Provided Tern are indeed produced in Taiwan instead of China, does this have an impact on quality and features?

Why wouldn't you keep the Birdy and simply add a dual drive hub for the extra gearing you want.. certainly the full suspension, stellar frame and Birdy ride characteristics would make one of the nicest compact folders for travel .. especially where roads aren't ideal..

Re dual drive.
On uk ebay, good used dual drive hubs, usually 8 speed, come up frequently for less than £100. You should be able to get dual drive finished on your birdie for less than £200 even if you get someone to build the wheel up. You might even be lucky and find a disc brake version.
You can sell the old rear birdie wheel, (pm me), or keep for spares or to utilise a different tyre set up.

Thanks for the info.

Because I don't have the tech skills to do it, and the retailer from which I bought told me it was either expensive or right-down impossible to modify the World Sport to add more speeds. Considering the price of the World Touring Disc, I'll pass and buy a Dahon/Tern instead.

But out of curiosity, what parts are certified to be compatible with the World Sport, ie. it wouldn't take more than a couple of hours of work for someone qualified to make the change?

http://en.r-m.de/bike/world-birdy-sport/

I'd like a bit more speed than the current 56x11 when riding on flat roads, and more speeds when going uphill than the current 56x30.

The Shimano CS-HG40 is crap anyway: Even though the new cassette only has 500km's and I always start in 3rd gear (6 on the shifter) to go soft on it (and my knees), the first cog (11) is already "ripping" (don't know the right word: Feels like missing some teeth while cycling.)

Edit: BTW, is this the dual drive mentioned above?
http://www.bike-advisor.com/reviews/...e-27-2011.html

That's the system. You don't need the whole system, just the hub and a 3 speed shifter for the IHG. You don't need to change your cassette, or your rear mech, or your shifter. You also need the click box for around £12, and a 3 speed shifter and cable.

Birdie used to produce a dual drive version, but they struggled getting enough hubs I believe.
Its all compactable with shimano shifters and any shimano sis kit.
You just need to use the same number of speeds. Ie and 8 speed will index with an 8.
The 3X8, will also take a nine speed cassete ifaik.

Any good bike shop should be able to build up a dual drive set up on your birdie. I have done 3 dual drive conversions. I got a
Get a used hub for less than £100 with luck on ebay.
+ click box £12+ wheel build£45 + rim 30 + shifter and cables £20.
£207 Total.

But.....
maybe as low as 100-150 if cheaper hub and/or sale of old wheel or reuse old rim spokes and sell old hub.

They make a disc version also. See the dualdrive on a mezzo thread or upgrading a mezzo.
Most dualdrive users love the system ime. The ihg shifts slickly under load and is miles better than a front mech that it takes the role of.
I might do it to my birdie, but mine needs some some general tlc first!!

Interesting retailer you have there... it's the same setup as the THIS ... Birdy offered the same setup for the legacy frame bikes (which I prefer) originally as the Sachs 3x7 and later the SRAM Dual Drive .. I'd look at dealers who are Birdy savvy ... I have a brand new complete Birdy wheel with Sachs 3x7 , cassette, shifters etc. in my parts department for under $300 complete ... shipping to France would probably be prohibitive, but that gives you an idea of how inexpensive the upgrade would be... old wheel out, new wheel in, add shifters and cables and adjust... pretty easy... I used to ride a Birdy Silver, same bike as yours, but with the Sachs 3x7 .. one of my favorite bikes...

sash
 

Come to think of it, I've got a spare sash 3X7 for £25 on ebay. Its in bits at the moment. But it was working fine before.
I prefer the sram version as it shifts under full load.
I Think if I still have my Birdie next summer I will put it on.

Thanks much for the infos. I'll stop at the store next time I'm in the neighborhood and ask them about installing either the Sachs 3x7 or a SRAM Dual, and see what they say.