Our Team Needs Help on the Drive!

Okay, today our team decided on a drive for, what is to be our chaise drive train. the layout looks like:
|-------/|
|-|-----|-| Front

o ---- o
–___/–
oo-----oo Back

(This is not to scale, the little - just is a space, so ignore them))

Okay the circles are the wheels, the back four wheels in the back will be powered on a chain, and the front two wheels will be none powered and will be held in a straight position, not free on a caster mechanism. Before our team dives into creating this, I was hoping for some feed back, this is essentially 2 back wheel tank drive. So here are my questions?

  1. What advantages do you see with this drive?
  2. Should the front and back wheels be powered or just the back ones?
  3. is there a thread, or some specifics to way a tank drive is bad for this competition?
  4. With the layout above, where would you put the wheels, which ones would be powered, and what kind of drive would you recommend for this base layout?

TY so much to all who respond, are team is rly in a fix on this, and can’t rly decide, and answering these questions, would help us SOOO much, ty again! :slight_smile:

I can answer some…

  1. All wheels should be powered. If you don’t, your available power drops. Worse, if you’re turning, the front wheels are pure resistance. With the back wheels not having a lot of traction, you’re going to have lousy turning capability.

  2. Experiment!

  3. I think there is, but I can’t remember where it is.

1.Not to be crazy harsh, but i see just about no advantage to this drive:(
2. You need to power every wheel touching the ground this year. Depending on how your weight is displaced, you will be giving up tons of traction by not powering wheels. For example if you have around 20% of your weight over the middle of the bot, you could be losing upwards of 20% of your traction, which is a lot in this game.
3.Ill tell you right now why. The latteral coefficient of friction is nearly twice that of the normal CoF.

well what is the big gap in the front of the bot going to be used for? the idea looks good but if your are going to put wheels near the front y not the very front of the bot not as far back that is the only thing that i think might help some. and i dont think you need all wheels dive with four or two M. if one slips and one of the others grip it may make the bot jolt to one side. i think they might got the right idea here. 2M/2wheel drive hmm . and resistance is kinda what you need in this game.

Question for you, or anyone who can answer, for the above layout, elt’s say the scale from the very, very front of the robot is (the two points you see in my pic) let’s say that’s 28 inch apart and the U cut out in the center of the bot was 14 inches wide.
5) How would you then get power to the 2 front wheels?
6) Is there another way to model this layout to have that U cutout, but still have a set of front wheels, that are powered?
7) Still not sure as to what drive our team should consider

the U cut out is to be a place for the opponent’s trailer (it doesn’t grapple with it) just to ease our shooting.

Coming from a guy who lives in Wisconsin and see’s his fair share of icy roads (dug someone out of a ditch the other night at 2 am in fact), I can tell you that you don’t want to do rear wheel drive.

There is also no advantage to doing dual rear wheels.

More wheels does not equate to more traction.

^^^

That phrase may just have surpassed “Water game!” as most popular words to be uttered in the past year.

Since I can’t refresh this thread, and don’t want to start a new one, I am going to re-post the questions I still have, ty to all those who replied to the above questions, when answering just refer to the numbers below not the ones earlier in the post.

Questions:

  1. What are the advantages/ disadvantages do you see with this drive? (be honest no hard feelings if it is all disadvantages)
  2. With the layout above, where would you put the wheels, which ones would be powered with this base layout?
  3. How would you then get power to the 2 front wheels?
  4. Is there another way to model this layout to have that U cutout, but still have a set of front wheels, that are powered?
  5. What drive train would be best for this layout? (ie: tank, swerve, omni, crab ect…)Any specific reasons to why, would be helpful.

ty again to any who respond.

  1. The disadvantages of this design are that the front wheels are not powered. You are going to want to power anything touching the ground. Also two wheels in the back provides you with nothing if not hurting you, if you are trying to increase the surface area you are only decreasing the amount of pressure over the surface area.

  2. try to get the wheels as far into the corners as possible and power all of them.

  3. Power to the front can be achieved either by running chains from you gear boxes or by having gearboxes on each wheel.

  4. you might want to consider driving a wide robot instead of a long robot.

I would first stop thinking about half the robot. You will need to think about the trailer joined to the robot. It doesn’t matter much what your half a robot can do. It will matter a lot how your complete robot (w/trailer) behaves on the regolith.

The two halves of the robot pivot horizontally on the trailer hinge pin. Study this carefully.

You are friction limited. The force available to move forward or backward or change orientation depends only on the Normal force (weight) on your driven wheels.

You’ve decided on an 8 wheel vehicle (2 on the trailer). You should have an explanation of why need each and every one of them.

The 6 wheel drive you illustrate may not even be able to turn with normal wheels on carpet. It depends where the center of gravity is.

Until you are prepared to do the math and physics, your best bet is to stick with the wide kit bot configuration and drive both wheels on each side. The longer wheelbase is your friend. You may want to buy another pair of wheels and build at least a dummy trailer that you can test with.

The last thing you want this year is a toaster that doesn’t move because it will end up being a scoring disaster. I would suggest that you get a simple and reliable drive working so that your drivers can practice while you work on mechanisms and software. Don’t bite off more than can chew.

Good Luck!

My team (as of the last meeting) is considering a drivetrain similar to this…


|==…/==|
|…|…|…|
|]…]|
|]…]|
|…|
|…|
|==…==|

The == are non powered horizontal wheels that don’t normally touch the ground.
They are only there to keep the chassis from hitting the ground during impacts and abrupt stops.
The ] are powered wheels.
This is essentially just a 4WD, but the wheels are as close together as possible for better turning.
They are slightly towards the front because that is where the majority of our weight is expected to be. That way we keep as much weight directly over the wheels as possible. It also helps balance things, so it has as little rock back as possible.
This (or something like it) may be a good fit for what your team is trying to do as well.

And we’re thinking of something like:


__          __
|]\_______/]|
|]         ]|
|]         ]|
---------------

With the center wheels dropped a bit, but all wheels driven. Depending on where the centre of gravity is located it begins to approximate a two wheel drive as the majority of the mass will act through the centre axles. As one approaches this state the transverse friction on the wheels preventing skid steering should begin to approach zero.

Of course, building in the wide direction does present other challenges, but also presents other opportunities. We’ve never built a “wide” robot before, but this year’s game has inspried us to try something new.

Jason

P.S. It is the code tag “#” on your editor window that lets you do ASCII art a bit more easily.

Might as well just use 2 rather then 4, you have the two on the trailer, so having them close like that does not do much for you in my opinion.

It doesn’t seem that anyone has directly answered your new questions so I will try

Questions:

  1. What are the advantages/ disadvantages do you see with this drive? (be honest no hard feelings if it is all disadvantages)

The ability to corner the oppenents trailer is a definite advantage however you will have definite problems with turning and traction unless you make design modifications. Every wheel needs to powered.

  1. With the layout above, where would you put the wheels, which ones would be powered with this base layout?

I like the layout and design, but it will make turning difficult. I would turn the design 90 degrees to make a wide robot. This will making turning easier because you have a larger lever arm to torque against while turning. Also a wide robot will be resisted more by inline friction and less by transverse friction when turning.

  1. How would you then get power to the 2 front wheels?

I would turn chain from the rear wheel to the from wheels (assuming the rear wheels are directly driven) If the rear wheels are not directly driven then I would place the transmissions near the middle of the robot and rune chain to both the front and rear wheels. This does require you to devise a tension system, but that won’t be too difficult. You can see our teams previous designs for chain tension or shoot me an email for more information.

  1. Is there another way to model this layout to have that U cutout, but still have a set of front wheels, that are powered?

See the above–it’s more than possible with a little chain and no design modifications.

  1. What drive train would be best for this layout? (ie: tank, swerve, omni, crab ect…)Any specific reasons to why, would be helpful.

If you make your front wheels crab turning will be much more efficient. by turning your front wheels 90 degrees you will be able to turn on a dime–With this design I would still recommend you power your rear wheels independently to assist turning. This would make a killer design which could turn easily and maintain your design objectives. For this design I would use a CIM run through a custom gear box direct drive for each rear wheel. In the front I would use crab modules consisting of an upward CIM and a miter gear set up. Two FP motors would rotate the crap modules.

An interesting strategy with this (Although of questionable legality) would be to spin another robot into the wall using your superior turning ability effectively locking them in place.

While everyone is thinking about weight distribution, don’t forget the tongue weight of the trailer.

Well throwing another question out there that I have, for crab drive, I have never really seen one so I am not sure how they work or how to set it up. From my understanding this is what it would look like (correct me if I’m wrong)

--------/
=
—__/—
-----------
oo------oo

(This is not to scale, (-) is just a space so ignore them)
The circles (o) would be the wheels powered, and pointed to the front of the bot, the (=) would be wheels facing <- at a 90 degree angle, and powered. Is my interpretation of crab drive front wheels correct?







It’s interesting to see some convergence. These are the drive configurations for our two leading design concepts.


 One Wheel           Two Wheel
      _                  _     

|    //    |       ||         ||

      _                  _
      O                  O

// = driven and steerable,  || driven fixed,  | and _  fixed freewheeling.
O = Trailer Hinge Pin

Design CG is over the single driven wheel for one wheel drive
and between the  two driven wheels for the 2 wheel drive. 
Driven wheels are dropped so that the non-driven wheels 
do not normally touch the ground unless external forces are in play.

Not quite, but close
If the front two wheels are in a crab modules then they are free to rotate as directed by separate motors. i.e. when turning the crab wheels (=) would be exactly as depicted above, but when moving in a straight line the wheels would be parallel to the rear wheels. The wheels can be turned at a whim using two additional motors. Each crab module consists of a drive motor (generally a CIM) and a turning motor which turns the module.
Here are some threads and images better explaining crab drive:

http://www.chiefdelphi.com/media/papers/1836 (he calls crab swerve, but they are very similar)

Crab drives are a challenge to build, but can dominate if correctly implemented.

Okay our team officially decided on the following:
--------------
|--------------------/-------|
|—0-----_______/----0—|
|-----------------------------| 28*
|-----------------------------|

–00--------------------00–

38*

So the (0) is a wheel pointing forward only, and it is a tank drive. The width is on the side, and the longer side, in the back. All wheels are powered by one chain on either side, one motor on either side.

This was what I tried to get our team to do:

--------------
|--------------------/-------|
|—=-----_______/----=–|
|-----------------------------| 28*
|-----------------------------|

–0----------------------0—

38*

The (=) is a crab drive wheel and the back two wheels are just pointed vertically, all wheels powered. Each wheel would have it’s own motor and chain moving it.

So if the first drawing is A, and the second one is B, then here are some questions:

  1. Which drive looks better for this competition?
  2. What are the pros and cons of A?
  3. What are the pros and cons of B?
  4. Suggestions that we should take under advisement, and any other comments.

TY to all who respond!:]