6wd with double rack-and-pinion steering

[spillerrec]

I haven't really built anything for ½ year or more, so now that I have a bit of free time I decided to start on something.
I have loved building cars from since I started with Technics yet I have never made a 6 wheeled one that was rack-and-pinion steered. (At least I don't remember having done one.) So I'm going to do one now. And why not make it all-wheel driven and steered both in the front and in the back at the same time? (I don't have the parts to make it both suspended and all-wheel driven though. And I'm a total noob with suspension anyway.)

So I have just started with the chassis. Is a differential for each wheel pair overkill or is it just perfect?

Other than what can be seen in the photo I don't have much planned. I don't know where the motors should be placed, nor the NXT (but that will probably go on the top or something).
So one motor will go to the steering and another one for driving the wheels. One motor more to use...
Any ideas, also for what this thing should actually become? Right now the only thing I have in mind is making it into some sort of driving table where I can put stuff on top of it. Or perhaps that adorable rover from Planet51? (I doubt can make it well enough though.) This is purely for fun so crazy or weird ideas are fine too : P

When the contruction is done I would like to try to make my computer and my NXT communicate. Properly just some sort of simple joystick-program using BT messages while having a program running on the NXT. In other words, not direct motor control, but communicating with an running NXT and sending messages like "set speed to X" and such.
Certainly overkill for controlling a car, but I want to get the experience in doing it so I can apply it to future projects. So does anyone know some C/C++ libraries which can be used for this? (Otherwise I will start a separate topic in the Software section.)


Well, the main challenge right now is to connect the differentials together.
I would normally like to have one long axle from one end of the chassis to the other however it doesn't go well with the center diff. However if I try to place it above the diffs, the mechanics to the steering is in the way. (I want to have a bit of clearance so I don't want to place it underneath.)
Having axles from the center diff to the outer diffs seems a bit overkill when considering the distance between the diffs...
If one extra stud of width is added between the diffs a simple gear train can be used:
diff - z24 - z40 - z24 - diff
and then the motor would be connected to the center diff. Seems fairly simple, but of some reason I really dislike the thought of connecting the front wheels with back wheels with a long gear train... Perhaps I'm just allergic over for gear trains?
What are your guys thoughts about the matter?
EDIT:
Quick prototype using gear trains, not really satisfied...

[themindstormman]

LOL. For me a 6x6 is nothing. I've done a 4x4 that could go down stars and transform into any drive like a 2x2 4x2 or even a simple differential. I've also made an 8x8. 6x6 pew thats nothing. Ok besides that i think it is very good and i really like how sturdy it is. my 8x8 tends to look more like a bus than a car. good job. but my 8x8 really had four wheel wheel drive and four wheel steering and it looks like yours is a 4x6. Also both my cars were geared up 1:3 but still had amazing traction.

My 8x8

[jwiger]

Turning the rotation 90 degs as you have is always difficult with the torque that the NXT motors produce. I would recommend seperating the front axle from the back axles. Then develop a solution to build each half, a front axle with steering and drive motorss, then a back end with a simple drive motor. After you are happy with the two halves it's easy to put them back together.

If you split the task of the drive motors you reduce the risk of the two of them working together and destroying some poor little gears.

If you want to maintain steering on the front and rear axles then you could adapt this steered/drive axle from Sariel. I managed to extend the vertical axles off the ends to make it into a portal axle, unfortunately I didn't take any pics of that. I bet you can figure it out though.

[spillerrec]

themindstormsman,
As far as I know it is: [amount of wheels]x[powered wheels]
So that would make it a 6x6 as all wheels are powered. Only the font and the rear is steered as I do not see much sense in turning all the wheels when using 3 wheel pairs.
It is just a prototype so it is not really that study...

Turning the rotation 90 degs as you have is always difficult with the torque that the NXT motors produce.

I'm not quite sure what you mean, could you rephrase this sentence?

I would recommend seperating the front axle from the back axles. Then develop a solution to build each half, a front axle with steering and drive motorss, then a back end with a simple drive motor. After you are happy with the two halves it's easy to put them back together.

If you split the task of the drive motors you reduce the risk of the two of them working together and destroying some poor little gears.

This would work fine for two powered wheel pairs, but for three? I don't want to have one motor connected to two wheel pairs while the other is only connected to one.
Right now the two drive motors are connected directly to each other. I know this isn't a good idea (since I didn't check if the motors had the same speed)... Well, it is a prototype...

It is a nice design by Sariel but I'm not really going to have pendular suspension in this.


I'm not sure if I want to bother remaking this, but if I do I will probably make it 2 studs wider and perhaps 4 studs longer.

[jwiger]

Turning the rotation 90 degs as you have is always difficult with the torque that the NXT motors produce.

Yeah, that was a bad way to say it, let me try again...

I see in the pic that you posted that your motors and your main drive shaft (if you were not using the gear train) would be perpendicular from each other. Two NXT servos together produce enough torque, that building a strong enough transmission here is a bit difficult. -not really advise, just an observation from me.

I would recommend seperating the front axle from the back axles. Then develop a solution to build each half, a front axle with steering and drive motorss, then a back end with a simple drive motor. After you are happy with the two halves it's easy to put them back together.

This would work fine for two powered wheel pairs, but for three? I don't want to have one motor connected to two wheel pairs while the other is only connected to one.
Right now the two drive motors are connected directly to each other. I know this isn't a good idea (since I didn't check if the motors had the same speed)...

At first I thought you wanted to have one steering axle on the front and simple tandem wheels on the back like a typical truck. As far as the motors having the same speed -this isn't a big factor if the motors are tied closely to each other. For example if you have a snort axle directly connecting the two of them, you won't notice any real problems, unless you go back and forth between running for a timmed duration, then running for specific degrees of rotation, then they may fight against each other a bit. Inserting a few float and position reset commands will relieve any stress on them.

With the combination of differentials on each axle, I would recommend a suspension of some kind. Unless you plan on running on flat surfaces only.

There is a good section on pendular suspensions here: Jennifer Clark's all terrain crane. You will have to scroll down a bit but it's a very good tutorial.

[spillerrec]

Ahh, that is what you meant. For these cases I always use those knob wheels as they can't slip out of each other without breaking the whole construction. I used those in my NXT motorized version of the Off-roader, where it raises the whole chassis 3 studs by rotating an axle 180 degrees. And since there was a NXT, 3 NXT motors, a PF battery box and a PF motor, that thing was quite heavy... Youtube link

As far as the motors having the same speed -this isn't a big factor if the motors are tied closely to each other. For example if you have a snort axle directly connecting the two of them, you won't notice any real problems, unless you go back and forth between running for a timmed duration, then running for specific degrees of rotation, then they may fight against each other a bit. Inserting a few float and position reset commands will relieve any stress on them.

I was just a bit worried if they where straining each other when directly connected like that. I'm not really sure if I'm going to keep using two motors as the reason for using two was, well, that there where room enough... I can always put a diff in between if I get enough paranoid.

With the combination of differentials on each axle, I would recommend a suspension of some kind. Unless you plan on running on flat surfaces only.

The main issue is that I don't have enough springs...
However you might be able to convince me of trying to do this with pendular suspension. Sariels design is nicely modular, so if I'm able to make something with steering I can just copy that in the other end. The center piece wouldn't be too difficult. And I can avoid the gear train. : D

I have a weird part which have an axle hole which can be turned 90 degrees to each direction and then there is a spring inside it which pushes it back to its center position. They might work well together with pendular suspension but perhaps a bit too weak. It is worth a shot though... Otherwise I might just buy the Technic buggy, it has 3 springs of an type I already have 4 of so it should be good enough for this. (The back tires rim is the same as the ones I use for this, however in gray so I could swap the yellow rims I'm using for the center tires right now.)

[jwiger]

Ahh, that is what you meant. For these cases I always use those knob wheels as they can't slip out of each other without breaking the whole construction.

Sounds good, that's what I would go with.

I was just a bit worried if they where straining each other when directly connected like that. I'm not really sure if I'm going to keep using two motors as the reason for using two was, well, that there where room enough... I can always put a diff in between if I get enough paranoid.

That is indeed an interesting idea. If you have a 5x7 open frame and a new diff with the 28z bevel gear you could try that out easily.

The main issue is that I don't have enough springs...

Thus is the point of a pendular suspension. I would build the center axle with no suspension and try to keep most of the weight over that. As long as the other two axles are free to move you shouldn't have too much trouble, unless you are going for real ambitious off-roading.

I have a weird part which have an axle hole which can be turned 90 degrees to each direction and then there is a spring inside it which pushes it back to its center position.

Ah you mean this... That is a very handy little part. I would love to have one (or two) -and I have an abundance of springs. I would easily trade if my stuff wasn't in a crate some where between Germany and Texas right now!

[spillerrec]

Thus is the point of a pendular suspension. I would build the center axle with no suspension and try to keep most of the weight over that. As long as the other two axles are free to move you shouldn't have too much trouble, unless you are going for real ambitious off-roading.

I have never thought of dropping the springs completely, that is indeed a nice solution if you don't have any. Since I do have 4 springs of same type I will try to use them on the front and back axle and keep the suspension on the middle axle but without the springs.

Ah you mean this... That is a very handy little part. I would love to have one (or two) -and I have an abundance of springs. I would easily trade if my stuff wasn't in a crate some where between Germany and Texas right now!

Yeah, that is the one. I happen to have these two sets: 3541 and 3540. Each set had the same price as a bag of potato chips after all, it was that cheap. (It was a store which buys up weird stuff and sells it at low prices. And they happen to have small Lego sets once in a while.)


So I have a plan for my next prototype now, or at least for the pendular suspension module. Thanks for all your suggestions so far.

[jwiger]

Not a problem, this thread was/is kinda fun. Good luck, and I hope you post some pictures soon. My stuff will be boxed up for the next 2 months so I have to live(build) vicariously through this board.

[spillerrec]

I'm now done with the first prototype of a steered pendular suspended module:

Of a first attempt on something like this I'm pretty satisfied. This prototype was just to test if I was able to make the construction strong enough though.

Since I do not need to provide steering for middle axles the axle does not go all the way through. It stops and is continued on the other side with the axle from the steering. I did the steering pretty much like Sariel, just with an normal 16 tooth gear instead. Whether if I use the gear or axle for steering I do not know yet though.

My great worry when making this was that the body and/or the gear connections wouldn't be strong enough.
The wheel connection which allows them to turn isn't good enough though, so needs some work here.

The only other issue with this prototype is that the steering doesn't run as smooth as I prefer. It is not a problem however.
I also need to add something to connect the springs to. (I didn't add it since I just wanted to test the strength.)