Drive Wheel Sizes

I’m designing some custom chassis(es?) with traction wheels and I’ve been wondering what wheel diameter FRC teams prefer. I’ve been itching to go for 6" but I’m not sure if it has any real advantage over the stock of 4" wheels we have other than increased speed and ground clearance.

Please leave any information you think would be useful in the comments, thanks!

It’s going to depend on the game, and the robot design.

4" needs less gear reduction for the same ground speed than a 6" or 8", but is going to have the most trouble with obstacles (including plates under the carpet, in some cases). On the other hand, the inherently lower CG will help out if the robot somehow gets into a tipping scenario.

8" needs the most reduction to get to a given ground speed range, but will clear most obstacles easily. It also has the highest CG of the three main types.

6" is a good middle ground: some trouble with obstacles, but not as much gearing as 8".

It’ll depend a lot on the game. For a game like 2014, I would probably go with 4" wheels. For 2015, I would consider 4" wheels to be a gamble I didn’t want to take due to the platforms, so I’d be looking hard at 6" or 8", probably 6", or some other anti-high-centering device. For 2012 or 2013, which involved climbing ramps or avoiding plates, some careful placement or 6" wheels would be the way I’d choose.

Because this is an offseason custom chassis, you could also design it to use both types. Or design for whatever you happen to have handy–working on the cheap and all.

Let’s see
2015: (moving half of tetherbot): 4WD - 2 omni, 2 traction, all 4" VexPro wheels
2014: 6WD 3.25" colsons
2013: 6WD 4" VersaWheel
2012: 6" Mecanum - AndyMark HD
2011: 6" Mecanum - AndyMark Standard
2010: Crab Drive 6" Wedgetop Traction
2009: Crab Drive 6" Lunacy Wheels
2008: 3WD H config 8" omnis
2007: 3WD H config 8" omnis
2006: never seen the thing
2005: 4WD - 10" (I think) pneumatic

I voted for 4" because we have been using small wheels as of yet. Why? 2013 easy to buy. 2014 low COG. 2015 only VexPro omnis that could fit in our pancake of a robot.

EDIT: hmm we seem to have a peculiar drive selection pattern… just noticed it after posting this.

It depends on the game and the nature of the field (obstacles, plates, etc) as previously mentioned.

For a given frame length, consider the length of the wheelbase you would get by using 3 1/4 in, 4 in, 6 in and 8 in wheels. As described, 8 inch gives the highest clearance (and probably center of mass), but also gives the shortest wheelbase, not necessarily a happy combination.

Smaller wheels have smaller angular momentum so they spin up more quickly.

You can look at wheel size in two ways. The first is ground clearance. If you have a bump or feild obstaclesthat you need to traverse then you want to goconsider wheel size so you do not bottom out. When choosing a wheel size for clearance I would always go with the smallest size that works.

The second way to think about wheel size is it is the the final stage of gearing. For a give gearbox a 8" wheel will go twice as fast as a 4" wheel. So the smaller the wheel the less you have to gear down. The less you have to gear down the smaller the gears, and smaller gear and wheel weigh less.

I voted “Other”. Of our four competition robots, 3 had 6" wheels, and one had 4" wheels. Our air cannon has about 8" wheels. I agree with above comments that, in most cases, the best wheels are the smallest ones which allow you sufficient mobility over the obstacles you are likely to encounter.

Have you quantified the efect of the rotational inertia of the wheels vs moving a 150 lb robot?

Assuming a 150lb robot traveling at 18ft/s (5.5 m/s) with 6WD configuration
I calculated the kinetic energies of 4 different wheels based on these assumptions
I did not do anything pertaining to gears though.
Colson wheel is uniform cylinder of radius 2"
Traction wheels are uniform cylinders combined with thin rings with radii 2, 3, 4"
All wheels calculated were 1.5" wide and I used masses given on the Vexpro website

The kinetic energies stored in all 6 wheels of the 6wd combined are
4" Colson - 21.7 J
4" Traction - 22.7 J
6" Traction - 35.6 J
8" Traction - 43.3 J
150lb Robot - 1028.5 J

Why did I use kinetic energy you might ask? well because ultimately your motors are imparting energy into the system at some rate. The motors must expend the listed amount of energy (plus all the losses reaching that energy) for the robot to reach that speed.

To be fair, larger gearing (well, more stages at least) will accrue more loss that wasn’t accounted for, but I’m not going to draw any conclusions.

Smallest wheel you can get away with easily is optimal - less gear reduction needed means less weight spent in the drive gearboxes, not to mention lighter wheels.

If COTS gearing options or field obstacles dictate a 6" wheel being easier to get away with than a 4" wheel, then go for it. But otherwise, there’s almost no reason not to go as small as you can.

Smaller wheels also put a smaller moment on the axles when the wheels scrub during a turn. Less force on the axles is usually a good thing.