Derailleur
 

I was playing with brother's bicycle some, and I was wondering if anyone knew, offhand, of a place where I could find something more than a general description of how the derailleurs work?

The more detail, of course, the better.

I think I've managed to see *how* it works, but I probably ought to start prototyping now if I want a working one of my own design by January ;)

michael, you know you are coming back to smithtown.... ;)

Okay - the specific concern I'm having right now is two-fold.

First, the teeth on bicycle gears don't appear to have the traditional sort of involute tooth profile I'm used to. . . and bicycle chain is some weird kind of pitch.

I'm wondering if the longer, pointier involute tooth profile of traditional ANSI-friendly sprockets would hinder the 'walking' of the chain to another sprocket. But then, according to this year's rules, wouldn't bicycle sprockets and chain be legal on the robot?


Second - I'm still having trouble conceiving of ways of indexing the derailleur so that it'd only shift one gear at a time. A pneumatic or servo driven system would be easy to implement, but that'd only work for two speeds. What if we wanted more? :)

Do any simple mechanical solutions exist for semi-accurate indexed movement?

you might want to go check out/buy a pair of Shimano Rapidfire shifters - they have 2 levers on them - one to shift up, one to shift down. I'm not sure how they work (nor am i willing to take my $1200 mtn bike apart :)) but i'm sure there is something online about how they work.

Good luck.... if you get this to work i can't wait to see it :)

Tom

I would just take the whole sprocket and derailler (spelling?) assembly off a bike and use a servo to pull the cable to shift it. That way you don't have to go through all the trouble of designing your own and making that work. If I'm not mistasken, these systems have a great deal of complexity and preciseness to make them operate correctly.

well, i went on a short bike ride this morning (i do odd jobs around the neighborhood, this week i'm feeding a dog, cause the family went away). as i was riding, i was thinking about how the deraileur works, and this is how it works on a bike (or so i think)...

first, you have your cable, which connects the derailleur to the controls (only slightly important ;)). next you have the derailleur, and finally the controls.

so, let's say you have a 7 speed bike (for simplicity), and you're in gear one, and you want to go to gear 2. you hit/pull/push/twist/turn/etc. the controls (every bike has a different way of doing it, but i'm going to work with mine from now on). one my bike, hit one button, you go up a gear, hit the other, you drop one. this is the kind of control we would want, because then with a small piston, and a servo, or two pistons, or two servos, or however you want, you could make a shifter. what happens is when i hit the button to gear up, it pulls the cable, which pulls the derailleur, which moves the chain, and when i het the other button, it releases a bit of the cable, and the spring moves the derailleur back. now, if we were to rip the derailleur off of my bike, i know i could somehow hook it up in the robot to work, and work it would :D. so, i think as long as we have at least 2 servos, and two small pistons, i could make a derailleur work, if it had the right controls.

if however, you find a derailleur that works because you turn something, and that pulls or releases cable, that won't work, because you'd need a motor, and why waste a whole motor to turn a small amount?

so michael, if you can find the right kind of derailleur, we could make it work. it wouldn't look nice, but it would work. :)

also, all of the above could be completely wrong. heck, i'm a programmer, not an engineer! but, i do know some about the mechanical side, and this has got to be the most i've ever thought about the mechanical side :D.

The basic concept of the derailleur doesn't seem too complex, however, I am worried about what may be important dimensions and way may not.

From what I gathered, the derailleur is really just a spring loaded four bar linkage with an attached tensioning mechanism.

So, the parallelogram formed by the four bar linkage skews to the side to line the sprocket up over the various gear ratios, and it drags the chain along.

The wire is used to keep a tension spring within that linkage stretched, to hold the derailleur in place. By increasing or decreasing the tension in that wire, like you mentioned, Ian, you move the derailleur.

A piston or a servo, however, only has two definite, reliable positions. There are ways of stopping each midstroke or midturn, but it'd a bit inprecise, and may result in a half-shift, or missed shift.

So, the key is a simple mechanical index that gives exactly the right number of positions. We can build one of these using the small electric motors included in the kit. :)

We can't buy a bicycle derailleur, I think, because it designed for a different sort of chain and sprocket than we'd be using in all likelihood. Plus, since it's not a legal kit part, we'd have to design our own anyway :)

reverse?
 

Have you thought about how it will behave in reverse? As we all know, bicycles are only meant to be ridden in one direction and the only time there is any real force on the chain is when you peddle forward (peddling in reverse has almost no resistance). Obviously you won't be able to shift when going in reverse because the derailer can't push the chain to a new sprocket. But will you even be able to drive in reverse? The force on the chain will try to fold the derailer up instead of transfering the torque to the sprocket. Just something to think about.

Mike

In all the years that i've been on my team I can count 3 times that a derailler idea has been brought up. Each time there were two reasons that we didn't do it. First was basically what Mike said. But I think someone on our team said that they came up with an idea to keep this from happening (I have no idea what that was though). Second is that the sprockets have to be relatively similar in size in order for it to shift. If the difference is too great the chain will get caught a lot, especially when downshifting. I've even heard that they have to be so similar that it's not even worth the effort. You could make several speeds like on a bike, but its probably not worth the number of servos/solenoids/cylinders/etc....
Just some food for thought/

I think driving backward with a derailleur would be out of the question. It would probably be easier and work better to make a shifting gear transmission. Also, most good servo's can hold any position accurately.

Okay, I'll concede that, unless I'm about to devise derailleur: part deux, shifting while moving backward is out of the question.

However, I don't think that moving backward itself is impossible.
The problem, as best as I can see, is that when there's resistance against the back sprockets due to, say, the wheels, the torque transmitted by the chain is taken up by the torsion springs in the tensioner part of the derailleur. Thus, you don't move.

But, there's no written rule that says, really, that tensioning mechanism needs to be a part of the derailleur, right? It serves to pick up the slack that is given off because of the smaller diameter sprockets. What if this slack were taken up elsewhere?

There are lots of robots with simple, similar tensioning mechanisms, and *they* can go backwards. I think a derailleur sort of mechanism doesn't preclude the ability to go backwards. We'll see, I guess.

I haven't ridden my bike in a while but as I remember the rear derailleur/sprocket/wheel assembly has some sort of one way bearing. Would the sprokets on the derailleur mechanism be your primary drive gears right off the motor or would something else be driving them?

I've really got to dig my bike out of the garage; this is getting interesting.:D

I've attached a *quick* sketch of what I have in mind. Bicycles have a one way bearing. . .our robot would not.


I've kept the sprocket that traditionally acts as a tensioning sprocket, but I've fixed its position relative to the actual sprocket that causes the gear shift. Thus, there's no tension taken up at that point, and the torsion springs aren't necessary. So, going backward, this ought to keep tension in the right places and make the sprockets turn.

To take up slack, I moved the tensioning mechanism to some other place outside of the derailleur mechanism. Where it goes, really, is irrelevant, so long as it goes somewhere.

Does this make sense?

It still couldn't shift in reverse, but it should be able to drive backward.

pretty colors.....

*thinks it sounds and looks kewlies*

Now if only Small Parts sold bikes, you'd have an idea. :D :p