it will be "mountain biky", similar to a 28/30 chainring on 29er. I ride 32 or 36T on my gravel and it is fine. I think it will depend of the tyre choice. On the same gravel bike, when fitted with Conti GP5000 TT tyres, I have to use a 46T chainring

T Line rides better than other Bromptons. The ti plus carbon really smooths things out.

Also,jus sayin',Brompton has been around for 50yrs,built 1M bikes,and has a dedicated dealer network.

Bring me a Helix to ride,I want to see how much smoother it is than my T. Also,my range is 28-114" with the stock setup,25-101 w/44t ring. Ultralight Helix is 25-95,Standard is 27-90,so neither has the 12spd Brommie range.

You may be right; The Helix has larger diameter tires, but a more rigid frame and no suspension like the Brompton rear. Without the directional layup like carbon main tubes, it can be difficult to get longitudinal frame flex for a good ride, while also torsional stiffness so not like a noodle when climbing standing.

I am assuming the Ultralight has the Hong Kong gearing option? I thought HK option had at least 400%. The problem with Brompton gearing is that low-low or high-high and you're not running direct drive middle range, so you have friction losses inside the hub, which are enough to be noticed. With the larger wheels for better ground clearance, the Helix should have no problem easily exceeding 400%, perhaps even 500% with a standard RD and wide 1X gearing.

In daily use,I really don't have any issues with riding and climbing with my BWR hubs. I'm sure that the guys in Le Tour would,but I really don't think normal people will notice the reduced efficiency from an IGH as much as they'll notice weight(esp in the wheels/tires) and gear range. When I was working at the old shop I would swap between my Pt Reyes and S6 for commuting and hub drag just didn't register.

I saw this Helix at a bike shop in Midtown, probably in for service. Maybe there's an online group where you can find a Helix owner in town that will let you try the bike out.

I had a 3x3 gear system on my Brompton. I ride mostly on flat ground. I so seldom used first or third on the IGH I switched it out to 3-speeed external This was mainly due to the extra complexity, particularly when dealing with flats.

For me it nonsense to use the slow, inefficient, heavy 4x3 transmission with a BWR on a lightweight, reactive bike like the T-line.

This bike should have an efficient lightweight 11s speed derailleur+cassette transmission like the one of the Ultralight Helix.

I wouldn't say it wobbles but when hands free, it will start to list left (fork side). I'm guessing it's to do with single fork design. So, no. You can't ride hands free, at least not for me. But if you have a core strength & balance of an acrobat, and are riding on totally flat smooth surface, it could be done? I dunno.

My sentiments are with you. But on 349-35, that could be tough. Let's see what the numbers look like.

52x10-40 gives a good range, 21.6-86.3 gear inches. Might be possible with one of those mid-cage 3-roller RDs for more chain takeup on the high cogs. If not, with a 36 low that gives a low of 24 G.I.

Going with a 9 tooth high cog (special parts, I know), 52x9-36 gives 24-96 G.I., so if you don't need that high, that leaves room for a smaller chainring to help the lows; 46x9-36 yields 21-85 G.I., that's nice in my opinion. If a 36 cog will work with a good RD on 349s, what you want is definitely doable.

The Ethirteen has 11s 9-34, 9-39 and 9-42 like mounted on the Helix HK specs.

And SRAM has 12s 10-36 and 10-44t like mounted on the Bike Friday All-Day.

Oh the parts are available (and I think Helix offering the HK spec is awesome), and fit fine on 24" and 20"/406. But skinny 349 as you recommend lightweight external gearing on the T-Line... I'm with you, sounds better to me, but will it "package" with the RD not scraping the ground? I think perhaps yes, if using the style RD with a third pulley that I saw pictured on the Dahon Curl D9, that cleverly takes up more chain slack without a longer cage. Maybe also some of the newer 1X RDs with I think an additional pivot on top or a dynamic B-screw pivot or something, I haven't messed with any of those yet.

I went 20" folder (no smaller) specifically to avoid needing an IGH for high enough top gear, as my 1st gen Dahon which I got for $10 because it only worked in 2nd gear, I finally got tired of walking up hills so pulled it off to check, it seemed to shift into 1 and 3, put it back on the bike, and the cones loosened up after riding 10 feet, multiple times. I goofed up something, probably with some retaining tab or washer, who knows, but I wasn't going to spend $100+ at Aaron's Bike Repair (IGH specialists) on a well-worn $10 bike.

So I'm with you Jipe, I'm singing the same tune, light external gearing on a T-Line would make a handy and carryable urban assault and travel tool, if it fits. Brompton's 4-speed external just doesn't have the range.

The Riese & Müller Birdy Touring is equipped with a 10s Deore long cage derailleur and has ETRTO355 wheels with ex-factory 40mm wide tires and its derailleur cage doesn't scrap the ground.

This begs the question, why have'nt they?

Well there you go! See below.

Brompton has been notoriously slow to innovate, even to adapt their design to use available off-the shelf systems. It was more than being conservative. I think it was because they were already selling everything they could produce. In recent years, under new leadership I think, and seeing more competition in the 349 trifold market, Brompton has been advancing technically. New frame material options, new wheel size options. But, for example, no Shimano 8 or 11 IGH, which requires wider dropout OLD, so a Scottish framebuilder offered his own custom rear triangle for such conversions, but at significant added cost for that and a new wheel. Though a long cage rear derailleur would not drag the ground according to Jipe above, I could still see Brompton being nervous if it was at all close to the ground; Brompton RD arm/cage is mostly forward, which also fully takes up the chain slack when folding. I'd be curious if the mount for that RD is same as for standard RDs. If so, and with a 135mm rear OLD, conversions should be easy for those desiring it.

I think there are two main reasons.

The first is the small, not standard OLD of the 16" Brompton that hasn't enough width to put a standard cassette. To use a standard derailleur and a standard cassette (9, 10, 11 or 12s), the OLD should be increased to 135mm what Brompton is not willing to do.

The second is the chain tensionning when the rear triangle is folded. The derailleur cage extension arms whatever their design proved not to be reliable on the Birdy 1 and Birdy 2 (the one of Riese & Müller and also lots of different ones from alternative suppliers who tried to solve the problem). The solution came with the Birdy 3 with a totally different concept = a tensionner arm around the chairing.

The 4s derailleur of the P-line and T-line is proprietary with a proprietary mounting on the rear triangle in front of the wheel axle, it only does the change of sprocket, there is second arm placed closer to the rear wheel axle that does the chain tensionning which is very similar to the one of the previous 16: Brompton.

Just curious,have you ridden a T Line? Cuz I own one. It's not a heavy bike,nor is it a slow bike. A full derailleur drivetrain would mean a larger folded size,and there would be serious limitations. My gear range is 28-114". What derailleur setup is going to give me that? And fit into 112mm spacing? And not have a cage that hits the ground or has issues with dropping the chain?

I can't think of a single 20"+ folder that I'd own,because none of them fold as small as a Brompton,and that's one of my top priorities for a folding bike. I've had a Mu SL and a Speed Pro TT and didn't keep either because they were way too big when folded(and didn't fold as easy). I also had a Mezzo D10 at the same time I had my S6,the Brompton stayed,the Mezzo got Craigslisted. The Mezzo was a touch lighter with a normal 1x10 105 group,but had limited gear range,dropped its chain,and had a crap fold.

Yes, I tested a T-line 4s and I had a 6s superlight Brompton with the BWR hub which is now upgraded with a Rohloff hub.

The original titanium rear triangle has been enlarged by Ben of Kinetics Glasgow, the enlargement of the rear OLD from112mm to 135mm doesn't change the folded width at all because its done at the end of the triangle where its width doesn't define the folded width.

For the gear inch, the Rohloff has a wider range than the Brompton 4x3s solution.
And there are cassettes compatible with small wheels with a similar or wider gear inch range like the 11s 9-39 and 9-42 (like mounted on the Helix HK specs) or 12s 10-44t.

BTW. the replacement of the original 6s transmission BWR+Brompton 2s derailleur by a Rohloff with some lightweight parts on it almost doesn't increase the weight of the bike (I measured a weight increase of only 150g that was compensated by using a 11s chainring, a lightweight X11SL Gold KMC chain and a singlespeed chain tensionner).

And yet you said:

So the Rohloff is that much more efficient than a BWR? I have a little trouble believing it's that much better.

I don't see a derailleur system that can handle a mondo cassette like those playing well with 16" wheels. 20" yes,but not 16".

I'm calling balderdash. We have a BWR here,and my Park Tool scale says it's .95kg. Rohloff's site says their hubs are between 1.7 and 1.825kg. That's a pretty big difference,over a pound and a half,pushing two pounds between the two hubs. Even with the 11spd parts,I can't see how you only saw a 150g difference.

Again, the Riese & Müller Birdy 3 Touring with ETRTO355 wheels is factory equipped with a long cage Shimano Deore derailleur that can handle cogs up to 42t and it works without any problem

So using a cassette with a 39 or 42t biggest cog (BTW these two cassettes, 9-39 and 9-42 give a wider gear inch range than the 407% of the Brompton 3x4 system) is not a problem with ETRTO349 wheels.

The comparison I made is not just comparing the weight of the naked hubs but the weight of the whole transmissions, so the complete 6s Brompton transmission including the derailleur, the levers, the cables, the cogs..., everything I dismounted and replaced by the Rohloff transmission (the weights you mention for the Rohloff are not the weight of the hub but of the complete Rohloff transmission, same as what I listed for the 6s Brompton transmission).

That said, I wouldn't mount a Rohloff on a T-line what would result in the same weight as the current 12s T-line.

That's pretty close to the ground. And those are 40/355. 35/349 are a lot smaller and lower.

From that link it looks like that's just the hub weight. And that hub has got to weigh way more than a BWR. From Google's AI:
A Rohloff Speedhub generally weighs around 1.8 kilograms (4.0 lbs). This weight is for the hub itself, without shifters, cables, or other components like a belt drive or cogs. The total weight of a complete Rohloff system, including the hub, shifter, and other components, can be roughly 2.4 kg (5.3 lbs).

Sorry,I'm not convinced.

Yes, the overall wheel diameter difference between 40x355 and 35x349 is huge: about 16mm, i.e. 8mm for the ground clearance.

So without a driver on the end(I assume that's what's missing from the left) that hub is 1.3lbs heavier than a BWR(1545g vs 950g) w/cassette driver,anti-turn washers,and nuts.

A quick update on the status of my order. I ordered April 3rd. Another member on the Forum with the next bike number in sequence already received their bike a month ago. We thought it might be the bike rack and box which was causing the delay. This may well be and my bike production was pushed out to coincide with the rack and box delivery.
The status of my order have just changed from the non-descript "In Production Queue" to "Main Triangle Welded - In Production Queue". Yay some visible progress at last!

Well, I wasn't expecting that. Checked my email this afternoon and was pleasantly surprised to see that my order has already been shipped via FedEx.
Within 24 hours, it has made it's way through two hubs from Ontario to Memphis Tennessee, then flight to Stansted UK. And now it looks like it's already departed the airport and I assume now on its way to my nearest hub. Pretty rapid!!
Although slightly miffed that it’s gonna take five days to travel the next couple hundred miles to the final destination as my scheduled delivery is on the 17th of June.
Knowing it's so close, yet so far, the anticipation is killing me.