This is our first time at VEX, we started three weeks before the skills match as the only participating team from Egypt. Unfortunately we just scored 10pts as we had some problems with the mobile goal gripper and the second robot was a joke. (Hopefully we figure it out before the global tournament)
We have used six wheels on the drivetrain with two standard wheels at the back to provide stability. 4 motors 200 rpm with a gear ratio 3:5.
We thought that boosting the torque would be better on short distances since we need high acceleration to reach high fraction of the maximum speed.
I need your comments and I have the following questions:
1- Why don’t people usually use standard wheel and build their drivetrain with omni wheels only?
2- Why is it more common to use smaller wheels while the bigger ones would provide more speed at the same angular velocity?
3- Is is better to use power transmission so that one motor transfers its power to another one, then the other motor provides the drivetrain with power or to connect both motors directly to the drivetrain?
Some drivers like the way the drivetrain moves side to side (drift)
Smaller wheels have more options for gearing the speed
Direct drive is not optimal as the speed is usually not good and the wheels on each side don’t move in synch.
By the way, if you have pneumatics or 5.5 watt (half power) motors, a six motor drivetrain is better. And also your drivetrain is currently very slow compared to other teams’ robots. Catalogue of Drive Gearings
I HIGHLY recommend going 6 motor 450 rpm drive (3:4). This is what we (and many other) teams use/used. It provides high speed while also having lots of torque from 6 blue motors (even though they might not have much individually, their combined torque is pretty good).
To answer your questions:
Omni wheels make turning significantly easier over traction wheels.
Never build your drivetrain purely out of traction wheels. Due to some weird geometry stuff, when you turn the outer wheels will travel sideways a bit. Traction wheels… well… are not supposed to do that, and will cause your robot to take a lot of time to turn.
Smaller wheels = more accuracy for odometry/autonomous stuff, lower center of gravity (less tipping and better for defensive play), and in some cases a better gear ratio.
It’s better to connect both motors to the drivetrain directly. Less parts = more reliable & less slop.
Using all traction wheels means the robot can’t turn since the wheels are going to be moving non perpendicular to the direction of rotation.
When you use gear ratios, the angular velocity doesn’t really matter, just the linear speed. Smaller wheels have better cog, are lighter, and happen to usually have more competitive speeds.
Most teams just gear all the motors on 1 side of the drive together to ensure consistent power output, even if one of your wheels loses contact with the ground.
I have a couple youtube videos covering drivetrain construction you might find helpful.
Many people are talking about how Omni wheels turn better but I think the OP means that two traction wheels is better than none and is asking why some teams use none anyways.
the way I see it is, if someone’s gonna push you, they will do it whether you have traction wheels or not, so don’t limit your movements to fail to stop the inevitable.
In VEX U the most popular drive I’ve seen (I could be wrong) is 8 motor 600 rpm 2.75inch wheels. The easiest way to do this is on each wheel connect it to a blue motor, some teams don’t even gear them all together. As long as you can drive that it will be good. If you want a slower speed look into 450 on both 2.75 and 3.25, or 480 on 2.75
Omni wheels give better turning and the “standard wheel” you’re talking about (traction wheel) where it won’t turn as well.
It can lower down the bot and it can allow for some gearings that you can’t achieve on standard 4" wheels.
A PTO can be used, where it can control other game mechs to share motors without using many more. It would depend what you’re going for, like a 66w drive, 11w intake and conveyer, and 11w lady brown. You can power-share the intake and conveyer together, and just use 11w for the lady brown on a slower gearing.
For a VEXU team, consider using more motors for your drive, as it gives more power to the drive and you could still power share between the intake and conveyer. There isn’t a limit to how many motors you guys can use.
In VEXU, you can print your own gears and use them for your drive. You can design your own gears and use them on your drivetrain for some special gearings. If there isn’t access to any other wheel size than 4". Try using smaller wheels and using the 48 tooth gears that VEX sells for a 3.25/2.75 drive.
A good ratio would be 36:48 which would give 450 on 3.25/2.75. You can test whichever’s better for your robot.
If you don’t want to be pushed side to side but still want your robot to be better at turning I suggest using one traction wheel in the middle, that’s what my team has done this season and it has worked pretty well for us.
