FRC Blogged-Kit of Parts Drive System Option

[Ether]

Originally Posted by Madison

Two stages of gearing, each 90% efficient -- .90 * .90 = .81

Are you sure that's not just a coincidence?

The "Speed Loss Constant" in cell I5 is independent of the gearing in cells C12 thru D15, or the Drivetrain Efficiency in cell J5.

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[Chris is me]

Originally Posted by Ether

Does anyone know where the 81% number came from?

If I remember correctly, I once read a really, really old CD thread that said 81% was determined by taking one of 229's drive bases, running it to top speed, and measuring that speed. Ratio of that top speed to actual speed became the default "speed loss constant". I'm sure this number is somewhat correlated with efficiency, wheel size, tread type, etc etc. For most practical purposes it's "good enough".

Efficiency in JVN's calculator is used to calculate motor loading, not speed.

[Ether]

Originally Posted by Chris is me

If I remember correctly, I once read a really, really old CD thread that said 81% was determined by taking one of 229's drive bases, running it to top speed, and measuring that speed. Ratio of that top speed to actual speed became the default "speed loss constant".

I seem to recall reading that somewhere too, but I wasn't sure.

I'm sure this number is somewhat correlated with efficiency, wheel size, tread type, etc etc.

It's a lot more than "somewhat". In the real world, the top-speed to theoretical-speed ratio is correlated with playing surface, wheel type (e.g. pneumatic or solid) and tread type, chain vs belt, chain or belt tension, wheel alignment, wheel runout, torque-dependent friction in the gearbox and drivetrain, and speed-dependent friction factors such as large airfoils.

For most practical purposes it's "good enough".

It's quite useful as a rule of thumb. Whether or not it's good enough depends on what you're using it for. Students need to understand the limitations.

Efficiency in JVN's calculator is used to calculate motor loading, not speed.

That is true. In a physics-based model, the torque-dependent friction (gearbox "efficiency") would affect top speed.

[Nemo]

Originally Posted by Racer26

If its 6" wheels, with the 10.71:1 ratio configuration, JVN's calculator drops out 10.51fps as the max speed.

Looking at the blog again, I see that they're calling that an "estimated final drive speed," so I shouldn't have been looking at maximum theoretical speeds.

As an extra piece of information, last year they said this about the 2013 kit drive:

The 2013 gearbox is a ToughBox Mini with 10.71:1 reduction. The estimated final drive speed is about 10 feet per second.

That drive had a 42:39 timing belt reduction, so the ratio was 11.53:1. If they used the 0.81 factor last year, they'd get around 9.75 ft/s, which they might have rounded up to 10 ft/s. Based on that I'd agree that they're very likely doing 6" wheels with 10.71:1 gearboxes. If not, they're estimating the speed differently this year.

[BBray_T1296]

Originally Posted by FrankJ

Wild speculations about what this means about the game.

First big assumption: You can build a competitive robot using the KOP chassis as designed.
Second implied assumption 6 in wheels based on stated speed.
Third implied assumption direct drive places the gear box center line on the wheel.

So with a 3" high nano box & 6" wheels, you have 1.5" ground clearance at the center of the robot. 8" wheels give you 2.5" of ground clearance in the center of the robot.

Leads to Big assumption based on this is no big obstacles the robot has to go over.

Wheels & stated speed also implies no water game. Again. Sorry guys.

What if the water is only 1" deep?

[Whippet]

Originally Posted by BBray_T1296

What if the water is only 1" deep?

Then that drastically reduces the coefficient of friction between the wheels and the playing surface, reducing the robot's ability to accelerate to the maximum drive speed, and possibly making it impossible to accelerate to 10 ft/s within the length of the playing field. However, they never said that the robot would be able to achieve that speed on the field. That was just the estimated speed.

[ErvinI]

I'm hoping that the dimensions are increased this year. The dimensions last year severely limited what teams could design (although it meant less clutter on the field). The fact that they somehow fit 6 wheels in a wide configuration makes me hopeful the dimensions are increased this year.

Has anyone tried modifying last year's wide 4-wheel KOP drive base in a 6 wheel base, keeping the same dimensions? This could answer the question of dimensions; if it's unfeasible or useless, then chances are that the dimensions could have been increased for next year.

[Akash Rastogi]

Originally Posted by ErvinI

I'm hoping that the dimensions are increased this year. The dimensions last year severely limited what teams could design (although it meant less clutter on the field). The fact that they somehow fit 6 wheels in a wide configuration makes me hopeful the dimensions are increased this year.

I liked the new dimensions. The new limitations were really cool and forced kids and mentors to think harder and iterate more. It also made transportation easier, made more room on the field, and I felt it made everything a bit more fun.

I hope the new dimensions stay.

[Gregor]

Originally Posted by Akash Rastogi

I liked the new dimensions. The new limitations were really cool and forced kids and mentors to think harder and iterate more. It also made transportation easier, made more room on the field, and I felt it made everything a bit more fun.

I hope the new dimensions stay.

Agreed. Seeing the little robots beside the 2012 and earlier robots really showed me how inefficient teams were with their space.