Drop center with gearboxes in the back

Hello all,

Historically, our team has always used the same drivetrain. The left and right wheels would both be raised 1/16th of an inch, and the center wheel was always lowered 1/16th of an inch to achieve an eight-inch drop center. The gearbox would also always be in the center, with the thought process being to help us achieve a more central COG.

This year however for a number of reasons, we have decided it would be best to move our gearbox towards the back wheel. We also can to the conclusion this year that we wanted to use the more optimal drop center (achieved by using the formula Wheel Drop = Center to Center Distance/14 * .125), and that has added even more problems onto our list. The bearing block we had used beforehand (custom made with a 1/16th inch drop) will no longer work. It was easy enough to modify the existing part to accommodate a new drop-center, but we’re not fully convinced that we have the expertise to mill them out at the current time.

One idea that was brought up was raising the gearboxes only up to the ideal height (about 1/10 of an inch offset), and leaving the middle and right wheels directly in the center, and using some of the vexpro bering blocks without a drop center. I had some concerns about only having the drop center on the back wheel not creating enough of a drop (i.e. the robot wouldn’t rock enough). It’s very possible I’m just overthinking this and the drop center would work just fine with only the back wheel raised.

If I forgot anything please feel free to let me know. I’ve been a lurker for a while but never made a post myself before.

Tl;dr: Are there any specific issues to watch out for with having gearboxes in the back as opposed to the center? Will a drop center work just fine if only a back wheel is raised, and the two front wheels are left at y=0?

When you say that you want your gearboxes in the back, I assume that you’re aiming to have your CG at the back of the bot, so that its regular “resting” position is on the back four wheels with the front two off the ground?

If so, then:

  1. It should work just fine
  2. Instead of raising some wheels, I would lower some. This gives you better ground clearance without having to do any extra work.
  3. If you want your normal resting position to be on the back four wheels, why raise up the back wheel? That means that your normal resting position will be angled back a bit. I would lower the back four wheels and keep the front at y=0.

As a side note, you were planning on using a custom made bearing block that you’ve used before? Sounds against the rules to me.

None that I can think of. I will typically two separate chains from the back wheel to the middle and front wheels, rather than chaining the front wheel to the middle wheel. This prevents any double loading.

Yes, this is effectively just running half as much drop. Whether 1/16" is enough depends on your wheelbase and wheel type.

As far as I’m aware, if you are only running 1 belt from your rear wheel (with gearbox) to the middle wheel (and then 1 from the mid wheel to the front), your rear belt will be transmitting the torque for both the middle and the front wheel. Will this cause the belt to fail? I don’t know. You will also result in having a pulley on the front wheel be somewhat cantilevered (since the middle wheel will have 2 pulleys), which may cause some long term issues, but again, I’m not totally sure.

If your rear belt snaps (which is possible), then your raised rear wheel may effectively freely spin in midair in some cases on this field. In the past, 254 has run a chain run from the back to the middle and the back to the front in order to combat this (the long chain run may have been on top of a normal middle->front chain run).

I’m absolutely unsure of the exact physics behind this, so if I’m blatantly wrong, I would appreciate corrections.

This shouldn’t be an issue. We did an 8-wheel drive last year, which meant running pulleys like this, and didn’t have a problem because of it.

Sure. It can work out fine. But I would guess it depends on the torque being transmitted to the pulley. I don’t know how much cantilever would be required before something in the system starts acting up.

Note: I have run several years worth of drivetrains like this without issue. But it’s something to keep in mind, I feel.