4" vs. 6" + 8" Diameter Wheels

[Gdeaver]

There is a difference in the contact patch. Also the carpeting is 3 dimensional. The wheels sink into the carpet. So how do the different size wheels affect the contact patch for traction and the ability to turn?

[Jared Russell]

Various teams have done experiments to measure CoF with different wheel configurations. The data that I've seen (it was given to me by another team, so I will let them post it if they choose to) was quite surprising - wheel diameters and widths can make a big difference! Definitely worth experimenting with.

[James Critchley]

Still not clear.

Originally Posted by James Critchley

Then I'm spinning the wheel under no load...

No load means no load. The robot is on jack stands. If radius drops out here, then it's not coming back as the model becomes more complicated.

The bottom line is that an increase in rotational inertia of large wheels alone is not a valid reason to choose smaller wheels. It is an increase in total mass of the robot or (EDIT) increase in rotational inertia attributed to (END EDIT) mass distribution (EDIT) and mass (END EDIT) of the wheels that contributes to any performance degradation. These and all of the other excellent reasons cited in this thread push for smaller wheels.

[Ether]

Originally Posted by James Critchley

It is an increase in total mass of the robot or mass distribution of the wheels that contributes to any performance degradation.

YMMV, but I think it's clearer to explain it the following way:

The equation for vehicle acceleration in the PDF file attached to this post can be re-arranged as follows:

a = (tau/r) / (I/r² + (1/4)*(M_v+4*M_w))

or, substituting K*M_w for I/r²

a = (tau/r) / (K*M_w + (1/4)*(M_v+4*M_w))


Since tau/r remains unchanged when you change the wheel radius (if you change the gearing accordingly), what remains is

a) the total vehicle mass M_v+4*M_w, and

b) the mass of the wheel times the mass distribution factor of the wheel (K*M_w)

Since neither (a) nor (b) is likely to remain the same when you change wheel size, there will be a change in vehicle acceleration.

The equation also gives a way to estimate the change.

[Paul Copioli]

All,

While the calculations in this thread appear correct (I haven't checked them against mine thoroughly, since mine are done a little differently), they really aren't the main reason for going to small wheels on our team.

It is all about the huge constraint placed on us in FRC for max robot weight. While the wheel weight effect on overall robot acceleration is low (if you do the math you will see), it is significant to help meet the weight restriction in FRC. The max weight restriction is really the driving force for us to go to smaller wheels. Second is the smaller packaging. Robot acceleration is not on our list of reasons.

Paul