This year we are planning to make a custom chassis for mecanum drive but we have a question in our minds. Our main goal is to add one more motor for each wheel so that we can increase our performance. But, while doing that we don’t want to increase our weight so want to customize our chassis to make it lighter. But we are lacking experience in designing drivetrains so do you have an idea if it would save us some weight or not?
A second motor would theoretically allow greater acceleration, and there are ways to build a lighter chassis than the AM14U4. But let’s pause a minute.
I don’t have any photos of 6985’s robot from 2019, but I’m seeing on The Blue Alliance that the team went 5-6 in qualifications this year, 19th of 35 and semis as a second-round pick. The year before, a hot run to the finals as the 3 seed and a trip to Championship where the drivetrain speed couldn’t keep pace with the competition.
If you’re lacking experience in custom chassis building, especially mecanum, with the résumé you currently have, you are playing a dangerous game by using it as your competition drivetrain. I’d strongly consider building the stock Kickoff kit chassis this season, then optimize it for the game using tools like ILITE’s drivetrain simulator once you know how you intend to play the game. You’ve got a whole off-season to try custom chassis systems and get them right.
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To the question you asked about - In general, you might be able to save a few pounds by using square stock and gussets, rather than the heavier-gauge aluminum and blocks used in the kitbot. I think it would be hard to get four motors worth of weight out though. Of course, depends on what type of motors you’re talking about. 4 CIM’s? not a chance. Four 775 pro’s? Maybe…
If weight is truly the highest priority, you could start looking at things like aluminum gears, taking material out of the gears, using smaller plastic gearboxes… each of these will have some tradeoffs with strength and robustness though.
While investigating "higher performance mecanum ", check out Team Titanium’s 2017 robot for some winning design ideas.
To the question you didn’t ask, but really should be asking:
I assume this is intended to be for the 2020 competition?
I would strongly recommend avoid making the decision prior to knowing what the game actually is. Choosing mecanum carries some strong tradeoffs in your ability to play defense - there’s a lot of threads around here expounding on the limitations.
Not saying you shouldn’t do this - Just saying don’t “put the cart before the horse”.
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What do you mean by this? Different answers to this question will lead you in different directions.
Taking material out will reduce weight. In general, it will also reduce strength. If you are intending to add more power to your drive train, reducing the strength of the chassis may not be such a good idea. Unfortunately, there is no easy way to determine how strong is strong enough. It depends strongly on the game you are designing for and the role that your robot is being designed to play in that game. Successfully implementing a light AND strong chassis depends on you and your team having a lot of prior experience.
Please note that it is total robot weight that you need to be concerned about, not just the weight of your chassis. It is also possible that weight distribution may be a significant factor in your robot’s on-field performance.
Taking a step back, I would personally recommend West Coast for your first custom drivetrain instead of using mecanum. 6502 has been using custom west coast since 2018 after using mecanum in 2017, and we’ve never really found a need for the agility that mecanum provides. West coast is also a lot easier to design for, so I would start there and move forward to mecanum if you so desire.
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Mecanum wheels aka vectored intake wheels belong on intakes 
But to answer your question…
If you’re talking about doing this in the remaining time before kickoff I think that’s definitely a valuable endeavor. I would recommend against building your first custom chassis during the build season. 2X1 tubing drivetrains can certainly be made lighter than AM14U2, but you have to determine if your resources are better spent on that versus designing a custom mechanism.
The kop chassis is well designed and usually a really good idea!
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Every year, I see a bunch of robots with under-developed scoring mechanisms that have no choice but to play defense. Note, mecannum robots are often viewed as poor choices to play defense.
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Mecanums are almost always heavy. 4 motors, 4 gearboxes, 4 heavy wheels. They’re cool, but 99% of the time, your driver is not going to be using the mecanum functionality.
I say stick with a west coast or 6 wheel drop center or some sort.
Also, you don’t know what the game is. Don’t plan on building anything yet
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I’m going to chime in here as a team that has had success with mecanum drive. Unlike many of the other commenters, we used mecanum last season and found two important things: 1) we did use the strafing ability of the drive extensively to allow us to align to the target when acquiring and placing hatches. It saved a great deal of cycle time for us, so we were able to get up to a 8-9 hatch/match average by the end. 2) The defensive disadvantages are often over-emphasized for mecanum. It all depends on how you power it and what you want out of it. We used miniCIMs and 12:1 CIMSport planetary gearboxes, which still gave us a reasonable top speed and a whole lot of pushing power (using the Andymark HD mecanums, which have a better grip than the Vex versions, though they are about twice the weight.) We were able to stand up to tank drive bots and actually push them back while not being pushed off-target ourselves. We did sacrifice some speed for pushing power being geared at 12:1, but it allowed us to play effective defense (though our main role was as an offensive bot, given that we had a highly effective mechanism) when we needed to. The mecanum even proved useful there, since it made us more nimble than many of the bots we were up against.
Having said all that, the question of how to design a custom chassis for mecanum depends on a host of issues. What are the gearboxes and motors you’re looking at? What kind of ground clearance and wheelbase are you wanting? How do you want to mount other components (electronics board, pneumatics board, mechanisms, etc.), plus a whole set of other questions and restraints that have to do with the particular rules for the new season. Then there’s the question of capabilities. Do you have machining or CNC capabilities? Welding capabilities? Both of these will have a strong effect on what and how you will be able to execute any frame design and thus will tend to dictate many aspects of that design. Let us all know a bit more about what your team is thinking and what you can do already and we can probably give you better answers to your questions.
So to inform you, we want get a better performance from 8 in. andymark mecanum chassis (by torque and speed) and at the same time we don’t want to increase our weight. We want to put our electronics on a metal sheet under chassis and our machining capacities include CNC , welding and much more. These are all that are in our minds currently.
All drivetrains are limited on how much force they can put into the ground by the friction between the wheels and carpet. Mecanum drives, which usually have a CoF around 0.7, will have a much lower limit than a similarly weighted traction drive, which generally have a CoF of 1.0-1.2. It won’t matter how much power you throw at it, your wheels will just start slipping. Now you can put that extra power towards a higher top speed, but once you get above 18 ft/s or so the robot becomes very difficult to control.
I’d definitely consider whether adding more motors to your drivetrain is going to accomplish what you want it to. And if it does, then consider whether the performance gain you get outweighs the added time it takes to design and manufacture your own chassis. Which is more valuable to you, a higher top speed or more time to work on your scoring manipulators and drive practice? You need to answer that as a team, and then you’ll have the answer to the title question.
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I’d advise against adding additional motors in your case. Instead, perhaps changing to using better motors is a better option. It looks like you guys used CIM’s last year. I would try switching to using Neo’s or Falcon’s.
First you don’t know what the challenge is. we had that kind of discussion before I think stronghold and then got some tank threads instead and built a tank to negotiate the obstacles. We have used mecanum extensively with all kinds of gear ratios 6 in and 8 in and always with 1 motor per wheel as the wheel will loose grip well before you get into the range of blowing a fuse or overheating the motor so a 2nd motor might give you a little more acceleration. If this year is not going to need delicate adjustments like strafing to position a panel etc. then a tank drive might be a better solution. Heck we might have to dig out our tank threads again or maybe this is the first year where we have to teach our robots to walk like a star wars walker. If you can build the custom chassis between now and kickoff and it works well before then then by all means do it - if not - do what worked for you in past seasons
I am going to go out on a limb that your team would benefit from and see more “success” by going to a simpler drive system, like the KoP chassis or a WCD.
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First off I’d recommend only 1 motor per wheel for mecanum to make sure you’re not killing your battery. Putting 2 motors per wheel is comparable to swerve especially when swerve is a better option than mecanum. This is coming from a team who’s had some experience with mecanum. It’ll get the job done but at the cost of defense and traction. This year (3rd year using a mecanum drivetrain) we underestimated defense and screwed up our strategy badly and payed. If you’re planning to use a custom chassis for 2020 I’d recommend if you’re still content on doing mecanum doing AM-14U4. Easy to make and design. Unless you’re fast at machining and rapid design, having a 2020 mecanum chassis is not the best idea.
I will also at least try to attempt to push you towards a WC drivetrain. Having a west coast chassis means you have the power to do and change a lot of things that other teams with mecanum. (Wheel [type, size] gear ratios [fast, torque] Shape). It gives you the option to make the chassis design yourself and custom tailored to your robot.
Last resort, look at Einstein matches for the past few years. Only Swerve and WC. Best options hands down.
That is one issue we were thinking about, since we have no experience about one wheel two motors we were wondering if it’ll allow us to use the robot with full battery for one match or not?
Definitely wouldn’t recommend it. The power draw especially on CIMs basically make the 2 motor 1 wheel idea not the best working one. You wouldn’t get the best performance for the time/money/materials.
I built a mecanum chassis on an AM14U4 frame - it was quite straightforward. The only negative thing about it is that you really need to do 8" or larger wheels, because with 6" wheels, the chassis is just over 1" off the carpet along most of its length, and significantly less than 1" off the carpet where the TB-mini mounting area dips down - and there is not a wheel there as there is with the 4" skid steer configuration. That’s way too easy to high center on a ramp or other obstruction.
Are you aiming draw so much current you will fully discharge the battery during a match?
Discharging the battery deeply will cause the voltage to decrease. At some point, you will start experiencing brown-outs and some of your control system components (RoboRio & radio) will reboot.
That would be quite a flimsy limb. We’ve used both in the past, more than once, but we very specifically chose the mecanum drive and had some of our best performance in competition ever. I know too many don’t like mecanum, but it was exactly the solution that worked for us last year. We had neither the experience nor the funds to use a swerve drive, yet we needed more agility than a tank drive (even with omnis) could produce, so the solution was mecanum drive. As it was, we did exactly what we set out to do with our robot, which is where we needed to be. I understand that for a team like the Killer Bees, with 23 seasons behind you, we may seem naive, but we knew what trade-offs we were making and found that they worked for us.
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