2 vs 4 motor drive

Hi There,

I haven’t been here for a while, and missed this explosive conversation. I’ll try and sum up a few things that I’ve noted, as well as put in a few of my own views (instead of studying for exams)

  1. Holonomic or Not
    In my opinion, it’s never ben necessary to have a holonomic drive, and with v5, it has become increasingly popular to create holonomic drives (especially in turning point), because the limited field space and resistance to defense, as well as integrated velocity PID on the motors made creating drives with mechanums much more viable, however, for TT, you really need consider whether or not you need the extra mobility from side to side - the same motion can quickly be achieved with a good driver on a tank drive setup, and would only require a little bit more space, watch 574C or 929U lining up for shots in TP.
  2. Power of the motors in pushing each other
    Yes, v5 motors are more powerful than 393 motors, but there is still a limit to how much each motor can push - you won’t have as bad of a situation of let’s say turbo drives stalling against HS ones, but you will still lose a pushing battle against a stronger drive, but then I beg the question: how often are you going to push head to head, when there’s so much open space. If you think that you’ll be doing it a lot, go for a stronger drive, otherwise, you can use less motors or gear down. If you think that your driver has the ability to control the robot well, and use physics to their advantage (pushing from a side / on an angle), a 2m drive will be just as effective as a 4m drive, it goes down to which driver can get the upper hand in positioning.
  3. Speed
    More motors does mean that you can get more power down - this means that you’ll be able to accelerate quicker, reach top speed quicker (if at all), and keep travelling at that speed all the time. With a 2m drive, and a heavy robot, especially with quick direction changes and defense, I can quickly imagine that it’ll become difficult to control. Think about it this way, if you have a bowling ball rolling down a track, it’ll be a lot easier to stop it with your arm, rather than with your finger, because your finger can only do so much work. Same with the motors: both your finger and your arm can start moving the bowling ball, and both can stop it, but which one will take longer - do you need that time? or will you be able to make up for it by building a quicker scoring mechanism elsewhere.
    Do also note that with the increase in power, you can gear up your drive - go past 200rpm to 250, 280, 330, or even 360 rpm on 4" or 3.25" omni wheels, which will allow you to move around the field quicker than the rest of the other robots. Obviously, on 2 motors, this would mean you take a while to get up to speed, and on 4 motors you would still be at a disadvantage compared to a 200rpm ‘base’ drive, but then you think about how many small motions you make and how many bigger motions you’ll make, and you’ll have to make a decision to see whether you want faster acceleration to top speed or a higher top speed - for this season, you’re going to want some decent acceleration to get around defense, but i can imagine that there’s going to be a fair amount of straight up driving between towers, scoring zones, and rushing to stray cubes.
  4. Other scoring mechanisms
    Generally, the debate seems to focus a lot around how fast you can “cycle”, or how long it takes you to score x amounts of points. If you think you need more than 4 motors to create a super fast stacking thing, and then only 2 motors on your drive to get it there, that’s all the better for you, but generally, you can make things work with 4 motors. Even in turning point, where there were so many different objects to manipulate, teams managed to create wonderful ratcheted systems with even just 2 motors to do all their shooting, intaking, scraping, and cap lifting. with that in mind, given 4 motors, you should be able to complete at least 4 individual tasks, and looking at TT, you need to consider if you need more than those 4 tasks, and if you want more than 1 motor on each task - for example, you might want 1 motor to intake the cubes, 1 motor to stack them, 1 motor to out-take the stack, and 1 motor acting as an intake lift so that you can get the towers.
  5. How to build the drivetrain
    This is something that has been covered extensively throughout this post, where people have been talking about the advantages and disadvantages of chains and gearing. 2381C and 2381X used chains and gears to transfer power throughout the entire season, and we can confidently say that chain will work just as well as gears, you can look at our reveal if you want to see what we did. Chain will allow more versatile mounting, and it can snap, but it rarely did. The only time 2381X had chain snap was when 2381C literally broke the chain (they snapped an actual link in half by hitting us from the side, catching the chain, and pulling us around the field via the chain). Chain will have a little bit more slop on the drive, but you can use tracking wheels to easily counteract this - think pilons (5225a) in itz. Gears are more direct, but will also have a certain amount of slop - in fact, even direct driving has some slop - it’s just how much you think is acceptable, given the number of field aligns you do, and the motions that you need to make. In driver control, the slop is basically not noticeable. I know that a well built chain or geared drive will have similar amounts of friction, so don’t worry too much about that. I think that, for you guys, if you want to explore, there is another option for the drive. Usually this type of drive isn’t done, because historically it has introduced a LOT of friction into the system, but as build quality generally gets better, it’s something that you can start to explore. This is a 3m drive, where 2m power the wheels, and 1m powers a shifter to switch between high torque and high speed settings, allowing you to have good pushing power under defense, or high speed on an open field. The actual gearing ratios might be a little bit weird, but generally 5:3 and 3:5 back makes sense, on let’s say a 200rpm drive. If you want to explore this option, be prepared for a lot of pushback from the community - it’s difficult to do well, and nobody has done it well in VRC in quite a while. This type of drive would probably put one wheel as direct driven by gears, and the other wheel chained to the first wheel, similar to what 2381X for TP. In the same vein of that idea, let’s say you only needed 3 motors for your scoring mechanism, then you could either create a 5 motor H drive, or a 5th motor as a transmission, to allow your robot to go between 3:5 and 5:3 on 200rpm, or perhaps 5:3 and 7:1 on 600rpm.
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3 wheeled drive. (Or in this case, 3 motored).
image

(Not mine, I just found it online):wink:

Front 2 wheels are main power, and make it turn.
Also the back wheel would have to be an Omni wheel.

Probably not very viable, but it would work if you just had to have access to an extra motor. I think it’s also known as an a-drive.

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Do not do a 2 motor drive. Vex is about what you can do, but also speed. If you can do something well and really fast and then play defense, you will do really well. I am using a 4 motor drive but this year I hope we see some tray stackers with 6 motor drives.

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Are there any teams that do this?

little spring-loaded sprocket like a car’s belt tensioner?

why not 2 motors to a diff in the rear and 1 motor up front for ackerman steering? Or is that too complicated?

I got one of those for ya https://youtu.be/ux8r3FH2Zj8, probably not worth is, but it is definitely doable

I’m a little confused here, is there any reason rubber bands pulling the sprocket down as a tensioner is more efficient than just a axle on pillow bearings holding it? Doesn’t the pillow bearing design allow for a more constant tension on the chain rather than the rubber band method?

Not really, usually a static tensioner does it just as well. That’s why I said it’s probably not worth it.

What mechanism do you need 6 motors for?

This would give the pushing power of 3 motors, rather than using the 3rd motor to steer.

This would be pretty pointless, as you would still have the same power as a 2m base. You could do it, though. It would just kind of be a waste of a 3rd motor, since you can already turn a lot better with only two wheels.

we have our drive 4 motors but 100rpm. Should we put it at 200rpm or not? We kind of like the extra torque, and speed wont necessarily be a big problem since the cubes are spread out across the field. I would just like to hear your opinion.

My team generally prefers speed in our drivetrain, but it depends on what your robot’s main functions are. You could always use 200RPM motors and gear the drivetrain for torque or vice versa. In general however, the best robots are usually fast.

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you could use a ratio to get something in between, but I personally would go for the 200RPM

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Speedier drive trains are typically better if you want to compete on the higher levels. While the torque is nice the small amount of time you shave off here and there with a faster drive can make all the difference. My experience has shown me to never underestimate the value of a little extra speed.

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4 motor drive is always the best, less strain on the motors. You also have more pushing power if you encounter defense.

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Not gonna lie I used to be a strong advocate for 2 motor drive trains. Then I built my bot. After all of the added weight, we went from uncontrollably fast to “are the motors even running” and “the motors are getting really hot”. So after some design changes, we went ahead and removed a piece from our bot using 2 motors and put them into our drive. Our best decision ever. Even though we tried gearing the 2 motor drive train for both torque and speed (during prototyping), the shear speed and power from a 4 motor drive train is something that I doubt will have me ever going back to a 2 motor setup. For terms of torque and speed, I feel that speed (as long as its not geared for speed at an insane level) is king over torque as unless you are a pushbot or wallbot, you will never quite need a torque ratio to fend off other bots. As mentioned before, the small increase in speed also helps you compete as this essentially means you can score and defend much faster than your opponents. Remember, defending doesn’t necessarily mean winning every single push battle. As long as your robot is fast and can stand in front of others to defend, you are totally capable of winning.

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@MostLikelyAHuman ostLikelyAHuman I don’t think you understand how to use chain because if done right it is easy to obtain the right tension. Also someone else said you lose power when you chain your drive that is kinda of true but isn’t 30 present it is more like 5ish imo just from friction. Also I don’t think you are understanding the competitive side of VEX robotics because you have to have a 4 motor drive. You will be slower to all the cubes you won’t be able to play any defense even against the high gear ratios the good teams are running.

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2 motor drive = no

only do 2 motor drive if you want to be a skills bot

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