Motor Placement Pros and Cons

[Pattie]

Hi,

We need an outside opinion as to pros and cons of this motor placement design using two transmission boxes (shown) with two CIM motors each - each side will have one short chain to the nearest wheel and a longer chain to get to the further wheel on the same side to make 4 wheel drive. Forgive the artistry - not quite talented in that department yet......

Thanks,

Pattie

[Max Lobovsky]

Could you explain why you are considering this design as opposed to one that is more traditionally symmetric (symmetric over the line down the middle of the robot)?

[Pattie]

Hi Max,

I wish I could explain - thats why I'm asking if anyone has pros or cons. This was something that happened yesterday and nobody can explain why they are thinking of this configuration that makes any sense to me. There may be a valid reason and I'm just missing the boat on this one.

Pattie

[Jeff K.]

Well, for this year's design, our robot has 4 WD with the gearbox placed as far back as we can. We are doing this because our arm is going to be in the front, and since the battery and gearboxes are the heaviest things on the robot, it'll keep the robot from becoming unstable and tipping over. Pros of this is stability. There are no cons. A con with having the gearbox placed closer to the front and having the arm hanging over the front is that the robot will tip over. But the placement of the gearbox depends on your team and where you plan on mounting the arm.

[Pattie]

I think I'm concerned that it may be easy to turn one way, but not the other and also will it be more difficult to move straight. And, as you mention, the weight won't be countered by mounting the weight in the back. Also seems like a pretty long chain to worry about. But I'm sure there has to be some really great reason to choose this design given the risks involved.

Pattie

[Greg Perkins]

Hi Pattie,

Let me offer my POV on this..

are you guys going to use this as a direct drive to one wheel on each side??

If so, it might be worth your while, bit the instant you try and turn, you will be doing donuts easily. my advice is mirror your robot, make each side symmetrical, it makes for much better weight distribution as well as power effeiciency.

One mroe thing, in that current setup, you will also have some frame warp, this will happen when your robot goes to pick up something and your left wheel is trying to get traction and the other is going to be slipping (kinda the same as a posi-trac rear end)

all in all a forum is merely a tool for suggestion, and we utterly cannot swing a teams vote...

good luck!

[Pattie]

Hi Greg,

Thank you for the comments. This helps. I had never heard of a setup like this before and wondered if any other team had tried it or considered it. No direct drive was in this latest plan. I have visions of the the chassis moving diagonal. But I know I am biased from a programmer perspective - hoping this approach doesn't make the chassis veer off to one side or making one turn difficult that we have to get creative modifying code to make up difference for - so asking for other input. I really appreciate everyone's responses on this. I'm looking for the pro that makes this approach worth taking the risks. My concerns may be total nonissues. But warp on the frame is a big concern. This helps a lot.

Pattie

[CyberWolf_22]

My team had a design like this at one point. The main advantage that this design has is that you open up the middle of the robots chassis so that wiring and placement of other components is much simpler because you do not have to run wire and hose around the motors in the middle of the chassis. However, the electrical team captain and I (Programming, and pneumatics captain) designed this.

[Pattie]

Hi Cyberwolf. Thanks for the response. You say you had this design at one point - what were the reasons you did or did not use this design? Were there any fatal concerns with this design?

Pattie

[CyberWolf_22]

The reason we eventually decided against it was that we were going to go with a 6-wheel drive train if the chains connecting the wheels broke we wanted out center wheel to stay powered so in the end we decided to put the motor in the center of the robot. In addition, it put the weight above our center wheels.

[greencactus3]

well just an imput from a lego stance, i have used this in many lego models with much success. but lego is slightly different as most of you know. why i used it in lego, because haveing them in that config allowed me to leave the minimum width to 5 blocks rather than 10

[sanddrag]

I didn't take a look at the word document, but because our gearboxes are so large we are placing one at the front right wheel and one at the back left wheel. Not optimal but functionally the same.

[Pattie]

Have you had any issues with driving crooked at all?

[Al Skierkiewicz]

Originally Posted by Pattie

Hi,

We need an outside opinion as to pros and cons of this motor placement design using two transmission boxes (shown) with two CIM motors each - each side will have one short chain to the nearest wheel and a longer chain to get to the further wheel on the same side to make 4 wheel drive. Forgive the artistry - not quite talented in that department yet......

Thanks,

Pattie

Pattie,
There is no glaring reason against your idea. Most teams will center up the transmissions to balance the weight over each of the wheels. Your configuration may cause some steering instability as the wheels under the trans will have slightly more weight over them than the other wheels. The longer chain paths do have a little greater chance for problems as the loop developed as the chain stretches is more capable of throwing itself off a sprocket. There are ways to prevent that but they come at a cost of additional weight. No matter what you decide, plan on some device to take up the slack in the chain and check for slack before each match. Correct chain alignment is a must for efficient drive so make sure all your sprockets line up (in the same plane) before driving as well. Let us know how it worked.

[ChrisH]

Originally Posted by Al Skierkiewicz

Pattie,
There is no glaring reason against your idea. Most teams will center up the transmissions to balance the weight over each of the wheels. Your configuration may cause some steering instability as the wheels under the trans will have slightly more weight over them than the other wheels. The longer chain paths do have a little greater chance for problems as the loop developed as the chain stretches is more capable of throwing itself off a sprocket. There are ways to prevent that but they come at a cost of additional weight. No matter what you decide, plan on some device to take up the slack in the chain and check for slack before each match. Correct chain alignment is a must for efficient drive so make sure all your sprockets line up (in the same plane) before driving as well. Let us know how it worked.

We haven't been as extreme as your sketch, but we normally have one chain longer than the other when we're doing a tank type drive. Like Al says, keep your chains and sprockets in the same plain and keep the chains tight enough but not too tight and you should be fine.