DumplingsGalore

Before you go the custom route brompton -- have you seen the rumors of a 20-inch brompton with disc brakes reportedly being announced next year, including a supposed prototype being spotted in the wild? (I can't post links due to my post count, but look up "brompton G line"). Note: that image seems to have made the rounds previously, but it nonetheless seems legit to me for various reasons, including the standard brompton electric battery, a very different stem design, the carbon-looking fork, and unusually tidy fenders.

While it'll almost certainly fold bigger than the current brompton, with some frame tweaks it's possible it is not too much bigger. I haven't heard anything about it a titanium version, but it would at least check several other items off your list.

V1LLAGE_ID10T

I did in fact see that and I'm really glad to see Brompton finally start to innovate. However this is more the off road sort of cycling I'm less invested in. I'm looking towards more road than gravel bike if that makes sense. Like if the Giant Revolt Advanced was turned into a Brompton.

biosync

I know you said off-road, but there is a person on my local Brompton group who got a 20-inch fork and triangle from ti39. The quality looks okay, I haven't tried it personally though.

DumplingsGalore

Hmm, I'm not sure I understand you on this. The only thing "off-road" about it is that this particular prototype is that it has somewhat knobby tires, and who knows if those are even the only or final tires. Surely you could just install some road tires? You'd have so many to choose from at 20 inches

univega.duder

Wow that looks great - interesting cranks and chain ring as well! Can I ask where you found the setback seat post?

V1LLAGE_ID10T

Maybe I'm too quick to judge, but the rear triangle and handlebars don't quite look right to me. Yes Bike Friday pocket rocket has a similar gooseneck forward bending handle, and chances are knowing brompton (assuming this is an actual prototype not a random guy's build) I would still have to wait on an announcement and a price and what are the actual options and if swapping for a different stem is in the cards. Likely would be. You do have a point a tire swap would be next but the other thing is I want to go full titanium as well.

univega.duder

Have you looked at all at the forthcoming Siggi bike? It has 20" wheels weighs less 10kg has a "StoneWeave frame. A light and durable mix of carbon and basalt". I believe they're offering it as a single speed, 11 speed derailleur and 11 speed alfine option. I believe they're also offering a belt drive option and an electrified option. I'll be curious if it would be compatible with a Rohloff or Kindernay hub. The fold doesn't look as compact as the Brompton or Helix but I'm curious to see what the ride quality will be like. I like the the company seems more established the Helix and has a track record of other bikes that have been produced successfully,

StoneWeave frame.
A light and durable mix of carbon and basaltC

StoneWeave frame.
A light and durable mix of carbon and basalt

StoneWeave frame.
A light and durable mix of carbon and basalt

V1LLAGE_ID10T

I guess we will both wait and see while I'm building mine.

DumplingsGalore

Definitely don't want to sound like I'm telling you what to care about, but I suppose my own perspective is that Bromptons are far from optimized for anything other than a compact fold anyway-- I doubt things like handlebar positioning and the triangle design will make much of a difference in terms of road ride quality. That said, I get the impression that isn't the final rear triangle. It looks a little rough; probably the most "prototype-y" part of the bike.

And of course, it's not titanium. Hope this comes to fruition soon nonetheless.

univega.duder

Yes - well keep us posted as you get into building yours. I'll be interested to see what you come up with. Cheers!

V1LLAGE_ID10T

Just ordered the parts. Working with GR5 at 1.3mm. Should come in December. Meantime If anyone knows where to get a rohloff for cheap lemme know. I'm on good terms with my mechanics and one of them happens to have worked with folding bikes quite intimately. Hopefully that should cover the majority of the potential hurdles.

V1LLAGE_ID10T

I Just got an email back from them. Do what you will with this information.

vuurmot

Titanium 20 inch brompton clones are pretty common where I live
This is mine
Alfine hub
TRP disc brakes
Titanium frameset
406 wheelset

pros:
definitely can feel the difference in ride smoothness and speed with 20 inch wheels
less twitchy

cons:
bigger folded size
some racks are incompatible with the 20 inch frame
internal hub is quite heavy, and PITA to carry

V1LLAGE_ID10T

That's EXACTLY What I am looking for! I'm going to go out on a limb and guess you're in Singapore as 90% of the trifold bikes I have found with 20 inch wheels are selling there. I am looking at rear racks next not for carrying stuff but for rolling. I see many people favor the seatpost roll but I use mine with the bag as a cart when I'm grocery shopping/ bringing things to work so I do in fact need a proper one. 99% of my time carrying comes from the stairs and not much else. I've also been taking up weightlifting recently so this should be a non issue. I've been considering switching to Alfine, but I am concerning with hill climbing and reliability. Lot of steep hills where I go and I also live on a hilltop. Know any vendors willing to ship internationally in your side of the planet? Ones that sell a good rolling rack allowing for the mudguards?

silverx

How sigi fold mechanism looks similar with helix
But with when bike fold, helix wins in term of how more eficient folded.
helix with bigger wheels also more light 😊
for me, helix when fold the best compare any folding bike as front and rear is straight and have locking mechanism each others

vuurmot

Yeah I'm based in Singapore
Regarding the rear rack, I've had problems with a couple of custom racks, the rack angle doesn't allow the bike to be folded. It was a massive PITA.
I've found that only a certain rear rack work (as shown in the picture), as the rack is attached to the easy wheel and the angle can be changed.



I've not found a suitable mudguard configuration for 20 inch wheels. With standard 20 inch mudguards, it hits the ground when you fold the rear triangle, really janky in general.
The vendors here source their parts from china, and you would probably have a better time doing some shopping on aliexpress.
All these small problems should go away if you decide to go with kinetics and their custom brompton parts.
I highly do not recommend going to 20 inch route unless you want something unique for yourself
A brompton with custom 16 inch titanium disc brake triangle/fork 100/135 O.L.D with internal hubs is ideal IMO

If you want a 20 inch brompton clone with chinese parts, mudguards and rack will be annoying obstacles.
My suggestion is to source a ready built 20 inch brompton clone (unless you live in SG/HK/TW/CN, it is probably hard to find) or go with kinetics.

Or maybe book a flight here to buy it
This is carousell, our general secondhand/vendor goods marketplace. Singapore is a small place so you could reach out to all the vendors here in an hour max

Jonesandrew

It looks like you’ve done your homework on your build and you’ve mentioned concerns about the stiffness of possible carbon components in your OP but you haven’t commented on the mainframe stiffness. Note the following comment.





The above was the only reference I could find today but I know others have commented on the flexibility of the ride of titanium Brompton clones in this forum suggesting they are not as stiff as a steel framed Brompton. Compare images of the T-Line Brompton main frame tube with those for the Aliexpress titanium clones – It looks a lot larger diameter.



I’ve been down the “build a better Brompton” road using Columbus SL tubing where possible. I was happy with my first attempt (20” wheels, 21 speed SRAM Dual drive, asymetric main frame tubes to allow very compact fold, increased BB height, Vee brakes, Thudbuster seatpost instead of rear triangle bumper for suspension) but it was heavier than I wanted at about 12 Kg.

I tried everything I could think of to reduce frame weight for my second attempt including using smaller diameter main frame tubes, but I had to back track on that idea because the frame flexed too much when I tried it.

The final version is here: https://www.bikeforums.net/22405531-post38.html

In short, I suggest you concentrate very strongly on every possible aspect of stiffness otherwise you may end up with a superlight build which rides like it’s a basket of sticks.



By the way, why did you choose Grade 5 titanium? I’ve just looked at 6 different manufacturers who all quote Grade 9 for their frames. Only Bossi in Australia (frames made in China) quote Gr5 castings for head tube, BB and seat/toptube intersection – maintubes are still Gr 9.

Jipe

I fully agree with you.

Brompton redesigned the Ti frame to take into account the different mechanical properties of titanium vs. steel and this resulted in a frame with tube with a significantly larger diameter.

Moreover increasing the overall wheel diameter (by moving to 20" wheels or/and using wider tires) requires to increase the wheelbase to be able to fold the rear wheel under the main tube what will might increase the flex of the frame.

About the wheel size, I compared 3 possibilities on my titanium Birdy (because the Birdy uses disc brakes and its frame allow to mount wheels up to 35x406 without any modifications) 40x349 (with Greenspeed Scorcher tires 40x349), 50x355 (with Schwalbe Big Apple 50x355) and 32x406 (with Continental Contact Urban that are actually 35mm wide on my rims and are known to be fast rolling, strong and lightweight) and the conclusion was:
- 35x406 wheels were less agile wit more inertia that the two others while not being faster nor more comfortable. Schwalbe Pro-one 28x406 will be faster (not as fast as Scorcher) but less comfortable and known to be fragile, Kojak 35x406 are slow and fragile.
- 40x349 were the fastest due to the very low rolling Greenspeed Scorcher but fragile on trails and bad roads.
- 50x355 was the most pleasant to use, quite fast, comfortable, agile and usable off road.

One more info about fast tires for ETRTO406 wheels: I have also a Moulton Speed with TL ETRTO406 rims. I originally mounted TL ready Pro-one 28x406 but after many damaged tires (not puncture, but sidewall cuts and rolling band de-lamination) I eventually gave up and switched to the Continental Contact Urban 32x406 that appeared to be almost as fast and very strong. Even the slick Greenspeed Scorcher is stronger that the Pro-one (and faster) but the 40x406 Scorcher are too wide for my Moulton (and for he Birdy).

V1LLAGE_ID10T

In the worst case scenario I can alternate paths to 16inch for a lot of the reasons you outlined, but my parts are already ordered so we shall wait and see how this turns out long term. I had not ordered the rear rack and mud guards yet for these reasons

I was actually talking to a few vendors and asking for quotes. None of them replied when I started asking about rohloff hubs. I was tempted to spend my Xmas taking a plane ticket and going to the beautiful country but thought it best to just build exactly what i wanted for the fun of it.

Stiffness is exactly the reason I choose grade 5 for everything. I noticed the change in the frames on the official parts and saw that as a sign more strength would be required. I'm not a mechanical engineer but I assumed that using a stronger grade of titanium and as thick as I was able to get manufactured for me would take care of that. I also before pulling the trigger on the order asked around here and nobody here raised any concerns about this stuff until now (which i see as a good sign figuring my inbox wasn't spammed with a lot dude no wtf comments) as well as threw a few prompts to chatgpt about this sighting my concerns about using such materials. It sighted it would improve overall stiffness. Why other manufacturers don't offer grade 5 I can't say for sure. I'm assuming that this will not be a light as a feather build as much as lighter than it would if it were all steel with the benefit of no rust and being stronger than aluminum.

in a worst case I'll be returning a few parts and switching back to 16. Still this will be fun to figure out together. And I'll be about to document the journey along the way. I am not a mechanical engineer but my guess is the larger diameter tube design Brompton landed on was the cheaper way of making the frame instead of switching to a stronger grade and keeping the original shape of the frame at scale. That's my guess at least and definitely a wild one we shall soon find out about.

Jipe

Grade 5 is indeed stiffer than grade 9 but its mechanical properties aren't the same as the steel used for the Brompton steel frame.

The reason why frame manufacturers usually do not use grade 5 (or only for some parts like the bottom bracket or other CNC made parts, not tubes) is that tube manufacturing is more difficult in grade 5.

For the wheel size, the drawback of 16"/ETRTO349 is that its mainly used for the Brompton and its frame limit the tire width to about 35mm. For this reason most tires are 35mm wide or less. The widest available tire I know in ETRTO349 is the Greenspeed Scorcher (and TruBlu) that are race tires made for the Greenspeed race trike.

To have wider tires, you must switch to ETRTO355 (named 18" even if the rim is only 6mm bigger that the ETRTO349) for which there is the very good Schwalbe Big Apple 50x355 and many wide BMX tires.

V1LLAGE_ID10T

So it's uncommon to use grade 5 due to it's various difficulties. Makes sense. As I mentioned before in a worst case I can still return parts. Soon as the frameset arrives I'm going strait to a place that custom builds titanium bikes too. I initially asked for their help in building but thought trying to do folding bike parts to be out of their specialty. They said they'd be more than happy to inspect it. I think I'll check with them too on some details about this. Worst case I return the parts and trade for 16s

univega.duder

Nice comparison of the Siggi and Helix. I agree the Helix is excellent and more compact than the Siggi. It also manages to be lighter while having larger wheels. I'm curious how much weight a rack would add to Helix vs Siggi. One thing I do like about the Siggi is the option of a belt drive. Not many folding bikes offer that at this point.

V1LLAGE_ID10T

So after some more feedback, I'm asking the vendor for some certificates and going to also ask for additional pictures of the process to ascertain the level of quality and strength.

V1LLAGE_ID10T

So I got back from the vendor some certifications of the material which has restored my confidence quite a bit. More pics will come of the frame later. In the meantime though they're will be a lot of waiting until parts arrive. Meanwhile looking for recommendations on a decent bottom bracket for this ride!

V1LLAGE_ID10T

So I got back from the vendor some certifications of the material which has restored my confidence quite a bit. More pics will come of the frame later. In the meantime though they're will be a lot of waiting until parts arrive.