Index SRAM Dual Drive DD3

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Triple Geared

3 speed hubs are among the earliest versions of a bike transmission, and adding a cassette & derailleur (Hybrid) was almost a natural progression. Sturmey-Archer, Sachs and SRAM all have Hybrid models on the market although the SRAM Dual Drive DD3 seems to have the widest use in America. Coming standard on some of the more popular HP Velotechnik models they often eliminate the need for a front derailleur.

You'll be pretty busy shifting if you plan on having 90 gears.

Sachs was acquired by SRAM in the late 90's and as they say on Star Trek "resistance is futile" like most other corporate assimilators. SRAM made improvements on the original Sachs design including gear tooth contact and improved manufacturing techniques that reduced friction and extended life.

The SRAM DD3 comes in 24, 27, and 30 speed ranges depending on whether its equipped with an 8/9/10 speed cassette. Combined with a triple front, the DD3 with a 10 speed cassette offers 90 gears. Of course you're likely to leave the DD3 in the middle and only shift it to low or high on really steep hills. But its nice to have when you need it.

Inside the DD3 is a single set of planetary gears (in carrier, 8), a sun gear (on shaft, 11) and a ring gear (inside 8).

Cassette hub (16) drives the planetary carrier (7).

The sun gear (11) is fixed to the axle. The ring gear is fixed to the rotating outer hub(4).

In low gear the planetary engages the sun gear. Cassette 1.00 : Hub 0.73 rotation ratio.

In middle gear the hub and cassette are locked together with zero losses through internal gears; ratio is 1:1.

In high gear the planetary engages the ring gear. Cassette 1.00 : Hub 1.36 rotation ratio.

The screw shown here is item 19 above. the 3mm threads are fragile and screw into the carrier. Don't over tighten this screw, just snug it and it won't shake loose.

The hub has a range of 186%. In contrast a typical front triple with 30/39/52 chainrings has 173% range. Losses in the SRAM DD3 only occur in high & low ranges and are very little, on the order of maybe 1%. Internals are lubricated with grease and according to SRAM needs no service.

SRAM does not sell the DD3 hubs in the USA although there are a few sources who import them, and they are sold in Canada. Spare parts are hard to come by, even in Europe.

While many of these hubs have endured long lives few serious long distance touring riders will want one. Lack of parts and reports of failures when misused don't bode well. Most common failures are due to incorrect shifter adjustments. If the planetary is not positioned precisely it tends to jump - either 1-2 or 2-3, and when it jumps the gears only have the edges engaged. Over time gear-jumping causes the edges of the teeth to grind off from the sides.

That said, so long as the shifter is adjusted properly, its highly unlikely that an 8, 11, even 14 speed transmission will be as stout from the planetary gearing components. Fitting more gears translates to thinner gears. That's especially true when driving Gear Inch ranges into the teens.

SRAM uses a Clickbox with a window to see the adjustment point. On a bike or a folding trike the plastic Clickbox is vulnerable to damage.

The cable pull for the hub transmission is a bit unusual and does not match any common shifters, so you're pretty much stuck with the OEM options from SRAM. Some folks have tried using Shimano Bar Friction Bar Ends, but the lack of proper indexing will almost certainly lead to a reduced life.

SRAM offers a single twisty along with a combined trigger & combined twisty-thumb button shifter.

While it sounds pretty negative so far, the DD3 has some really bright points to consider. Shift at rest or while pedaling the SRAM won't leave you wishing you shifted into granny when its too late. Replacing the front triple with a Dualdrive will allow a short cage, which is a huge benefit for those with 20" wheels.

If you're looking to extend the range of a full derailleur system, the Dualdrive is a great option. A typical 27 speed with a 500% range is extended to 930% range. As far as Gear Inch ranges go, the only place SRAM makes any mention of chainring sizing is in the service manual with a range of 33-38 teeth. Lots of folks have dropped lower then 33 teeth, some running in the 24T & 26T range.

There is about 1 lb of weight penalty with a Dualdrive, which is close to what 2 chainrings & front shifter weigh. It seems that SRAM feels they make really heavy front derailleurs and chainrings as they claim the Dualdrive will actually cut about 250 grams of weight.

Bottom line is the SRAM DD3 makes an excellent choice for those who prefer the efficiency of rear derailleurs and want the widest Gear Inch range possible.