most "epic" motor ever

[Peter Matteson]

Originally Posted by MattC9

Why were the globes ever ruled out?

They've been discontinued for several years. We were still using them for a few years after they were out of comercial availability.

[theprgramerdude]

Originally Posted by artdutra04

2.5" CIM motors, followed closely by the Banebot RS775-18. While much praise has already been espoused about the versatility and durability of the CIM motors, we've found that the Banebot RS775-18 motors are close runners up. Unlike the other BB motors, the RS775's are champs and won't give up the ghost just because you told them to run at stall for a few seconds. During testing, we learned they'll trip out the Jaguars or circuit breakers long before they'll suffer any harm.

Putting 8 CIMs in a FRC drivetrain won't really accomplish anything other than to deplete one's battery faster. This is because of the major difference between your example of cars and FRC robots: Most FRC robots are traction limited, while cars are usually torque limited. Adding more powerful engines to cars yields better results because torque-limited cars have a large capacity void between what the stock engine puts out and the maximum power than can be utilized by the wheels to make the car go.

On the other hand, adding more power to an FRC drivetrain may actually lower a team's effective pushing power, as dynamic friction is almost always lower than static friction. (Think back to Lunacy). Thus, by having more motors in a drivetrain, they will increase the odds that the motors will be able to overcome the maximum static friction with the ground and start spinning their wheels, thus lowering their ability to push other robots.

Now even if they can overcome the traction limitations, they still have to deal with the relatively small field size. How likely is it that robots will have the room to accelerate to speeds beyond the top speeds of 15-16 ft/sec already seen on many FRC robots? Why do you think teams don't currently use three or four speed gearboxes that can theoretically take their robot to 20, 25, or 30 ft/sec? It's because the field is small enough such that gearing for that speed would never be useful.

Don't forget the huge voltage drop that can occur if the CIM's change their input voltage to fast; it's pretty easy as-is with 4 CIM's to brown out the system if you go from stop to full power. With 8, or even 6, the current limit starts to become a major factor too.

[Tristan Lall]

Originally Posted by artdutra04

Thus, by having more motors in a drivetrain, they will increase the odds that the motors will be able to overcome the maximum static friction with the ground and start spinning their wheels, thus lowering their ability to push other robots.

Doesn't that depend on whether the driver is blindly giving it full throttle (spinning the wheels), or modulating the output in proportion to what the robot is doing? (Or better; the cRIO is handling this for him.) And in the case where a robot is geared rather high, and therefore is using a high power setting to push as hard as it can, by having more motors, the load will be distributed among them. Although the output force is limited by traction, with more motors, each will each be operating at a more desirable point on the efficiency curve; that gives the robot more endurance.

So there's a trade: is the added complexity, weight and opportunity cost (in terms of not having those motors on another mechanism) worth it? In drivetrain-heavy games where robots are operating at high power most of the time, my sense is that this is often a good decision.

Originally Posted by artdutra04

Now even if they can overcome the traction limitations, they still have to deal with the relatively small field size. How likely is it that robots will have the room to accelerate to speeds beyond the top speeds of 15-16 ft/sec already seen on many FRC robots? Why do you think teams don't currently use three or four speed gearboxes that can theoretically take their robot to 20, 25, or 30 ft/sec? It's because the field is small enough such that gearing for that speed would never be useful.

Though I don't know how often this figures into teams' design decisions, I'd say that the current state of the art with FRC transmissions is such that none can shift fast enough to make more than two speeds really valuable. Though that's far from an absolute rule, I haven't seen any 3+ transmissions that can shift fast enough to make the unpowered time between shifts worth it for accelerating a robot. Even many two speeds are (counterintuitively) sacrificing acceleration for endurance (because the ratios in each gear mean less total power consumption, due to motors operating at more efficient points).

For example, a servo-shifted Super Shifter with default configuration, mounted in place of the Toughboxes on a 2010 kitbot probably should just stay in high gear most of the time: there's no point waiting a half-second for the shift to happen.

So, returning to the constraint of limited field size, and recalling the previous discussion about motors, I'd say the most practical way to achieve really high speed (in the current FRC game) is lots of motors and a high gear ratio.

[JesseK]

Arthur, I agree with you about traction limiting to a point. Pushing wasn't the concern -- high speed ramming is (hence, the highway safety analogy). More motors nets more acceleration in shorter distances, meaning the occurances of high speed ramming would go up. Could we adapt? Sure. Yet why should we have to adapt to that when the overall build season / competition experience in its current form is already stressful enough?

[nuggetsyl]

Add a poll to this. Then you will see the results teams think.

[11douglash]

Vandoor Motor, hands down.

Yes, it's old. Last included in 08 kit. But by god, there's a reason that we joke that is has more torque than a toyota camery in the camery's highest gear.

[galewind]

I'm going to have to say that size to punch, I am REALLY REALLY liking the BaneBots rs775-18 motors. They're just darn cool motors, and they're a lot more versatile than people think. Just make sure your power leads aren't grounded to the case.