WCD vs Standard

[Gray Adams]

Originally Posted by LedLover96

So, doing more research on CD, I have come across bearing mounts called "Sliding Bearing Mounts". What are these exactly. It seems as if they are used for chain tightening, but I have no details about how they do this.

The idea is that the wheel, axles, sprocket, and bearings are all part of a block that slides back and forth along the chain path to adjust tension.

Here is a section view from our offseason project last fall. If you look carefully inside the tube, you can see the space the bearing block has to slide back towards the gearbox and loosen tension (to the right).

Also, here's a view of how we kept the bearing block in place. http://i.imgur.com/VrDEJ.png
It worked, but it really wasn't the greatest method. It should get you thinking though, and we came up with it halfway through building it.

[smistthegreat]

Originally Posted by LedLover96

Okay, so in the 4th link, I saw that WCD's use a live axle, vs. a Standard using a dead axle.
First off, could some elaborate on the difference between the two? I found the posts kind of confusing as to the difference (the live axle is spun by chain or gears or motor while dead axle is just sitting there, not powered?)

Next: Why would you choose a live axle over a dead axle, if the center wheel is lowered, thus making your robot rock between the front and back wheels? It seems that live axles (if I have their definition correct) would make the 6 wheeled work like a 4 wheeled when it turns, porblematic. But, when observing many WCD's, I do not see slick wheels or omniwheels to offset the traction/turning problems associated with a 4 wheeled.

All of your information is correct, but you make a couple of incorrect conclusions. Yes, live axle is when the axle itself is powered, and dead axle is when only the wheel is powered. Also, you are correct about the drop center making it turn like a 4 wheeled drive. However, this 4 wheeled drive now has a shorter wheelbase, making it easier to turn. The whole point of dropping the center wheel is to shorten the wheelbase enough to where it turns smoothly without adding slick or omniwheels

[EricH]

Originally Posted by smistthegreat

All of your information is correct, but you make a couple of incorrect conclusions. Yes, live axle is when the axle itself is powered, and dead axle is when only the wheel is powered. Also, you are correct about the drop center making it turn like a 4 wheeled drive. However, this 4 wheeled drive now has a shorter wheelbase, making it easier to turn. The whole point of dropping the center wheel is to shorten the wheelbase enough to where it turns smoothly without adding slick or omniwheels

And the other implied conclusion--dead axles don't need a drop--is also an incorrect one. A 6WD, dead or live axle, without the drop will act almost exactly like a 4WD with all traction wheels (there are exceptions, like FRC25); so will an 8WD without any drop.

[Garret]

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).

How are you getting 86" I would think that you are only eliminating 74" of railing, though I have never built a WCD so I may be off. Also I have seen plenty of plain non-cantilevered 6WD robots with direct drive on the center wheels.

However I do know that with the WCD it allows for wheels to be removed much easier because the shafts do not need to be removed.

[Walter Deitzler]

Originally Posted by EricH

And the other implied conclusion--dead axles don't need a drop--is also an incorrect one. A 6WD, dead or live axle, without the drop will act almost exactly like a 4WD with all traction wheels (there are exceptions, like FRC25); so will an 8WD without any drop.

What does 25 do? why are they an exception?

[EricH]

Originally Posted by LedLover96

What does 25 do? why are they an exception?

6WD flat. I don't know how they do it, but they don't bounce when turning, even with high-traction wheels. I think it has something to do with how they groove their tread, but I'm not sure.

At least, they used to do that. I'm not sure if they still do or not.

[MichaelBick]

For weight it honestly depends: if you are running tube then WCD is a pretty good choice to save weight. If you are running sheet there is really no need to run a WCD. However wether you run sheet or tube you always should drop the center wheels and should direct drive the center wheel.

[Kristian Calhoun]

Originally Posted by LedLover96

What does 25 do? why are they an exception?

Originally Posted by EricH

6WD flat. I don't know how they do it, but they don't bounce when turning, even with high-traction wheels. I think it has something to do with how they groove their tread, but I'm not sure.

At least, they used to do that. I'm not sure if they still do or not.

Jared from 341 made a really good post about our drive train about two years ago: http://www.chiefdelphi.com/forums/sh...5&postcount=31.

[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).