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View Poll Results: What is your drivetrain's top speed?
<10 7 2.94%
11-13 (kitbot is here) 48 20.17%
14-16 96 40.34%
17-20 72 30.25%
>20 15 6.30%
Voters: 238. You may not vote on this poll

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Unread 22-01-2017, 12:51
jdaming jdaming is offline
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What is your drivetrain's top speed?

What is your drivetrains top speed in FT/S (FPS)? If you are doing a shifting drivetrain please comment with your low speed too! I would love to hear any other comments about what you think is "best" and why!
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Unread 22-01-2017, 13:01
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Re: What is your drivetrain's top speed?

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Originally Posted by jdaming View Post
What is your drivetrains top speed in FT/S (FPS)? If you are doing a shifting drivetrain please comment with your low speed too! I would love to hear any other comments about what you think is "best" and why!
Your poll has an issue; we clock the stock AM14U3 at 10.0 feet per second in the user guide (PDF link). (That uses 75% of free speed for calculations, to account for losses to friction.) If you meant it to be 10-13 (since the previous option is "less than 10"), you'd be correct.

As for Iron Kings, we'll be around 15 theoretical or about 13 in practice. 5.95:1 gearing, 4" HiGrip wheels.

(Full disclosure of the painfully obvious: I work at AndyMark.)
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Unread 22-01-2017, 13:06
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Re: What is your drivetrain's top speed?

Yes, that was meant to be 10-13. I did see that the kitbot was listed at but also thought (or hope) that most kitbot teams will elect to use two CIM motors rather than just one that the 10 is derived from.
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Unread 22-01-2017, 13:16
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Re: What is your drivetrain's top speed?

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Originally Posted by jdaming View Post
Yes, that was meant to be 10-13. I did see that the kitbot was listed at but also thought (or hope) that most kitbot teams will elect to use two CIM motors rather than just one that the 10 is derived from.
If you use two CIMs or one, your top speed will not increase--simply how fast you get to it (and how much current a motor will draw). To go faster, you would have to change the gearing or start integrating motors with a higher free speed (like a Mini CIM, whose free speed is about 500 RPM faster).
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Unread 22-01-2017, 16:58
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Re: What is your drivetrain's top speed?

Quote:
Originally Posted by Billfred View Post
As for Iron Kings, we'll be around 15 theoretical or about 13 in practice. 5.95:1 gearing, 4" HiGrip wheels.
We're running the same setup with wheels from 2015's kit since we used 6" wheels that year.

Quote:
Originally Posted by Billfred View Post
If you use two CIMs or one, your top speed will not increase--simply how fast you get to it (and how much current a motor will draw). To go faster, you would have to change the gearing or start integrating motors with a higher free speed (like a Mini CIM, whose free speed is about 500 RPM faster).
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?
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Unread 22-01-2017, 17:15
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Re: What is your drivetrain's top speed?

228's going 4 CIM, 14 / 6 ballshifters. For packaging more than anything else - the 2 CIM model is a really compact gearbox, but the 3 CIM is huge.
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Unread 22-01-2017, 17:21
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Re: What is your drivetrain's top speed?

14+ fps is drivable if your drivers get a LOT of practice. I would never gear a bot that fast without a practice bot.

Our fastest bot ever was 17.3 free speed but our driver had more than 50 hours of drive time before our first event.
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Unread 22-01-2017, 17:41
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Re: What is your drivetrain's top speed?

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... if your drivers get a LOT of practice.
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Unread 22-01-2017, 18:34
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Re: What is your drivetrain's top speed?

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.
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Unread 23-01-2017, 07:36
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Re: What is your drivetrain's top speed?

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.
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Unread 23-01-2017, 08:03
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Re: What is your drivetrain's top speed?

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?
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Unread 23-01-2017, 09:57
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Re: What is your drivetrain's top speed?

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Originally Posted by Gdeaver View Post
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.
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Unread 23-01-2017, 08:10
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Re: What is your drivetrain's top speed?

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Originally Posted by Lexeo View Post
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.
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Unread 23-01-2017, 08:44
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Re: What is your drivetrain's top speed?

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Originally Posted by Lexeo View Post
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).
Quote:
Originally Posted by GeeTwo View Post
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.
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Unread 22-01-2017, 22:47
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Re: What is your drivetrain's top speed?

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Originally Posted by Donut View Post
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).
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