WCD vs Standard

[MattC9]

Originally Posted by SuperNerd256

All in all, WCDs are lighter, allow for direct drive (more efficient), and the wheels a bit farther out for better turning. 6WD vs. 8WD is mainly based off of whether or not the robot has to traverse an object (climbing over the bumps in '12 and '10). 8WD also allows a smaller wheelbase, so that a robot can be more maneuverable, but the turning difference is often nothing a 6WD can't do, and the extra weight isn't worth the little bit of turning help on a flat field.

Do you have solid proof that WCD are lighter than any other drive?

Originally Posted by MattC9

Do you have solid proof that WCD are lighter than any other drive?

No outer framing. Basically eliminates about 86" of outer railing (assuming a 37" long by 27" wide bot). Center wheel is direct drive, so less chain (6 chains per average 6WD, 4 for your average 6WD WCD).

[BrendanB]

WCD's are known to be lighter but they aren't always lighter.

[Tom Line]

Originally Posted by SuperNerd256

No outer framing. Basically eliminates about 86" of outer railing (assuming a 37" long by 27" wide bot). Center wheel is direct drive, so less chain (6 chains per average 6WD, 4 for your average 6WD WCD).

Could you show the mathematics for that? West coast drives have to carry the same amount of force that a standard chassis chassis does. Keeping in mind that both have the same load requirements, I see no reason that one can't be engineered to the same weight as the other. West coast style introduces a rotational component that needs to be accounted for that astandard chassis does not require.

Nothing stops a team from direct driving on a standard chassis, and a standard chassis does not require bearing blocks.

[Walter Deitzler]

Okay, new questions to ask.

In my WCD design I have designed the wheels to be far enough from the outside to allow for interchangeable wheel sizes of 4in, 6in, and 8in. Is this a good idea? I have read somewhere that you use 4in wheels to save weight, go faster, and keep your wheels as far to the outside as possible. Is this true? Should I design my WCD to use one type of wheel?

Also, can you weld the AM Flanged bearings to standard aluminum wall? (6061 I think).

Thanks!

[Mk.32]

Generally speaking the bearings are pressed into the alum, or sometimes held in place with super glue if needed.

[Garret]

Also, can you weld the AM Flanged bearings to standard aluminum wall? (6061 I think).

I am pretty sure that those bearings are steel not aluminum, so welding them to aluminum isn't really possible (As far as I know). but anyways, there are far easier ways to mount bearings than welding them, as Mark already said they can be pressed in or super-glued. On our robots we usually hold the bearings in with a screw and washer that covers part of the flange.

The smaller wheels allow for smaller gear ratios and sometimes fewer reductions in the gearbox, this usually results in a more lightweight gearbox. I do not know about keeping the wheels as far out as possible as a motivation for smaller wheels, I would usually try to do that anyways regardless of wheel size (just my preference no real reasoning on my part behind it).

[Gray Adams]

Don't weld or weld near bearings. The grease has a bad habit of lighting on fire.

Originally Posted by LedLover96

So you do want to put your smaller wheels farther to the outside, to widen your wheelbase? Putting them in the same spot you would put an 8in wheel would be pointless, I am assuming, because the wheel base would be the same, but you wouldn't get better speed?

And what is "press fitting"? Could you elaborate.

You need less gearing for 4" wheels though, and they have a lower mass (and therefore moment), so it can save you weight even if you don't put them further out on the frame.

Press fitting refers to having a hole that's just slightly smaller than the diameter of the bearing, so when you press it in, it doesn't come out. It's kind of tough to figure out what size hole to make and then make it accurately (within a few tenths).

[Walter Deitzler]

Originally Posted by Gray Adams

Don't weld or weld near bearings. The grease has a bad habit of lighting on fire.

Well, good thing I asked, thanks!

[Gregor]

Originally Posted by LedLover96

I have read somewhere that you use 4in wheels to save weight, go faster, and keep your wheels as far to the outside as possible. Is this true? Should I design my WCD to use one type of wheel?

Also, can you weld the AM Flanged bearings to standard aluminum wall? (6061 I think).

Thanks!

Smaller wheels will in fact make the robot go slower at the same gear ratio. Wheel size shouldn't be considered in terms of speed, as you should be adjusting your gear ratio to account for the wheel size. However the 4" wheels will save weight and widen your wheelbase, as opposed to 6" and 8" wheels.

Typically, teams press fit bearing, as opposed to welding them.

[Walter Deitzler]

Originally Posted by Gregor

Smaller wheels will in fact make the robot go slower at the same gear ratio. Wheel size shouldn't be considered in terms of speed, as you should be adjusting your gear ratio to account for the wheel size. However the 4" wheels will save weight and widen your wheelbase, as opposed to 6" and 8" wheels.

Typically, teams press fit bearing, as opposed to welding them.

So you do want to put your smaller wheels farther to the outside, to widen your wheelbase? Putting them in the same spot you would put an 8in wheel would be pointless, I am assuming, because the wheel base would be the same, but you wouldn't get better speed?

And what is "press fitting"? Could you elaborate.

[Garret]

And what is "press fitting"? Could you elaborate.

Press fitting usually means (at least how I have interpreted it in FRC) the hole is slightly undersized such that the bearing needs to be pressed in quite hard, this will result in a tight fit that will hold the bearing in place.

[Adrian Clark]

Ok, multiple things:

1. If you need to hold in a bearing that is not a press fit the standard approach is to use bearing loctite, not glue or welding. Although glue is a much better idea than welding. But why are so keen on holding in your bearings?

2. A press fit is not necessarily undersized, it could also be exact size. The difference between press and slip fit depends on surface finish as well as hole size. If you have a hole that you made exact size via a rat tail file chances are that's not going to be a slip fit. I caution you if you press in your bearings, too tight a fit can make a bearing seize up.

3. I would not suggest having a drivetrain with swap-out wheels with the size variation that you're talking about. If you have clearance for a 8" wheel but you use a 4" you're just not utilizing your frame space, the idea is to get the wheels close to the ends of the frame. There is no way you can swap out a 8" wheel with a 4" without changing the gear ratio. The fact that you want to do this makes me think that you didn't fully plan out your ratios and wheel sizes to your desired torque/speed. I suggest you calculate your drivetrains speeds using your different wheel sizes and you'll see how big of a difference wheel size makes.

4. The advantage of small wheels is that they're light, they lower your CG and they require less reduction. Don't use them unless you calculate your robots speed using them, or you might end up with a very slow bot.

[R.C.]

Originally Posted by MattC9

Do you have solid proof that WCD are lighter than any other drive?

Matt,

I can't think of another drive that would be lighter than a WCD other than a super pocketed sheet drive. I'm curious to see the weight comparison and sheet drive.

But WCD is decently light due to the fact:

-No outer Railing
-Minimal amount of parts.
-Smaller parts: Wheels/Sprockets/Gearboxes etc....

We switched over to WCD in the fall of 2010. Never looked back (thanks 973!), its such a nice system that 254/968 has perfected over the years.

-RC

[sdcantrell56]

While not the traditional wcd, doing one with fixed wheels and belts would without a doubt result in the lightest drivesystem in FIRST.

That said there is really no right or wrong answer. Do what works best for you. If your main manufacturing resources are sheetmetal then WCD might not be the best choice. It is possible and we have actually run a sheet WCD before but it's not the best use of resources.