See this thread.

Suitcase for folding bikes

[MENTION=203914]Tourist in MSN[/MENTION] fits his in a S&S (26x26) case. But, not necessarily an easy task, and it would be similar to fitting a full 26" or 700c S&S coupled bike into the case.

I.e. the Airnmal Chameleon does and the Helix claims it will (once/if it get's delivered).

Much more info and additional links here:
24" advocates

I have not packed up a 20 inch folder into a suitcase so I can't compare, but I can say that it takes a lot of time and effort to pack up my 24 inch bike.

Also consider S&S coupled full size bikes or Ritchey Breakaway full size bikes. The Ritchey case is oversize but the owners that I have talked to were rarely charged the oversize fee.

Bikes that you can pack up for travel in a small case, are compromises between portability/transportability and ride-ability. Only you can judge how much time and effort you want to put into assembling and later re-packing a bike and how that compares to your enjoyment that you get from riding it.

I've packed my chameleon 4 times so far. In terms of time or difficulty to pack it is the same for a 24 as a 20. Either way the tires are coming off. Figure about 20 minutes on either side of the packing. Main difference is the suitcase dimensions. If you go 24 make sure you get the correct suitcase (more square than the 20 case). Mine weighs in at 38 pounds for the case, padding, and bike, so plenty can be added if needed.

When your suitcase approaches the 62" maximum dimensions for non-oversize airline travel, it's more important to make sure it's under the 50 lb. weight limit than to worry about oversize.

My experience traveling with folders is to never, and I mean never, exceed that 50 lb. limit. If you do, the agent will whip out a tape measure and see if you can be charged for oversize as well as overweight. I stock my suitcases to a max of 48 lb., just to make sure any differences in scale measurements don't bump me over 50 lb. at the counter.

If the suitcase looks close to the 62" limit but isn't over 50 lb., many agents - especially if they're busy with a long line - will skip the tape measure step. It takes more time to use a tape measure than to read a weight scale.

I have a Bike Friday Pocket Rocket, which I love. I wish I could travel more with it, in fact. Bike Friday makes great folding bikes of various styles. Mine is a road bike with drop bars, but they also make city bikes (Pakit), touring bikes (New World Tourist), and even a mountain bike (Pocket Llama). All fit in a Samsonite luggage case, which you can buy from them or cheaper on Amazon.

Bike Friday are highest quality but a bit more expensive than most other folding brands. If you can spare the money, you'll be happy.

https://bikefriday.com

This was how I packed my Dahon ios on my last trip. Using cardboard instead of suitcase, and dimension was longer and slightly tall, but definitely narrower. Easier to handle that way, I even connected a shoulder strap later on. Depends on airline, you maybe lucky enough just as myself 2 years ago, no extra charge from SQ, all the way from Balikpapan to CDG v.v.

Asia-Pacific airline carriers are much more accepting of bcycles as checked-in luggage. I've never had to pay more than a USD17 sports equipment fee. That was on a budget carrier that fleeces you any way they can. On regular carriers, no questions, no hassles, no fees. On my recent flight on Garuda from Bali to Seoul, i folded the bike, put it in a soft canvas bag, and wrapped it in plastic wrapping. I asked them to load it as separately as fragile, which they proceeded to do. No fees, no if, and or buts, no damage on arrival.

I can't comment on how fast a gate agent will pull out a measuring tape, but the 50 pound limit is critical. I think the weight scale is directly wired to the computer that says they need to ask you for your credit card.