pic: concept 775pro drivetrain

previous: https://www.chiefdelphi.com/media/photos/45181?

Re-arranged it a little bit, much happier with the way this looks.
The two motors on the top are chained to the rear axle.
The lower motor is chained to the middle wheel, and a chain connects the middle and front wheels.

The motors being closer together frees up a little more space in the middle at the expense of the valuable back corner of the frame. but it could allow for an easier setup of a cooling fan system if needed.

and there is still space for a 4th motor if needed

Still ~18:1 overall ratio, 4" wheels

any feedback?

If I’m reading this right, do you only have 1 775pro per side powering the middle and front wheels?

Nice progress.

Suggestion 1.
Connect all the motors on one side and use that to power all the wheels on one side of the bot.

Add a support structure across the front of the bot like the transverse structure in the back of the bot. To save space, you could use a bent piece of aluminum like the KoP. That way a front bumper can be securely supported.


There’s supposed to be a chain from rear axle to middle as well I just forgot about it not being connected

As pictured, do you have triangular chain runs, or do the rear and middle wheels each have three sprockets? What if you add the fourth motor to each side?

A double sprocket between the plates and a single on the outside of the plates behind the wheel

I guess it could be a triangle between the two motors connected to the rear wheel as well, that would eliminate a chain. But I’m worried about how many teeth would actually be engaged on the sprocket

Here’s my rule of thumb on chains. 120 degrees of wrap, minimum, for any sprocket that is transferring torque.

And, you’ll want some way to adjust the tension before its all said and done. Some are happy with blocks of Delrin. Me? I want to move an axle to get the chain tight and keep it that way.


Dr. Joe J.

Why don’t you just use gears to connect the VP together? I mean it’s not going to be great because it’s plasma cut but I don’t think there is a difference in it since it would be plasma cut for chain with no tensioners anyway.

Again I really don’t recommend building a drivetrain this way. Just because you can does not in any way mean you should. “Cool factor” is not a good reason to build something. 2x1 or 3x1 is a solid option to use instead and in my opinion would be faster on your mill than using the plasma cutter all things considered. Gussets and extrusion are king if you can’t put a flange on sheet metal, and even with your added “support” I still think this is a weak setup. Also more than 6 motors in a drivetrain does not make sense in most cases especially in a game like steamworks.

Even with 775pros? I’m wary of burning out the motors, and have only heard of 8x 775pro drives doing well just this year.

Several teams used 6 total, one team I saw used 4.

Thanks, that’s a good thing to remember. Still, it’s just a rule of thumb right? How would someone actually calculate the max load on a sprocket? After all, 120 degrees of wrap on a small sprocket is different than 120 degrees on a much larger sprocket. I vaguely remember Gates suggesting at least 6 teeth of contact on timing belts. Is there a similar thing for chain?

There is. I remember seeing some quick and dirty formulas for calculating minimum acceptable loading conditions in my textbooks. When I find some time, I can either post here or shoot you a PM about what I find. The thing to remember is that FRC is such an unpredictable environment that often uses mechanisms outside of standard industry practices with success. So even if you calculate it pretty well, real world experience trumps theoretical calculations every day.

I know of at least two teams that used 4, both with swerve.

+1, though my calculations indicated ten sprocket teeth rather than an angle. For small sprockets (12-20 teeth), 150-180 degrees is preferred. With sprockets of more than about 40 teeth, at least 90 degrees.

Just looking at the picture, I would have wanted to have the wheel go the opposite direction of the gearbox outputs, to enable a wrap-around chain routing.

What sort of results did those teams get? Were the results satisfactory to them? Would they do that again?

What sort of arrangements were you thinking of for using fans to cool the motors?

My argument is against using MORE than six motors in a drivetrain. With 775pros and the experiences i’ve heard about it’s sounds like 8 775pros is a good rule of thumb unless you want a ridiculously slow drivetrain (again I am against more than 6 in a drivetrain because it takes too many pdp ports aka half of them if you use 8)

Why can 6 775pros not be run with the power of 4 CIMs, similar to miniCIMs?
Everyone seems to be set on 8 775pros limited to the power of 6 CIMs, I’m just not sure why.

The people I talked too liked them. One team just burned theirs out at the last minute but they were only using 4, I think 6 will definitely do the job

Excessive current draw killing the motor is the my thought. You can’t run a 6-775pro drive as hard as you can a 4-CIM drive. I wouldn’t do it without safety checks in place.