Need to move CG

Lego makes special heavy blocks that you could use to re-balance your robot’s center of gravity; does VEX make anything similar to that? If not, what do people use for that purpose?

I’ve seen a lot of teams use wheels to do this, but you could also use metal drive shafts.

Yes, part number 228-2500-1402, 6x Pitch Flywheel (75 g).

Currently the only kit you can get this part in is the Trebuchet kit, which contains 4x of these. The Trebuchet kit also contains a few other cool mechanical bits, like a ratchet and pawl mechanism and a bunch of rope elements.

2 Likes

Check out the Trebuchet, it has the 75 gram 6x Pitch Flywheel (228-2500-1402) that are legal competition parts.

image

… jinx … :slight_smile:

6 Likes

Adding weight to a VEXIQ robot is never a good idea.

Can you describe your robot? Maybe a picture?

Until you do that, let’s take Clutch as an example.

When you load a Cube onto Clutch, it has a tendency to lean forward, almost tipping down. Now one solution would be to add weight on the back to help counter the force.

Plan B would be to extend the chassis to move the front wheels 2-3". This longer wheelbase gives you better balance, it’s moving the center of gravity. Two 8x2 plates weigh almost nothing.

Good luck!

4 Likes

Instead of adding weight, can you shift weight? The heaviest parts are going to be the brain, battery, and motors. The brain is probably is the “most” flexible one to place since you use longer wires to connect the motors and sensors. Can you place the brain more center and more towards the bottom? What about your motors?

8 Likes

Wow, good point, didn’t think of that, thank you!

We came to the same conclusion and ended up extended the wheelbase by ~2 inches but that made turning virtually impossible (it’s a 4WD with 200mm wheels). Is that expected? Our previous drivetrain was based on one of the standard builds (Clutch or Flex…) and had no problems spinning around. We’re going to switch to RWD with omni wheels in the front and see if that makes things better.

You should still plan to do AWD. Unpowered wheels are rarely a good idea, especially on a field with obstructions. An unpowered omniwheel getting caught on one of the orange rivets would cause havoc on an autonomous run.

3 Likes

As @mtaylor pointed out the omni’s are the way to go, but they need to be powered (chain, gear train, etc.) to get the best utilization out of them.

Remember that the gear train needs to be an ODD number of gears to get the wheels going in the same direction. In Clutch you are driving the rear wheel. As long as the gear on the rear wheel and the front wheel are the same size you’ll get the same speed

OOOOO works with all the gears the same
OoooO works with the smaller gears as idler gears will also work.
OoOoO also works
OOooO also works

So there are lots of ways to get all wheel drive with the gear sets you have. Good luck!

4 Likes

I won’t be able to try it out for the next 6 days (which is going to be torture), but theoretically with a longer wheel base there’s going to be more turning “scrub” where the wheels are not so much turning as dragging across the field. So you’re saying we still want to power the (front) omni wheels and that’s not going to add more scrub?

2 Likes

With any tank style design it’s recommend to either chain or gear the front and back wheels together. That way you power all the wheels. Using omni wheels (either all omnis or omnis on front wheels) help reduce wheel scrub.

3 Likes

The omni wheels roll from side to side, so the scrubbing isn’t a big deal.

You want to power them because of the orange pegs getting stuck it the crack between the roller wheels and the supports.

6 days will fly by pretty quickly!

Reporting back - we ended up with a longer AWD chassis with omni wheels in the front and the bot is turning pretty smoothly and the pegs aren’t a problem; the only downside is that going straight in autonomous has degraded somewhat but is still acceptable.

1 Like

Yay!

To fix the drift you can either use motor rotations instead of time, add a gyro to sense the drift amounts, or a combination of both.

Good luck with the rest of your season!

Yeah, that’s the drawback of all omni wheels. Some teams have opted for omni wheels on front fo reduced wheel scrub, with regular 200mm rubber tires on the back for more accuracy in autonomous. You can try that, too.

1 Like

This topic was automatically closed 365 days after the last reply. New replies are no longer allowed.