Ideal Wheel Diameter: 4 or 8 Inches?

What are the benefits/disadvantages of 4- or 8-inch wheel diameters?

I will start you off by saying consider the weight and space that each wheel takes up.Then consider the weight and space that the specific gearing for each wheel takes up.

Hint: Smaller wheels take less gearing to achieve the same speed as larger wheels

It depends… And I say the ideal diameter is 6 inches. (But that’s just me.)

The real answer is, it depends. What the terrain is, what the gearing before the wheels is, how many wheels there are, and what type of wheels/orientation are all going to be big factors.

4" wheels will need less gear reduction off the gearbox for the same speed, and are lighter, in general. However, the tradeoff is ground clearance and climbing ability (2" obstacle max without some sort of assistance, which could be a 6WD drop center rocking back). I’m not saying that a 4" wheel can’t climb, mind you–however, placement and number of wheels need to be considered before evaluating climbing ability.

8" wheels need more gear reduction for the same speed and will weigh more than a 4" wheel. However, they increase the ground clearance and climbing ability–at the expense of a low CG. Again, this can be designed around.

6" wheels are in the middle in terms of gear reduction and weight and climbing ability and ground clearance. They’re better than a 4" at ground clearance, and better than a 8" at weight and gear reduction.

The real question is, what does the game require?

Everyone’s opinion will differ. I’m a strong believer in choosing your wheel to fit your design, not vice-versa. A completely flat-floored game like logomotion would be a good example of a game where a team could use four-inch wheels to save space and weight. Rebound Rumble, on the other hand, seemed to have called for larger wheels to make traversing the midfield barrier an easier task. Gearing also has a lot to do with wheel choice. In 2011, team 3556 used eight-inch wheels to achieve a theoretical maximum speed of around 17Ft/s but switched to six-inch wheels in 2012 to both lower their robot’s speed and to increase the pushing power of their 'bot. This allowed their robot to handle the bridges and midfield barrier more easily.

I hope that this makes a little bit of sense, and helps answer your question. Feel free to correct anything that I said that may not be correct, or just seems nonsensical!

  • Chris Nettles

In general small wheels make for a lighter drive system. They use less gear reduction to get to the speed you want them to be at and are lighter. In addition, you can usually use less stages in your gearbox which means a more efficient drive.

I don’t think that 8" wheels are necessary(keep in mind this comes from a team that used 8" wheels on our 2012 robot). You can usually get by with using slightly larger wheels but more of them(still lighter and more efficient). For example in 2010 many teams used 8 or 10 wheel drive with wheels no bigger than 4.5". In 2012 many teams got by with relatively small wheels, an 8wd, and some mechanism to get over the bump(ramps under the robot and or push down wheels). 971 even used 3.5" wheels with a 6wd and their push down skids.