I’m new to designing Drive trains and wanted input. I’m debating whether if I should use a WCD drive train with a gear box connected to an axle at the back end of the robot and run a chain to the front axle for both sides(With a cam for tension). Or just power the back axle on each side of the robot. If you can it would nice to know why one is better over the other. Thanks
Hi! Before I answer your question, how many wheels are you intending for your drivetrain to have? Unless your chassis is very short and wide, I would strongly recommend against having any less than 6 wheels. In answer to your question, you should be powering all of your drive wheels. Having the robot supported by non-powered wheels lessens the amount of tractive force your drivetrain is able to produce. I would to suggest you look at Simbotic’s Mobility Resources. You can read more about the actual physics behind drivetrains and there’s some great tips on design.
Also, here is an example guide for a West Coast Drive made from COTS parts. If you have any more questions, you can post here or PM me.
Good luck, and happy designing!
It is preferable to drive all the wheels if possible. It might not be absolutely necessary, but for a small amount of extra work, driving all your wheels will greatly enhance your robot’s performance.
Someone can correct me if I am wrong, but if you are driving all of your wheels and your robot weighs xxx pounds, you get all of that weight on your drive wheels which will give you more traction. If only half or even a third of the robot’s weight is on the drive wheels, you just significantly reduced the amount of force with which you can push.
Additionally, if you drive only with your back wheels, you will be easier to push and turn by a defensive bot. And if you get in a pushing match and your drivetrain lifts slightly, your drive wheels may lose contact with the floor.
Edit: I was too slow. Cothron beat me to it. And to reiterate, in case you are unfamiliar with the WCD, it has 6 wheels, with the middle set dropped ~1/8" to 3/8" to improve mobility (see Cothron’s links). In this case, you would at minimum need to drive the middle wheel, since the back set may not always be on the ground.
Thank you very much. Those resources were helpful and answers was insightful. By the way I am using a 4 wheel drive train.
Assuming by “WCD drive train” you meant West Coast Drive, then the wheels would be powered from a center axle of the drive chain and not the back. As I understand from this thread:
Running chain to power the front axles would probably be best because more axles would be turning at the rate of the gearbox instead of just one set providing more ease of turning and more even driving.
Do you already know how long and wide your chassis will be? I would strongly suggest you use 6 or 8 wheels unless your wheelbase is wider than it is long.
EDIT - I always suggest 6 or 8 wheels, but if you have a strong reason to only use 4 wheels, you can get a way with it if your wheelbase is wide and short enough to keep the turning scrub down.
Why would wheel height matter? You should adjust your center drop to be bigger with a longer base, but I don’t see how wheel height comes into play at all…
I’m referring to the length and width of the wheelbase as seen from the top of the robot. The OP said (s)he was planning on making a 4-wheel drivetrain, which I think is a poor decision unless the wheelbase is sufficiently wide compared to its length to reduce turning scrub.
The current wheel base of the robot is about 19" by 25". Since it is not very big of a robot go you think wheel scrub is negligible?
Turning scrub is never negligible. It’s always a factor, and can be a crippling factor if not designed correctly.
https://www.chiefdelphi.com/media/papers/3022
That link is to a very rigorous explanation and calculation of the physics behind a 4-wheel drive. I’ll let you run the numbers and figure out if doing a 4-wheel drive will be an issue.
If the wheelbase (front to back distance between axles) is 25", then you will be unable to turn unless you have pebbletop tread or something similar with a better coefficient of friction in the drive direction than sideways.
Even if the wheelbase is 19" with a 25" track, my memory of the numbers puts you right on the edge with isotropic (same CoF both ways) tires - run the calculator before you build anything, and even then, I suggest building the chassis and trying to drive it with enough weight to simulate the full robot before you go any farther with the build.
One way to skip both chains and belts and still have 6WD:
CIM driving each wheel via a VEX single reduction clamping gearbox on 1 x 2 in frame.
Extra gears add weight, but likely offset by what you save by eliminating belts and chain and pulleys/sprockets.
Simpler system than the KOP drivetrain/chassis system.
No center drop is the big challenge here as you have to deal with scrub. There are teams out there who succeed with 6WD with no center drop. They pay a lot of attention to wheel selection including tread design. And I’ll guess that they also test carefully for voltage drop challenges.
Just to make absolutely clear, when people are talking about 6 wheel and 8 wheel drives they mean with center dropped.
Simply adding in more wheels will help slightly, but the center drop is where 90% of the scrub reduction comes in when adding wheels
I’ve never heard this before, could you explain it? The height of the center drop doesn’t seem like it should matter very much to me
While this would work, if you have a drop center at least two wheels will be off the ground at all times. You’d have two CIMS running for no reason (unless you use a sensor to only power the wheels in contact with the floor), and you would only have the torque of four CIMS in a pushing match. If the robot is pushed onto its back wheels, you now have the torque of only two CIMS, rather than 4 or 6 in a traditional drivetrain.