These are full-frame 18-speed bicycles. No cheap little tall seat posts and handlebars. If you didn't know what you were looking for, you'd swear that it was a non-folding. The tubing is standard wall chromoly and they use middle level Shimano gearsets. I bought one at OSH last year (~$450) and it is the best. I had previously been considering getting a folding bike, but I also wanted a good mountain bike. These Montague bikes are so good I didn't buy the MB.
The front wheel comes off, and the bike folds in half along the seat post (with a clever locking mechanism). Thus, the bike becomes half the fork-to- rear tire length, and it fits into my Tiger very easily.
Another alternative for a 4 place Grumman is to buy regular size mountain bikes as opposed to the foldables. You can fit a full size mountain or road bike in the luggage compartment (back seats folded down) without any problem.
Now, if you really want to impress your friends and shock the Piper/Cessna drivers, put a full size bike rack in the luggage compartment with bikes. The Rhode Gear Euro Shuttle works quite well for this, and I hear it works well on automobiles, too. Fold the Euro Shuttle so it's flat, and place it in the luggage compartment with the fork mounts facing toward the front of the plane. Take the front wheel AND the seat post off the mountain bike, and roll it into the luggage compartment and mount it on the bike rack. You will need to reposition the bike rack so the handlebars don't make contact with the windows or someone's head. You can then pack all your gear around the bicycles. You will want to double check the roof clearance if you require a mountain bike frame larger than 18", and you will also need to remove any rear racks on the bike. This won't work with road bikes.
I, too, was ready to purchase a couple Schwinn Montague's, until I discovered this combination. It really makes the Piper/Cessna people envious.
You make two cuts to the main tubes on the bike, The cross tube near the seat and the diagonal tube near the cranks. The metal tubes reconnect the tubes using a small socket driver and to cotter pins. I have been using this system for over 10 years with no problems.
I have fit the two bikes in the back of the Traveler, Piper 140 and even got one of them into the back of a Cessna 150! I have no idea if the company is still in business, I think they were in Ohio.
Also, I met someone who used standard pipe fittings to hold their bikes together. He said all the hardware cost him about $15. I would not recommend cutting an expensive bike in half, but you can probably pick up a nice used 10 speed really cheap.
rec.bicycles.something has a FAQ about folding bicycles which is well-worth looking at for anyone considering one.
A summary is that the Montague (not sold by Schwinn anymore) and the Bike Friday seemed to come out on top.
When you get to the airport, call the hotel to pick up your luggage, and hit the road right out of the plane.
I would just like to add that you can fit TWO full size mountain bikes, camping gear for two and a good size cooler full of drinks and food in the back of a Tiger. I also had a full size Kawasaki KLR 250 motorcycle with camping gear in the back. The motorcycle weighs 257 lbs and needed to be taken apart to fit inside. It takes about 1.5 hours to put the bike in or take it out including assembly. A picture of the bike can be seen here.
Remove the front tire and seat and my 26" frame (not seat) standard 10 speed fits in my Tiger no problem. Bonnie's fits with removing the tire only. Great fun 'till we had kids and have to leave the seats up!