So do all tandem derailleur bikes have undished wheels ?
No. It depends on the frame. Either the rear dropout spacing has to be wider than a solo or the dropouts have to be assymetric in order to have an undished rear wheel with derailleur gears. Tandems were often spaced wider than solos at the rear to accommodate an extra drum or disk brake.
Is a tandem wheel simply a rim with more spokes? Or is it wider like an Andra 40? I guess the hub has to match the number of spokes
Again not necessarily. There are light tandem wheels and super-heavy duty tandem wheels and various nuances in between. But unless you want to try an esoteric spoking arrangement, the hub has to match the number of spokes.
On my tandem, the front hub was spaced 100 mm and was pretty standard. The rear wheel was spaced about 135 mm (wide for the time, when 5-speed freewheel blocks were the norm). This was because it incorporated a large drum brake in addition to the cantilever brakes front and rear. I upgraded the rims to Mavic 4, which were wide for the 1980's, but much narrower than some current rims, such as the Andra 40.
I know it’s totally overkill but I’ve been looking at getting 48hole CSS rims . Mainly because I’ve always wanted CSS rims and they are the only ones I can find online . Son make a 48hole tandem hub so I think that would be fine with the rim
But the thought of extra strength and bombproofness appeals to me. Saying that , I am almost 17 stone so I like the thought of a super strong rim. I don’t really care about weight . I’m not going to be going very fast especially when loaded up.
I do think 48 hole is overkill on a solo bike. Well-built 36 hole front and rear wheels with fairly wide rims and 50 mm or wider tyres should be sufficient even for 17 stone with luggage. 17 stone is about 108 Kg, adding 20 kg of luggage, which is a lot unless you are doing real expedition touring, makes a weight of 128 Kg. With the weight of the bike itself, say about 20 Kg, that makes a total all up weight of 148 Kg, so about the same as the tandem that I used in the 1980's. And unless you are a very powerful rider, you aren't going to be putting in any more power than the combined power of myself and my wife when we were both young and fit.
A non-standard 48 hole wheel will be difficult to replace on tour if it does break, which might be more likely because the holes on the hub flange are closer. I once had a 36 hole hub crack at the flange during a long tour after crashing a loaded touring bike on an icy concrete cycle path about 500 kms from home. I was lucky, the wheel finally collapsed less than a km from home when a section of flange with 3 spokes broke away from the rest of the hub, so I only had to push for a short distance.
And you can always risk having a rim break by hitting a large pothole at speed. 36 hole will be much easier to find if you do break something.
CSS has gone out of fashion. So, although I have them on my Raven Tour, I wouldn't consider getting CSS rims now, as the special brake pads are getting hard to find.