6WD Chassis Stiffness vs. Maneuverability

[JVN]

In my experience it is VERY possible to do a 6WD with 6 high traction wheels. If you find yourself reducing traction on some wheels to increase turning, you're negating MANY of the virtues of using a 6WD in the first place.

There have been lots of arguments about optimizing a 6WD configuration, especially centered on how much "wheel drop" you should use. In my experience this isn't as huge a factor as many people seem to think.

I believe that the key to a well performing 6WD lies in the robot's CG. A lower CG will result in much better performance than a higher CG. Placement front-to-back is also important and will greatly change the way a drivetrain performs.

Don't believe me? Take one of your old robots and add some weights to it in different places, observe the change.

Good Luck! Looking forward to seeing Team 33 at IRI.
-John

[JesseK]

We have had this problem as well in the two years we've done a 6WD. Even in '08, when the front wheels were omni and and two rear traction wheels were spaced only 12" from each other, we still had issues ('07 was a rocker). Come to think of it, our ramps were mounted on the outside of the frame in '07 and the mass of our elevator in '08 came down right on the inside of each wheel.

This makes me wonder if the mass distribution left to right on the bot should look fairly centric rather than outward.

[Ken Patton]

Originally Posted by JVN

I believe that the key to a well performing 6WD lies in the robot's CG. A lower CG will result in much better performance than a higher CG. Placement front-to-back is also important and will greatly change the way a drivetrain performs.

I think John has it exactly right. The lower CG machines - and the ones mentioned were lower CG than the 33 machine - were able to make the turns in a smooth and fast way.

I don't know this for sure - it would be interesting to get some comments from the 1114 or 254 people - but did they bias their 2008 fore-aft CG so that the bot is running on the back wheels through the turns? My guess is that they did.

One other item that you mentioned in the title is chassis stiffness. I have always thought that 33 machines have had a somewhat flexible chassis. Maybe this is causing an unintended wheel contact through the turns as the chassis flexes?

The previous 33 design, with short wheelbase center+rear drive (like you had in 2002 I think) was amazingly fast and maneuverable. I was surprised you guys didn't use that layout for 2008.

Ken

[desi_technology]

We haven't had any problems with 6WD drive for speed and turning ability. We used 6 4x2 IFI traction wheels with the center wheels have a 5/32 in drop which was just enough to allow us to turn well yet retaining pushing power as well. All six wheels had the gum rubber thread from mcmaster so it really stuck to the carpet well. I think the key here is what john mentioned earlier is your robot CG!!!, we had 60% off our total weight four inches off the ground which made a great difference in our mobility. If done right you don't need any omni wheels in the front or back to turn smoothly.

[NickE]

254's 2008 Robot had an extremely low center of gravity. Also, the vast majority of our weight was in the rear of the robot mainly due to the following things:

• All elevator mounting points were within 6" of rear wheel axle
• Battery positioned slightly to the rear of center axle
• Compressor (and most pneumatics) were on rear of robot
• Due to open front of frame, all electronics (except for speed controllers) were mounted rear of the center axle

I believe that our center wheels were dropped 3/16", and all wheels are 4" diameter and 1" wide, using black roughtop tread

Due to the weight distribution on our robot, the robot rarely had any weight whatsoever on the front wheels. When changing tread throughout the season, the front wheels rarely required a change (they probably could've gone the whole season on one set of tread). However, the center and rear wheels burned through tread much quicker (during competition, we typically changed the tread about once per day of competition, but always had a spare set of treaded wheels ready)

During play, the robot was probably on the rear/center wheels over 90% of the time, and only really rocked during quick stops and reversals of direction.

[IKE]

Thanks. I really appreciate the great feedback so far.

Here are some observations of our chassis developement. There is a bit of a monkey wrench in the "CG" height thing.

During the build season, we took it for a test drive when it was still a bare chassis and balasted with sand bags up to competition weight. We used the sand bags to change for-aft weight distribution, but the "CG" height could not have been more than 7" off the floor. The same dynamic issues that we see in the fully prepped machine were there with the bare chassis.
The omnis were added to improve cornering. When we had omnis on all 4 corners the chassis predictably had a lot of whip. If the "CG" was not balanced for aft, it was extremely loose (unstable). When I ran the turning delta power calculations it was under 50 Watts (delta side to side) for a relatively high speed tight turn. This can be achieved by just the slightest release of inboard power with a 2 chip per side powertrain. When we had 2 KOP wheels on the back the chassis required a lot of reverse inside torque to turn even at high speeds (approximately 300 watts calculated for the same turn that was 50 watts above). This was correlated with motor temps as example 1 the inner motors stayed cool, while the motors were significantly hotter in example 2. For competition we went with 1 omni (inside rear) and 1 KOP outside rear, high traction wheels in the center and omnis up front. The calcualted turning power (again for similar turns) was about 150 watts. This set up has been good for our machine for this year, but would lack in 2007 games.
As JVN stated, I know it is possible to run with 6 high traction wheels with relatively little rock (I have seen these machines and talked to them about their "rock" or "drop"). It just isn't "working" for us. That is why we were thinking stiffness plays a big role.
("Working" is relative term here in that we were able to repeatably get 5 and sometimes 6 lines in Hybrid on clean runs, so it does handle pretty well. I am really looking for turning good into great refinement.)

Ken,
The 2 traction 4 omni set up performed similar to what the 2002 did (so I am told, I have only 4 years with the Bees). With the high speeds of Overdrive it was just a little too unstable.

Tom- Talk to Jim Z. and he can give you some really cool tips on how to drive straight. With a gyro and some tuning he can help with the straight equation.

[JVN]

Originally Posted by IKE

Thanks. I really appreciate the great feedback so far.

Here are some observations of our chassis developement. There is a bit of a monkey wrench in the "CG" height thing.

During the build season, we took it for a test drive when it was still a bare chassis and balasted with sand bags up to competition weight. We used the sand bags to change for-aft weight distribution, but the "CG" height could not have been more than 7" off the floor. The same dynamic issues that we see in the fully prepped machine were there with the bare chassis.
The omnis were added to improve cornering. When we had omnis on all 4 corners the chassis predictably had a lot of whip. If the "CG" was not balanced for aft, it was extremely loose (unstable). When I ran the turning delta power calculations it was under 50 Watts (delta side to side) for a relatively high speed tight turn. This can be achieved by just the slightest release of inboard power with a 2 chip per side powertrain. When we had 2 KOP wheels on the back the chassis required a lot of reverse inside torque to turn even at high speeds (approximately 300 watts calculated for the same turn that was 50 watts above). This was correlated with motor temps as example 1 the inner motors stayed cool, while the motors were significantly hotter in example 2. For competition we went with 1 omni (inside rear) and 1 KOP outside rear, high traction wheels in the center and omnis up front. The calcualted turning power (again for similar turns) was about 150 watts. This set up has been good for our machine for this year, but would lack in 2007 games.

The fact that you are calculating turning power requirements is a cool thing. We never go into that level of detail. Turning is one of those things that we do experimentally (for better or for worse) and based on prior experience. (Guess and Check is probably a bad way to do it, but it seems to be working out so far. Negative reinforcement of the worst kind.)

What motors were you running in your drive this year?

-John

[kramarczyk]

A low CG certainly does help you withstand higher turning moments, but it does not help you generate them.

For those that have read Chris Hibner's whitepaper on turning (http://www.chiefdelphi.com/media/papers/1443), which is everybody I hope, it looks at the individual contribution of each wheel to the turning moment, so I suggest we start here. The moment is generated at each wheel by the coefficient of friction (CoF) and normal force (Fn) at each wheel. I think that the CoF is largely understood as demonstrated by the unusual wheel choices cited above. So what changes the Fn at each wheel? Simplifing the frame as a series of beams for discussion we can reference some stock solutions for the beams with 3 supports; based upon a uniform load the 3 moment equations show that the center axle would support 62.5% of the load with 18.75% for the outer axles. A snapshot of this eqn is in a previous post here.

[kramarczyk]

Originally Posted by kramarczyk

A low CG certainly does help you withstand higher turning moments, but it does not help you generate them.

For those that have read Chris Hibner's whitepaper on turning (http://www.chiefdelphi.com/media/papers/1443), which is everybody I hope, it looks at the individual contribution of each wheel to the turning moment, so I suggest we start here. The moment is generated at each wheel by the coefficient of friction (CoF) and normal force (Fn) at each wheel. I think that the CoF is largely understood as demonstrated by the unusual wheel choices cited above. So what changes the Fn at each wheel? Simplifying the frame as a series of beams for discussion we can reference some stock solutions for the beams with 3 supports; based upon a uniform load the 3 moment equations show that the center axle would support 62.5% of the load with 18.75% for the outer axles. A snapshot of this eqn is in a previous post here.

Apparently that got submitted early... whoops. To finish my point...

<theory>
When driving on concrete (or a similar surface) my observations have been that the chassis drives very similarly to how this distribution suggests, however, when we put them on carpet it can be very different. Many attribute this to the CoF change on the carpet, but I think that the Fn also changes as the carpet compresses under the bot like a spring. (See attached spreadsheet.) This is why lowering the center wheels improves the turning of a chassis. Yes, the center wheels can be lowered to the point of rocking, but my 3+ minutes of bot driving time suggest that rocking between 4x4 cg aft and 4x4 forward is not a pleasant experience. A lot of folk do manage it though and they are amazing.

So how does the stiffness of the frame play into this...

Assuming the carpet is spring 'like', the deflection of the frame limits the difference in deflection of the 'springs' under each wheel which eats into the wheel drop. The stiffer the frame, the less wheel drop lost.

In working terms, a stiffer frame requires less wheel drop to move the weight to the center than a less stiff frame. Consequentially a stiffer frame is also more greatly influenced by a change in wheel drop than a less stiff frame. i.e. a more flexible frame is more robust to wheel drop.

</theory>

Unfortunately, the situation is statically indeterminate so I don't have a set of equations to conduct an optimization study with.

[doukjin]

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

[=Martin=Taylor=]

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

[Jonathan Norris]

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.

[IKE]

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...

[Cory]

Originally Posted by IKE

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.

[IKE]

Originally Posted by Cory

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.