Wow - that's an interesting idea. Can you (or anyone) tell me what the advantages/disadvantages would be over switching out rear derailleur, cassette and chainrings? Sorry, again, I'm not tech-savvy. According to specs, the rear wheel is 135 mm.

If you were going to build a SRAM Dual Drive disc wheel, you wouldn't have to switch out anything and would have a fairly large range with 54 gears and plenty of overlap.. you would need an additional shifter for the hub.. so you then would have three shifters (hub, rear der, front der).. probably overkill.. most dual drive users use a single chainring.. so you could use your existing setup, lose the front der and shifter, lose one chainring, reuse the rear cassette and you have a Dual Drive Formula with big gear range.. downside would be added weight in the rear wheel ( you'll feel that ) , but other than the perceived efficiency difference of the hub combined with cassette, it would work well and solve your gearing issues.. but it will cost more than $250 installed..

Thanks, Bruce. I assumed if I put a hub on the rear wheel I'd dispense with the 2nd chainring. Info your provided is helpful.

Do you have the bike yet?

I'd take it out for a spin before planning hundreds of dollars worth of upgrades. You may find the small tires gives you a significant low gearing boost that you hadn't anticipated.

I have an Air Friday that came with the Sachs Dual Drive (which has a 7s freehub, but mine came with a 9s cassette cut down to 8s, and Ultegra 3x9 brifters).

However, I've only done about 20 miles. My brifter setup still seems a bit awkward.

According to the specs (p 18), the DD3 hub has gearing: 73%, 100% (direct drive), and 136%.

One of the advantages of it is the "Overdrive" which is a significant benefit for folding bikes that often suffer from too low of gearing.

If going with the IGH hub upgrade, I'd also consider maybe an 11s Alfine.

Thanks for your input, Clifford.

I did a test ride at the store in New York. At the store location there is a fairly sizable hill for New York City, and the stock low gear handled it fine, but where I live and work, hills are steeper. I am basing my target low gear-inch on my experience with my Brompton. It has a Schlumpf Mountain Drive installed, which gives the bike a range of 12 evenly spaced gears - 16.1-87.2 gear-inches. I can dispense with the two lowest gears, and am aiming for around 20 or low 20s. Now you just brought up something I hadn't though of. My Brompton has 16" wheels, while the S18 has 20" wheels. I didn't think that made any difference in comparing gear-inch range, though. Please correct me if I'm wrong. We're comparing distance travelled in one turn of the crank.

The distance travelled is a function of the circumference of the wheel, where (circumference = pi * diameter)

The tire sizes are approximate outer diameters.

So,

16" vs 20" would make some difference... you can multiply the Brompton calcs by 20/16, or 1.25, or the equivalent of 1 or 2 gears higher.

Comparing it say 27", one gets:

27/16 and 27/20 or ratios of 1.7 and 1.35 respectively. (or the inverse, depending how you think of it).

So, a 11/50 gearing would be 1.7 times harder on the road bike than on the Brompton.

And the equivalent gearing of a 11/50 on a road bike would be a 11/85 on the Brompton (and thus the use of some very large chainrings on folding bikes, or alternatively, very small cassette sprockets such as the 9/26 Capreo.

Sorry to be dense, Clifford. I calculated gear inches for each bike in two ways: 1) Sheldon Brown's Gear Calculator which takes the different wheel circumferences into account, and 2) physically rolling each bike one crank turn and measuring the distance travelled and dividing by pi (3.14). Results were identical within fractions. So aren't I comparing apples and apples when I compare the two bikes' results?

Yes,
If you are using calculated gear inches... I think. Although an earlier discussion seemed to indicate a discrepancy in one of the calculations somewhere that I didn't track down.

If you are just going by cassettes and chanrings, then you need to have some kind of a fudge factor for wheel size.

I believe gear inches are as you stated and is a calculated value with respect to all parameters which is why you can compare different bikes with same cadence and distance traveled.

Putting in the 3 speed rear hub without FD is back to a Mu P24 without disc brakes and slightly wider tires. However the Mu P24 is apparently discontinued and I had a hard time finding my second one for my wife.

Test rode the bike again at the bike store today, followed by a test ride of a Dahon MuP24 at another store.

I have to say, the gearing on the MuP24 is excellent. It's gear range is huge - more than I need - I can't imagine ever needing the top two gears. Shifting was smooth, and I do love being able to shift the hub gears without having to pedal. However, all that said, despite being able to adjust seat and stem height (and handlebar angle), the bike puts me in a position that just doesn't feel as sporty as I like. The handlebars feel wide and contributed to that. I felt like this is a nice bike for liesurely cruising, not the semi aggressive riding I like to do. It's a really nice bike - just not the right one for me.

The Formula S18 feels like a different animal. It feels light (it is lighter) and nimble. You can tell it was designed for speed. One thing I did notice this time was that, while riding on the street is smooth, riding on a path made of pavers was quite harsh. You do feel every bump. I'd stick to street riding with this bike, which is its main function for me. (I do wonder if I could switch to cushier tires if and when want to ride a pathway made of pavers, as is one of my favorite weekend leisure/exercise rides). I also would need to practice modulating the disc brakes. I love how lightweight the bike is.

While at the bike store I inquired about the Vector P24 some have mentioned here. Gearing is wide and includes the low gears I'm looking for. Apparently they have one in their warehouse. I made plans to test ride it next week.

That's where I am now. Leaning more and more toward keeping the S18 and making the mods.

My Speed Pro TT has a DualDrive setup(stock). It does give a nice wide range,and you can shift the 'triple' at a stop. But it's heavy(3lbs 11oz for the wheel w/cassette),you need to carry a 15mm wrench for the wheel nuts,and you have a shift box that sticks out the side. Between the weight and the box sticking out the side(not covered by the fold),I really kinda wish it had a double or triple front instead.

Sachs 7x3 speed dual drive (made before the SRAM Buyout) used their pull chain to shift the 3 speed ..

they were also the first supplier to Brompton for the 2 cog 3 speed , driver used the 3 spline AW3 cog interface.

so the 3/32" thinner cogs work on the 3 speeds too I use that 15t

to shift 1 in 5 teeth are shorter ( 4 full height. )

What size tires can you fit on the S18? That will make a huge difference in comfort and very little difference in speed.

Since writing my last post I checked: I can put tires with a range of 1" to 2" width. I'm thinking if I find the ride a little too rough on my commute I'll buy a set of Big Apple tires which come in 20" x 2". Do you have another suggestion?

I also had a dualdrive with my Mu p24 and felt the extra weight in the rear hub. Sold it. There's a sacrifice though if you switch to chainrings because you lose top end gear inches. A 60t chainring on 406 wheels would max out only around 100 gear inches (unless you use a Capreo). With the DualDrive you get greater top-end.

My kids recently started with 20" tires. To celebrate the occasion, I tested out some decent quality BMX tires that tend to be much less expensive. In short, if you're looking at wider tires, there are lots of options.

I picked up sets of Tioga Powerblock (20x1.75) because of the rim width on their bikes. Long story short, they're supple and fast. My fat ass has them only pumped up to 35-40 PSI. No flats but I'm not putting big mileage on these days. They come in lots of sizes in both a wire and folding version. I see them in wire versions for $15-18 a tire. It's hard to go wrong, IMO.

Re Dual drive. If you do the maths comparing it to a tripple it's not much heavier. it depends on set up but it can be only 1/4 to half a pound if you opt for the dual drive single dual shifter.

Just thought I'd post an update. My bike mechanic did the mods on my S18. We were both happily surprised to discover that he did not have to replace the rear derailleur after all - it works just fine with the 11-32 cassette - not just fine, but well. In addition, he was able to swap out the chainrings (39-53) with no problem - front derailleur works perfectly. Shifting works well and I'm super happy with the gearing now - exactly what I was going for - 22.8-90 gear inches - perfect range for me, and gaps between gears are fine. My mechanic also upgraded a few things on his own while working on the bike - replaced derailleur cables with slicker ones and replaced the ferrules with better ones less likely to cause friction. Also fixed how the bike store had mounted my kickstand. They had used a mounting plate which was totally unnecessary because there is a kickstand mounting hole in the frame which they ignored. Anyway, so far I'm very happy. Thanks to everyone who posted suggestions here, and an extra thanks to you, BruceMetras, for excellent info you provided. I'm loving riding this bike and just sorry it's over when I come to the end of my commute. Oh - and I really love the disc brakes!