Here are threads in which all your questions may be answered:
Do a folding bike forum search for "choose", "decide", "newbie", "choices", "folding" etc. You get the idea. Search only titles, an option to be found in advanced search.
here are some examples:
If after reading all that as well as some of your own search results, THEN please go ahead and post your own particular question. Bear in mind newbies post questions about which folder to buy at the rate of several per week. You are welcome to do this but only after doing some homework. Please pose your question in a new thread, not in this one, as it may not be spotted here. Also, try not to resurrect zombie threads by answering old questions in long-dead threads where the original poster may have moved on. Same for posing questions in ancient threads.
There also web sites to study:
http://www.foldingcyclist.com/index.html (this one contains a comprehensive table of manufacturers and a buyers guide, too.)
When asking for advice, the following info is useful for determining your needs:
* your weight (bikes have weight limits)
* how tall you are (almost all folding bikes are size medium, ie 22" (55cm) equiv. top tube length)
* your budget
* your riding experience
* what you intend to use the bike for (eg touring, commuting, casual park riding, shopping, MTB-ing etc)
* what country you are in, as countries tend to have different choices
Q: Don't 20" wheels suck?
A: It is largely a myth that small wheels are a lot worse than large ones. This is only really relevant if the wheels are going to encounter road irregularities of similar size to the wheels. Eg, if you want the bike to roll well over gnarly roots while off-roading, a 20" wheel isn't the best performer. But for road and path and most gravel riding, small wheels are just fine and on paved roads, there is evidence to suggest that the difference is insignificant. But this is a controversial subject and wars have been fought over this, so there are lots of opinions. Small wheels have other advantages such as packability and importantly, strength. There is also a very wide choice of tyres available.
Q: Won't I pedal like a demented monkey with those small wheels?
A: The small wheels are compensated for by higher gearing so no, you will pedal about the same as for any other bikes. However, small wheels do have the limitation in that gearing systems have been designed around big wheels, so getting high gearing on small-wheeled bikes is not as straightforward as on big-wheelers. But solutions do exist, some of them quite costly though.
Q: Aren't small wheels hard to pedal uphill?
A: No, that depends entirely on the engine and the gears, not the wheel size.
Q: What gearing will I need?
A: That depends. If you are experienced, even a single-speed would do, you would have to get off and push very rarely if ever. But for a beginner, you would like to have gearing ranging from say 30"-90" or lower still. (See www.sheldonbrown.com for an explanation of gear-inches.)
Q: The steering is twitchy. Will this be a problem?
A: The amount of twitchiness depends a lot on the bike geometry, but yes, by and large, folding bikes have very responsive (ie "twitchy") steering. However, experience shows that you can safely ignore this, since you will get used to it in a very short time, and indeed, after getting back on your big-wheeled bike, it will feel like a truck and you won't want to ride that big-wheeled monster again.
Q: Won't I look like a clown?
A: Yes. Get used to it. People will make comments ranging from "Cool bike!" to "Get a real bike!" with everything in between. But comment they will, and get ready for the question, "How much did it cost?"
Q: Will I look like a circus bear?
A: Only if you are wearing a bear suit.
Q: What bike can I get for $150?
A: New, a pile of junk. Used, some quite nice deals. As always, if you buy cheap, you will get corresponding quality. Known value-for-money inexpensive bikes are the low-end Dahon range, Downtube bikes, Citizen, and some others as well. But that does not mean more expensive bikes are over-priced. On the contrary, folding bikes are just as well-priced as road bikes. Most experienced cyclists wouldn't flinch at $2000 for a road bike, but strangely when a folding bike approaches that sort of price, people start getting very reluctant.
Q: Folding bikes are little more than toys, why are they so expensive?
A: Don't be fooled by the small wheels. Yep, some folders are toys but good quality ones are not - they are used by experienced and some very well-known riders all over the world. They are used for all sorts of things big-wheeled bikes are used for, and a lot of things big-wheeled bikes can't be used for. So folders are actually more versatile than big-wheeled bikes. As such, folders are not expensive per se. As for the fun factor, small-wheeled bikes are more fun to ride than big-wheelers, so in that respect they are closer to toys than big-wheelers.
Q: Can I take it on a plane as carry-on luggage?
A: Maybe... Most may be too large or too recognisable as a bicycle. Most folding bikes will only fit into a large suitcase and you can't take these as carry-on luggage.
However... The Brompton has one of the most compact folds, and when folded is generally not recognised as being a bicycle outside of the UK. There are numerous reports by people who have taken their Brompton into the cabin by gate-checking it. Basically you arrive at the gate with your Brompton and ask staff to gate-check it. They attach a label and carry it into the cabin and store it in a space where wheelchairs and baby strollers go. On disembarking, the bike is waiting at the gate for it to be picked up.
Some people have done this numerous times without a problem on a variety of flights. There are even cases where people have taken it into the cabin themselves and put it in the overhead locker. So it can be done. Gate-checking is meant for people with mobility aids. Doing this procedure with a Brompton, while possible, is perhaps not what it was intended for. Savvy staff may chuck a wobbly and force you to check it in as luggage. There are no cases that I know of where this happened but it still prevents me from doing it.
Q: Can it fit into a airline-legal suitcase?
A: Most do, but you will have to do a certain amount of disassembly. The wheels usually have to be removed and depending on what bike, some other bits will need to be disassembled as well. Some bike such as the Dahons have an optional suitcase that will accommodate the bike just folded, but these are LARGE. Bike Friday is probably the easiest for this since these have been specifically designed for this purpose, so generally the rear wheel of a BF does not need to be removed. Bromptons have one of the most compact folds of all folders but even these won't pop straight into a "legal" suitcase.
Q: Can you recommend a 26" folding bike?
A: There are several on the market such as from Dahon and Montague. Unless you have a immovably compelling reason for needing a 26" folder, consider opening up the options to include 20" and 16" folders as well. There is a very large range to choose from if you do.
Q: I am 6'6" and 300 pounds of sheer muscle. What folding bike should I buy?
A: Not any folding bike, you're out of luck. Folding bikes in general have long stalks (seatpost, steering riser) and 300lbs is above the weight limit for known folding bikes, although Bike Friday do build extra strong frames on request. 6'6 is also a challenge, as almost all folders are size medium, so a 6'6 person would be quite cramped.
You should contact Bike Friday, perhaps they could build something for you. There are no other known choices.
You could consider a Raleigh Twenty, a popular folding bike from the 70s. It is enormously strong, and if you are so inclined, can be modernized and also built up with very strong wheels.
Q: Is folding bike suspension a gimmick?
A: There is poor and good quality suspension, and big travel and small travel suspension. First, let's eliminate poor quality - that is a gimmick. That leaves small vs large travel. For road riding, large travel has to be argued as being not necessary.
The first line of defense against road bumps lies in tyres. Modern hype makes many riders over-inflate tyres leading to unnecessary harsh riding. Every tire has its optimum pressure setting, that should be used. Fat tyres employ lower pressure due to geometry considerations (see sheldonbrown and this), giving a plusher ride. In general, a tyre twice as fat needs half the pressure than a narrow tyre for simular sidewall stress- don't be tempted to jack up the fat tyre pressure! Narrow tyres need higher pressure to prevent pinch flats. This usually also leads to lower rolling resistance as there is less flexing in the casing. A tyre that is pumped even harder actually feels faster but careful testing has shown that to be not a real effect when the tyre is already pumped to a suitable pressure. A suitable pressure is one where you can see a small amount of bulging when seated - typically 15% drop.
It is possible to use narrow high pressure tyres and combine that with small travel suspension to remove the harshness. That can actually lead to lower rolling resistance over uneven ground, as only the wheels have to be lifted over bumps instead of rider+bike. That sort of suspension is no gimmick. It also gives greater comfort than tyres alone.
So, avoid buying a bike with poor quality suspension - it will give you info as to the rest of the bike's quality. The best approach is to start looking at bikes with no suspension - best bang for buck as good suspension costs lots. If the bike as-is is too harsh, the next step is to get some plush tyres, and here Big Apples is the final name. Others also work very well, such as the Greenspeed Scorcher.
Q: Can I do long distance on a folder?
A: Long distance on any bike basically equates to long time in the saddle. To stay for many hours in the saddle of any bike, and folding bikes are no exception, the saddle needs to be comfortable enough, and you need to be comfortable on the bike as a whole, not cramped neither stretched out. The grips also need to be comfortable, preferably with multiple hand positions on the bars. Once you have these 3 items sorted out, you can do long distance on the folder, any folder.
Q: Why do folders often have a chain guard on the crankset? Why does my bike drop its chain when I shift to a smaller cog at the back?
A: * When chains wear, they become more able to flex sideways.
* When shifting to a smaller cog, the top run of the chain gets a sideways impulse as it drops abruptly on the smaller cog.
* When the chain is worn enough, the sideways impulse travels forwards along the chain and will tend to derail the chain when this sideways snake arrives at the chain wheel.
* This is the most important reason a chain guard is needed when there is no front derailer - to prevent that snake from dropping the chain. Keeping your trousers clean is a secondary benefit. However, I suspect that few folding bike manufacturers know this.
To work best, the guard must be as close as possible to the chain to arrest the chain snake. In most cases imho it is not close enough. You want a few mm at most. My Birdy also dropped its chain, until I fitted custom guards that I spaced as close as possible without chain rub when the extreme gears were engaged.