Single Flywheel Gear ratio

VEX V5 General Discussion Technical Discussion
#1

Hey Forums,
I’m seeing alot of the best teams out there with single flywheels have some form of 4’’ wheels on their flywheel. I tried a 25:1 and a 21:1 with 4 inch wheels but wasn’t getting the results distance wise. At full power, I hit 1-2 tiles away from the Net. Last week I tried 5’’ wheels, and got the distance, but however 5’’ wheels don’t package well into my robot. Any advice on what gear ratio to use on 4’’ wheels to get a full court shot and a high fire rate ?

Thanks

#2

How many motors are you using?

If the 5" wheels worked well, here is an over simplified way to convert it to 4" wheels

5" Wheels: C = 15.71"
4" Wheels: C = 12.57"
15.71/12.57 = ~1.25 or 5/4!

That would mean that with the 4" wheels you would need 1.25 times the gear ratio you had with the 5" wheels. However, this is way too simplified, and there are many more factors that come into play, most notably the compression of the ball and traction with the wheel. Have you tried playing with changing the compression to see if it makes a difference?

#3

Thanks for the reply.

I’m using 4 motors on my 4’’ wheels.
I’m using barely any compression on the flywheel, with the antislip mat for extra grip. I have tried higher compression, but my fire rate goes really low. Currently I have around the same firerate as 323z’s robot, but I can’t do full field with low compression.

I’m also not sure if its possible to fire full field with my current setup of
4 torque motors on 4 4’’ wheels geared 25:1.

#4

You could use rubber bands on the wheels.

#5

I have also tried rubber bands, and it only slightly helps the distance.

#6

So yeah 25:1 on 4 inch wheels is too low a gear ratio, especially with little compression. One quick change you could make could be changing the torque motors to high speed motors, resulting in a 40:1 overall, which will definitely get you full court. I’m not sure if this is too high or not. We have used 35:1 with great success, which is the equivalent of a 28:1 on 5 inch wheels.

#7

one thing Kevin forgot is that KE, kinetic energy, is also dependent on the mass. The formula for KE = (Mass* velocity^2)/2.
Calcs for gear ratio:
(note that I use metric)
The big 5" wheel weighs in at 160g, while the smaller 4" wheel weighs in at 90g.
The big 5" wheel has a circumference of 39.9 or about 40cm.
The 4" wheel is at 31.9 or about 32 centimeters in circumference

KE= (Mass* velocity^2)/2
Velocity = meters/ seconds
At one rotation per second, you would have a rotational velocity for each wheel equal to .4m/s for the 5" wheel, and .32 m/s for the small 4" wheel

So lets plug those values in: ( All values in standard meters, kilos, seconds)
4" wheel has a KE at one rotation per second of:
(.090*.32^2)/2=.004608 joules

5" wheel has a KE at one rotation per second of:
(.160*.4^2)/2= .0128 joules

Dividing .0128 by .004608 Gives us the ratio of the required gearing- 2.78 times the gear ratio is required to run one 4" wheel vs one 5" wheel.
This is why you commonly see people run two 4 inch wheels- it brings the required gear ratio to only 1.39 times the required ratio.
Hope that helps!

#8

I’d recommend 33.6:1, 35:1 or 40:1 on 4 inch wheels.

#9

Thanks for all the answers!
This gives me a much better understanding to approach my flywheel!

#10