6 Wheel Drives Without Dropped Centers
 

I've been looking at different drive systems lately and I have tried to come up with something that our team could try. I have heard of teams using 6 wheel drives without a dropped center so that it is truly a 6 wheel drive and not just a 4 wheel drive with some extra stability.

I have heard that 25 has used this concept before and looking at some threads from the early 2000's I found some information on it but I still don't quite understand how it would work.

Can anyone enlighten me as to how this would work? Everything that I can think of says that a 6 wheel drive robot without a dropped center shouldn't turn without a special 7th wheel mechanism or omniwheels or something of the sort.

Re: 6 Wheel Drives Without Dropped Centers
 

Hey Cam!

Someone from 25 will probably have to pop in to clear up the details, but here is what I know. 25 does use a 6WD with no drop. In the past, they have even gone up to 8WD, still with no drop. They use what has been termed as "Brute Force" turning; no tricks, no mechanism. It's all drive train for them. From what I recall, the secret lies in the wheels. Those massive drum wheels 25 has used in their drive system for years are from Skyway. 25 sands down the wheels so they are slick, and then cuts into them and adds tread as necessary. Here's a little more info on it: 25's Tires. While the thread is over 10 years old now, I seem to recall a thread in more recent years where the process is verified to still be alive.

Like I said, an actual member of 25 is going to need to provide the details, but that's the just of it.

EDIT: Here is a more straight-forward description of the design from a member of 25 - Link

When it comes to drive systems with speed and power, you'd be hard-pressed these days to find a team who surpasses 25 in both respects.

Regards,
Leeland

Re: 6 Wheel Drives Without Dropped Centers
 

4 traction wheel in the back or front and 2 omnis in the front or back.
Great traction and mobility.
Our team uses this all the time. It is very effective.
I have also heard of teams that put 2 omnis in the middle.

Re: 6 Wheel Drives Without Dropped Centers
 

Omnis in the corners. Traction wheels in the center. Very maneuverable, fast, with a lot of pushing power.

Re: 6 Wheel Drives Without Dropped Centers
 

Last year I did my grade 12 physics final project on this topic. To super-simplify it (ignoring center of gravity, wheel coefficient of friction in x and y, gear ratios, etc) the rule of thumb is that you want your wheel base to be at least as wide as it is long.

There are multiple ways that a 6wd, no-drop center drive train can be effectively executed. The two most popular as far as I know are the following:

Wide Base 6wd:
This drive base is exactly what you describe, only the drive base is in wide configuration. The reason this works is because as our rule of thumb dictates, the base is wider than it is long.
This is a kit of parts base, which does have a drop center, but it looks just like one would without the drop, just pretend :rolleyes:
http://www.chiefdelphi.com/media/photos/37486

4 traction + 2 omni:
This drive train is probably more like you are envisioning, a standard long orientation, 6 wheel drive robot. The thing that allows this one to turn without issue is having omni wheels on one end. Omnis have very little friction sideways due to the rollers, essentially serving the same "cut the drive base in half" that a drop center does.

Something like this, except with omnis only at one end:
http://www.chiefdelphi.com/media/photos/31039

Re: 6 Wheel Drives Without Dropped Centers
 

Yes, thank you Leeland, you have come through for me again! This is exactly what I was wondering about. I hope someone from 25 could come in and provide a little bit more detail about this.

Just wait till FLR next year when we have a 6 cim, 2 speed, 6 wheel no drop drive train ;)

Re: 6 Wheel Drives Without Dropped Centers
 

The process we use to cut our tires hasn't changed significantly over the years, so a lot of the information in those old posts is still relevant. We first shave down Skyway Bead-LOK wheels using a table saw (a couple of years ago we created a machine to help us with that process - see this video), and then cut the treads using a wooden jig.

Here's a thread from 2010 that generated some good discussion about dropping the center wheel: http://www.chiefdelphi.com/forums/sh...ad.php?t=85763.

In particular, this post by Jared from 341 does a good job explaining some of the specifics of our drivetrain.

If you have any more questions, feel free to ask (or come on down to Brunswick Eruption in November to check it out in person :)).

Re: 6 Wheel Drives Without Dropped Centers
 

Just curious if you guys have ever had experience with an 8 wheel drive train with no drop. it seems like if you are doing 8 wheels then all 8 are contributing friction while turning as opposed to 6 wheels where it seems like only 4 are.

Re: 6 Wheel Drives Without Dropped Centers
 

Although not quite as much pushing power as having four traction wheels on the ground.

Re: 6 Wheel Drives Without Dropped Centers
 

Our team had four traction wheels in the back and 2 omnis in the front. Does that count?

Re: 6 Wheel Drives Without Dropped Centers
 

Cheap option: zip ties around the treads of the 4 corner wheels?

Re: 6 Wheel Drives Without Dropped Centers
 

We ran an 8 wheel drive with no drop in 2007, which was detailed in the Behind the Design book from that year. From a driving perspective, it was noticeably harder to turn than the six wheel configuration, but not by much. Since Rack N' Roll was such a defensive game, the added stability from the longer wheelbase and extra resistance to turning worked to our benefit. It was fine for that game/year, but we haven't returned to an 8WD of the same type since (even when we built Evil Machine 5's twin in 2011).

As a fun side note, we blew one of our drive speed controllers at Battle O' Baltimore that year (and didn't have a replacement with us to swap it out) and were still able to drive around pretty well.

Re: 6 Wheel Drives Without Dropped Centers
 

The turning ability of a robot with no drop-center and all traction wheels is dependent on the normal force experienced at each wheel. That is to say, it depends on how much force each of the wheels on the robot supports.

As Jared mentioned in his post that was linked to, a 6 wheel drive with no drop center is statically indeterminant. This is because if you isolate the physics on one side of the drive train (assuming a perfectly symmetrical robot), you have the force for 3 wheels as variables but only the sum of moments about the wheels' axes and the sum of forces in the vertical axis as equations. That's 3 variables and 2 independent equations, so this mathematical model cannot solve for how much weight each of the wheels supports.

This makes it complicated to approach from a design perspective, which is why the over simplification of "a long wheel base robot without drop center wheels will not turn" is often made. In reality, as you approach the limit where all of the weight is supported by the 4 corner wheels, the robot will behave like a 4WD long robot and have difficulty turning. As more and more of the weight is supported by the center wheels, this reduces the normal force at the corners, thereby reducing the tractive force applied at the corners and reducing the robot's resistance to turning. More weight at the corners = Harder turning. Less weight at the corners = Easier turning.

When the center wheel is dropped on a robot, much of the robot's weight is supported by the center wheel at all times in addition to shortening the wheel base, which makes turning easier (another generalization, but a widely accurate one so this is part of why drop centers are so popular).

Team 25 REALLY knows what they are doing, which is why they can pull this off. My guess would be that criteria that lead them to this type of design decision are:
-Makes it harder for defense to turn them
-Eliminates rocking, so stability isn't an issue

I personally cannot perform the math necessary to prove that a drive train without a drop-center would turn. I also have not done the necessary testing and experimentation to have the experience needed to design such a drive train that I will guarantee can turn well. Therefore, I would hesitate to build this type of drive train. This is where it's important for teams to build within their means and recognize them.

Re: 6 Wheel Drives Without Dropped Centers
 

Does anyone know if versa wheels would work for this sort of drive train? They have really good traction going forward and backwards but not the best laterally because of the W tread on them. I think that this sort of wheel would almost be perfect for this scenario.

Re: 6 Wheel Drives Without Dropped Centers
 

I think the diagonal pattern that 25 uses is important to their success. This is obviously something that versa wheels don't have.
It might be worth trying with colsons though...

Re: 6 Wheel Drives Without Dropped Centers
 

Someone on CD said versawheels tend to dig in to the carpet like cleats when pushed laterally

Re: 6 Wheel Drives Without Dropped Centers
 

I heard other wise from the highest traction wheels thread:

Re: 6 Wheel Drives Without Dropped Centers
 

What T^2 said is true given your robot remains completely flat on the floor, parallel with the carpet, and doesn't weigh too much. In the event you're not parallel (ie. being pushed from the side and slightly tipped upwards on the pushing end), the W tread will grip to the carpet like cleats. It won't need to be a significant lift, but anything to change the robot from parallel with the floor will engage cleat mode.

Source: Experience with cleat mode this year.

Re: 6 Wheel Drives Without Dropped Centers
 

Quite interesting, Miss Daisy used the VersaWheels this year, and I witnessed them getting pushed sideways. However, I'm not certain if they dropped the center wheel or not (6wd, 6cim), It would be interesting to hear about their drivetrain.

Re: 6 Wheel Drives Without Dropped Centers
 

48 has been running no drop center, 6WD West Coast setups since 2011.

2011: two 4" omnis up front, four 4" performance wheels in rear, roughtop tread. Long wheelbase.

2012: six 8" AM pneumatics, square footprint

2013: six 6" dia. 1" wide aluminum performance wheels, blue nitrile roughtop tread. Very slightly wide-biased footprint.

All three were powered by some variation of the A-M SuperShifter, 2 CIM's per side.

2012 was not surprisingly the least-maneuverable of the three setups, but it still was good enough to get the job done in one of our most productive seasons ever.

The 2013 iteration is very nimble yet able to provide enough punch to apply solid defensive pressure when needed.

Re: 6 Wheel Drives Without Dropped Centers
 

Just to clarify for Travis, 48's 2013 drivetrain ran 6-in custom performance wheels with 1-in roughtop tread riveted in place. Due to our relatively square frame, this put our wheel base at 24" between centers (front to back) and about 26.5"-27" between the center tread contacts side to side. It was an all-around quick cycle runner with no problems doing zero-radius turns in either low or high gears.

Our 2012 robot's wheelbase was only an inch wider than it was long, so with the much larger contact patches of the pneumatic tires, it definitely did not turn as well as this year's bot (as mentioned above).

Re: 6 Wheel Drives Without Dropped Centers
 

We had 8 wheel drive with the middle 4 axles 3/32" lower than the outside axles (approximately 26.5" long x 29" wide frame). Note that because of our center of gravity, the rear 6 wheels (and in particular, the 3rd pair from the front) bore most of the load, and wore the fastest. As a result, at various points in the season we had 3/32" +/- 1/8" of effective drop.

It turned like butter (if anything, a little more scrub would have been nice), and it was possible for a high traction robot to push us sideways a little bit. But with fresh-ish wheels our traction in the forward/backward direction was incredible (we inadvertently tore a couple holes in the carpet at the Las Vegas Regional during pushing contests).

Re: 6 Wheel Drives Without Dropped Centers
 

Ah, thanks for the clarification on the drive. I didn't have too much of a chance to look closely at the robot this season.

Is 8WD a new drivetrain standard with you guys? Just wondering...

Re: 6 Wheel Drives Without Dropped Centers
 

It will be interesting to see what the rules have to say about those wheels in 2014.

Re: 6 Wheel Drives Without Dropped Centers
 

I have also seen roughtop tread tear a hole all the way through the carpet in a similar fashion (1676 attempting to push their whole alliance up a bridge at Brunswick Eruption). In both cases it was a pretty anomalous situation. In our specific case, we were slot loading because our ground loader wasn't working, and the driver didn't realize he was still holding down the sticks when we were against the wall.

Re: 6 Wheel Drives Without Dropped Centers
 

So what immediate and post-event remedial action, if any, is taken when a bot tears a hole in the playing surface?

Re: 6 Wheel Drives Without Dropped Centers
 

Black Gaffers tape, lots of it. 2809 got stuck under the pyramid at GTRE, and kept spinning their wheels. They left us 4 nice holes to tape up.

Re: 6 Wheel Drives Without Dropped Centers
 

What about post-event. Any attempt at a more elaborate repair?

Re: 6 Wheel Drives Without Dropped Centers
 

Carpets are not reused event to event. Most are given away to teams.

Re: 6 Wheel Drives Without Dropped Centers
 

We tore a few holes at Bayou this year. Lots of gaffers tape is the only fix. The FTA will also normally have a nice talking to with the team telling them to stop damaging their field. (Sorry Eddie)

Re: 6 Wheel Drives Without Dropped Centers
 

I am seeing a lot of conflicting posts about versa wheels so could someone clarify a little bit? As far as I know the most defining characteristics are:

1.) Great traction in the standard forward backward drive directions
2.) Poor traction if robot is being pushed sideways while parallel to floor
3.) Great traction if robot is pushed sideways but tips up so that the treads "dig in" to the carpet

If anyone could confirm or deny these that would be very helpful. And if these are true then it appears that these would be perfect for 6wd w/o a dropped center since they should have minimal scrub due to their poor lateral friction.

Re: 6 Wheel Drives Without Dropped Centers
 

Here's some video evidence. At 0:54, 2834 hits 469, accelerating from about 4 feet away, and shoves 469 about 3 feet. 469 uses VersaWheels. (I got a good look at their drivetrain Saturday evening, about two weeks ago.)

As to your question about their use in the corners of a no-drop drive, I'll reaffirm what I said in the other thread, that I don't think that's a great idea. Consider a 6-wheeled no-drop robot with VersaWheels in the corners. For simplicity, assume the drive base is a perfect square with the horizontal center of mass at the center of the horizontal plane. Thus, the turning circle should touch the contact points of the VersaWheels. Now think about how the tread interacts with the turning circle. The tread looks like this:

-----------
~~~~~~
-----------
~~~~~~
-----------

more or less. And the turning circle at the patch, when superimposed on the tread pattern, looks like this:

-----------/
~~~~~/~
------/-----
~~/~~~~
-/----------

sort of. (Imagine the slashes as being a continuous curve.) When the wheels exert force on the ground, the easiest way for them to slide is sideways. But they instead want to slide along the turning circle! This means a significant portion of the force is wasted trying to turn against the "W" tread.

Of course, this is all theory. The best way to test this kind of drivetrain is to build it. I just don't predict that it'll work out too well.