Drive train questions

My team knows that there are multiple variables that could affect our base. Our general question is if it is possible for a 17-18lb robot to run off of 6 393 motors geared 3:5(speed)? Also how about a 24-25lb bot with the same ratio? The second question is due to the amount of sacks were planning to hold? Would we stall our motors? Also most of the weight is in the back with no sacks, but with a large amount of sacks in the bot, the weight is mainly in the front.

I think you’re pushing it at 24-25lbs but 17-18lbs on that number of motors and ratio sounds reasonable. You should look to distribute weight better by using 6 wheels if possible. Of course, without having your exact robot in front of me, I can only speculate off my own experiences, so make sure to thoroughly test yourself under tournament/match conditions (duration and intensity).


A ratio of 3:5 (speed) is 1:1.67. I am assuming you have your motors internally geared for torque? Have you considered swapping the motor internal gears to speed ratio, as it is almost the same at 1:1.6 ? This would allow you to directly mount the motors on the wheel axles, possibly giving you a more compact and reliable drive train.

Regarding weight, we are at 19lb and run the drive with 4 x 393 motors set to speed ratio. We carry a maximum of 12 sacks. To run this configuration, our programmer ( /driver/builder/designer :smiley: ) has found he must code acceleration ramps to keep the peak load on the motors down and help prevent them from stalling. This configuration is close to the edge of the cliff in terms of stalling drive motors, but I think you should nudge this cliff edge to maximise performance.

Merry Christmas to all and thank you for all the learnings and entertainment :slight_smile: during the year.
Cheers, Paul

So your robot is 19lbs with sacks on the bot, or without sacks? Is the acceleration ramps for only your drive motors or for your whole bot? Is the acceleration code, challenging to create?

Actually we haven’t thought about just having the 393 in speed mode.
Is the link an example of how your code works?

The following code is by 24c.
[ -= 1; //Subtract 1 from left side motor power.
//Same thing for right side… .

wait1Msec(2); //Wait for a couple milliseconds to allow motors to speed up gradually.

19 pounds without sacks. As my mentor said, the robot at 19 pounds, and carrying ~12 sacks with four 393 motors on internal speed gearing (1:1.6 for speed) is very close to being too much for the motors.

I agree with Sweetmochi, in that a robot weighing 24-25 pounds with 6 motors at 3:5 is probably borderline, depending on how many sacks you plan on carrying.

We have also experienced the same issue. This becomes an even bigger problem if the front wheels aren’t driven, when all the robot’s weight is being put on them. If you use 6 wheels instead of 4 it might help with your weight distribution a little. Also, the further forward your front wheels are, the more weight that will be put onto the back wheels (I think?)

Good Luck with your drive! Let us know what works out :D!

So we changed our ratio on our base to a 1:1 with the 393 motors in torque. Currently we are having trouble pushing objects. We placed a box filled with 15 sacks, and the robot couldn’t push it. Currently we have the lift and manipulator not mounted on the bot.
The drive train has 2 high traction wheels in the back, as close as possible to each other, and then a free a spinning omni wheel is 4-5" away from the second traction wheel. (First traction is the back wheel, and the second is the middle wheel) The motors, cortex, and battery are in the back. Besides the box, I tried holding the base while I drove it backwards, and the driven wheels were just spinning in place. Lastly we are testing it on the foam tiles.

Surprisingly, Vex omni wheels have higher forward-backward traction than traction wheels. Try driving omni wheels.


My opinion is that designing a robot that pushes sacks is not a good idea as it is making your drive motors do a lot of work that is not really necessary. The sacks have a lot of friction on the foam tiles. I think it is better to pick up sacks in front of you or just drive around them.

However, if your strategy involves pushing other robots, then more weight on your robot will give your wheels more traction on the foam tiles, so traction will improve when you mount your lift and manipulator. If you want to have a great pushing robot, it needs to be very heavy and have as many drive motors as possible geared down as low as is practical. Problem is it will be slow.

The six wheel configuration we like is to have omni wheels on the four corners and traction wheels in the centres. This allows the robot to turn well, as well as have some resistance to drifting sideways or being pushed sideways.

Cheers, Paul

If you are building a 6-wheel 6-motor drivetrain, I suggest you look at the drivetrain on my team’s robot. You can find some pictures on our reveal thread:

It is about as simple as a drivetrain can get and is very fast and manuverable and has quite good pushing power and can easily drive over sacks.

Don’t worry too much about maximizing pushing power. The most effective defense is played by quickly putting your robot between an opposing robot and where it wants to go, not by trying to push it around the field.