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Unread 03-07-2008, 17:46
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Re: 6WD Chassis Stiffness vs. Maneuverability

to improve maneuverablilty/decrease turning radius, our team [1002] has moved away from standard wheel setups and gone with 6WD, but with the middle two wheels offset by 1/16th of an inch from the other four wheels

this way at any given time, we have 4 wheels on the ground... usually... and one set of wheels 1/8th inch above the ground

you might not get the best results with this however though [as i said before] it does improve turning radius [and power expenditure for turns] considerably
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Unread 03-07-2008, 17:46
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Re: 6WD Chassis Stiffness vs. Maneuverability

I'm not too good with numbers, but I've built a lot of prototypes....

I've discovered one important thing. The closer the traction wheels are oriented into a square the better the robot turns.

This is why 4 WD robots in the "long" configuration turn horribly. Because the wheels are in more of a rectangle than a square.

Many teams have solved this problem by adding another set of wheels in the center to essentially make their robot into two 4WD trains. As the robot tips back and forth the wheels remain in a square (all six are never touching the ground). I believe this is why 254 is so speedy.

Another way to correct this problem is to place two omni wheels on one end of the robot. Since the omni wheels offer no side-friction, the robot will turn just like a 4WD machine with all four wheels in the back of the robot.



We've always built robots with the rocking syle drive train.... and then gone back latter and added omni wheels
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Unread 03-07-2008, 18:26
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Re: 6WD Chassis Stiffness vs. Maneuverability

I totally agree with JVN, ect. that say COG is the most important factor to a smooth driving 6-wheel drive. just try driving only your base drive system, it will drive like a dream, once you lump stuff on top and raise the COG it will never drive the same. comparing your robot this year to 1114, 254, etc. your COG is probably a good amount higher because of your scoring mechanism, thus leading to a tippier rougher driving robot.
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Unread 03-07-2008, 19:04
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Re: 6WD Chassis Stiffness vs. Maneuverability

Looks like we will have to run an experiment. I will do a tilt test to estimate actual CG height.

I really think that the poster talking about carpet stiffness vs. chassis stiffness is on to something. I think we will build an experimentally stiffened chassis.

NICKE/254? You said that you had to retread wheels daily on your middle and rear wheels. Was the tread-wear even? More on the inside? More on the outside? I do a little amateur racing and you can tell a lot from your tires...

Last edited by IKE : 03-07-2008 at 19:05. Reason: better clarification and typo
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Unread 03-07-2008, 19:17
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Re: 6WD Chassis Stiffness vs. Maneuverability

In 2006, when I was still with 1293, we used the kitbot--stock ratios and all--feeding one IFI wheel in the center and Skyways on the corners. The videos I've seen all show us whipping around pretty hard, and we were quick enough to do our job (defense) effectively. It didn't get us into eliminations, but it was good enough to be the best finish the team has ever achieved in qualification rounds (before or since).

In 2007, I took the concept with me to 1618. This time, we used skinned-and-roughtopped AndyMark FIRST wheels, driven by a Gen2 AM Shifter using the big and small CIMs. We used the second speed to blitz across the field, important when stopping folks on the far side of the rack. Observation from off-season testing and Brunswick Eruption showed that this design was a little harder to turn. It doesn't like to turn in place, though it'll do it when you push it hard enough (as I found when one of the pre-rookies on 2458 about spun the numbers off of it at Brunswick Eruption).

This year, we switched to the AM Super Shifter with two CIMs per side for lack of the large CIMs. (And, truthfully, we would only have used other motors on drive if we really needed a CIM elsewhere.) Instead of roughtop, we used wedgetop cut with the long diamonds going around the wheel (as opposed to how IFI cuts theirs) for a little less CoF since we were flirting with the upper limits of current draw if we stalled a motor. (That was before we never got to a manipulator and weighed in south of 75 pounds.)

The more revolutionary change for us was riveting the frame together instead of using bolts. The result was the tightest frame I've ever had a hand in (well, excluding Bob in 2004 where we sent out the 2x4 aluminum of that year's kit to get welded together). The weight was slightly skewed towards the rear, with virtually nothing more than a foot off the ground. When driving both, I've found Speedy Debris to be a little easier to turn than Uppercut before it, though it may be a function of the 30-pound weight difference. An apples-to-apples comparison of bolted frames against riveted frames would be interesting.
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Unread 03-07-2008, 19:26
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Re: 6WD Chassis Stiffness vs. Maneuverability

294's base for 2007 and 2008 (along with the 2007 summer prototype) were all extremely stiff by the nature of their design. All had 3/16" of center wheel drop, and were all very maneuverable (in the hands of a good driver; I didn't make the 2007 base look great). I hope this provides some anecdotal evidence for you.

I think your idea of stiffness changing the manueverability makes a great deal of sense.
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Unread 03-07-2008, 19:43
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Re: 6WD Chassis Stiffness vs. Maneuverability

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Originally Posted by IKE View Post

NICKE/254? You said that you had to retread wheels daily on your middle and rear wheels. Was the tread-wear even? More on the inside? More on the outside? I do a little amateur racing and you can tell a lot from your tires...
The outside wheels wore down probably 3-4 times faster than the inside, due to the left hand nature of the game.
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Unread 03-07-2008, 20:46
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Re: 6WD Chassis Stiffness vs. Maneuverability

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The outside wheels wore down probably 3-4 times faster than the inside, due to the left hand nature of the game.
Very interesting. Our inside traction wheel wore down the fastest. Just an observation.
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Unread 07-07-2008, 03:25
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Re: 6WD Chassis Stiffness vs. Maneuverability

I'd love to pick the brains of someone familiar with team 25s 6WD. If I recall correctly, they ran an extremely maneuverable 6WD totally chainless setup with no centre wheel drop whatsoever for a number of years. I'd bet someone there could tell us whether the secret to achieving this was through superior chassis stiffness.
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Unread 07-07-2008, 08:17
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Re: 6WD Chassis Stiffness vs. Maneuverability

25 was a 8WD chassis. They had a good article in the 1st Behind the Design book. I just read through the one for their 2007 chassis (2nd book), and it was limited but did remind me that they turn down the treads on their wheels and then carve a specific pattern. I would love to hear more if anyone from 25 is offereing up adivce/opinions.

kramarczyk-
Thanks again for posting that paper. It is a really good example and applies really well to a drop-center or rocker style 6WD. This paper really should be included in the KOP. While the overarll results seem more like common sense to most vetrans, many rookies don't have their machines running until someone helps them program it at their first competition. That is a horrible time to learn taht you have a dynamics issue.
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Unread 07-07-2008, 09:00
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Re: 6WD Chassis Stiffness vs. Maneuverability

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Originally Posted by IKE View Post
kramarczyk-
Thanks again for posting that paper. It is a really good example and applies really well to a drop-center or rocker style 6WD. This paper really should be included in the KOP. While the overarll results seem more like common sense to most vetrans, many rookies don't have their machines running until someone helps them program it at their first competition. That is a horrible time to learn taht you have a dynamics issue.
I swear, that white paper is one of the best kept secrets out there. I did a spreadsheet to save some of the 4x4 users from the minutia of the math. http://www.chiefdelphi.com/media/papers/1917 I've got a beta version for 6x6, but got stuck on some of the issues we're discussing here.
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Unread 07-07-2008, 11:10
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Re: 6WD Chassis Stiffness vs. Maneuverability

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Originally Posted by kramarczyk View Post
I swear, that white paper is one of the best kept secrets out there. I did a spreadsheet to save some of the 4x4 users from the minutia of the math. http://www.chiefdelphi.com/media/papers/1917 I've got a beta version for 6x6, but got stuck on some of the issues we're discussing here.
This is a good spreadsheet, but it assumes the center of mass, center of area, and all forces of the wheels are perfectly coplanar and are at a height of zero. It also assumes the center of mass is right/left centered on the bot, which sometimes isn't the case. Also, for a 6WD rocking chassis to rock, the CoM must break the vertical plane of the middle axle, which inherently assumes a CoM above the axles.

Since the act of rocking helps a turn anyways, perhaps the first iteration of an improvement to the spreadsheet could simply be to account for a three-dimensional CoM? Even turning in place, a CoM that isn't centered will cause each wheel to contribute a different turning moment. Then we could split the 6WD rocking chassis into two separate 4x4 arrangements with different CoM heights based upon the "default" and "rocked" states.

Of course, early in the build season we would have to estimate CoM height based upon desired manipulators (elevators, arms, etc), but later in the season for tweaking it can really help determine the best placement for mount points and heavy-component placement.
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Unread 08-10-2008, 08:33
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Re: 6WD Chassis Stiffness vs. Maneuverability

Here are some "tilt table" pics. By balancing the robot, the CG appears to be about 12 inches off of the floor centered side to side and for-aft. I measured this with the Ball in which adds about 7 lbs. 26 inches off of ground-line thus moving the CG up and forward a bit.

The math on this is pretty simple trig. Balancing the robot on various edges will give you planes that the CG is along. The intersections of these planes will then show where you CG is. The assumption there is that the "robot" is a rigid body and there are no large shifts in weight (i.e. fluids in a tank, or arms that flop around).
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Unread 05-12-2008, 08:12
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Re: 6WD Chassis Stiffness vs. Maneuverability

Not sure why this didn't occur to me earlier, but I think I can vocalize it better now. We have danced around the concepts, but some simple assumptions show a lot.

1. Assume an inifinitely stiff drop center 6WD chassis with 6 traction wheels. As long as the drop is more than the carpet compression, only 4 wheels can touch the ground at a time. More importantly, the middlewheel will be on the round and then only the front, or only the back can touch. This means you can use the above whitepaper to analyze whether or not your bot will turn well.

2. Assume now a noodle chassis. Same base geometry as the above chassis, but it is so soft that all wheels touch even though the center is dropped. This will require a lot more torque to turn because of all the additional lateral grip.

Now to answer my torsional stiffness question.
3. Assume that the rails of the chassis are very stiff, but connected with a swivel. While only two wheels of each rail section can contact the ground, opposite corners can contact during a spin manuever. If you sum the moments, you will find that this will require about double the torque of case 1 to spin(turn).

Case 3 is probably the most similar set up as what Team 33 ran last year. The sheet metal construction had relatively stiff rail and axle systems, but the stringers between the rails sections allowed for a lot of torsional flexibility.
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Unread 05-12-2008, 09:10
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Re: 6WD Chassis Stiffness vs. Maneuverability

That seems like a valid analysis of what happened. We're going to experiment with C-Channel for the rails, which will give us very stiff side rails and alot of open room in the middle. It sounds like we shouldn't skimp on the cross supports at all.
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