Converting AndyMark Mecanum Chassis to 4WD Tank

Does anyone know if it would be possible/practical/beneficial to swap out the mecanum wheels on the AndyMark mecanum chassis kit for traction wheels while still using the 4 toughbox micro gearboxes in the corners? Would scrubbing be an issue in a 4WD using 4 traction wheels?

Probably be worth considering if you could put one traction and one omni wheel per side if concerned about turning.

But it depends on why you are changing in the first place.

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Traction and speed. Do you think it would be worth the swap?

Your drivetrain dimensions (more accurately, your wheelbase dimensions) will depend here too. If you have a long and narrow wheelbase, you’re going to have a harder time turning than a short and wide wheelbase with traction wheels.

If your superstructure design allows for it, I’d just make the swap back to the belt driven 6 wheel kitbot config. From there, you have a lot of flexibility for different wheels and gear ratios. (as well as always having your full motor power available even with wheels off the ground)

If you really are constrained to toughbox micros and corner wheels, I would recommend going with @ngreen s idea of combination traction/omni. You are going to have a very tough time turning with 4 tractions and no center wheel unless you are running a very wide and short configuration.

As for your reasoning for swapping:

Traction: yup, swapping for traction wheels is probably going to net you more traction than mecanum wheels.

Speed: Not really going to change unless the wheel diameter is increasing. In fact, your overall driving might slow down if your driver is already used to mecanum.

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If you do stick with 4 wheel and solve the scrubbing while turning issue, I would highly recommend running a belt between the two wheels. You don’t want to lose half your drivetrain power if one side of your robot lifts up during a pushing match.

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Running the AM14U5 with the mini output shafts will give you a wheelbase of 20.00". As noted above, you will want your track width to be a fair bit larger than the wheelbase for traction wheels so you can turn. If you cut your front and rear plates about 27" or 27.5" long (max to keep frame perimeter under 120") you should be OK. Reference:

Note that with a 20" spacing, these wheels may be easier to connect with (inch-based) chain than with (metric) belt.

3946 ran 4 traction wheels a couple of times, but we had a much shorter wheelbase; we cut the back third of the KoP chassis and had a wheelbase of less than 12". Those worked great!

Scrub while turning (or trying and failing to turn) is an enormous problem with 4WD tank. I would seriously consider (a) belting/chaining the wheels together and (b) making your back wheels omni wheels.

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I would not try to convert the mecanum drive into tank, your better off buying the toughbox mini instead if you don’t have one and do 6 wheel wcd. We found the scrubbing will wear out your tread really fast and drain your battery. Also it’s really slow at turning/ not accurate for auto code.

Why the back ones in particular? Other than with unpowered omnis, I’ve never worked with a mixed traction/omni drivetrain.

My intuition (caveat - not calculations or experience!) says that if the CoG is significantly forward or behind the geometric center, the wheels closer to the CoG should be the traction wheels. If they’re near each other, rear fishtails are easier to drive out of than front fishtails, so front should be traction as you indicated.

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This is really important if you want to turn quickly. In 2018 we ran a front traction rear Omni drivetrain, but the center of mass ended up being way at the back of the robot. This made autonomous tuning really difficult since you had to apply a considerable amount of turning force to get the thing to turn, and once it started turning it was hard to stop. The driver got used to it, but I definitely would not do it again.

Because front-wheel drive is way easier than rear-wheel drive, especially for someone who has to learn it on the fly.

We learned that the hard way ten or so years ago.

Beyond the if you should do it, and more to the how. Yes, this is possible.

We did this last year. You could use the Andymark Mecanum kit.

You could use any wheels you want for this.