Advantages/Disadvantages of West Coast Drive

FIRST General Forum
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Hello Mentors/Teams/Students. I’m hoping to learn more about the advantages and pitfalls for a West Coast Drive type of chassis/drive train compared to the traditional drive train. Which of you have recently transitioned from traditional drive train to WCD? Please share your experience… the good, the bad and the ugly!

What are unique design challenges? Maintenance issues? Challenges with buying WCD parts during the rush of build season? Cost? Reliability? Since we dont yet know what the 2017 game: is WCD drive better for certain playing field surfaces but perhaps not-so-good for other surfaces? Discuss some of the varoius versions of WCD.

Anxious to hear your thoughts!

Thanks

Jim

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What would be your definition of a traditional drivetrain? I thought that WCD was the traditional drivetrain…

3

Maybe 6-wheel drop-center KoP drive is traditional?

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4

First and foremost for me, WCD is a tube and gusset construction technique that is handy for making the full frame. VersaFrame as a system makes it simple without needing complex tooling.

Making bearing blocks used to be the toughest part for us. Again VersaFrame COTS parts have fixed that.

Mounting the gearboxes is the next challenge. There are COTS solutions for some of the drive train gearboxes. We have a milling machine to make custom mounting plates for any gearbox we use.

Design challenges: You can quickly design your base frame shape to fit the scoring section configuration. Cut and rivet the tubes… (to borrow a phrase) Boom. Done.

Maintenance: Wheels are easy to change if your tread wears out.

Buying parts: We commit to WCD style before seeing the game and buy tubing, gussets, and bearing mounts before the season starts. We’ve pondered swerve drive but haven’t yet committed the resources to develop one.

Surface: I usually consider WCD a 6 or 8 wheel skid steer system. There are several variations on this theme. However, the wheels don’t care which construction technique you use. Skid steer driving performance comes down to designing for scrub / traction forces, good construction, good control, and good weight distribution.

David

5

Pros:

  1. Reliability. If you have a 6-wheel WCD, you drive the center wheels directly off the output shafts of your transmissions. Even if every chain to the outer wheels fail, you should still be able to drive your center wheels.
  2. Maintainability. Because the wheels in a WCD are cantilevered, removing a wheel is as easy as removing a snap ring. No need to take apart your frame to get at a wheel.
  3. Simplicity. No outlandish parts or software required to use.
    *]Drivability. Driving a good 6-wheel WCD is incredibly smooth, thanks to the dropped center wheel.
1 Like
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I’ve found a good WCD to be far more rigid than other drivetrains. This gives it better, more consistent driving characteristics and allows you to run less drop.

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While my team has never built what I would consider a true WC drive train, we have incorporated at least one element of WCD in all but our rookie year. First of all, my understanding of the definition of WC vs traditional skid steer:

Common elements:

  • Both traditional and WC normally have 6 simple (non-roller) wheels with a drop center, though variants of each with additional wheels and/or omni on the corners exist.
  • There is no notable difference in software required; you have two banks of motors, one on either side. Either may have one, two, three, (or occasionally more) motors per side.
  • Either can be used with a single speed or shifting gearbox.
  • [Edit]Any articulated drive train does not properly fit either category, though an articulated gearbox may be extended from either design.

Distinctions:

  • West coast drives feature cantilevered drive shafts; traditional has a bearing on either side of the wheel.
  • West coast drives provide direct drive to at least one wheel on either side, usually (if not always) the one closest to the center of gravity of the robot; traditional usually had chain or belt to all wheels, though direct drive is now common in non-WC drive trains, including the 2014-2016 KoP chassis.
  • WCD usually employs tube (though occasionally channel with the opening on top or bottom) in order to support the cantilever; traditional usually employs channel with the opening inboard or outboard, or vertical plate, most commonly bent inboard or outboard at the top and/or bottom.

Note that there are also drives which incorporate these features mix-and-match. The common example which comes to mind first is the (recently discontinued?) Andy Mark nano tube chassis. It has direct drive on four cantilevered axles, but these are the two corners, NOT the one usually nearest the CoG. Our second and third year drive trains also featured direct-drive wheels on cantilevered axes, but for similar reasons were clearly not WC drive trains [Edit: Our 2013 robot only had two driven wheels and two idle omnis, whereas our 2014 robot had mecanum drive; we learned a lot from these mistakes].

Advantages of WC:

  • Easier to change wheels by removing a single lug screw, snap ring, shaft collar, or clip.
  • Reliability despite failure of chain, as the axle nearest the CoG is direct driven (though this feature is now on some newer “traditional” drive trains)
  • Track width can be a greater proportion of robot width, as there is no need for an outer plate/channel (other than to meet bumper support rules)

Advantages of traditional skid steer:

  • You are much less likely to bend an axle. (I never saw an FRC axle with a bearing on both sides of the wheel bent until STRONGHOLD.)
  • Requires less precision in machining (though this can now be mitigated with dollars by buying a COTS WC chassis kit).
  • [in recent years] Available through the KoP chassis program at little or no dollar cost, as a kit which can be assembled in a remarkably small number of worker hours.
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Well, let me tell you all about our experiences at 2363!

Quick Build Drive Train Concept

Triple Helix West Coast Drive Trains

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Here is my only question for you, has 254 ever not done a WCD?

I’d bet $100 that 254 does a WCD next year, and another $100 that that is what we settle on as well.

It is quick, easy, and robust. Most of the design drawbacks for a particular game can be designed around with WCD without much effort, this year it was wheel spacing.

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One of the great things about it is that it’s almost formulaic in design, which allows for super quick design and construction. Many of the problems have already been solved so you can focus on the harder elements of robot design. Obviously there are some exceptions (this year was a year of drivebase exceptions), but in a flat floor game, it becomes much easier.

1 Like
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The primary reason to build or not build a WCD versus a “traditional” skid-steer drivetrain is what your resources are. Some teams resources lend themselves to a more traditional drivetrain, some towards a West Coast Drive. It’s worth noting that time is most definitely a resources. Probably the most important one. Just because you have the fabrication capability and money to make something doesn’t mean it’s worth time investment during the season if it’s not an efficient use of your time.

The primary disadvantage of a WCD over a more “traditional” drivetrain is that it usually requires more precision machining (yes there are COTS options like the VersaBlocks that can let you build one with hand tools). WCD requires live axles and does not offer as many simple solutions for chain tensioning. Dead axles tend to be a little more forgiving, but nothing that can’t be achieved with time and attention to detail. Bumper mounts also takes more thought.

The primary advantage is typically maintenance and access, especially with replacing wheels and worn tread. There are some marginal performance benefits that can be gained by putting your wheels a little farther outboard than usual.

Sheetmetal construction methods are also generally not well suited to making WCD either, which is why you see many teams with sheetmetal sponsors opting for other drivetrain styles.

It’s worth mentioning that many of the challenges associated with building an effective WCD are obsolete due to new COTS options and the proliferation of exact C-C belts and chains. Machining the bearing blocks, developing a good chain tensioning method, wheel retention, ability to make custom wheels with hex bores and building a transmission with the correct output speeds on a live axle shaft are all aspects of WCD that are now available as COTS or able to be fabricated without a mill/lathe. These weren’t available even a few years ago, which made WCD out of reach or not worth the resource investment for many teams. At the same time, it also means that many of the advantages of a WCD are now available to “traditional” drivetrains as well, so the lines are blurred a bit.

From a COTS standpoint, if you’re not doing any precision fabrication yourself it’s still likely cheaper to take your KOP chassis in the kit or purchase the VexPro 2014 Drive-in-a-day (both of which are “traditional” drivetrains) rather than spend money on the various bearing blocks, bearings, etc for a WCD.

Don’t make drivetrain decisions in a vacuum, especially if your team already has a particular drive style that you like using. I’m not saying don’t switch, but be sure you’re considering what the cost of switching would be (time, money, learning curve, etc) and what problems you’re solving by making a change. Is it cheaper? Maintenance? Better performance? Fewer or easier to make parts? Then figure out how switching to a different drivetrain style addresses that.

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While WCD maintenance, on the whole, is quite nice, one very annoying aspect is access to the belts or chains, which generally are driven by pulleys/sprockets inside the gearbox. In the case of belts, the gearbox has to be taken apart for any maintenance that requires removing the belt or putting on a new one.

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Do you not run into this issue in other cases but with having to take wheels off instead?

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I don’t recall having to take a gearbox apart to change belts or chains on a traditional drive train. Robot chassis, but not the gearbox. Then again, I have seen drive trains which were described as WC which had easily replaceable chains, and others where it was apparently impossible in an FRC pit period - or six. I would list this more as a “gotcha” of too-well protected chain and belt, not a WC feature per se.

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Yes, but it’s usually somewhat easier to slip an axle out than to take a gearbox apart.

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As a matter of fact, yes, I believe they have.

BEFORE the WCD was developed (2002-2004 timeframe), of course, but seeing as 254 is one of the developers of the WCD…

The WCD was officially unleashed in late 2003/2004 season, the result of a collaboration between 254 and 60. There are photos in CD-Media from 2001 showing both teams running elements of what is now known as a WCD, though 254 for some reason ran their chains outside their frame from a central gearbox that year (either one of which would, under the traditional definition, disqualify the drive as being WCD), and neither had more than 4 wheels on the ground (also disqualifying as a WCD under the traditional definition, which requires 6 (or more, depending just how traditional you want to get)).

So, more detailed answer: yes, but it’s been quite a while and they were developing WCD when they did.

And now back to your regularly-scheduled discussion of the pros and cons of the WCD.

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Here is my only question for you, why does “254 does it” make something a good design decision for a team without 254’s resources?

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Because it does not take many resources to do a WCD, and by 254 doing it, they are proving that it is a good solution. Which means that is a good solution for not much resources. (Man my grammar was bad in that sentence.)

We are a low resource team, like really low resource, yet we do WCD with success.

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I think our definitions of “not many resources” differ. I’ve been on teams where the build budget was maybe $1,000, the machining resources stopped at that of a disused wood shop, and the mentor expertise didn’t really include machining. I would wager my dollars to your donuts that that situation describes More Than A Couple Teams. I would also say those teams are less likely to have an inspiring season if they reached for WCD without more of a grasp of the situation than “254 does it and does well”.

20

Then that would have been a better argument than “254 does it”.

I’d disagree with that argument since it’s not a function of raw number of resources but more a function of what type of resources and the allocation of them. For teams with tight budgets and resources using the KoP or VP DIAD may be better uses of tight resources. I don’t see how 254-copy describes advantages or disadvantages for the purposes of resource allocation.