Help with drive train

I am a mentor from Team 1422 and last year was our rookie year. We had MAJOR problems with our drive train and this year, we hope to fix it so we aren’t stranded in the middle of the field. Sigh…so, I’m asking teams for advice on what we can do to help our little dilemma. I appreciate ANY suggestions or advice. Thanks everyone!

Does that mean you are trying to find ways to fix the old style of drivetrain or are you looking for a completely new design. If you want to modify the old design, it would help to know what the old system was.


Here are a few suggestions:

  1. Use the CIM motor (if it is in the kit), not the drill if you want to simplify your system. Many teams have successfully used the drill motor, but if you want a robust and simple drive use the CIM motors.

  2. Do not shift. That can be your next step after you get comfortable with drive train design.

  3. If you are designing 4WD or are using chains at all, then use 3/8" (also called #35) pitch chain because it is much more forgiving.

  4. Experiment with different wheel materials. Many teams have been successful using the skyway wheels and putting extra tread material on them.

  5. Look in the white papers section about the different designs and the different analysis techniques used to aid drive train design.

  6. Ask a lot of questions on this forum.



We have found that with the drill or CIM motor you will have a drive system that runs reliably if the following conditions are met:

  1. Use chains or spur gears, do not use the right angle gearbox.
  2. Final rpm of the wheel, calculated from the free speed of the motor, should be approximately 200-250 rpm for an 8" wheel. Example-bosch motor and gearbox had an output speed of 450 rpm in low, a 2:1 sprocket/chain reduction to the 8" wheel equals 225 rpm at the wheel. If you use a different size wheel than change the sprocket ratio (3:1 for 12" wheel).

We have used the skyway wheels every year. Wheelchair, 3x8 and 2x8 foam and the pneumatic. Your goal for this coming year should be simple and reliable.

Im going to chime in also…

Given im relativly new at designing geartrains, Ive been around FIRST for a while and i know what works and what doesnt.


Being (as i assume) a sophmore team, you kind of get the idea that your rookie year you would see what works, what doesnt work, and what other people have done.

I like the Atwood motors. They are easy to mount, and you can mount it anywhere.

Go for a simple reduction. Estimate the FPS you want to acheive by figuring out what wheel diameter you want to use. then lets say you want a 6" wheel with a desired output FPS of around 8
Input your drill using a 14tooth 20dp.
From the 14tooth go into a 64tooth 20dp.
thats just gearing
then on your box’s output shaft use a 10tooth #35 sprocket
to a 28tooth #35 sprocket on your wheels

THis would give you 9FPS

PM me if you would like a copy of this spreadsheet i used.

I would recommend using the skyway wheels that come in the kit. the 6" ones. and cut grooves in them for added traction.

and please if you have ANY questions, feel free to ask anyone of us who post here regularly for help…i have been helped my Andy and John. They aer very helpful for questions.


  1. Use the CIMS
  2. If 4 wheel drive have a caster mechanism to pick the 2 front or 2 back wheels up! It is very hard to turn in 4WD.
    3.Place your tranny at the back of your robot. Take the output of the trannies to each back wheel. Then take a chain from the back wheel to the front wheel.

Last year we used #35 chain on the output to the back wheels. We then used #25 to connect the front and back wheels. We never had a problem with the #25 chain. If the #25 chain does break then you probably just saved something in your tranny from breaking; you will still even be able to drive around (three wheel drive) YAY!

I have more up my sleeve just can’t think of them off hand.

Wow, thanks with all the help guys! Here is what we did last year:

First, we used 4 drill motors with the transmissions to obtain a 4 wheel drive. We figured out that it didn’t work very well at all, we couldn’t turn worth crap and the trannys kept slipping. We also used 2 of the big 12" pneumatic tires and the 2 other wheelchair tires.

Next idea, we replaced the 2 wheelchair tires with 2 more 12" pneumatic tires. We did this to try to get up the step. Another problem, we did not have the power or traction to get up the step, scratch that idea.

Final idea, completely take off the front two systems and replace them with good ol’ casters. No getting up the step but it helped us maneuver a little better. Sigh…another problem, we were not used to the whipping action of the bot so it eventually threw out our battery. Very embarrassing.

So, as you all can see, we had a wonderful time learning what works and what doesn’t, but we still haven’t found what works. Thanks to all of your help, we should prevail over the dilemma of the drive train.

Thanks a bunch and I hope to see all you guys in upcoming events and meet you guys.

To give you an idea of what works. Here’s a simple 2 speed transmision. You’ll need an additional reduction after the output shaft but it’ll give yah enough power to turn in 4wd if that’s what yah want.

I would not call the Technokats transmission simple (and this is coming from someone who was responsible for all the machining for an almost identical copy)

The principle is simples, however, it is most definitely not simple to make some of the parts without a CNC mill, something my team lacked. it can be done by hand, however it is VERY time consuming.


It is good this did not work. Per the rules you would have only been allowed to keep two of those motors on the robot at the competition, this would have left you with a robot with serious drive problems and no time to fix it.

The rules only allow the type and number of motors supplied in the kit. Two drill motors means only two on the robot. Of course more can be bought as spares in case you fry. This is a common mistake by rookie, and sometimes veteran, teams. I remember a couple of years ago the kit list said we had a van door motor when we did not. FIRST updated the list but one team showed up at an event with the van door motor on their robot :frowning: .

You went through the motions so many rookie teams have gone through. (and a lot of veteran teams)

  1. Four wheel drive (tank drive) works great when going in one direction but you can’t turn on carpet so you need a method of lifting the two wheels off the carpet or turning them as part of the steering.
  2. Using large tires can accomplish certain goals but the circumference of the tire needs to be considered in the final gear ratio choice. It is difficult to get good control with large tires at normal speeds in the torque range of the motors supplied. There are so many trade-offs.
  3. Casters work OK but give the same problem that grocery carts have and that is the need for incredible steering torques to be stable and maneuverable. They also give a varying response when changing direction due to a fixed turning diameter of the caster.
  4. Battery must be secure and protected within the robot. No battery=no play.
    All in all you have learned a lot your first season out. Keep using the people here on CD as a resource, they are the best.

Our team usually bends a small little sheet metal slot to put the battery in and puts a few strips of velcro on. But every year, it comes down to one thing that keeps the battery in there… the good ol’ zip ties. Go get a bunch of the longer ones (or however long/short you’d need it) and tie the two battery connectors together and then again to some sturdy point on your bot (this year we did it to our air tanks) It never fell off after that… but then you get the times when you got like… 10 seconds to get the thing on the field and you’re all scrambling, “WHERE’S THE ZIP TIES!!!1 OGMGOMG”

Regarding the drive train, if you’re lucky, might get a game like 2002 where having casters would work alright. My team in '02 had the drive wheels at the back and casters at the front; no problem turning with that.

All the help is really great guys! My mistake, we used 2 drill motors. sorry for the mix up.

We used ball casters - available from Mcmaster carr in 03 and did not have the swerve problem as bad -
This is considering we are now a 6 year rookie team.

just to chime in, watch your wheel base, we had the problem of getting too much traction and had too much distance between axles therefore we could not turn.

hope this helps
good luck, the more you play with it the more solid it will be


First off…Woo for my first post!
Now that that’s out of the way, I just thought I’d ask for any suggestions. Seems like my team (2nd year rookie) is having problems turning. I can go in a straight line though! lol. We’re using the FIRST transmission with a sproket on the output shaft with a single chain which turns two wheels on each side. Essentially a cheesy 4wd. The sprokets for each wheel are keyed to the axle, as are the 12 inch pneumatic wheels. Our problem seems to be one that others have experienced in the past. Seems to me like we’ve got too much traction when trying to turn. I think we’re going to try moving to a front-wheel, rear caster type setup. Hopefully that’ll solve our problems. Any suggestions from you fine people out there? Personally I wanted to have a more sophisticated drive system, but the skill level of the team isn’t quite where it needs to be for such a task. Any help would be greatly appreciated.

i have a question then i’ll put my two cents in…are you using one sprocket to power both wheels?

Ok, for one I would never suggest using casters, especially the swiveling kind, my team used them in our first year (b4 i was there) and the robot could not manuever very well.

We used the 12" pneumatic wheels last year on our bot and also had trouble turning. with those wheels, you are usually about 6" off the ground so have a wide wheelbse to keep you stable when turning. Now, for a solution, our team bought zipties (like foot long ones) and put about 10-20 on each of the back wheels, and we were able to turn better, we still shook (?) when we turn but that is because we had a scissor lift that would go up and down as we turned. but anyway, try the zipties, they should work, if not post again!

here is a pic of our robot from last year, and be sure to poke around the website:

just some thoughts

We used a center wheel design last year with the 12" wheels and it worked out very well. Here’s some examples of last years prototype and the actual bot and in action .

The center wheel has a 6" pnuematic from Skyway and was offset by about 1/4" depending on air pressure. We are using a center wheel again this year because it makes turning so make easier on the drive system. However, one lesson learned from last year is that it is a lot of weight placed on that little 6" wheel, and very little on the 12" wheels. But last year we wanted to easily get up those steps - so it proved useful. I’m not sure how critical it is to use such large wheels in this year’s competition. We’re using 6" wheels this year. Good luck.

Just to clarify, yes, we’re using one sproket to power two wheels. If you look at it from the side the chain makes a bit of a triangle shape. One sproket on top from the transmission, and the other two on each side sharing the axles for each wheel. Anyways I’ll be sure to try the zip ties tomorrow when I get to school. I do remember seeing that tactic used last year now that I think about it. Seems like those 12" pneumatics have caused some problems for other people too. Oh well, robotics wouldn’t be fun if it wasn’t challenging.

The easiest solution would be to use the gearboxes that are supplied with the kit. They are superior to last year’s worm drives shudder. The worm drives were made of a plastic housing, which cracked, and our team had to use enough oil on the gears to power a small city. The CIM motors are the most powerful, but you could buy some Bosch drill motors from last year, those worked beautifully.

Mike F.
Team 1279