# Mecanum drive or omni with strafing wheel

Our team is testing out the mecanum wheels, but would like to know if having a omni and strafing wheel is better.

Well, we already bought mecanums… and you saw how poorly the strafing wheel worked out for 1581A.

Another question our team has: Do the 45-degree rollers on mecanums reduce pushing power at all? One of our team members is convinced that mecanums cut torque in half…

I would prefer Mecanum because a strafing wheel uses up room in the center, as well as a motor. The advantage with strafing is that you can gear all the wheels together, but that shouldn’t be necessary if you have the center of the mass of the robot in the right place (in the middle).

Hes wrong. You still get full pushing power with mecanums.

Excellent. I thought this, but I wasn’t sure. Thanks!

There are some advantages and disadvantages to each.

Mecanum drives have maximum power when going forwards and backwards. This is helpful because these are the directions you usually want to push in. When strafing they use all the motor power available to them but some power is lost through roller friction. When travelling diagonally they use half their motor power because two of the wheels don’t spin.

Omni H-drives (tank drive with a strafing wheel) don’t have full power when going forward - they only have the power of the motors on the forward-facing wheels. Imagine an H-drive with 4 393s on the forward facing wheels and 2 393s on strafe - it would use 2/3 of its motor power when going forwards, 1/3 of its motor power when strafing, and all of its motor power when travelling diagonally at some angle between straight forwards and 45 degrees from forwards.

Some results of this are:

• in a head-on pushing match, a mecanum drive will beat an H-drive if they both have the same number of motors (and the same gearing).
• to get full speed out of an H-drive you should both drive at full speed and strafe at full speed, meaning you travel towards your destination at a weird angle. Most drivers don’t do this meaning H-drive is in some ways “wasteful” of motor power because it doesn’t use all its power during its usual driving mode.
• if another robot tries to push you on a diagonal when using mecanums or from side on using H-drive, it isn’t worth fighting because your breakers will trip.

Some other things:

• if you overheat your strafe motors with H-drive, you can keep going pretty easily using the forward-facing component of the drive. If you overheat a wheel on a mecanum drive, the consequences are more severe because the other wheels will have to drag that wheel around, increasing the strain on them and the probability of them also tripping.
• mecanum wheels are massive and difficult to physically fit onto a robot. It’s possible that this was done deliberately by VEX to “balance” them, but speculation aside a major reason not to use mecanum wheels is if your chassis design doesn’t leave room for them.

The third option is X-drive, which behaves similarly to mecanums in most ways with some subtle differences.

Thanks for your input!

Having competed with tank-drive against both X-drive and H-drive robots, we have found that pushing from a diagonal angle is a very difficult thing to do.

The heavy mass of the mecanum wheels may explain their success in the video that snowpenguin linked. This gives them more downforce than the other wheels.

Mecanums are nice, but in my experience with our worlds robot the weight had to be quite evenly distributed across all four wheels (although this is really a thing with all holonomic drives)

A much more major factor is that mecanum wheels eat sacks for breakfast. This being said, I don’t believe that mecanums are going to be wheels of choice for sack attack.

I presume you mean they drive over the sacks easily? I haven’t actually been able to test driving over them yet because I don’t have any sacks, but if the mecanums do that well, that’s certainly a bonus to using them (assuming the ability to drive over sacks is actually needed).

Also, if you don’t think mecanums will be the wheel of choice, then what DO you think will be?

The sacks actually get pulled inside the wheel, then everything jams omni’s drive over the top easily, which is probably better than trying to push them around.

Probably just tank drive. Having said that, I have a working transmission at the moment, which i will be showing on the forum in the next few days. It’s a different type to all the ones i have seen here, i think it’s much better

I see something like the nona-tread being useful this year. Teads for getting over mounds of sacks, and omnis for strafing.

Darn… What about if there was an Omni wheel at the front, and then a mecanum at the middle and back? If the omni already initially made it over the sack, so the mecanum didn’t have to do any “climbing”, do you think the sack would still be pulled into the mecanum?

I can’t see myself using a transmission unless its very space efficient, but I’ll be interested to see it anyway :D.

Yeah, probably you can’t really go to a really low chassis either, as you will probably end up stuck on a sack. I imagine most people will be driving over them this season

I had an idea where there’s a 4WD, chained together. The chain is supported by idlers on the bottom, and might even have tank tread links on it if there’s space in the drive. That way, the undercarriage of the robot won’t get stuck on sacks, effectively being a tank tread drive if the wheels can’t touch the ground.

Depending on how it pans out this year, there might be a lot of bots that hoard everything until as late as they can. Not sure if it’ll actually turn out like that; some teams might actually decide to play fairly. But if there is hoarding, I wouldn’t really anticipate driving over any sacks because then you’re doing it wrong by passing up an extra sack. It would be nice if you could intake literally everything that the robot just drives over instead of making an effort to intake anything.

So why don’t you?

4886’s rollers look like they could do that (if they sped them up a little and made them wider). It would be a little harder with a scoop, but if it’s fast enough delays shouldn’t be too bad.

So the sacks get stuck in the Mecanums? That’s a bummer…

Putting Mecanums in the back only would do all kinds of bad things for strafing, but there’s got to be some way of making a cattle guard-type thing to keep sacks from getting stuck up in your drive train.

My team actually used a drive train with 6 wheels for a part of last season, where the front wheels on each side were omni wheels, and the rest were mecanums. It actually worked fairly well, the strafing travelled in a shallow arc and this arc changed depending on the arms height as the COG moved around, but that was really easy to fix with programming or the driver could just compensate.

The only reason my team took the mecanums off is because they tended to cause overheating in the drive motors after any pushing whatsoever. On a lower drive ratio, or with more motors, its a feasible drive train. It allows the chassis to be narrower at the front, allowing more room for an accumulator at the front when the arm is lowered (last season it allowed us to “engulf” goals).

What was your gear ratio when you had the overheating problem?

The motors were geared for high speed internally (1.6:1), and the robot weighed about 16 pounds from memory.

What you have to realise is that with a mecanum drive, there are situations where only 2 of the drive motors are actually moving the robot (when you are going on a diagonal). 2 motors carrying all the robot’s weight is pretty hard on them, and causes overheating really quickly.

The speed of the intakes is actually good. They give enough torque to intakes multiple sacks at a time. I bet we could intake sacks as we drive into them, but we only have 2 sacks right now to test with.