Jipe

I think that the most compact folded and easy to fold with 20" wheels is the Birdy that even if most Birdy have 18"/ETRTO355 wheels has a frame that accepts 20" wheels and there is one Pacific Cycles model factory equipped with 20"wheels and 32x406 tires.

The advantage of the fold of the Birdy is that going to 20" wheels doesn't really increase the folded size while a Brompton with 20" wheels like the one of Kinetics is inevitably bigger folded than the Brompton with ETRTO349 wheels.

Schwinnsta

Bernds, but I think only in Germany.

Winfried

Looks a bit messy

Jonesandrew

It's possible to build a nice compact 20" bike which folds exactly like Brompton by making the main frame tube(s) asymetric as I've done below.

Jipe

This is exactly the concept of the Birdy: the rear wheel folds besides the main tube, not right under the main tube like the Brompton.

To keep the folded width as small as possible, the main tube of the Birdy isn't round, it has a complex shape, thinner on its bottom part.

On the Birdy III the main frame s made of two soldered hydro-formed aluminum plates.

On the gen 3 titanium Birdy its a tube with an asymmetric shape that has a partly flat left side.

Schwinnsta

Impressive work. Why did you choose to go with asymmetric top tubes rather than bent "hump" as with the Brompton?

tcs

Ah, snaking the main frame around the folded wheels in the manner of Dr. Hon's 2007 patent (never put into production).

tcs

More like the Hummingbird, in that the front fork/wheel does not fold, and not so much like the tri-fold Andrew Ritchie design.

tcs

Different ways to measure, different critical parameters, different definitions of 'easy', etc. Factory folded dimensions:

Birdy 72x60x33
Caracle 65x48x33
Nyfti Raio v2 75x47x35

Jonesandrew

Yes, I had high hopes that the Curl would be built with 20" wheels, but after waiting about 10 years for my hopes to be dashed I decided the only way to get what I wanted was to build it myself.

Jipe

It seems to me that such a snacking tube frame will be unbalanced with more weight on one side than on the other.

Could this influence de ride ?

Could-it be a reason why Dahon didn't use this concept on the Curl ?

Now, about the wheel size, I have had the same bike, my Birdy, with ETRTO349, ETRTO355 and ETRTO406 wheels and honestly, there is almost no difference between these 3 wheel sizes using the same tires type (in my case, Continental Contact Urban 35x349 and 32x406 that appear to be 35mm wide on my rims). With the 32x406, I was feeling slightly more inertia than with the 35x349, the Birdy was slightly less dynamic/responsive (but the difference is really small).

What really make the difference, is the tire width, wider tires are the best. Of course, like always, tire performances has also a major influence.

So, the fastest solution was ETRTO349 not because of the size itself but because I was using Greenspeed Scorcher 40x349 tires (much faster than supposedly very fast Schwalbe Pro-one 28x406 road race tires).

The best tradeoff is with the Big Apple 50x355.

tcs

Not by enough you'd notice.

Jonesandrew

Because this way yielded a more compact fold than the "hump" arrangement. In this comparison shot I think you can see the bike is only slightly bigger than the Brompton.

This is the second bike I built. It came out about 1 kg lighter than the first one.

Jonesandrew

I certainly haven't detected any problems with the ride which I could attribute to the additional imbalance. The additional off-centre mass is very small in comparison to the combined mass of the rider and the rest of the bike.
I've no idea why Dahon didn't continue with the concept. I (obviously) thought it was a great way to achieve a really compact fold!
The main reason I wanted to use 406 wheels was to get somewhere near a 500% gear range. I happened to own a Dahon with SRAM Dual Drive set up, so that became the donor bike for the first build. I went with full derailleur on the second one in an attempt to reduce weight.

2_i 

I have 818% on a Brompton with 16" wheels, so the range is not a good argument . Trying something new obviously is.

tcs

Brilliant work.

Jipe

OK, I understand.

Now, the Dual Drive and spare parts for it are not available anymore and the clean and easy solution to have 500% is to use a Rohloff hub (526%) that fits in ETRTO349 and ETRTO355 wheels (see the Kinetics Rohloff upgrade and the Birdy Rohloff sold by both Riese & Müller and Pacific Cycles). The gear inches obtained with a 54x13 transmission and ETRTO349 or ETRTO355 wheels are excellent.

So ETRTO406 wheels aren't a must to have 500% gear range.

Jonesandrew

Firstly, apologies for continuing the deviation from the thread topic, but I'm intrigued to know how you managed to achieve 818% range on a Brompton. Can you share the basic details?

I'm not an athlete and I'm not a particularly strong rider either, so I'm always very keen to convert as much of my energy as possible into forward motion. Consequently I like to avoid using less efficient internal gear systems and prefer derailleur transmissions which also help to keep the overall weight of the bike down. In addition I wanted to get a top gear around 100 - 110 inches from the derailleur which would have been much more challenging with the smaller wheels.

Jipe

The Rohloff efficiency measurements have shown that it has an efficiency very close to a derailleur solution (especially when taking into account the fact that derailleur transmission efficiency drops when the system isn't perfectly clean and lubricated) and much better than the Dual Drive in gear 1 and 3 (only gear 2 = direct drive has a high efficiency).

The Rohloff also weight less than a Dual Drive + derailleur transmission.

Its only drawback is its high price. But for riders using their bike daily, on the long term, the Rohloff become also economically valid because it last much longer than any derailleur and the unique cog and chainring are much cheaper and last much longer.

tcs

Although the Sturmey-Archer CS-RF3/CS-RK3 hubs and spares are available. Sturmey has offered hybrid (hub+derailleur) gears since the 1930s.

tcs

One German report I read listed Rohloff @ 87% in 1st gear and 94% in 8th gear. Another listed 92.5% in 1st and 97% in 8th. In an entirely different report, the Sachs three-speed hub was measured @94.5, 95, 94%. There seems to be disagreement in results among various test rigs, disagreement over input power used to measure efficiency, and disagreement over using continuous power from an electric motor or pulsed power to mimic a rider's legs. If you have a link to a published test of the Rohloff and the Dual Drive in the same trial on the same test apparatus by the same personnel, I'd invite you to share. Thanks.

Jipe

Actually, the loss on these transmission has an absolute part independent of the transmitted power and a relative part which is a percentage of the transmitted power.

If the transmitted power is low, the absolute part is a higher percentage of the transmitted power and explains lower total loss in percentage on some measurements. Some other measurements give two curves, one at low power, another one at medium power.

All reports showing several IGH show that the Rohloff is far above the other common hubs like Shimano Nexus 8, Alfine 8, Alfine 11, Enviolo (this one is y far the worse) and the the Rohloff is not far below a clean derailleur transmission.

Indeed, the efficiency depend of the gear but its the same with a derailleur, smaller cogs have a lower efficiency.

Now the advantage of the Rohloff over other IGH is that it allow to change gears under load (not as smoothly as with a modern derailleur but it works) which is not possible with other IGH with which the load must be interrupted a short time while changing gears what represents another kind of loss of efficiency.

The drawback of the Dual Drive concept is that it add the losses of a 3 speed IGH and a derailleur.

The current Sturmey Archer company has little to do with the historical one since the company was bankrupt and take over by Sunrace.

Indeed, SA has an equivalent hub to the old Sachs Dual Drive discontinued by SRAM who took over Sachs but it doesn't help much Dual Drive owners who need spare parts for the Dual Drive (like the push rod).

What I see is that there were many bike brands using the Dual Drive (on recumbent for instance) and only few of them replaced the Dual Drive by the SA hub, maybe because this concept is less needed with the current wide ratio big cassettes ? Riese & Müller had a Birdy with Dual Drive replaced by the current Birdy Touring with a 9-32t cassette that gives 356% gears ratio which is enough for most users (Pacific Cycles moved to the SA hub combined with an obsolete 8s 11-30 cassette).

2_i 

There is a thread here on the primary development. The essential part is a triple crankset 50-34-20. A Suntour derailleur is mounted on a Litepro adapter, both highly tweaked. It is operated by a SunRace shifter. The drivetrain modification fits into the regular Brompton fold. The added low gears are needed, in my case, for winter and for a mountainous surrounding I take my bike to.

In what way do you think your bike allowed you to expand the gear range better compared than Brompton? (I admire your bike anyway, of course .) How good is the standard derailleur in gathering slack, when you have the two rings in the front? When I tried to put a standard derailleur onto Brompton, I found its tensioner to work marginally in the needed gathering of slack, compared to the standard Brompton tensioner. I tried chain catcher, as in Birdy, but it was largely leading to nowhere. Humbled, I went back to the Brompton pusher arrangement

tcs

Well, other than continuing to build many of the same designs and offering repair parts for hubs going back to the 1930s.

Jipe

The Birdy catcher, a kind of extension of the derailleur cage, wasn't working well neither on the Birdy. The chain was often falling of the chainring when unfolding the bike.

Since Birdy III there is a totally new concept of tensioner dedicated to the tensioning of the chain while the bike is folded, the derailleur or a Shimano or a Rohloff chain tensioner being used to tension the chain while riding.

This new concept is a parallelogram type arm attached around the bottom bracket box of the frame. It works perfectly.

The same concept could work on a Brompton with rear derailleur.