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.

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.

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

Greg McKaskle

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?

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…


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.

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.

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.

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:

View of the outside of the prototypes:

Wheel assembled with a lexan plate showing how it goes together:

Inside view of a wheel plate:

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.

[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.



[/off topic]


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?

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.

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.

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. :stuck_out_tongue:

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.

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.

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.

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

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.

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.