pic: off season chassis

[IKE]

I am with JVN, give it a spin (pun intended) and shoot some video. I would suspect it would have a very crazy behavior if you had a slightly rear biased CG. Take the 2008 game, where you are trying to do laps. If the weight is rear biased, going into the turn I would suspect the robot to initially push (understeer), but then as the weight shifts to the front, and those wheels grab, it would have crazy snap oversteer. With a lot of practice, you could probably make an "FRC Tokyo Drift" video...

[,4lex S.]

Interesting, I think this wheel configuration would be handy in a way. If you ended up pinned, you could shift CG and slip out of it in a lot of cases. My old team designed mechanisms to make the robot 'slippery' in '09 and it worked fantastically. If you can shift CG and need to escape pinning this could be viable.

[Brandon Holley]

Unless theres some sort of optical illusion going on here, it seems that the omni wheels are smaller than the traction ones. The sprockets on all the wheels appear to be the same size as well, meaning the ground speed of the omni and traction wheels will be different. I still think its worth a shot at trying and showing everyone here your results, just wanted to point that out.

-Brando

[PAR_WIG1350]

Originally Posted by Brandon Holley

Unless theres some sort of optical illusion going on here, it seems that the omni wheels are smaller than the traction ones. The sprockets on all the wheels appear to be the same size as well, meaning the ground speed of the omni and traction wheels will be different. I still think its worth a shot at trying and showing everyone here your results, just wanted to point that out.

-Brando

I measured the wheels and the omni wheel seems to bee about 6/7x the size of the outer wheels. Good observation.

[jsasaki]

here's the video a day early.

[Chris is me]

Well that's unexpected. It looks like it's driving exactly like any other drivetrain. Is the omniwheel touching the ground at all? Is it dropped lower than the other wheels, or are all the wheels collinear?

[jsasaki]

Originally Posted by Chris is me

Well that's unexpected. It looks like it's driving exactly like any other drivetrain. Is the omniwheel touching the ground at all? Is it dropped lower than the other wheels, or are all the wheels collinear?

its the dropped 1/8 center. it rocks just a little bit eventually I would like to put on supershifters and run it with that.

[IndySam]

Originally Posted by Chris is me

Well that's unexpected. It looks like it's driving exactly like any other drivetrain. Is the omniwheel touching the ground at all? Is it dropped lower than the other wheels, or are all the wheels collinear?

But it's on a hard surface. Most any drive will work well on that. Carpet will tell the real tale!

[theprgramerdude]

It looks like the front wheel isn't being used at all, like it's acting just as a 4WD with omni's on the front for sliding.

[Chris is me]

Originally Posted by IndySam

But it's on a hard surface. Most any drive will work well on that. Carpet will tell the real tale!

What seemed most odd was that the drive rotated roughly on a dime despite conventional wisdom saying it would have a very odd center of rotation.

[Chris Fultz]

From the video, it looks like most of the weight is on the back, and the robot is acting like a 4 WD, with the omni's being the "front" wheels. The other "front" wheels do not appear to be in contact with the surface.

When it spins, it is turning around a center somewhere between the back and middle wheels - hard to tell by just doing stop frames on the video.

As others said - try it on carpet. The carpet will give a little and absorb the wheels, whick may allow all 6 to have some surface contact.

[PAR_WIG1350]

It did seem to oversteer a bit and hit the pole in the beginning, but that could be due to lag or driver error.

[JVN]

Interesting. Not quite what I expected.

My guess as to what we're seeing:
Is the CG very close to the center? This would put so much weight on the center wheels it would offset the "zero" lateral traction affects of the omni wheels and shift the CG back towards where you'd expect for a "standard" 6WD.

-John

[IKE]

Very cool. Thank you so much for the testing and videotaping. Another really cool attribute to the video was the voice explaining what the driver should do as opposed to random driving around.

As far as turning goes, I was impressed that the turing center was so closely loacted to the center of the wheels (most notable at the spin near the end of the clip). I would tend to concur with JVN that maybe most of the weight is on those center wheels.

As fat as my over-steer under-steer comments from before, it would likely take a little more speed for these behaviours to show up. The bot appeared to be traveling at or below 4 ft/s.

Published lateral grip on those wheels is supposed to be around 0.2. I would expect the oversteer understeer stuff to come up when attempting at turn that would have lateral forces around 1/5th a G to 0.5 G. Lateral acceleration is equal to the centripetal force. V^2/r for 4 fps, this would be 16/r = 0.2 to 0.5 or r of 3 to 1 ft.
I will need to review the clip again. From what I remember, most of the turns were done after it had stopped.

Thanks again for trying this.

[artdutra04]

Originally Posted by PAR_WIG1350

I measured the wheels and the omni wheel seems to bee about 6/7x the size of the outer wheels. Good observation.

The original photo showed the same size sprockets for the chain runs between the omni wheels and the traction wheels. If this was unchanged, and the wheel diameters of the omni wheels is significantly different (I'm guessing the outer wheels are 8" AM wheels and the omni wheels are 6"), this could be adding another variable into this equation.

If this is the case, the surface velocity of the traction wheels will be faster than the omni wheel, so the omni wheel will experience longitudinal scrubbing (collinear with the omni wheel roller shafts) which may yield weird results when driving or cloud the the expected results of what would happen with equal size OR equal surface velocity wheels.