Hi everyone,
I have a few questions about H-Drive.
1- A total of four omni wheels on the Right and Left sides would it be better, six omni wheels?
2- Would two 3in omni wheels be better for the middle wheel or one 6in omni wheel?
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I haven’t personally implemented an H Drive before so I can’t comment on the # of wheels, but what I can say is that under the currently worded rules H Drive will be ruled illegal for the 2024 season since you need a minimum of 5 motors for H Drive and the new rules will limit you to 4.
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You could still do H-drive with 3 motors. 4 or 6 wheel omni tank driven off a single CIM-esque motor, with a 3rd for the horizontal wheel. I dont recommend it, but it could be done.
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I don’t think it’s worth it to do H drive after the rule change, however, if you are, the direction you go depends on the level of complexity you want to design to, and if there are obstacles on the field you want to drive over.
To answer the first point, I don’t believe there would be much benefit to additional wheels, if any at all. You will get pushed around regardless of the number of omni wheels on your bot due to the nature of the wheel itself.
As for the second point, the single wheel setup is good because it’s really simple, however, unless you make a mechanism to engage and disengage it, the wheel will contact the ground the whole time. So say if you have an obstacle like the cable bump or something similar, that wheel will take a decent side load when you drive over it.
The double wheel setup is different. Generally*, the wheels actually sit off the ground, and only engage when the motor spins due to the torque from the driven shaft. I’ve known it as a rocker pod, but I’m sure there are other names floating around.
It’s not really that complicated, but it’s certainly more complex than a stationary wheel.
Here is a good video of the rocker pod in action on 148’s 2014 robot.
118 also ran a rocker pod in 2019, here is a link to their CAD.
*I say this as I’m sure someone has done H drive with 2 stationary centre wheels
I hope this is what you were looking for, if you have any questions let me know, and I’m sure people smarter than myself will step in if I’ve gotten something wrong.
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for the off season
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Last season (2023) we did H-Drive. We use 6 inch omnis on the tank portion and 4 inch for the lateral. Lateral is attached to a piston so we can retract it for going over the cable cover.
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It depends on when the H drive is used. In this past seasons game, it would be valuable for lining up more accurately before scoring. There shouldn’t be any concerns about defense at that time.
Yeah, but you’ll still get pushed around plenty if you ever want to cross the field. The trouble is that you will always have omnis, even when you aren’t moving sideways. (Yes you could design an H-drive that lifts the normal wheels completely off the ground allowing for high grip wheels but that is relatively rare.
DO NOT HDRIVE 
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Could H-Drive still be ruled legal if only 4 motors are applying power to the ground at one time? If one set of driven wheels could be shifted into a neutral gear when strafing, and vice versa, its possibly still legal.
Not at all worth it, but maybe still legal?
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Without the actual kickoff released rules, it is unknown. Based on what is written in the blog post, when a motor is defined as a propulsion motor is not determined (likely in inspection). Seems to me like this could potentially be similar to when the inspector makes you show that your extensions can’t violate the extension rules.
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Yeah that seems possible, which could mean software controls are valid as well to prevent more than 4 motors activating at once.
Maybe when strafing is activated 2 of the 4 forwards driving motors could be switched to coast mode?
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Very unthought out solution, keep the 5 motors and no software control. Have the wording say something about 4 motors for a single direction. The 5th strafe motor can’t contribute to the propulsive power of the robot so it doesn’t count sorta thing.
Ofc this opens the “well we offset our swerve modules 1 degree so it isn’t the same direction” and run 5 motors per module and just have some very slight inefficiency, but I’m sure there are ways around that
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Going back to OP, and (for now) ignoring the new 4 motor rule:
For skid/steer drive (including omnis), there are two reasons for more than two wheels per side:
- To reduce scrub in turns and while spinning (not applicable to omnis)
- To reduce the chance of high-centering by more closely approximating a tank tread.
So for H-drive, unless it’s high-centering you’re worried about, there should be two wheels to a side.
Wheel size (assuming compensated by a gearing change) is more a matter of ground clearance and packaging than mobility on flat ground; pick wheel sizes according to the terrain and desired ground clearance.
Unless the new 4 drive motor rule is going to be more complicated to inspect than I’m hoping, I can’t think of any game other than 2015 (Recycle Rush) where H drive is a viable option. If you do this as a non-competition project, I concur that the slide wheel needs to have a defined contact force with the floor, implemented with (for example) pneumatics, springs*, or a drive-motor-torque actuated rocker pod. While I never built such a rocker pod, I did a theoretical paper on it a few years ago (mostly so I could understand it).
* Not useful for getting over cables and other obstructions
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