California Drive

[LondonBoy29]

In a California or West Coast drive system, how much lower should the middle two wheels be relative to the two in the front and the two in the back? What are the pros and cons of using California vs. Tank?

[Chris is me]

Never heard it called "California Drive" but considering your non-US location, it certainly makes sense as a name.

The "drop" part of a West Coast style drivetrain is actually common to most high traction tank drives, six or eight wheel. The center wheel or pair of wheels is lowered a small amount (between 3/32" and 3/16") to aid in turning while maintaining high traction.

[thefro526]

Just to add onto what Chris said, drop center 6WD's and 8WD's don't always have to be 'West Coast Drives' but the numbers stay true. Drop center six and eight wheel drives are extremely common - and very effective - using about any build method.

If you do a search on West Coast Drives or drop center drive trains you'll find all the info you could ever want and then some.

[LondonBoy29]

Thanks a lot. I have been reading about it, and I am wondering what is a live axle and what is a dead axle?

[Jon Stratis]

Originally Posted by LondonBoy29

In a California or West Coast drive system, how much lower should the middle two wheels be relative to the two in the front and the two in the back? What are the pros and cons of using California vs. Tank?

Using standard tank drive, where all wheels are the same height, leads to trouble turning. Picture your robot from the top. Now draw two circles representing the direction you want to turn - one will hit the middle of the left side of the drive train, the other will hit the middle of the right. As the robot turns, you'll realize that the wheels have to move sideways (called scrub). This motion is high friction, and reduces your ability to turn quickly or sharply. See the attached example for details.

Now, change this to a West Coast drive train with 3 wheels and the center dropped lower than the others. If you imagine adding a center wheel to the tank example, it now rests entirely on the circle it's traveling on. That means it can travel along the circle without any scrub*. By dropping the center wheel, you can essentially pick up the outer wheels. If your center of gravity is balanced over your center wheels, you'll probably see the robot rock back and forth on it a little as it turns - that's the small amount of friction when the outer wheels hit the ground causing them to pop back up.

If your center of gravity is NOT balanced over the middle wheels, then two of your wheels (either the front or the back two) will essentially be off the ground the entire time - giving you a standard tank drive between the middle wheels and the other two outer wheels. The advantage here is the distance between those 4 wheels is much shorter - in effect, they are closer to the ideal spot for turning, giving you less scrub. Having those two wheels off the ground all the time isn't a bad thing - there are still situations where they'll be used, like when you hit another robot, or hit an uneven surface of the field (like around the towers this year).


I've seen high traction tank drive robots that couldn't turn. West Coast robots can easily spin in place, giving you a turning radius of 0. I've also seen normal tank drive robots that do just fine... but they do require more power to turn than West Coast systems, power that can often be used elsewhere on the robot.


* In an idealized example where the contact surface area between wheel and floor are points. In reality, it's more of a rectangle and you have a small amount of scrub from one side of the rectangle to the other... but it's really not worth considering.

[EricH]

Originally Posted by LondonBoy29

Thanks a lot. I have been reading about it, and I am wondering what is a live axle and what is a dead axle?

A live axle has the axle rotating and used to drive the wheel. A dead axle is where any rotation of the axle is inadvertent; it's your standard chain-sprocket-wheel setup.

[thefro526]

Just to add some clarification on terminology to prevent confusion:

'West Coast Drive' is a term used to describe the type of drive train pioneered by 60 and 254 in the early 2000's (2004, if memory serves me). It's distinguishing features are rectangular tube construction, live axle setup and cantilevered wheels. Almost all WCD's have a dropped center to aid in turning (only notable exception was 2009 - the lower COF between the wheels and floor negated the need for a dropped center in most applications).

That being said, a dropped center drive train isn't always a WCD. Referring to all dropped center drives as WCD may lead to some confusion.

Also, on the same line of thought, 'Tank Drive' really refers to a type of steering and not a specific type of drive train. Technically, 95% or more of the Robots in FRC are Tank Drive, meaning that they don't have any sort of active steering - the two most common types of live steering in FRC are Car Style Steering (Front and/or rear wheels mimic those on a car) and Swerve Drive style steering - so referring to a drive as a tank drive isn't really all that specific. When discussing drive trains it helps to refer to them by wheel configuration since it reflects the performance of the drive much better, for instance, 4WD Tank, is very specific and easy to understand and visualize - anyone familiar with basic drive trains should be able to predict the performance of the drive if they're also given the wheel type.

[EricH]

Originally Posted by thefro526

'West Coast Drive' is a term used to describe the type of drive train pioneered by 60 and 254 in the early 2000's (2004, if memory serves me). It's distinguishing features are rectangular tube construction, live axle setup and cantilevered wheels. Almost all WCD's have a dropped center to aid in turning

I want to say that it was even earlier. I remember seeing it before the '04 season; I believe it was the '03 SCRRF Fall Workshops. 60 used 4WD in '03; not sure if 254 used 4WD or 6WD.

Competition debut was in 2004, when the aforementioned teams built identical robots.

[Travis Covington]

Originally Posted by EricH

I want to say that it was even earlier. I remember seeing it before the '04 season; I believe it was the '03 SCRRF Fall Workshops. 60 used 4WD in '03; not sure if 254 used 4WD or 6WD.

Competition debut was in 2004, when the aforementioned teams built identical robots.

Team 60 used the cantilevered output shaft/wheel design for many years prior to the 254/60 collaboration. I would argue that those designs were the foundation of today's “West Coast Drive." The addition of six/eight wheels and a lowered center wheel/wheels was the next obvious iteration. The elegance and simplicity of the design, however, was something that had been done by team 60 for many years before the prototype 6WD drive train that was used at Cal Games in the Fall of 2003 and the collaboration between 254 and 60 that occured in 2004 and brought about the now well known 6WD West Coast Drive..

[Trent B]

Originally Posted by EricH

A live axle has the axle rotating and used to drive the wheel. A dead axle is where any rotation of the axle is inadvertent; it's your standard chain-sprocket-wheel setup.

To elaborate.

Live axle is where the axle is suspended in bearings and the sprocket and wheel are rigidly attached to it (direct drive is also a live axle design using the gearbox shaft as an axle)

Dead axle is where the axle is any rod (in some cases just a standard bolt from a hardware store) and the wheel is on bearings with a sprocket bolted to it.

Another note, 6WD is often much better than 4WD especially in a drop center configuration because it takes an incredible amount of torque to rotate a 4WD system due to the long wheel base, whereas 6WD with a drop center configuration has an effectively shorter wheelbase.

[Ether]

Originally Posted by eagle33199

Using standard tank drive, where all wheels are the same height, leads to trouble turning.... As the robot turns, you'll realize that the wheels have to move sideways (called scrub). This motion is high friction, and reduces your ability to turn quickly or sharply.

Originally Posted by Trent B

it takes an incredible amount of torque to rotate a 4WD system due to the long wheel base

A force calculator (and, for those interested in the physics, a complete explanation with the equations) can be found here.

Just by seeing the name of this thread, I want to create a parody of Katy Perry's California Girls. Just imagine it: Cal-i-fornia drive, it's so incredible! No outer frame, live axel setup.

DSK, if you're listening, and have a female vocalist, PLEASE make this song!

[Akash Rastogi]

Originally Posted by SuperNerd256

Just by seeing the name of this thread, I want to create a parody of Katy Perry's California Girls. Just imagine it: Cal-i-fornia drive, it's so incredible! No outer frame, live axel setup.

DSK, if you're listening, and have a female vocalist, PLEASE make this song!

I'd rather have it go along to Tupac's *California Love....



*-(real music)

[MagiChau]

Originally Posted by SuperNerd256

Just by seeing the name of this thread, I want to create a parody of Katy Perry's California Girls. Just imagine it: Cal-i-fornia drive, it's so incredible! No outer frame, live axel setup.

DSK, if you're listening, and have a female vocalist, PLEASE make this song!

I will never be able to view West Coast Drive the same way ever again.

[Jim Wilks]

Originally Posted by Ether

A force calculator (and, for those interested in the physics, a complete explanation with the equations) can be found here.

Nicely done!