for those who coming from road bike, you already well know bike but just want a smaller size foldable one, please kindly consider those 16" 349 wheelset folding bike, for 20" you don't get the portability that you want and you don't get the speed that you want too, so it's neither here or there. and the amount you invested will never get you the same performance which could give you in road bike.
for those who coming from mountain bike, just don't expect the same comfortability and the versatility of all-terrain. the tyre and folding hinge of folding bike really can't withstand those abuse. the best "suv" type of folding bike that "maybe" meet your expectation might be Birdy GT 10SP. What? too expensive? try a look at Rhine R20.
for those who really don't know where to start, so blur how to choose a folding bike? just buy 1 that you feel comfortable to your budget, digging into the rabbit hole and start the journey from mistakes. or you may refer to those experienced rider and ask what's the nice budget to start with and which model to get based on your requirements.
some tips for considering a folding bike by parts:
1. Frame
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which frame to start with? set your desired wheelsize first then only shortlist the available frame model.
14 inch (254): small folded size and portability is the key here. but i don't suggest newbie starts with this wheelsize.
what's the popular frame model here? Dahon K3, Fnhon Retro, Dahon Dove 412, Fnhon Breeze 1401, Fnhon Freedom 1402. For 14 inch, i recommend Dahon Dove 412 and Fnhon Breeze 1401 single speed setup. 3-speed setup better go for Dahon K3, Fnhon Retro, and Fnhon Freedom 1402.
16 inch (305): newbie should start looking from here. you could use those 14 inch frame i mentioned above and mod into 16 inch (305). The wheel hub O.L.D. (Over Locknut Distance) for 14 inch frame normally will be 74mm for front and 85mm for rear, while the wheel hub O.L.D. (Over Locknut Distance) for 16 inch frame normally will be 74mm (rim brake) / 100mm (disc brake) for front and 130mm (choose this because more choice of wheelset) / 135mm for rear. what's the popular frame model here? many choices from Dahon.
16 inch (349): bigger than 16 inch (305), technically it's around 17.8 inch. consider this wheelsize as it's getting popular. i would say this is the best balanced wheel size for folding bike. what's the popular frame model here? plenty of it. Brompton, Element Pikes (or a.k.a Camp Pikes), Fnhon Zephyr with C-Brake frame, Fnhon Gust.
18 inch (355): the only bike you will want to use this wheelsize is Birdy.
20 inch (406): most popular wheel size, but the started to lose portability of folding bike too. normally people will recommend this wheel size for newbie because hardly go wrong. what's the popular frame model here? too much. same wheel hub O.L.D. (Over Locknut Distance) mentioned above. the O.L.D. greatly affect the wheelsize choice and the folded width.
20 inch (451): for speed purpose. newbie don't start from here.
after select the wheelsize then there are frame welding quality, frame geometry, frame folding mechanishm, frame and fork material, frame hinge mechanism, brake mount (v-brake, c-brake or disc brake) front derailleur bracket availability, frant or rear rack bracket availability, kick stand bracket availability, seat post compatibility, cable routing, color and paint finishing to look at.
2. Handlepost/stem
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- inner fold or outer fold
- locking mechanism (folding and handlebar)
- material
- welding or 3D post
- adjustable or non-adjustable
- if non-adjustable, what's the suitable height for you? taller for upright riding position for comfortability, lower for aerodynamic purpose.
3. Wheelset
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- wheel size like mentioned above
- disc brake or rim brake hub
- wheel hub grade
- wheel hub sound
- spoke material and quantity used
- spoke design and pattern used
- rim profile
- cassette supported speed
4. Tyre
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- tyre material
- tyre size and profile width
- tyre thread
- tyre sidewall (foldable or beaded)
- rolling resistance
- puncture resistance
- road feel
- tube size
- tube valve (presta or schrader)
- supported air pressure
5. Headset
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- the smoothness of the bearing
6. Handlebar
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- length
- height
- tube diameter
- material
- pattern (straight/M/butterfly/dropbar/etc)
7. Grip
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- material
- locking mechanism
8. Brake Lever
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- material
- spring and feel
9. Brake Mechanism
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- compatibility
- v-brake, c-brake, or disc brake (study the pros and cons)
- material
- spring and feel
10. Shifting Mechanism (Groupset)
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- shifter compatibility
- front derailleur compatibility (single or double chainring setup)
- rear derailleur compatibility
- cassette speed (8/9/10/11 speed?) (gear ratio range 11-25T, 11-28T, 11-32T, etc)
11. Chainring
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- single or double chainring setup?
- size to match gear ratio
- teeth compatibility and optimization (for single or double chainring setup)
- material
- BCD (Bolt Circle Diameter) compatibility
- crank mechanism compatibility (square taper or hollowtech)
- chainring nut material and length
12. Crank and Bottom Bracket (BB)
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- square taper or hollowtech mechanism
- BCD (Bolt Circle Diameter) compatibility
- BB size and width (chainline compatibility)
- bearing material
- bearing seal
- crank length
- crank design
13. Chain
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- speed compatibility
- brand compatibility
- material and design
- locking link
14. Seatpost
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- material
- tube diameter and length
- seat mounting and adjustment mechanism
15. Saddle
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- good luck in finding saddle that fit your ass
16. Cable housing and wire
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- lubed cable housing and structure
- wire material, coating and braided structure
- apply to shifting and brake cable
17. Pedal
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- foldable or non-foldable mechanism
- fixed or quick-release mechanism
- material and design
- bearing material and quantity
- effective surface area
- cleat platform and compatibility
18. Kickstand, botthe holder, fender, carrier rack, other accessories.
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- for convenience purpose, you might not include it to improve aerodynamic and reduce weight
- front light, rear blinker, and bell IS A MUST!!!
What else? How to expect newbie know all these? Just get a bike first that suits your comfortable budget then only we talk later.