Wheels: Metal or Plastic?
 

Last year we used the AndyMark plaction wheels to good advantage. In previous years, the metal IFI wheels were quite good as well.

I'm wondering what others think, in terms of pros and cons, of each of these wheels.

Plastic: They don't break, but the thin lip holding the tread on sometimes chips off. Riveting is not as easy. Driving on pavement chunks them out severely

Metal: Not inexpensive. Never had one bend, but tread *seems* to wear faster. Easy to rivet. Tread takes more brunt of pavement. Seem heavier.

Your thoughts?

Re: Wheels: Metal or Plastic?
 

I have thought a great deal about this, and I think plywood is a very suitable material for FRC wheels. Attaching tread is easy, manufacturing is cheap, and the strength to weight ratio is high.

I plan on doing some experiments with this if I have some free time, but I think 188 has used this design successfully in the past for sure. I have a CAD model somewhere which I believe beats most aluminum wheels I have seen for weight.

Re: Wheels: Metal or Plastic?
 

Sounds like you have the trade-offs pretty well figured out Don.

I'd sure like to see more wood wheels....they worked on wagons for centuries...

Re: Wheels: Metal or Plastic?
 

:eek: If it can support a hummer, it can take anything. But honestly the skill to make a good wooden wheel has been lost throughout the ages. Its probably better to pick up some carbon fiber wheels. They are the future:p

Re: Wheels: Metal or Plastic?
 

For one FRC season? You might need a few more than some. Probably 2 for every time you go over the bump and crack a wheel, plus 4/6 for each time you put it in the crate.

Re: Wheels: Metal or Plastic?
 

Now seems as good a point as any to bring this up. What material you should use is a function of what your requirements are. One of these requirements is the way you are loading your material. Another is your strength to weight ratio. Yet another is cost. For most FRC teams what tools you have is also a significant factor. Some teams have the ability to fabricate really nice wheels out of wood/metal that serve their purposes. Others, not so much.

As far as woodworking being a lost skill: http://www.chiefdelphi.com/media/photos/24424 I think not. Never underestimate the skill of an artist working with their preferred medium.

Re: Wheels: Metal or Plastic?
 

:p I guess I have been proven wrong. But it does take significantly more effort and skill to make multiple identical wooden wheels by hand than to make plastic ones (Or you are gonna prove me wrong again, I never made wheels before)

Re: Wheels: Metal or Plastic?
 

we have used custom aluminum wheels for several past challenges and found them to work very well.

Re: Wheels: Metal or Plastic?
 

Who ever said that hand was the only way to cut wood? If we're going to go by hand, then we've got to compare hand-formed wood to hand-formed plastic--and the plastic won't win by much. Making a mold or gouging plastic is going to take a lot of time.

If it came down to a race to build identical wheels, one set from wood, one from plastic, one from metal, and one from carbon fiber, from scratch, I'd go with metal being the fastest, and wood being a close second. In probable order of finishing:

Metal: Depending on the metal, cheap(ish), strong, and can be easily worked with given a lathe, mill, or CNC variant of the aforementioned tools.

Wood: Cheaper than metal, can be strong, and could also be done on a lathe, mill, or CNC variant of those two. Just make sure you have wood bits in when you do, cut it a little bit big, and sand it down the last couple thousandths.

Plastic: Once you have the mold (cheapest way to make a lot of identical wheels) or the CNC program (guess what else can be formed on a CNC?), pretty fast. It's the mold that'll take a while, if you're molding. Reasonably strong.

Carbon fiber: High strength to weight ratio--but low strength to wait ratio in this application. Also rather brittle if you hit it wrong. Great for airplane wing spars, not so much for wheels of any sort, and can take a while to shape and set. Also note the cost... The other wheel types are driving merrily around long before the first wheel comes off the first mold.

We'll assume that everything else is equal for this setup.

Re: Wheels: Metal or Plastic?
 

With the everything being equal...I can machine wood and plastic a LOT faster than aluminum. Maybe because my lathe is small and old, maybe because plastic is a lot softer?

Re: Wheels: Metal or Plastic?
 

Carbon fiber might surprise you with how durable it is. Check out this company that makes CF wheels for cars: http://carbonrev.com/impact_strip.html

Re: Wheels: Metal or Plastic?
 

Using COTS wheels, and keeping the environments in application for each wheel identical, I prefer metal. AM performance wheels are nice in that they are easy to cantilever on live axles and there are only 2 pieces to the wheel.

This thread begs the question: does it matter for omni vs. skid drive trains?

Re: Wheels: Metal or Plastic?
 

We have used all kinds of wheels in the past. For a while we were even making wheels out of lexan, cut on a laser cutter. Had a good grip on carpet but melted with a lot of slip. We have used aluminum with tread (belting)attached also manufactured. The shaft design and implementation changed over the years but it allows us to customize for wheel encoder mounting and bearing implementations. I would stay away from plywood particularly in a game like this past year. The constant stress coming over the bumps would have stressed the laminations and left little slivers of wood all over the field. I would guess that plywood would weigh more than the aluminum version we are currently using.

Re: Wheels: Metal or Plastic?
 

I posted a picture during build season of the wheels we were using this year:

The thread with the picture is here and it has all the information on how we went about fabricating them: http://www.chiefdelphi.com/media/photos/34720

The wheels worked surprisingly well and with a few more tweaks this will be a viable option for us going forward. We may be looking at making a 4 cavity mold so we can pump these out even more quickly. I estimate in one saturday we could have 20+ fully completed wheels done and ready.

As for other wheels I've used in the past, metal (aluminum) has always been our go to wheel because of the stability. We've always made our own wheels so we can control the dimensions that are critical to us and cut down on interfaces (hubs, extra bolts, spacers, etc.)

-Brando

Re: Wheels: Metal or Plastic?
 

Making a wheel out of composites (carbon fiber) just seems excessive and difficult. It's just a wheel, no need to go overboard. Especially considering how the material fails when you drill into it for tread attachment and stuff.

I'd be tempted to use more aluminium wheels, since you would need more material in a plastic wheel to make it as rigid as an aluminium wheel, and I can't think of a time I would want a wheel to be "less rigid". Machining it from solid seems time consuming; I wonder how long it would take to make some with outer "plates" and AL tube though. I think 1625 does something like that.

If I'm buying wheels, AM Plaction wheels are great. The right price point, they worked well this year, tread changing was easy enough, and light enough.

Re: Wheels: Metal or Plastic?
 

We've used both metal and plastic wheels in some of our previous drivetrains and I haven't really noticed a functional advantage from either material.

Back in 2006 and 2007 we used the IFI High Traction wheels (Now sold under the VEX Pro brand) because they were one of the only COTS High Traction wheels at the time.

Since then we've used AM Kit Wheels and Plaction Wheels and all of them have held up fine. We'll probably continue to use Plaction Wheels in the future because of their low cost and durability.

I guess if we were making our own wheels we'd probably go with some sort of Aluminum though, just because we have more experience with it than plastics.

Re: Wheels: Metal or Plastic?
 

I know that CF construction SEEMS difficult, but when you explore your options, gain a little experience working with composites, and perhaps settle on a design that isn't 100% carbon fiber you might realize that it's not so bad to work with.

Re: Wheels: Metal or Plastic?
 

We won't be leaving aluminum anytime soon; Our wheels are very light, and the runtime on the CNC is pretty short.

Re: Wheels: Metal or Plastic?
 

Shaker has used pultruded fiberglass for two years; we know it works. It just seems simpler to buy wheels or to make them out of aluminium than to figure out a way to make them out of composite materials. We'll be happy to hook those AL wheels into a fiberglass frame, though. :D

Re: Wheels: Metal or Plastic?
 

That's not all that dissimilar to what the IFI traction wheels are.
And it's even more similar to how 612 used (still does?) make their wheels. Except they use(d) PVC rather than aluminum. They gave the wheels great reviews the first time they used them (2007, iirc) and I know they've used the concept again. I do remember a drive failure they had in the semi-finals at VCU in 2007, and it might have been a wheel, but I'm not sure.

I'm interested in a different take on composite wheels. Not wheels made from a "composite material" (carbon fiber, etc), but rather a wheel made from multiple materials. In order to reduce weight, many teams use less material in locations that experience less stress. I'm curious about using a different material in those locations could possibly lead to even greater weight reductions. Or course, this requires more machining and some system to attach the two materials, but it's an area that I don't think has really been explored yet.

Re: Wheels: Metal or Plastic?
 

It's been a while since I checked with the guys fabricating them, but with a CNC running at decent speeds the runtime for our wheels is less than 10 minutes, for sure.

Re: Wheels: Metal or Plastic?
 

One could take some tubing, machine a rim profile into it, then use radially oriented bolts to connect it to a spider (hub and spokes) made of a different material. This eliminates the (IMHO) silly practice of milling a huge chunk of billet into a wheel, which is just not practical in the real world.

Re: Wheels: Metal or Plastic?
 

When racing teams want to save weight on wheels, they go with magnesium or "mag wheels" -- similar to the cRIO housing.

Greg McKaskle

Re: Wheels: Metal or Plastic?
 

Plaction wheels have worked well for us. However, more important than the hub is the material that touches the playing surface. The conveyor belting works but, we are looking into better materials. Has anyone tried other tread material that they like better than the conveyor belting?

Re: Wheels: Metal or Plastic?
 

My original question was intended in the context of "buying" wheels, not fabricating them. They ARE just wheels.
Or, for those concerned about the impact resistance of Carbon Fiber, how about some metal reinforcements in critical spots? If we were inclined to make wheels, this is the direction we'd be heading. A clear focus on the required capabilities of each part of the wheel, and then choosing the best material to meet those requirements, and finally figuring out how to join them together...

Re: Wheels: Metal or Plastic?
 

Really?

There are many industries where machining a wheel from billet is *exactly* what they would do, hands down, everytime (it'd certainly be cheaper and faster than carbon fiber, which those same industries would also not hesitate to do).

For most mass production purposes, they might not be cost effective (really depends on the size, and the value to the user the wheels provide), but they easily could be.

Also, they're clearly cost effective for AndyMark, priced at a point verrrrry tempting for teams.

Re: Wheels: Metal or Plastic?
 

Can you please be specific about what those "many industries" are?

And AndyMark, an excellent company, supplies wheels almost exclusively to FRC (as far as I know they are not an OEM supplier to any manufacturer), and at $26/whack the performance wheels are far from cheap. The Skyway wheels (injection molded I believe) are less than half the cost with integral tread. If you were picking wheels for anything other than a 1-off robot you would definitely pick the skyway wheels over the "performance" wheels because they would be much more cost-effective.

Machining from solid chunks of metal is very costly because there is a large amount of wasted material and a lot of time is spent removing that material. These are both bad for your bottom line, and bad for your carbon footprint.

How many machines made in the real world use wheels machine from machined billet? Cars (including race cars), bicycles, airplanes, roller blades, skateboards, office chairs, commerical robots (i.e. packbot) etc. all seem to use wheels not machined from solid chunks of billet. Sure, you can buy car wheels machined from solid billet, but they're more expensive (by a factor of 2 or more) and heavier than forged wheels.

Machining wheels, or any part of reasonable size for that matter, from a solid material is typically found in 1-off, prototype, or development scenarios. I can't imagine these types of machines accounting for anything other than a very small fraction of any market.

Re: Wheels: Metal or Plastic?
 

You'd be surprised at the amount of production-run items that are fabricated in machine shops on milling machines.

Go to any local machine shop, and chances are 99% of the parts there aren't onesies and twosies for prototyping, but rather part of a 50, 100, 500, 1000, or 10000 part production run. Why?

Injection molding is cheap, but it's not a universal solution. It's good for cheap, low-tolerance stuff. But there are a lot of components out there that require high-precision, high-tolerance components in order to function properly. For these parts, they are often fabricated in machine shops. But because these products are often industrial-to-commercial/industrial or industrial-to-military parts, you won't see them in Walmart anytime soon. But these products do exist, and they do keep machine shops humming.

But precision isn't the only reason that machine shops remain in business; often times injection molding or sandcasting is much more expensive than milling out of billet stock if you're talking about low-volume or intermittent product runs. Injection molding has a lot of tooling and setup costs, and doesn't become cheaper than milling until you're made quite a bit of parts.

And even then, a sizable chunk of business I've seen at a local machine shop was taking forged/sandcasted parts, and then milling critical features into it, like facing off flange surfaces, drilling and tapping mounting holes, etc.

Besides, there isn't that much of a waste from machining out of billet stock; the chips get recycled. That, and (without seeing numbers) I'd rate machining stock out of aluminum much higher than plastic injection molding on the environmental/CO2 scale. The former is infinitely recyclable and does not need petroleum as a basic ingredient, whereas the latter can only be "downcycled" a few times and needs petroleum-based hydrocarbons as a basic ingredient.


P.S. This post was typed up on a MacBook Pro computer, which has its entire frame/case milled from a billet of 7075 aluminum.

Re: Wheels: Metal or Plastic?
 

177 has been do this since 2007 I have even posted several pictures on the forum showing how the design evolved.

We use 4" OD 1/8" wall 6061 tubing for the rim and 3/16" 6061 plate for the sides.

We actually put the rim profile in the pocketed side plates by having them extent past the tube, which makes assembly easier and reduces machining and rim weight.

I hope this saves anyone looking at doing this a few design iterations.

The evolution of how the wheels assembled:
http://www.chiefdelphi.com/media/photos/31102

View of the outside of the prototypes:
http://www.chiefdelphi.com/media/photos/31103

Wheel assembled with a lexan plate showing how it goes together:
http://www.chiefdelphi.com/media/photos/31105

Inside view of a wheel plate:
http://www.chiefdelphi.com/media/photos/31104

Re: Wheels: Metal or Plastic?
 

I'm not saying that machining doesn't have it's place, I am also not saying that injection molding is end-all, be-all solution. Many parts do have some machine work done to them, but milling something out a solid piece of billet is generally inefficient.

The wheels Peter Matteson shows are a good example of a wheel with much less machine time invested, and much less wasted material than machining from solid billet.

Re: Wheels: Metal or Plastic?
 

[off topic]

A big up and coming thing right now are bioplastics like PLA (polylactic acid), which are made from corn as opposed to being petroleum based.

I've tried to introduce these types of earth-friendly plastics into a bunch of the projects I've been apart of.

PLA

Bioplastic

[/off topic]

-Brando

Re: Wheels: Metal or Plastic?
 

Thanks a lot for posting this. This method is really cool and something I'd like to try and duplicate this year.

One question: Are the side plates held on with a press fit in the last iteration, or are they eventually welded?

Re: Wheels: Metal or Plastic?
 

While they are a slight press to assemble the final hold is a series of fillet welds along the lightening pattern of holes. The lightening holes expose the edge of the tube to facilitate this.

This years modifications included adding hex drive hubs to some of the wheels.

Also the miss-alignment of the lightening patterns on both sides is to provide tool clearance to get a ratchet inside the wheel to add a lock nut if the threading in the Al side plates is stripped.

Re: Wheels: Metal or Plastic?
 

Aerospace companies and defense contracts would not hesitate to machine from billet.

There is a Packbot wheel sitting on my desk, it's machined from solid plastic in a very process that requires multiple ops.

Re: Wheels: Metal or Plastic?
 

Oh, great. Instead of making things out of fuel, make them out of food? Yeah, that's a big help. Now instead of just making a luxury like energy more expensive, now everyone ends up having to pay more in order to do something absolutely vital like eating. :P

I'm [mostly] joking, of course. Renewable resources are almost always better than fossil ones, and disruption to the food economy from corn-based ethanol production has pretty much been worked out by now.

Re: Wheels: Metal or Plastic?
 

True, some parts are machined from billet when they have to be. Though those are two industries known for spending quite a bit of money.

Re: Wheels: Metal or Plastic?
 

Considering the amount of land devoted to the production of corn, the solution is arguably not-that-much better. Now, if they could reduce the ammount of corn-based blah in our food supply, replace it with natural stuff (such as cane-derived sucrose instead of high fructose corn syrup), and remembered the revolutionary practice of crop rotation, we could have better tasting, more diverse food and whatever corn isn't sold as corn could be used just for a select few applications such as the production of bioplastics and ethanol.

On the topic of wheels, small wheels for light duty applications can be easily made from layered disks of strong corrugated cardboard (shoeboxes made of corrugated cardboard are often made out of 'strong' material [the edges and corners don't crush easily]). If properly laminated and reinforced**** & covered with roughtop or wedge top, this design could be somewhat competative with other wheels, particularly those around or below 4 inches in diameter.

**** Honestly, lamination and reinforcement is probably the what will doom every attempt to implement this, but I could be wrong. Integrating lexan, nylon, fabric, carbon fiber, etc. into the design could help.

To give anyone who looks into this a tip, look into tripple layered corrugated cardboardike the kind used to ship large items such as pianos (some where I heard about a cardboard boat that was rather successfull and made out of the stuff. The people who ran the team owned a piano shop.

EDIT: I should probably have mentione that direct attatchment to the axle is a bad idea for these wheels, the bore would probably get destroyed. Using some sort of hub to distributre the stress around the wheel should be a much better solution.

Re: Wheels: Metal or Plastic?
 

Peter, how do you hold your sprockets and bearings in place on those wheels?
Thanks
Bruce

Re: Wheels: Metal or Plastic?
 

This decision depends on the game but personally, I will always go with alumnium for wheels over plastic, especially with terrain obstacles. This is for a variety of reasons but mostly becuase our drivers beat up our robots and we find alumnium just holds up better. Not saying that ABS or UHMW won't hold up becuase I've personally used both and seen other teams use them perfectly. Generally, we'd rather not take a chance on failure in our drivetrains and like to over engineer them. We've had only 2 drivetrain in match faliures in 3 yrs. 2008 nothing went wrong (in fact that dt would still be in tact with the original chain installed 01/08 had we not dissasembled it.), 2009 we lost a chain once, and in 2010 we had one of our tracks break but it was due to manufacturer defect. As far as the material on the wheels, we like the wedgetop, but have tried gum rubber and roughtop. Wedgetop while not the higest friction of them all is easier to get on the wheels then the gum rubber and the friction diffrence between roughtop and wedgetop is pretty much negligible from the data I've seen.

-Drew D.

Re: Wheels: Metal or Plastic?
 

We used a press fit and then designed our wheel spacers to ride against the inner race as a back up. It was really basic but easy to assemble.

Re: Wheels: Metal or Plastic?
 

Don,
Our history with IFI alum. wheels and AM plaxtion wheels sounds pretty similar to yours. Aside from all the discussion about materials and homemade wheels, to your original question, I would have to say after using plaction this year, we prefer them. They took an incredible pounding with the way we often came off the bump, and held up to shocks that I think would have bent an IFI wheel. The slight compliance of the plastic makes the bearing press-fit a breeze, no tweeking the hole size. The split assembly means you can easily widen the wheel to any width you want, not limited to stock widths. If you cut your own treads from bulk belt like we do, that is nice. Clamping a slightly over-width tread between the wheel halves really locks it in. No tedious assembly of the wheel, and it doesn't fall to pieces when you remove the screws. Yes, it may be slightly harder to rivet through the thick rim (although plastic is easier to drill), but 1/2" long rivets carefully located between spokes workes just fine.

So let's see.....lighter, cheaper, stronger, variable width, easy bearing fits, no assembly. Plaction.

Re: Wheels: Metal or Plastic?
 

Where do you get your bulk belting and how do you cut it to width?
Bruce

Re: Wheels: Metal or Plastic?
 

Mcmaster search "incline conveyor belting"
and tread down on a horizontal bandsaw works great for us

Re: Wheels: Metal or Plastic?
 

Bingo Aren.

This stuff http://www.mcmaster.com/#incline-conveyor-belts/=9gagfi cut on a bandsaw with a wood blade

Re: Wheels: Metal or Plastic?
 

We typically cut our tread on a table saw, but have used bandsaws and even a foot-actuated sheet metal shear in the past (with mixed results).
I believe Pink (233) cuts its tread on a waterjet.

Re: Wheels: Metal or Plastic?
 

I prefer going to a shop that specializes in conveyor belting because they have a wider variety of materials to choose from than McMaster.

We have used razor blades, hacksaws, tin snips, a bandsaw and other methods but what still gives us the cleanest most precise cut is a pair of scissors that are very sharp.

Re: Wheels: Metal or Plastic?
 

When you buy this belting from mcmaster, which material do you order? I see the rough top comes in several varieties. We used the rough top tread that comes with the plaction wheels this year to great advantage but I am not sure exactly which material in mcmasters catalog it is.
Any thoughts? I'll probably send an e-mail to andymark if you guys dont know.
Thanks
Bruce

Re: Wheels: Metal or Plastic?
 

I'm 95% sure the material is nitrile.

Re: Wheels: Metal or Plastic?
 

I believe AndyMark sells natrual rubber tread. The Nitrile is usually blue or black.

I would suggest trying different options to find what works best. The answer is not always obvious, and we went through many materials to arrive at the material we swear by now.

Think about how the material is used and what it does. Don't just use what everyone else does. Treat this the same way you would treat any other engineering problem and look at trade offs between materials and what the requirements are. I have rarely found a team that comes up with the correct reasons for choosing a tread material.

Re: Wheels: Metal or Plastic?
 

The roughtop my team got from AndyMark was blue.

Re: Wheels: Metal or Plastic?
 

You should change that when you're on the red alliance.

Re: Wheels: Metal or Plastic?
 

I stand corrected. I've only noticed them with the tan colored tread shown on the website.

Re: Wheels: Metal or Plastic?
 

we use mechanum drive for our competition bots