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I am planning to build an electronically controlled automatic bike transmission for my senior design project. I'm personally more of an electrical guy, so dealing with the mechanics is somewhat new to me. I would have loved to use an off-the-shelf electronic derailleur, but alas, they are too cost prohibitive (the school has allotted a budget of $115 per student and we're a group of two, so we have $230 to spend). So I'm going to need to make my own like several other people have done.

I am planning on using the bike's current derailleurs and replacing the hand shifters with motors of some sort to pull on the cables. Of course, we're going to need a certain amount of torque to shift up (and much less for shifting down) and a holding torque to ensure that we don't accidentally slip down a gear.

I have not bought a bike yet, I'm looking at a Diamondback bike that I found on Craigslist (it looks like a hybrid bike). So I'm wondering if there are any average measurements for the required torque.

And of course, I'm sure there are many people more experienced in bicycling than I am. So if you have a better solution, please feel free to shout it out. I'm all ears.

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One of the IEEE publications, I'm thinking "IEEE Computer", included a "hobbyist" article maybe early this year where a guy did an electronic shifter. He managed to put a stepper motor on the derailer itself without much difficulty. You should try to look it up. –  Daniel R Hicks Oct 3 '13 at 22:45
    
Maybe I'm just being dumb, but if you're pulling a cable linearly, wouldn't it be an issue of force rather than torque? –  amcnabb Oct 3 '13 at 23:07
    
amcnabb: yes, but even that approach will almost involve a winch drum, so it comes back to torque in the end. But I suspect the questioner is on the steep end of the learning curve so hasn't thought that far yet. –  Mσᶎ Oct 4 '13 at 0:01
    
How will you shift gears when the rider is applying pressure to pedals? What will happens when the gears shifts while you stand up on the pedals or going down hill on flat pedals? –  Look Alterno Oct 4 '13 at 9:32
    
Its just a design project for school, so there's no need to for it to be practical. You might consider a rachet mechanism to hold the cable in place when not shifting-- that's your "holding torque". This is the basis of "indexed shifters". Alternatively you could hold the cable with enough friction that it doesn't slip and the motor then just has to provide enough torque to overcome the friction only when shifting (that's how "friction shifters" work). –  Angelo Oct 4 '13 at 11:36
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From working on bikes, the amount of force needed to pull a gear cable is more than I can apply gripping the cable with my fingers, but is easy to apply gripping the cable with pliers. So probably about 10-20N. But almost all that force is required to oppose the return spring in the derailleur. Remove that spring and the force drops by an order of magnitude. And as Kibbee says, you can use leverage to change that force (but you still need the same amount of energy).

The easy way would be to use an existing gear cable and gear lever attached to the rear derailleur. That way you don't have to mount your motor right down next to the rear axle, where it's vulnerable to shock and dirt.

If you use a winch drum attached to a stepper motor it'll be fairly easy to make everything work. The issue with that is that the holding torque will almost certainly be greater than the stiction of the motor, so you would need power to stay in gear. Or gear down the stepper motor, or use a mechanical brake with a solenoid to release it when you want to shift. But that's possibly more complex than a student project can easily build.

A different way would be to use a common two-lever shifter with a motor to push the levers when you want to change gear. That's a low-precision task, but you will need considerable force. If you used a fairly hefty RC servo motor I think it could work, and within budget. Then drive that off a microprocessor that has a PWM output to make the servo work (there are Arduino libraries that do this OOB, it's not hard)

A much more elegant way would be to modify a derailleur directly, removing the return spring and using a motor to move it backwards and forwards. This is what the electronic shifter setups do. If you use a stepper motor and drive "off the end" to position it, you should get reasonable reliability at low cost and without having to build sensors and servo rigs to locate the chain exactly on the drive cog. I fear you might need a brake again, but I'd build the first version without it. At worst, you can do your final report saying "it works but needs a lock to hold it in position over bumps".

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You can use pulleys or some other mechanical mechanisms to decrease the amount of force needed to change gears. Think of it like this. If you're controller is operating a standard down tube shifter, it has to provide less force if it's pushing against the top end of the shifter, than if you're pushing against the bottom end of the shifter close to the axis of rotation. The smaller force will be applied over a longer distance, so the same amount of work will be done, but requiring less force. –  Kibbee Oct 3 '13 at 23:28
    
comment deleted –  Mark W Oct 4 '13 at 13:03
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If you cut the spring in your mechanical gear system, gear will change without an important torque. But i don't know the strong needed to move the derailleur.

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What do you mean "will change without an important torque"? –  Batman Jan 10 at 3:26
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