow width and a short wheelbase makes it easy to get onto two wheels.

A recumbent will be much safer, more expensive but less chance of falling being lower to the ground and having a longer wheelbase.

Mikefule

If the axles "just sit in tubes" and you cannot identify any bearings, it will be hard work and very unsatisfactory to ride. It sounds like the "derailleur" is an ad hoc way of tensioning the chain. If you can get the chain the right length using patience and possibly a half link, you may not need it.

TugaDude

Please tell us the exact model you purchased so that we can look it up online. That way we can have an intelligent conversation rather than speculating about what you do or don't have. Safety is paramount and I'd hate to give advice only to find out it was wrong because I misunderstood the situation.

Even cheap bikes are going to have bearings and such. So at this point we're really just guessing.