Outboard vs Inboard Wheels

Technical Technical Discussion
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We have been experimenting with a drivetrain this last offseason that was everything we never do in a 6WD: chain instead of belts, super small frame (>500 sq in), corner omnis, and outboard wheels as opposed to inboard wheels. Inboard wheels are within a rail of the frame, whereas outboard wheels have nothing between them and the bumpers.
Example of inboard wheels:

image
image1283Ă—744 381 KB

Example of outboard wheels:
image

My question regards how these two perform different. Why one over the other? What are the tradeoffs associated with each?

Thanks

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I think first you should know that the first type of design is the Kit of Parts Frame, also part of the AndyMark AM14U series. The second type which your team developed is known as a west coast drive or WCD for short.

The benefit of a WCD as far as I know, is that it allows greater customizability as you can really design the frame for how you want to design your robot. You generally have more bellypan room. You are able to customize to a greater degree your wheel size, wheel type, number of wheels.

The benefit of the KOP frame is that it works. It’s a tried and tested solution that works for the majority of teams.

Depending on how you approach your frame design, a WCD can be lighter than a KOP frame or vice versa. In the end it really depends what your team can do better. If you are comfortable with a WCD, by all means go for it. If your team already opted out of the KOP frame, you might as well go for a WCD otherwise it would be better to stick with a KOP frame.

The other thing is that a WCD is not necessarily smaller, you can make it larger or smaller depending how you want. Adjust the width and length.

In the end a WCD offers greater customizability over a standard KOP frame. If you google “West Coast Drive vs Kit of Parts Frame” you will find numerous threads discussing the advantages and disadvantages of each.

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I seemed to have interpreted the question differently.
first off: both examples are WCD. WCD is generally defined as a traction drivetrain with the center wheel dropped.

Correct me if I’m wrong @F3LINE, but the question here seems to be more of a debate between parallel plate/clamshell drivetrains (one plate on each side of the wheel, 971 2016, 610 2013) versus cantilevered drivetrains (the outboard one referenced). generally, parallel plate style drivetrains are better, since they are able to support both sides of the axle, and therefore balance the load of the wheel better. also, depending on the construction method, it can be lighter than WCD (I have no idea what the weight of 971’s 2016 chassis was, but it looked real light.

However, the hard part is that generally, parallel plate style drivetrains are much harder to machine, and more complex to design. many of these drive trains use CNC machinery, including CNC press breaks, because these parts need to be very accurate if they are bending them to put together, like how the KOP chassis is. On the other hand, cantilevered WCD’s are almost always made of rectangular tubing, is very simple, and can be made using a manual mill (you might be able to get away with a drill press and a jig, but milling is preferred).

However, if you are looking at comparing purely the KOP to a traditional, cantilevered WCD, then that a whole other ballgame, and is explained pretty well by @ProPain37’s post.

Overall, if you are looking at what kind of chassis to use this season, my recommendation is to go with what you are most comfortable with using, and what will cause you little to no headaches. the reason why many top teams use a cantilevered, rectangular tubing WCD is not just because of the advantages pointed out in this and the previous post, but because these teams have done it year, after year, after year, and can build one relatively quickly without issue. If your offseason project went well, and you feel like you could do that again with ease, then by all means go ahead, but otherwise a KOP might be better, and the rectangular tubing WCD can be an ongoing project for seasons to come.

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Just to get some definitions straight:

What you’re calling “outboard wheels” are usually referred to as “cantilevered wheels” because they are cantilevered off the drive tube. What you’re calling “inboard wheels” are known by a number of different names, but your best bet is probably “non-cantilevered wheels”.

 

Over the years the definition has gotten looser (no longer usually 2-speed gearbox, can use belts instead of chain). But cantilevered wheels is still definitely a requirement.

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Entirely avoiding the question of definitions (since, as Ari already mentioned, they’re kinda all over the board on this post so far):

Thoughts on Inboard:

  • More material: Heavier, but possibly more rigid without requiring a stiff bellypan
  • increased protection for chain and wheels
  • Larger surfaces for mounting other mechanisms
  • Motors (appear) mounted slightly lower - lower center of gravity is generally good.

Thoughts on outboard:

  • Less material: possibly cheaper, lighter, requires a bit more creativity to keep rigid
  • Fewer pieces of metal near the wheels = fewer constraints on wheel size
  • Potential for easier maintenance- wheels can come on and off easier (again assuming properly designed)

Most gearboxes will tend to mount to one or the other more easily. Yes, you could probably make any gearbox mate to any chassis, but it’s a question of how much custom manufacturing work you want to do.

Both inboard and outboard designs could be built up from mostly COTS parts. It’s worth noting this wasn’t always the case. The “WCD vs. KITBOT” discussion is heavily influenced by history.

In all cases, bumpers will go around the outside of the wheels, so they’ll have some protection. Both cantilevered and supported-on-both-sides designs have been shown to have high performance, assuming they’re manufactured properly. Due to this, I’d say selecting the drivetrain based on wheel position isn’t a relevant design criteria.

I generally would call the outboard-wheels picture “better” due to the fact it only puts metal where you need it. But, I’m also assuming you’ve got the manufacturing ability to do it well.

The conservative advice: Do what you know how to do, don’t try something new during the season. If you have proven experience building both, great! You have two options. Wait until Jan 4th, then pick based on the game (or flip a coin if you really have no idea). If you’ve only done one type in the past, do that again this year. Investigate building the other type in the offseason.

The aggressive advice: go for whatever looks best/cool/whatever, and have fun and learn some stuff!

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Hello F3LINE, this (old) thread may have useful information for you about the advantages of a West Coast Drive over a non-cantilevered wheel design like the modern kitbot.

A short summary of advantages on the 2nd image (a WCD design):
-pushes wheels to the edge of the frame, giving greater stability if designed well
-easier to maintain the wheels; you can take them off and replace them without removing axles
-lighter weight
-increased interior space for electronics and components

Disadvantages:
-requires customization, design, and perhaps special machining (such as a mill or cnc router). Your top picture of the kitbot can be put together with common hand tools and no expertise. This is less of an issue today than it once was, since products like Versa Frame exist.

The main tradeoff is an increased performance potential with a WCD vs a very well designed, durable, quick, and easy to build kitbot that is good enough to get most teams all the way to Einstein.

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One more thought that I haven’t seen made explicit: you will need to find an answer for bumper mounting within the (usual) rules. Simply tacking them on with big voids behind it isn’t going to fly, so you’ll need to think on how you can demonstrate backing with some additional (if maybe vestigial) framing.

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Really can’t say on the other drive train but with the am14U3 drivetrain you have displayed I can provide some notes
-Standard FRC Drivetrain and part of the KoP teams recieve
-Provide more protection given the outer rails although that’s what bumpers are for
-wheels are really difficult to remove once you have it together, even more so when you’ve got a robot already together on it
-really just a heavier duty drivetrain

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There’s a simple solution to that: think very carefully on what you attach to the outer rail. The last time I would’ve entertained an argument for mounting something besides bumpers and such to the outer rail on one of my teams was 2017 for extra fuel capacity, and even we kept ourselves on the inner plate.

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As noted above, chassis with outboard wheels are harder to provide a proper FRAME PERIMETER, especially with 8" and larger wheels. They also require two separated points of support (e.g. opposite sides of a tube) to support the cantilevered wheels. Inboard wheels are usually supported from both sides, which is a more conservative and robust answer. The advantage of outboard wheels is that they can be closer to the FRAME PERIMETER/inside bumper perimeter, providing additional width stability and a more advantageous length-to-width ratio for turning the robot.

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