Flywheel trouble

I was wondering if anyone can look at my flywheel and see why my rpms are low. I have a 1:49 external gear ratio with the turbo gearing and two motors.

Do I need to add more motors? And if so, do I add them to the same axel?


First, I would set the motors to torque, as the setup you have now is effectively a 118:1 ratio, with a free spinning speed of 11800 rpm, which is much too hard for the motors to turn. Second, I would recommend building a more stable structure for testing as to help with friction. Also, you can add motors however you want, whatever best makes sense for your size constraints.

Hi nalobots,

As sazrocks says having the motors on speed is probably a bit overkill.

Also, from looking at your video it looks as though you have one motor on each of the 84 tooth gears - is this the case? If so then this could bring a lot of issues into the system as one of the motors is powering the wheel on a 1:7 ratio, and the other on a 1:49 ratio.


I agree. You can’t use a plate like that and hope the warping of your structure is going to be acceptable. The warping will put stresses on the bearings and shafts that generate tons of friction. If you must use a plate, then stiffen it with some C-channels somewhere. Otherwise get some 5-hole C-channel or figure out a way to connect your motors to regular C-channel and build everything so it’s rigid. When the motors are physically disengaged, your flywheel system should be able to spin freely for at least 5-10 seconds when you spin it by hand, even when you put some stress on the structure.

When it comes to these speeds using Vex parts, building quality is really going to matter.

Especially for safety of people and objects (expensive objects!) around the flywheel.

This advice from Vex Vortex is pretty helpful too:

We’ve found that a 3:49 single wheel with high speed motors (which is what 8059 was using) is perfect (a tad too strong with 4 motors full power) for full court shots. It’s at about 2600 RPM.

  1. your motor layout is not going to work, because the motors are attached at different parts along the gearing chain this is going to make on motor have to drag the other one along, potentially damaging motors (we shredded a motor earlier this year by doing this)

  2. you need a really solid structure when doing this, a very well built low friction system is important, because at these speeds losses dues to vibration and friction are significant

  3. you are asking way too much torque from your motors, the problem is not that your gear ratio it too low, it is that your motors don’t have enough power to spin at their full speed. the gear ratio we have found that work is speed motors(1:1.6 internal) with a 1:21 external gearing, using the five inch wheels

We also found this to be the case.

A 1:50.4 flywheel (1:21 on turbo) was tried by someone at our school and that ratio was also much too high.

Thanks for the replies. I will adjust my flywheel when my new motors come in.

  1. Make sure all of your bearings are in place, it sounds like an axle is hitting metal. Make sure your bearings are exactly in the center.

  2. Use highspeed motors, not turbo. It is easier to mess around with those gearings. I recommend 3:49 if that is a 4 inch wheel

  3. Strengthen your structure. Also, it does help to test a flywheel with both wheels and a ball. (Assuming you are doing a double flywheel)

  4. Is that a 5-inch wheel? If so, i recommend 3:35 ratio.

Good Luck! :wink:

As others have mentioned, your gear ratio is too large. I would use 8059A’s gear ratio as a starting point for yours.

In addition to adjusting the gear ratio, you need to reduce vibration. Vibration creates friction between the axles and bearings which wastes energy. Depending on how secure your robot is, it may also cause the robot to move which will reduce the accuracy of your shots. The plate that the motors are mounted to needs to be secured in order to reduce vibration; this can be done with standoffs.

In addition, while some teams have had success with the 5 inch wheels, they cause vibration which also wastes energy. At the high speed a single flywheel design requires, the rubber tire starts to flex, the wheel becomes imbalanced, and it vibrates. Zip-ties will reduce this effect, but not remove it completely.

A better flywheel choice would (surprisingly) be an omni wheel. These are solid, and hardly vibrate at all. One of the teams I mentor built a flywheel launcher with 2 motors and an omni wheel that was quite good. It could consistently score from the opposite corner of the field, although the delay between shots was quite bad (about 2 seconds). More motors would help with this. (I’ll leave it up to you to figure out the gear ratio.)

Finally, another way to improve the efficiency of your flywheel will to be to add some kind of high-friction surface to the backboard opposite the flywheel. Anti-slip mat works well. This seems counter-intuitive, but imagine there was no friction at all on the backboard. The ball would simply spin in place, slipping on the backboard. Without a high-friction surface, your ball will slip against the backboard, energy will be wasted, and your ball will not go as far.

Hope this helps. Good luck!

Well for starters the whole gear system is overkill, 2nd it’s not stable at all, using flimsy to keep the gears meshing isn’t good. Make a solid structure out of some c-channel or fat C and bring the ratio down a lot.

Just a thought –

Would it be feasible if you used a flywheel a 5-inch high traction wheel, except that you don’t use the rubber tire? It would make the wheel a bit less than 5" wide, but it would make it a real solid wheel.

I would try this idea out if I can manage to get the rubber tire out of the wheel

Good question. There is an issue, however. The friction between foam and plastic is not very much. So when the foam ball is launched, it will likely slip against the plastic flywheel, and not go as far as it could have. This is only my best guess; I encourage you to try this design to see how it works in the real world.

another thing to remember is that the rubber provides a lot of mass to the momentum of the flywheel so you would lose out on that. Not that this isn’t fixable. but its just something to be aware of

Speaking of mass

Do you know how to add more mass to the current 5-inch wheels? Our team has tried stacking 2 5-inch wheels together but it seems that it is too heavy (and too bulky for our robot design)

While adding mass might seem like a good idea, it actually isn’t. Adding more mass to your flywheel (from what I can tell mathematically) will increase the time between shots, not decease it. Yes, a flywheel with more mass will slow down less when it fires a ball, but the motors will also have to work harder in order to spin the flywheel back up to speed. As it turns out, the effect of slowing down the flywheel is more than the effect of speeding it up.

It seems that you may have already verified this, but you seem to still think adding mass is a good idea. May I ask why?

As stated above, increasing your flywheel mass is, from what I can tell, not good. So this is not an issue.

as I understand it, in theory, adding mass has no effect at all, accelerating a ball to X velocity take Y energy, and the motors must put Y energy back into the wheel.
So while a heavier wheel might slow down less, you need to put more energy into it to speed it up, a lighter wheel will slow down more, but needs less energy to speed up.

Im speaking from personal experience when i say mass is better than a faster speed up. the flywheel I’ve prototyped has less vibrations and the accuracy is much better the heavier the flywheels are. This may be because the ratcheting mechanism we are working on may exaggerate vibrations and the mass counters that. simply put it relies on your design but Ive found mass is better.

I was speaking about the time it takes for a flywheel to get up to speed. This should be less for a lighter flywheel. In my experience, it is totally possible to make a very accurate flywheel that is not very heavy (one four inch omni wheel). Also, a properly designed flywheel should not vibrate at all. I think you have some other issues with your flywheel you should look into.