(Mid)west coast drive?

Hi all!

This year, our team opted with a 6 cim 6 wheel west coast drive as our drive system. Because our shop monkeys didn’t want to work with chains, we are using belts, and because our shop monkeys wanted easy access to our belts, they are outside the sidewall tubes, making our cantilevered shaft rather long. The FEA indicates this will be fine.

One of our mentors thinks that our drive system’s pushing power could be improved by eliminating the belts to the front and back wheels and using the space where the pulleys used to be on the center axles for … more wheels.
Thus the center axles would have triple wide performance wheels, and the front and rear axles would have a single performance wheel each. Only the center axles would be driven.

His explanation of why this would still give us excellent traction (6 wheels in contact with the ground) while improving turning (we have enough weight this year to get our CG more or less directly above center with a specifically lightweighted steel underbody plate) made a lot of sense to me.

I haven’t seen anyone else try this, though, so I’m curious as to what the community might think. Are we just being silly?

Unpowered wheels rob you of potential traction. You want every weight-bearing wheel to contribute to motive power.

Your maximum pushing force is equal to,

Mu * M * g

Where Mu is the friction coefficient of your wheel material on FRC carpet, M is the mass of your robot carried on driven wheels, and g is gravity. This assumes your 3 CIM gear boxes produce enough torque to slip the wheels. If that is not true, then your max pushing force will be lower. In either case, the number of wheels touching the carpet is irrelevant unless that number is less than 2 in which case you may have other problems.

Your mentor’s suggestion has the potential to reduce your maximum pushing force, but it might be the lesser of two evils if your cantilevered shafts aren’t up to the task.

As long as you are on ONLY the center wheels… You should have similar traction to the situation in which all of your wheels are driven.

The moment one of the blob driven sets of wheels touches the ground and stays carrying weight, you have less traction than if all were powered. The reason being that your weight is now being split between the two sets of wheels. So your coefficient times weight goes down, because that only refers to the weight on driven wheels.

There are 2 interactions occurring at the wheels that determine traction: ‘simple’ friction, and the more complex interaction of the engagement of the carpet and the tread. The former is dependent on the normal force applied on the driven wheels and the coefficient of friction. The latter is dependent on, among other things, contact area of the driven wheels.

The driven contact area will roughly be the same for both configurations (six wheels worth of contact), so the maximum traction from that interaction with only the centers driven is AT MOST as high as that as when all wheels are driven. Traction from friction will (almost) always be less when only the centers are driven. So, as far as traction is concerned, all wheels driven is preferable.

I don’t know how it would help with turning, but you could test it if you have time to see if there is any improvement.

Now, traction and turning aren’t the only factors involved. Weight, reliability, and maintenance should also be considered. When all factors are accounted for, it might benefit you, or it might not.

Another option is to only drive the middle and back, and shift your weight rearward. Assuming the center wheels are lowered, the front wheels would usually be off the ground so your traction would normally be the same as it is with all wheels driven. there would be brief periods where the traction is reduced, but it during forward acceleration and pushing (when it is particularly important) it would be maxed out.

Think about it this way: if you don’t power the outside wheels, they will spin freely. So when someone pushes you from the front, they won’t resist, they will just spin. However when somebody pushes you from the side, they will resist in the same way that a power wheel would. However, this also means that you will have more trouble turning, because when you turn the force is perpendicular to your wheels, resulting in the same effect of having it not matter if the wheels aren’t driven. On the other hand, if your robot is light and you don’t weigh it down, that will hurt your pushing power in all directions significantly and increase your turning ability.

It’s up to you to decide how this info will affect your decision.