Drop-center drivetrains: Why?
 

So, this may sound extremely n00bish of me, but I can't seem to see the advantage of a drop-center 6 (or 8) wheel drivetrain over other types.

I understand the point of dropping the center wheel: to reduce friction by making the robot rock so that two wheels are off of the ground. But I can't see why you wouldn't you just have a 4-wheel drivetrain, only 4 wheels are going to actually be touching the ground anyway.

We went with a 6 wheel drive this year, but we lowered the friction by putting omniwheels on the corners of the robot.

Is there some secret purpose that you could enlighten me of?

Re: Drop-center drivetrains: Why?
 

With a fixed robot width and fixed wheels all facing forwards, the shorter your wheelbase is, the easier turning will be. This is because the wheels have to scrub less to make the turn.

With a 4WD robot, the wheelbase is quite long, causing difficult turning.
When a 6WD dropped-center robot, only the front four or back four wheels are on the ground at any given time, effectively halving the length of the wheelbase, resulting in easier turning.

Re: Drop-center drivetrains: Why?
 

With a dropped-center 6 or 8 wheel robot the wheels contacting the ground when turning are closer together than on a 4 wheeled robot. Because the wheels are closer together it makes it easier to turn. This is one of the reasons it is easier for wide base robots to turn.

Re: Drop-center drivetrains: Why?
 

All of the below assumes a roughly centered CG and even tractioned wheels (all roughtop or wedgetop or kit)

With four equal and high traction wheels on the robot in the long orientation, turning on a dime is basically impossible. It can be done with large wheels and a lot of "jumping", but it's close enough to impossible that the robot's basically worthless.

Adding another set of wheels with no drop fixes this a little bit, but generally not enough without some specialization. 25's drivetrain is an exception due to a lot of subtle reasons I don't really want to go into here.

Dropping the center wheel means that effectively you have shortened your wheelbase by half, since only 4 are on the ground. This makes your wheelbase wider than it is long so you can turn just fine. As a bonus, when you're pushed on one side it's very easy for the robot to "lean" on those wheels to make you harder to spin than, say, the same drivetrain with omni wheels, leading to great pushing resistance.

With a 6 wheel no drop omnis, you get comparable performance and even better turning at the cost of being more easily spun yourself. 4 wheels with 2 traction and 2 omnis exaggerate this effect further. 6 wheel drop is basically the very best of most worlds, for most games.

---

In the case of a wide body robot, Lunacy demonstrated you don't _need_ 6 wheels to turn well.

Re: Drop-center drivetrains: Why?
 

Ah, I see now, of course!
I guess I underestimated how much friction the 4-wheel drive created...

Thanks a ton for the insight!

Re: Drop-center drivetrains: Why?
 

No problem.

I felt really, really bad not including "the math" supporting the claim, so here's a link to an excellent paper on the topic of drivetrains that explains all of the phenomena. It never specifically addresses the case of the 6 wheel drop, or what happens if you lower the sideways traction of one of your wheel sets (i.e. if you put omnis on one part of your drivetrain) but it shouldn't be too hard to see how a 6 wheel performs better than a regular drivetrain as a result.

If we're engineers, I feel we shouldn't be content with the what without the why...

http://www.chiefdelphi.com/media/papers/1443

Re: Drop-center drivetrains: Why?
 

ditto that !

Re: Drop-center drivetrains: Why?
 

We're not engineers, small correction. Statement is true otherwise.

Re: Drop-center drivetrains: Why?
 

Simply put, without dropping the center wheels, drive motors rapidly go to stall current in turns.

Re: Drop-center drivetrains: Why?
 

Do a little more digging. A common questiont that many 6x6 drop center guys will ask is "How much drop are you using?". While a fair question, the same amount of drop will have various effects on different chassis. CG placement and Chassis stiffness, and overall goals/objectives play a big part into what is considered effective amounts of drop.

Re: Drop-center drivetrains: Why?
 

As an example, 95 has used dropped center designs when the robot did not have precise manipulators mounted on the chassis, where the robot rocking back-and-forth did not adversely affect controllability. On two robots that DID have bigger arms that needed to be controlled with precision we used omni wheels on the four corners so that the robot was more stable and the arm was easier to control.

EDIT: To avoid confusion, I am talking about a 6wd platform with two traction wheels in the middle, and omni wheels at each corner.

Re: Drop-center drivetrains: Why?
 

Anecdotal evidence would suggest that rocking drive trains still maintain the maximum manipulator control aspect given a proper c.g. As an example from 2005, 254's robot had a rocking 6WD with a very long 2-PID arm. When fully extended the arm would shift the c.g. slightly, yet when it was down the tetras were easy to pick up because the c.g. stayed towards one side or the other when the robot was not accelerating.

Unfortunately it is exceptionally hard to integrate c.g. from scratch ahead of time; rough estimates are the best I've ever seen derived at the early stages of design. Since drive train is one of the three systems that MUST work (drive train, electronics, programming) in order to do anything effective in a game, most teams prototype like crazy on a pre-season prototype drive train. Additionally, finishing the production drive train early in the season is atypical of actuality for most teams.

Ergo a wise suggestion to anyone worried about c.g. shift on a rocking drive train is to have a pre-season prototype ready to go such that manipulators may be directly mounted to it during build season prototyping. This will give earlier estimates on c.g. and should produce constraints on shifting c.g. that may dictate where to put the 'heavy' elements (compressor, battery, drive train gear boxes, etc) on the production robot.

Re: Drop-center drivetrains: Why?
 

For all of the analysis that could go into a chassis like that it is simpler to use omni wheels on the corners and be done with it. That frees up all the time and energy saved in chassis analysis for use in more unique challenges.

Re: Drop-center drivetrains: Why?
 

It's simpler, unless the game requires pointing in one direction for any length of time (like 2 seconds to unload a game object). The defensive robots love 6WD robots that have omnis on the corners, something about they're easier to knock out of position rotationally.

A pneumatic center wheel on a drop-center is actually a pretty good idea: you can fine-tune the drop a little bit to get exactly the characteristics you want, even between matches.

Re: Drop-center drivetrains: Why?
 

Good point about the defense.

Changing pneumatic tire pressure would also change gearing and grip, but those might also be good things.

Re: Drop-center drivetrains: Why?
 

And when drive motors rapidly go to stall current, that means...

1. Robot doesn't turn
2. Breakers pop
3. or Worse

We've been using 6WD with dropped center wheels for years (with the exception of last year).

We like using pneumatic center wheels. Makes adjusting the offset very easy.

Re: Drop-center drivetrains: Why?
 

1276 built a launcher in 2008 with a 6 wheel drive with omnis in the corners. I was sure we would get knocked all over the place trying to launch, but it was never an issue. We would've done it again in 2009 were it not for the slippery floor, and the team disbanding in 2010.

That said, in 2008 there wasn't a lot of incentive to knock robots silly, so we may have just gotten lucky.

As a counterpoint to my own counterpoint, I can't think of many situations where if someone is trying to rotate you, you'd be much better off with the stickier wheels. In 2010 if someone was messing with you, you'd drive off before kicking the ball. In 2007, you were probably entangled in the Rack, so the squirrelyness (technical term) might help you swing around to another Spider. In 2006 if someone was pushing on you, it was probably enough to throw off your aim. Additionally, it cut way down on our distance (though our conveyor to our shooter and shooter wheels were all on one CIM, so this may have been less of an issue for others). In 2005, perhaps it could help you stick your ground for just an extra second, but this came turned out so offensively, I'd rather not be rocking back and forth with a 10 pound tetra 10 feet up in the air.

Re: Drop-center drivetrains: Why?
 

Let's look at this a bit more. It's not always as simple as you have stated here. There are situations, or game elements, that do not lend themselves very well to a robot with omni's on the four corners.
If you are playing on a flat surface, then you bet, omni's on the corners is a simple solution. Now, if you want to climb a ramp, or go over a bump, then omni's on the corners add a whole new level of difficulty. I'm not talking so much about traction, but the need to hit the incline squarely. If you are out of alignment by only a few degrees then the robot will tend to slide out, or fall back to the level surface. The steeper the incline, the greater this effect will be.

The bottom line is, make the design decisions based on what you want to achieve in the specific game you will be playing with the robot.

Re: Drop-center drivetrains: Why?
 

aspiring to be engineers :)

I really think "just use omnis" can be a pretty poor solution depending on the game challenge. If you have a long arm, a gentle tap moves your game piece several feet away and there's not much resistance to speak of. Really, it's not that hard to do "all of the analysis". I can tell you what the analysis will say right now: The more rigid your chassis, the better your 6WD can perform. Building a rigid chassis doesn't take tons of time and energy. Even if you do want to do careful and thorough analysis for such a drivetrain, it's not exactly a huge resource drain, and depending on the challenge it could result in a much, much better chassis for your needs.

While some games you probably could get away with a chassis with four omnis, in the past I think more often than not you couldn't. Being spun off target in Aim High, knocking you away from your spider leg in Rack 'n Roll, or taking a mid to long range shot in Breakaway are all situations when I'd rather have a bit more resistance to being spun.

Re: Drop-center drivetrains: Why?
 

I don't know about your reasoning here. It seems in every instance you brought up that someone trying to rotate you is prohibiting you from completing your initial task. In your 2010 example instead of kicking the ball you drive away because someones "messing with you". In 2007 you are swinging from the initial spider your trying to cap to a different one because your being rotated. In 2006 being lined up and not moving was key for 90% of the shooters out there, obviously. In 2005 getting to where you needed to get to was absolutely key.

Not meaning to nitpick here, but the suggestions you make are all alternative maneuvers your team can make to offset being spun easily. In all those instances, I would much rather not be spun and complete the task my team wants to, than come up with a new game plan because someone is playing D on me.

-Brando

Re: Drop-center drivetrains: Why?
 

Make sure when discussing "performance" to use key words like traction, ability to hold ground, ease of turning, manuverability...
Saying "XYZ will perform better" in a thread like this can lead to misconceptions. Many of the parameters for manuverabilty and the ability to hold ones ground are in direct conflict.

Re: Drop-center drivetrains: Why?
 

Although we did not use omni wheels this year, we did us mecanums, and we noticed that the robot wouldn't glance off inclines, but rather tend to align with them. There probably is some angle where the robot would tend to glance off rather than self-align, but my experience tells me that this angle is sufficiently wide to easily drive over a bump or up a ramp.

Perhaps you did not understand that I was referring to a 6wd chassis with two traction wheels in the center and four omni wheels on the corners. In my experience these have been fairly resistant to being spun and do not rock much, if at all; a good compromise in my opinion.

I was not referring to a chassis stiffness analysis, but rather keeping track of CG location for the purposes of reducing rocking while manipulating a game piece. While these are both straight-forward analyses to perform, not every team has a lab full of CAD-capable computers and/or trained operators ready to do them.

Re: Drop-center drivetrains: Why?
 

I caught that. 6 wheel 4 omni isn't terrible or anything but it is more easily spun and negates a lot of the advantages of a rocking drivetrain. The CG issue, while relevant, really isn't something you need computer precision to analyze. If you don't want to rock with an arm too much, you can adjust the drop a little, put the CG a bit more to one side fore-aft, or even run an 8 wheel drive if your CG is centered. You should be able to get a good enough estimate with just some napkin sketches of where the "heavy stuff" goes. CAD isn't required to figure it out.

Even if you do find you need more precision than "I think it'd be best if we put the battery... here", CG analysis is always beneficial on a competition robot.

Re: Drop-center drivetrains: Why?
 

People refer to "rocking" all the time and cite it as a problem with manipulators.

Think about the math here. With a 33" wheelbase (37" long chassis, with 4" wheels), that means you have 16.5" between wheels. With a .1875" drop on the center, you can rock 1/100 of a degree. Obviously that's exaggerated when you have a long arm sticking out, but come on. .010 degrees of rock is nothing. It's hardly noticeable. It doesn't affect precise positioning whatsoever.

Re: Drop-center drivetrains: Why?
 

An interesting idea inspired by this thread would be to (gasp!) raise the center wheel on the fly in order to reduce turnability similar to the nonadrive concept.

Hmm.

Re: Drop-center drivetrains: Why?
 

Agreed.

Even with a 10' arm on there, if your manipulator needs to be accurate in placement to 2" or less, you really have a poorly designed manipulator that would be difficult to score with anyway.

Re: Drop-center drivetrains: Why?
 

For what "benefit"?

Before people start going off and design new for the sake of new, they should evaluate what a well designed full treaded drop center 6wd can do. There is a reason 60 started doing it, 254/968 always does, and we always love to copy it.

Re: Drop-center drivetrains: Why?
 

I am aware that CG/tipping moments can be roughly estimated with napkin calculations. Chassis stiffness is not analyzed so easily.

We've also done a drive-train with two omni wheels in the middle and traction wheels on four corners :eek: these omni+traction 6wd robots, in either configuration, have all been successful.

I'm not trying to invalidate your points, Chris, they are indeed accurate. While the does trade off a little hit-and-spin stability, it also gains turning speed and virtually eliminates jitters from scrubbing the outside wheels and rocking. As with any robot design choice it has to be made in context of the teams strategy in that year's game. I feel that this disclaimer need not be made with every post though.

Re: Drop-center drivetrains: Why?
 

Perhaps you were blessed with better drivers than us. It just seemed that the vast majority of the time we were being defended, it was a better use of our time to leave and sneak to a different location rather than wiggle back and forth with someone slamming on us. :)

Cory, we had issues with rocking and the dropped center wheel, but we may have dropped more than .1875, it's been a while. In 2006 when shooting from the floor, rocked one way we would make shots, rocked the other way they went high (or low). We also had some tracking issues with our dead reckoning autonomous.

Re: Drop-center drivetrains: Why?
 

We have (on more than one occasion) used a dropped a foot under the front or back of the robot to assist in turning. This raises one set of wheels off the ground.
For some of the other discussion, there is no substitute for practice.

A common joke among music professionals goes like this...
A person on a street in New York is asked "How do you get to Carnegie Hall?". The response is "practice, practice, practice!"

Re: Drop-center drivetrains: Why?
 

If what I've suggested is "new for the sake of new" implementation of 6WD/WCD is "a solution searching for more problems". I made the benefit pretty clear, but perhaps a more in-depth explanation is required. In short, the benefit to raising the middle wheel on the fly is to maintain a level of functionality while removing the derived assumptions realized by a more advanced/coupled design. Indeed, I did not even imply those assumptions, but then again no one ever really talks about them.

Reduce the team in question to an average team who seeks to maintain most of the same capabilities as the 'best' 6WD while also maintaining other requirements they've set forth for their robot. Perhaps 4-6 motors and a COTS shifting transmission on the drive train is deemed a lower priority than having extra power/weight for other robot subsystems for a team. To be honest, this is a very reasonable assumption for any team.

If the extra power/weight in the team's manipulators were to pay off in on-field success, many other teams would come to defend them.

Thus the team would be subject to defense via turning due to its gearing choices and wheel base when it competes against another robot with more power/capability in the drive train. Adding a potential design to raise the middle wheel using simple pneumatics in order to remove the disadvantage may prove to be a superior design for that team's overall robot depending on their time, available resources and funding.

Re: Drop-center drivetrains: Why?
 

We had a variable drop center chassis this year. In simplest terms, it allowed our middle wheel to raise up while going over the bump (4-5"). This allowed us to have smoother and faster transitions with less impact on the robot/chassis. This was a key component of our strategy as we knew that for many matches we would start in the back and work our way forward and did not want to be limited by the potential tunnel pinch point. Our analysis (check white papers) showed a flat or rock 6x6 would have a considerable amount of problems going over the bump (not impossible, just not pretty).

The variable drop middle wheel also allowed us to flatten the chassis to "hold" directional heading (very important in 2005, 2006 and 2007). As it turns out, this year holding a particular heading was not as essential as those other years. You were either pointed at the goal to take a shot, or you were not. If we had been allowed to store multiple balls, this would ahve been a different story, but it also would have been a very different game.

This variable ride height middle wheel did cause issues with our collector.

Re: Drop-center drivetrains: Why?
 

We did something similar in our non-dropped 6wd (wide) for Lunacy. It was actually a pneumatically actuated wheel that lifted up the back four, which was great for both turning on a dime acting as a break on the regolith. We could even strafe on occasion, though certainly not as well as nonadrive.

Lesson being there are a lot of novel "dropping" options to achieve your desired results--it need not be just the center. (photo)