Recumbents and all feet forward machines
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I'm confused about setting up an internal gear hub, like the SRAM dual-drive, imotion or Shimano Nexus.
I see on the Greenspeed site that the SRAM DD is used in conjunction with a 9 speed cassette and derailleur to give (3X9)X3 - 81 speeds.
...but what do you do if you use a 9 speed imotion or Nexus hub?
Do you still use a cassette and rear derailleur and if so how many speeds do you use in conjunction with the hub.
I see the GT1 just uses a Shimano 9 speed, but I don't think I'd survive my rides with just 9 speeds to use.
I quite like the idea of a 243 speed trike (3X9)X9
Please bear in mind I knew virtually nothing about bikes (cars are my passion) till a year or so ago, so I'm on a steep learning curve here.
SRAM Dual Drive is different to other internally geared hubs in that it is designed to replace the front derailler ie the three speeds in the hub replace three chainrings at the front of the bike. So you have a (usually nowadays) nine speed cassette and rear derailler combined with three speeds inside the hub.
The others - Rohloff, Shimano Nexus etc are meant to replace deraillers all together, so only afford a single sprocket, so you have however many speeds the hub has to offer inside.
I hope that makes things clearer for you
Her beauty in the moonlight overthrew you
As has been said internal geared hubs are designed to replace an external cassette setup - giving cleaner lines with a single cog front and rear. The SRAM dual drive is a hybrid of both systems, with 3 internals and an 7/8/9 spd cassette on the outside. The idea behind internal gear hubs is to get close to the usable gear combinations of a tradition derraillier system - the Rohloff 14 spd internal is probably the closest approximation (and unfortunately also the most expensive) - I use a rohloff on touring and mountain bikes and don't notice any loss in available gear ratios when compared to a 3 x 9 (27 spd) derraillier setup. Working backwards from the rohloff in terms of performance general opinion seems to be the I-motion 9 spd, then Shimano alfine / nexus 8spd, Sturmey Archer 8 spds....
Apparently there is also a shimano 11 spd Alfine in the soon to be released category. If your trying for a broad range with internal gears on a budget one option is to set up a 9 spd i motion with a double chainring. I have set this up on a touring bike for some one ad as long as you spend some time with a gear calculator and play with rear cog and front cog sizes you can come up with a workable range with minimal crossover. Harris cyclery have a nice imotion calculator on their site.
To run a double cog setup with the imotion you will obviously need a front shifter mech and a rear tensioner to take up chain slack between front cog sizes. Shimano make a nice alfine tensioner, but any old short arm derraillier will do the job as long as you line it up.
Hope this is food for thought
Gears are all about ratios. If you have a 34 tooth at the front and another 34 tooth at the back, for every revolution of your crank, you get one revolution of the rear wheel. If you shift the rear to a 17 tooth cog, then you will get two revolutions for every revolution of the front. If you write down the numbers for the front chain rings and rear cassettes, you can work out the ratios. On a road bike with 50/34 at the front and a 9 speed cassette on the back, you really only end up with 12 gear ratios, because the other 6 double up (or near enough to) - eg 50 front, 25 back is the same as 34 front, 17 back.
On a mountain bike with "27" gears, say 48/38/28 front, you may only get 13 or 14 useably different ratios. Likewise with a dual drive hub and 9 speed cassette, you still may only end up with about 13 distinct ratios. So, an 8,9 or 11 speed internal doesn't do too bad, and while the steps between the gears are often larger than you can get out of derailleurs, they tend to be designed to be very even over the whole range, which is good. As a previous poster mention, a 14 speed Rohloff will probably end up giving you as many or more usable ratios than your typical 27 speed triple derailleur setup.
Yes, you can setup a dual drive with a 9 speed rear cassette and a triple front for "81" gears, but if you do the sums you'll find it is really about 18 distinct choices all up.
Your iMotion 9 basically has the similar spread of gears as your typical 18 speed MTB. The Shimano Nexus-8 is similar, except you lose either the top or the bottom gear depending how you set up your gear ratios. I did a brief summary article about it here: http://raleightwenty.webs.com/regearingthetwenty.htm Basically, I've listed all the available internally-geared hubs and given a basic description or commentary about them. It's not complete yet, but I hope to be able to publish something a bit more detailed in the near future. I'd also welcome more user feedback from people who have tried these hubs.
It's still incomplete, but I'm postponing any updates until after I've completed this semester's exams (about 3 weeks).
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