6 Wheel Drives Without Dropped Centers

[KrazyCarl92]

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.

[cmrnpizzo14]

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.

[Jeffy]

Originally Posted by cmrnpizzo14

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.

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...

[mman1506]

Originally Posted by cmrnpizzo14

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.

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

[cmrnpizzo14]

Originally Posted by mman1506

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

I heard other wise from the highest traction wheels thread:

Originally Posted by T^2

They have good grip in the forward axis due to the "W" pattern, but horrendous grip in the sideways axis. This was confirmed for experimentally for us at Championships. (It wasn't our robot, mind you.)

Originally Posted by cmrnpizzo14

I heard other wise from the highest traction wheels thread:

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.

[coalhot]

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.

[Travis Hoffman]

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.

[M. Mellott]

Originally Posted by Travis Hoffman

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.

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).

[Jared Russell]

Originally Posted by coalhot

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.

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).

[coalhot]

Originally Posted by Jared341

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).

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...

[Ether]

Originally Posted by Jared341

tore a couple holes in the carpet

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

[Jared Russell]

Originally Posted by Ether

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

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.

[Ether]

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

[Gregor]

Originally Posted by Ether

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

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.