Two derrailours on one shifter

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xarvox   100 mW

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Two derrailours on one shifter

Post by xarvox » Feb 09 2014 6:03am

Hi all!

Im building a social tandem trike with two rear wheels and i have some questions that needs answers.
The project is for a family who´s daughter has special needs, so she cannot use any brakes or gears due to congnitive dificulties, witch further complicates the project.

Since it will be a rear wheel drive, it will require some constructive thinking regarding the power transmission and gears, but also a way to brake both wheels with one brake lever.

The setup is as follows:
Two sets of pedals rotates an axle (located under the seats), one or both sides has a freewheel. I call this axle a collecting or front axle.
The rear wheels are both chain driven and connected to the rear output axle (no freewheel).

Option A means installing a internal gear hub with aditional sprocked fixed on its output part (instead of a rim), the gear hub would be installed between the both above mentioned axles, so it would power both wheels regardless of who´s doing the pedaling.

Option B would be dual external gears on the rear wheels, possibly with only one axle between the rear wheels and the pedals.
This single axle would have 4 sprockets, 2 for the pedals (one or both with freewheel) aswell as 2 fixed sprockets for the rear wheel chains.

Option B would mean alot (!!) less work, lighter weight and simpler mechanics exept from one big problem; how to control two derrailours from one shifter (the handlebar mounted gear controller, whatever its proper name is).

My logic dictates that STI derrailours are indexed so i could, at least in theory create a Y-cable to solve this.
But its not totally unusual that reality fails to follow my logic so therefore im writing to ask if anybody else has tryed this.

Regardless of what option i build, the rear wheels will have freewheels.
This will allow only the slower wheel to be powered when turning or while driving on slippery surfaces.
This means that the inside wheel will be powered while turning, rendering the turning radius a bit big, but i simply cannot afford a rear differential that probably would be a better option.

The second, lesser, problem is the rear brake assembly.
I would very much like to control both rear brakes from one lever, since i would need the other lever to control the single front wheel brake.
The idea is similar, a single wire from the handlebar that splits up into two behind the seats.
This could be done in two ways, eather fix the wires with a cable lock (two screws with paralell plates that locks two or more wires onto eachother) or with one cable between both rear brakes and a pulley system that pulls on the cable with a wheel or a hook as found on many 90´s mtb brakes.
Cable lock:
the cable hook (top of picture)

I know that i wont be able to perfectly balance the brake forces but i think that ok is good enough, since this wont be a speedster.
I just dont want it to turn in any direction while braking.
This would probably mean constant adjustment of the individual brakes but i could live with that as long as i dont have to use two levers.

As a side question, im looking for pushrods, connectors and such that i could use for steering, where do you guys purchase such things?
and whats the required dimensions on such things? i figure the forces arent huge, but still, i would hate to loose steering due to pushrod failure..
The trike will have two handlebars and both can control steering, so the control mechanics looks like a T when looked on from above, with front facing down.
..see the 3d-pdf below for specifics..
Has anyone used pull-pull wires for steering?
I figure it could be a good idea to use pull-pull-wires since that would allow me to better hide the control mechanics..

Im including a link to a 3d-PDF that shows my project with the above mentioned option A gear solution, save the PDF and open with acrobat reader since 3D wont work on any of the browsers ive tested..

As i mentioned earlier, im building this thing, here´s a recent build picture :)
the whole thing was created in cad so if anyone wants to build one for themselves, just drop me a message and i´ll share my work :)

Tench   100 kW

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Re: Two derrailours on one shifter

Post by Tench » Feb 09 2014 7:32am

You could use hydraulic brakes on the rear from a single master cylinder which would work or connect the 2 brakes with a single cable and pull this with a single cable from the lever. Hydraulic brakes does not mean they have to be discs! Magura do a rim brake.
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xarvox   100 mW

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Re: Two derrailours on one shifter

Post by xarvox » Feb 11 2014 9:25am

Hos would one connect the pipes for a hydraulic brake system?
I assume that not many have this problem, so i doubt there is a over the counter solution available..?

I think hydraulic brake systems are a better option but unfortunately they are a bit beyond this projects budget..

But still, the brake problem is the lesser problem, my biggest, and most pressing problem is the gears and shifting.
the build isn't exactly lightning fast, but we're soon getting to the point of no return with the gears.
So if anyone has any experience, or the facility to test my single gear shifter with two deraillours, pretty please (!) let me know!

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teklektik   10 GW

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Re: Two derrailours on one shifter

Post by teklektik » Feb 11 2014 10:54am

Here is a quad built by Lightfoot Cycles for an accident victim that uses dual derailleurs.
lightfootcycle_annotated.png (191.1 KiB) Viewed 740 times
It's also mentioned in this this thread. You might get their comments on the design issues that you are having.

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Herrsprocket   100 W

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Re: Two derrailours on one shifter

Post by Herrsprocket » Feb 11 2014 10:55pm

Or you could do something I am doing now.

For the rear derailleurs, use a device from Problem Solvers that takes a single line from your shifter, and splits that off to two shifter cables, going one to each derailleur. The device will pull evenly for both derailleurs, keeping the shifting synchronized.

Problem Solvers also has another solution for the dual braking issue. They sell a single brake handle that will pull two brake cables simultaneously and equally. This a fairly inexpensive fix for the brakes. The derailleur fix is a bit more.


xarvox   100 mW

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Re: Two derrailours on one shifter

Post by xarvox » Feb 19 2014 3:28am

Herrsprocket, the only thing i can find on that site is a cable splitter, that connects 2 cables with one (or the opposite)
Alltho their solution looks fancy, i fail to see why it is better than the wire(cable) lock i mentioned earlier.

Thank you guys for helping me decide, i now feel confident that two derailours are possible without any big issues!

I had an idea regarding brake balancing, all though its very difficult for me to explain it in english.
But its pretty much like this: a brake cable (from the handle bar lever) is fastened in the middle of a aproximately 5-8cm long pin, and the brake cables that connects to the brakes are connected to the tips of the pin, one on each side, so if one brake is heavier to pull, the exess force is directed to the other brake.
This would work similar to a balancing scale (as the justice symbol).

Since the brakes would be exactly the same, with the same, but mirrored cable installation, i would assume that it would be nearly identical resistance on both sides.

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