Re: 4WD Turning Difficulties
 

I think you would get better performance with the front steering wheels. If you drove the back wheels like a tank drive (not opposite directions though, I expect you would want to program in a motor differential) and power the front wheels in the direction of the turn you should have greatly reduced resistance when turning.

Re: 4WD Turning Difficulties
 

Give it a little bit to finish clearing the upload, but:

http://www.youtube.com/watch?v=03tYPM1Bw4c

Videographic evidence of what "standard" 4wd bots will turn like. Bot is weighted down (with about 60-80lbs on the bot and some unknown amount on the trailer).

Before anybody asks, yes, our tile flooring is similar to the regolith. We had a mentor who works for 3M bring in their version of Regolith and we did traction testing (tile floor vs. 3m "regolith" and we got the same results, 10lb breakaway force, 8lb dynamic pull @60lb 2wd robot).


(What you don't see, after the video: The robot's wheels just about fell off and it took a good hour or so to get them working completely again. Oh, the joys of thrown-together prototype bots)

Re: 4WD Turning Difficulties
 

Dave,
Is it possible you have a career ahead of you in some kind of R/C drifting competitions????? ;)

Re: 4WD Turning Difficulties
 

To be completely honest, this didn't look as bad as I thought it would...

Re: 4WD Turning Difficulties
 

Is that video on real Regolith?

Our testing (with a stock kitbot weighted 150 lbs and a stock goal) shows it is worse than this video would lead you to believe.

Re: 4WD Turning Difficulties
 

Yeah, we got the same results agreeing with John. We had to spin our wheels for a good 5 seconds to get our bot to rotate (without trailer), on Glasteel FPR.

Edit: To be fair, our robot wasn't loaded to full weight, it was at about 35-40 lbs.

Re: 4WD Turning Difficulties
 

this video appears to be on a school type tile floor, which is different from the field material.

Re: 4WD Turning Difficulties
 

This discussion has helped me think through more on our plans. Any videos or real world experience would be helpful.

Re: 4WD Turning Difficulties
 

While we didn't use exact weights, we drove a 4 CIM AM Shifter in Low Gear (with a long drive base) at about 30 pounds and about 180 pounds (yes, I know this is 30 pounds more than an actual robot!). While noticeable, the difference in turning ability was nothing to write home about.

Re: 4WD Turning Difficulties
 

Four-wheel tank drive on a ~140lb robot seems about as easy to turn and maneuver with as a car towing a small trailer on a very icy road... Unreliable at anything approaching speed, but something you can definitely live with -- especially if there are no ditches into which one might slide, and the intactness of one's body is not on the line.

I didn't notice that skid steering was any better than the steering of an actual car on 1/4" of ice... and I had a chance to test that out in the school parking lot just this past Wednesday! (We had a snow day because of an ice storm, so I did a little 'research' with my car.)

Patrick

Re: 4WD Turning Difficulties
 

I took a few minutes to update the spreadsheet from the beginning of the thread to include the forces from the trailer in the FBD. At present I still do not have the physical properties (i.e. an adequate trailer) to validate the spreadsheet, but it does seem to agree with what I have seen in videos from other teams. If some folks concur that the physics are correct I'll put it up in the white papers. Let me know if it is worthwhile.

Re: 4WD Turning Difficulties
 

a = f/m
friction force = normal force * mu
normal force = mass * gravity

a = ( mass * gravity * mu ) / mass

a = mu * gravity


so, a 30lb robot and a 180lb robot should be able to change direction equally fast.

perhaps the 30lbs and the 180lbs were distributed differently, changing the moment of the center of gravity?

Re: 4WD Turning Difficulties
 

Has anyone seriously considered a robot with two wheels in the middle as the drive wheels and having it pivot? We're thinking it would get good traction, as all the weight (well most of it) would be on the two drive wheels, and the trailer would help balance out the wheels to keep the center of gravity in the middle. Also, the simplicity factor would allow us to spend more time making a way to score.

Re: 4WD Turning Difficulties
 

I've seen that discussed on here before, I think that the concensus was that the hitch allows too much vertical movement for that to be feasible. You'd probably rock the hitch up and down enough for your team to get called on the rule that states the hitch needs to be a certain height

Re: 4WD Turning Difficulties
 

This would give you no more traction than a 4WD robot as the downward force (max ~150lbs) is the same.

It would reduce your force required to turn (scrubbing) significantly.

I would be concerned that when you are fully turned (60+degrees difference between your robot and your trailer) you could possibly tip enough to touch the floor (breaking the rules) or tiping outside the envelope.

Also, I believe the robot needs to fit within the sizing box without being supported by the sizing box, and a 2 wheel bot may not do that on it's own.