Surface area of a flywheel

Does the amount of surface area of a flywheel help factor into how far the ball goes?

It can, a bigger (read: more massive) flywheel often is more stable than a smaller one (but usually requires more energy to get it going).

The biggest factors that contribute to the distance of a flywheel is:

  • RPM. Should be at least 2000 RPM, probably higher
  • Compression. Especially important this year with hard balls that won’t really compress on their own
  • The efficiency of power distribution. I noticed I go shoot much further at lower motor levels once I successfully implemented my control loop. You should make sure that your flywheel is on its own power expander, so it has its own breaker and battery

Surface area itself matters for gripping the ball. So where that matters and within the limits of how it matters, yes, it will factor in.

Surface area otherwise doesn’t matter in and of itself. However, many other factors that do matter will likely go alongside surface area. If you use a wheel with a larger diameter, the outer edge will have a greater tangential speed than that of a smaller wheel spinning at the same angular speed. For the same width, that larger-diameter wheel will have more surface area. Theoretically, you could build a new wheel with a larger diameter and the same surface area, but we’re limited in what we can use. If you have more wheels to increase rotational inertia, the wheels won’t slow as much when firing the ball, so the ball will speed up more. More wheels will increase the surface area. Theoretically, you could build a new wheel with a greater rotational inertia and the same surface area, but we’re again limited in what we can use.

Becasue we’re talking about mass, what about screwing steel slide tracks (the heaviest piece I think VEX sells of that variety) onto the sides of the flywheel in equal mass around the spokes of the 4” wheels?

Would that do well to increase mass of the flywheel? Or would it result in too many variables being present to be useful when compared to the time it would take to get the surface area spread equally? (Not building one, just want to know in case I do in the future.)

Would it not be better to just add more wheels? Most discobots teams in NbN had four wheels on their flywheel. I don’t think that’s quite needed for this year, you could probably get away with only two or three

I’m sure it would be.

But I’m just thinking for less lateral space. It’s probably too complicated for the purpose it would serve.

You’re a little confused. We’re not talking about mass. We’re talking about rotational inertia, the rotational equivalent to mass, but not mass itself. Rotation inertia depends on the mass and the distribution of mass, the further from the axis of rotation the greater the contribution to rotational inertia you get from any bit of mass. Distance from the axis of rotation actually matters more than the mass itself. The nice thing with the wheels is that much of the mass is concentrated at nearly the full radius. If you’re adding mass by attaching metal roughly at half the radius outward, you’ll need to add somewhere in the vicinity of 2.5 times the mass of the wheel to get the same rotational inertia you would get just by adding one more wheel (adding 1 times the mass of the wheel).

I wasn’t confused, I just worded it poorly. But I see your very valid point. It wouldn’t do much good. Thanks.

I would say people obsess too much over increasing the MOI of their flywheel. 3 4" wheels or 2 5" wheels works just fine, no need to add extra pieces. That adds drag and not much else really; you’ll also have a longer spin up time at the beginning of auton and driver control.

I’ve never actually built a competitive flywheel, so please forgive my ignorance. I have yet to take any physics courses, so I’ll just shut up and listen to what you all experienced builders have to say. :slight_smile:

No, no worries at all. I didn’t mean to come off that way. Having a high moment of inertia to a certain extent is good so that the flywheel doesn’t slow down while it’s in the process of shooting. But when people increase their moment of inertia too much, then spin up time is much longer, and there’s more wear on the motor every time you start up the flywheel. I’m happy to try to answer any other questions you may have @Got a Screw Loose

I’ve seen teams with two 4” wheels spinning at 4200 rpm or something like that. How would that compare to your 3 4” wheels, or 2 5” wheels? How many rubber bands should be coating the flywheel? Are they there to add grip or to actively increase MOI? Or both?

Rubber bands don’t add too much mass, but it probably depends.
When I did a double flywheel (it was of poor build quality, but it worked) during NbN, we needed the rubber bands to get the correct spacing/compression between the four wheels (each flywheel was made of two stacked 4" traction wheels). They also acted as good grips, but the rubber traction wheels do basically the same thing whether there are rubber bands or not.