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I'm looking at a folding bike for commuting.
It doesn't need to fold to the smallest possible size as it only has to either go in a coach luggage locker for me or a car boot for my other half.
I've tried a Brompton in the past an absolutely hated it so they're straight off the list.
I've been looking at something with 20" wheels as I'm hopping they'd ride a bit better than the smaller Brompton size wheels.
Cheap Btwin folders are an option, they seem to be dirt cheap second-hand. Cycle to work is also an option for buying new.
The Tern looks pretty good and disc brakes are a bonus
https://www.ternbicycles.com/uk/bikes/471/link-b8
Anything too look out for or avoid?

Have a look at Dahon, they do various wheel sizes
I’m probably a lot out of date with this but tried a few folding bikes for the reasons you list. Mostly Dahon. I didn’t like the creaky somewhat flexy nature of the lightweight alloy model (Mu P8) with a telescopic handlestem (or whatever they call that thing) - But the 20 inch wheels were just about doable - I remember thinking that would improve on these potholed roads with more tyre -clearance.
Also tried a Dahon D7 (4130 steel), which were fairly hefty to hike about folded, yet more comforting to ride for a biffer like me.
Eventually I set sights on a Dahon Speed P8 with balloon tires (Schwalbe big apples if I remember correctly) and a solid (non-telescopic) handlepost. Unfortunately I never sourced one of those - and I believe they’re long discontinued so whatever/if there is the new version of that then I would recommend 🙂
I’d be cautious about buying older VGC (hardly/used/unused) models as there were some product recalls if I remember correctly (both Dahon and Tern) owing to failures.
One would hope those early failures have been ironed out now?
I still would like a single speed 20” cromo folder with Big Apple tyres
I have an old Dahon and a Brompton. The Brompton is great for London traffic, it's comfy and there's something I love about the construction and quality, plus how the bag attaches - it's a keeper and one of the most useful bikes I've had. But I've never been a big fan of how it rides. Too skittish to be a confident ride and unless you get a firm / solid bumper they suck pedal power through chain tension squat like a 90s FS bike.
The Dahon gets used for the 7-8 mile ride on lanes to the station and it's pretty good for that sort of commute. Comfier, faster and more confident overall.
If I was replacing the Dahon now I'd get a Tern and ideally it'd have an IGH like my Dahon. The thing to watch on Tern/Dahon/etc folders are the hinge clamps, make sure they stay adjusted and clamp firmly. A loose main clamp fatigues the frame much faster.
I don't think discs add much to a folder. Small rims spin fast and clear water fast plus I'm rarely at the speeds where I need better brakes. They're just a bit cleaner running overall.
Always fancied a moulton. Looks like it shouldn't hold a human with those skinny tubes, but clearly they do 🙂
https://www.moultonbicycles.co.uk/models/XTB.html
I love my airnimal rhino. Very capable off-road too for sneaky post work rides on the trails.
but bloomin heavy if you need to carry it for any distance.
I was saving up for an Atto as I liked the idea of a lightweight carbon job,then read a few reports about creaking noises (which would bug me),now getting a Brompton from a mate for a few test rides.
Tern also look good.
More niche, and more packable than folding, but a Bike Friday rides like a proper bike. 3x9 dualdrive gearing on mine, bought on a whim of an ebay bargain during lockdown, but might be a victim of n-1 soon, the garage is a bit full right now. I've enjoyed riding it the few times I've taken it out, but it'll never be the first bike I reach for, I suspect any folder would be the same, though it does fit in the boot of my MX5
My mate rides a Moulton most of the time, in fact he just blew this year's C2W budget on a second one. Now, I wouldn't ride one all the time but if I needed something very rideable that could also be considered portable, I would seriously consider one.
While the Moulton looks pretty cool, I'd be much more inclined to go with a Velo Orange Neutrino build if I were spending that kind of money. The problem with both of these is that they aren't folding bikes so don't really fit my requirements.
If the Neutrino was able to collapse down and fit in the boot/coach I'd already have one ordered 😂
If the Neutrino was able to collapse down and fit in the boot/coach I’d already have one ordered 😂
1x Neutrino + 2x S&S couplers + 1x hacksaw?
https://www.halfords.com/bikes/folding-bikes/carrera-intercity-disc-9-speed-folding-bike-536878.html looks great value for ~£400 with BC discount, 9-speed (Which has a decent range despite the large chainring due to small tyres, lowest gear is under 30 gear inches iirc) and hydraulic brakes.
Btwin single speed is great value. And for a few extra pennies you can have this. Mine is set up fixed wheel with marathon plus tires. It’s not light, but copies my road position (almost, saddle is a bit low), fold with an allan key on my key ring (to avoid theft really) and rides very well indeed. Seat post was the longest I could source, bars are one inch diameter as I originally had them simply replace the flat ones in a clamp. Brake levers are V-specific. Saddle is a flite titanium to save some weight 😉
Ride is better than my previous Brompton (can't make fixed) and better than my Tactic (both with 16 inch wheels). Warning, when you cut off the handlebar clamp, the diameter is not 1 1/8 or 1 1/4 inch. It’s in between. So some shimming is needed and so far I still have my teeth. Needs a better solution but I’m pleased. It could also do with a folding drive-side pedal to lie it on its side. The bars fold down, but it’s not compact. But it’s more Bike Friday than Brompton. All in, it owes me £200.

You tried a 20" Brompton? Or a 16" with Big Apples?
Expensive but definitely going to ride differently to the original. Kinetics in Glasgow make the kits and they seem well regarded by those that use them.
I'd be tempted to try the Brompton again. They're so good and such fun when you get used to them. Don't know what type of riding you do, but around town they're pretty unbeatable.
Used to commute from home to station, then Victoria to Clerkenwell. People in the office didn't know I cycled cos the bike hid under my desk. And the fold meant that it always had space on the train. Some of the terns and dahons would block doorways. That small fold might be useful in coach lockers....and in the boot when there's shopping or dogs to come along too.
The utility, ride and fold is the reason that second hand values are so strong, they're getting something right that others aren't.
Go Cycle G4?
I had one of the B'Twin Tilt 120's which I used for about a year, then sold on. I seem to remember paying about £150 for it, price went up to £170 while I owned it and I sold it for £140
I was very impressed for the money - getting a set of mudguards and even some basic lights included was really useful. 6 speed gripshift worked well and it folded down easily with a few nice design touches when folded
Only negatives were needing a spanner to remove the wheels to change punctures and the seatpost would have been too short if I was much taller (I'm 5'10")
I'd happily buy another one if I needed a folding bike
Me too re pimped cheap Btwin folder. Solved problem with seat post length. Uses same diameter as Brompton so fitted a Brompton extending post. Replaced many other components with better bits out of my parts crate - brakes, bars, chainset etc. I increased the gearing. Rides well and as it doesn’t look expensive I am okay about occasionally locking up outside the station or shops. Serves its purpose well.