What is your drivetrain's top speed?

[Richard Wallace]

Originally Posted by IndySam

... if your drivers get a LOT of practice.

^+1!

This is the key.

[Type]

Originally Posted by Richard Wallace

Our robot will be zippy. Cruising ratio will be less than 5.7:1. Not saying exactly how much less.

Did your team end up going with shifting gearboxes this year?

[Valkonn]

This year, I designed a modular gearbox because I wasn't sure what the drive team would want to use or what would be best for the game. We've got speed ranging from 8 ft/s to 20 ft/s. I started the first config at 15 ft/s and this thing is already burning rubber. Any more than 15 and I think it gets uncontrollable.

[dirtbikerxz]

Originally Posted by mrnoble

EEP. Okay.

My question to you, oh CD Community, is this: do you have any students who have driven a robot that travels 17 fps or faster? 14-15 fps is on the edge of control for most drive teams, and the last two years had games in which something between 7-11 fps was usually ideal, so unless your drivers are seniors with experience in Aerial Assault as freshman, this will be a significant challenge.

For teams that are looking at doing speeds such as this, I would suggest taking your bot out into an open area (if you dont have a dedicated field that is), and just setting up obstacles such as cones etc, and letting your drivers practice at that speed maneuvering around etc. IMO It shouldn't take a good driver too long to get used to such speed and control without too much practice.

[Richard Wallace]

Originally Posted by Type

Did your team end up going with shifting gearboxes this year?

Yes. We are evaluating two options: DS and EVO. Should have a decision soon.

[dirtbikerxz]

Our gearboxes: http://www.vexrobotics.com/vexpro/mot... if anyone cares

[GeeTwo]

Originally Posted by Donut

I understand the theoretical max speed won't increase with additional CIMs since they all have the same free speed, but won't the effective speed you can reach change because of the additional torque available at the same point on the motor torque/speed curve? If the kitbot is rated at 75% of free speed due to friction losses on one CIM per side, then I would think the additional CIM per side would add enough extra power to run at a higher percentage of free speed before friction becomes the limiting factor again. Is there a significant amount of friction added from the extra gear for the second CIM?

It's definitely a second order effect. Essentially, by doubling the torque available at the highest speeds, you are going to be able to get closer to free speed. That is, the "speed loss constant" in JVN's calculator will nudge a bit closer to 100%. (or more likely, move farther away from 100% on the 2 CIM side, because I'm pretty certain that 4-6 CIM geartrains are far more typical of the ones used to determine the 81% value).

[Chris is me]

Originally Posted by GeeTwo

It's definitely a second order effect. Essentially, by doubling the torque available at the highest speeds, you are going to be able to get closer to free speed. That is, the "speed loss constant" in JVN's calculator will nudge a bit closer to 100%. (or more likely, move farther away from 100% on the 2 CIM side, because I'm pretty certain that 4-6 CIM geartrains are far more typical of the ones used to determine the 81% value).

Some empirical speed loss constants based on drivetrains I've built (west coast "belt in tube" style drives):

6 CIM: 90%
4 CIM: 84%

Your mileage absolutely will vary. Measure an old robot's actual speed sometime to get an idea what your team's speed loss constants are.

[ShIfTiNgBoT]

Luckily, we have a senior driver that was around for 2014. Our max speed will be 18 fps, low gear will be 7. For anyone curious, we're running the WCP DS gearboxes, with custom standoffs to fit sprockets inside (still smaller than the WCD version of these gearboxes). For any teams that haven't finalized cad yet and have the money to spare, I highly recommend them.

Back in 2014 (or so I've heard) we were extremely fast, looking at actually reaching our 19 fps speed with considerable acceleration.

[Lexeo]

The 4272 chassis would theoretically be capable of hitting up to 30ft/s (although the field isn't long enough for that to actually happen. We did the math).

We will realistically be able to hit a top speed of between 17-20 ft/s, (takes about 3-5 seconds to accelerate to that point).

We are using 3-Cim Ball shifters, with low gear at 7.5, and high gear at 2.81. We are using 4" wheels. In order to maximize acceleration, the students are working on some automatic shifting. Low gear hits about 10-ft/s.

I will also say that we have a driver with 3 years of experience, and this chassis has already been built for a little while. Also...we are building a practice bot.

[Gdeaver]

Everyone is focused on high speed getting across field. Don't forget that there are some very precise movements needed to align with the feeder stations and place the gear. How does your drive train handle precision?

[GeeTwo]

Originally Posted by Lexeo

The 4272 chassis would theoretically be capable of hitting up to 30ft/s (although the field isn't long enough for that to actually happen. We did the math).

We will realistically be able to hit a top speed of between 17-20 ft/s, (takes about 3-5 seconds to accelerate to that point).

We are using 3-Cim Ball shifters, with low gear at 7.5, and high gear at 2.81. We are using 4" wheels. In order to maximize acceleration, the students are working on some automatic shifting. Low gear hits about 10-ft/s.

If I understand you correctly, we're getting across the field faster with single speed 2-CIM gearboxes based on our time trials with a tote full of spare parts to simulate full robot weight. As strange as it sounds, you'd probably get across the field faster with slower gearing; I suggest you do more math.

[pilleya]

Originally Posted by Lexeo

We will realistically be able to hit a top speed of between 17-20 ft/s, (takes about 3-5 seconds to accelerate to that point).

Originally Posted by GeeTwo

If I understand you correctly, we're getting across the field faster with single speed 2-CIM gearboxes based on our time trials with a tote full of spare parts to simulate full robot weight. As strange as it sounds, you'd probably get across the field faster with slower gearing; I suggest you do more math.

A robot with single speed gearboxes and 4 CIMs total each can cross the field in under 4 seconds. Choosing a high-gear of 2.81:1 sounds a little bit much, something around 4-5:1 could be a better option. It also means that you don't to worry about tripping your breaker if you accidentally start off in high gear.

[mrnoble]

Originally Posted by Gdeaver

Everyone is focused on high speed getting across field. Don't forget that there are some very precise movements needed to align with the feeder stations and place the gear. How does your drive train handle precision?

We are theoretically geared for approximately 15.5 FPS high, and 5 FPS low. Alignment and pushing power both count this year, maybe as much as or more than top theoretical speed.