4 wheel drive vs. 2 wheel drive

[Josh363663]

we have been debating whether all wheel drive or front wheel drive or rear wheel drive was better.

[Collin Fultz]

General rule of thumb is that you want any wheel touching the ground to be powered. If not, you're cutting into your team's available pushing force.

[Mike8519]

Originally Posted by Collin Fultz

General rule of thumb is that you want any wheel touching the ground to be powered. If not, you're cutting into your team's available pushing force.

Indeed. If you are imparting robot weight on a wheel, make sure it is powered.

[scottyh2006]

all wheel drive will be the best

[Nuttyman54]

That being said, you really want your weight as far back as possible so that your pivot point is as close to the hitch pivot. That reduces the moment that the trailer imparts on you robot, which it needs to counteract to turn, and will allow you to turn better.

[eNyoron]

Between rear and front wheel drive, it's really not viable to use a rear wheel drive unless you don't plan on ever exchanging an empty cell... or being pushed into a corner. That portion is carpeted and if you use rear wheel drive to get there, you're going to end up with the front wheels on the carpet with high traction and the powered wheels on the regalith with low traction, making it incredibly difficult to move. And backing into the corner wouldn't work either, there's a trailer in the way.

[smartkid]

Where your weight goes is up to you but please know that any and every wheel touching the ground needs to be powered! Also know that your bot should weigh 119.99999 lbs (without battery and bumpers).

[IKE]

My recommendation would be to run some tests with the kit chassis. We subbed some wheels out on a 4x4 chassis last night and ran it on the FRP and carpet. We did a long wheel-base and a short wheel base. We also shifted the CG from 50/50 to almost all on the rear or the nose. All of these are without the trailer so far, so we will try those tonight.

All I can say is I would recommend doing a few tests for yourselves. The dynamics are very different on the carpet vs. on the FRP. Set-ups that were nice and controllable on the FRP wouldn't turn if two side wheels got on the carpet, and set-ups that were easy to turn off the carpet were very squirelly on the FRP. Also, the bot lateral grip seemed a lot lower than the inline grip.

That brings up a very important point, there is a hierarchy when you design:

Ideas are great, but can often be flawed.
Fully thought out theories are better, but often involve a lot of assumptions.
Models (calculations and 3D models) are often made of some tests and measurements and some assuptions.
Test based data is even better yet especially if it validates your model assumptions.
Prototypes can be even more accurate.

My point on this is I have read through a lot of threads on how these bots should be built and how they will behave. I have a lot of vehicle dynamics experience, and I was still surprised by a couple of our tests last night.

[Betty_Krocker]

Originally Posted by Nuttyman54

That being said, you really want your weight as far back as possible so that your pivot point is as close to the hitch pivot. That reduces the moment that the trailer imparts on you robot, which it needs to counteract to turn, and will allow you to turn better.

I am going to have to respectfully disagree. Having lived in snow/ice conditions for most my life, the best option is FWD with Rear wheel steering. Weight must be over the front wheels and can be done due to the trailers weight so you font tip over. It is much better to pull weight than push when it comes to this subject. Transverse friction is used by the rear wheels to guide the bot to turn. Also as for the trailer, since sway bars are not used (i tow trailers all the time) all of the weight will be centered and pulling at the tongue of the trailer. So putting the weight at the back would epically screw you over since it would want to lift the front wheels off the ground thus taking powered wheels off the ground (in any set up)

Edit: All wheel drive might help but might be wasting power, motors, and battery life...
(i have done some tests and this is what i get, but its up to you)

[Bongle]

Originally Posted by Betty_Krocker

Edit: All wheel drive might help but might be wasting power, motors, and battery life...
(i have done some tests and this is what i get, but its up to you)

All wheel drive absolutely helps.

The physics reason for all wheel drive is this:
1) Let's say your robot has 4 wheels
2) Each of these wheels supports some fraction f_i of your robot's weight. The sum of all F_i = 1.
3) Each wheel will thus have a normal force of m_Robot*f_i*g = 9.81*m_Robot*f_i
4) Each wheel will thus be able to generate (if powered) a frictional force of
F_f = u*F_n
F_f = 0.06 * (9.81*m_Robot*f_i)
F_f = 0.5886*m_Robot*f_i
5) Your robot's total potential frictional force is the sum of the frictional forces of all of its driven wheels.
6) For an all wheel drive robot, F_fRobot = 0.5886*m_Robot (because F_i sums to 1)
7) For a 2WD robot with its weight evenly distributed, we know all the F_is are 0.25 (due to equal weight), but only two of them are driven. So F_fRobot = 0.2943*m_Robot, or half of the AWD's robot full force.

So based on 5), 6), and 7), we know that by choosing not to drive some of your wheels, you are throwing away potential friction. Wheels that are touching the floor while the robot is driving and are not powered are completely wasting potential friction. A robot with its weight evenly distributed over 4 wheels that only drives two wheels will be two times slower than an identical all wheel drive robot. The two wheel robot could potentially employ a very complicated suspension so that its weight shifts onto its driven wheels when acceleration or decelerating, but it would be a waste of time when you could just drive all four wheels.

[Betty_Krocker]

Originally Posted by Bongle

A robot with its weight evenly distributed over 4 wheels that only drives two wheels will be two times slower than an identical all wheel drive robot.

if you are going to attempt to prove me wrong, at least understand my argument. I never said an even weight distribution, in fact i did say WEIGHT OVER THE FRONT DRIVE WHEELS, plus my system cuts down on weight thus allowing you to allocate that precious weight to other areas of the robot like the heavy control system, or ballast weight that you can add where needed...

[Bongle]

Originally Posted by Betty_Krocker

if you are going to attempt to prove me wrong, at least understand my argument. I never said an even weight distribution, in fact i did say WEIGHT OVER THE FRONT DRIVE WHEELS, plus my system cuts down on weight thus allowing you to allocate that precious weight to other areas of the robot like the heavy control system, or ballast weight that you can add where needed...

Yes, but even with an uneven weight distribution you're losing pushing power. Even if you stack absolutely everything over your driven wheels, you're probably still losing 10-20% of your pushing power from the weight that will inevitably be over your undriven wheels. Plus you're adding a large design constraint in that all your heavy mechanisms must be in a column over your driven wheels to maximize force.

I understand your idea (though I did gloss that over in my original post, sorry) and it is definitely the most optimal way to implement a 2WD system, I just think that _any_ 2WD system is far from optimal this year.

[Betty_Krocker]

Originally Posted by Bongle

Yes, but even with an uneven weight distribution you're losing pushing power. Even if you stack absolutely everything over your driven wheels, you're probably still losing 10-20% of your pushing power from the weight that will inevitably be over your undriven wheels. Plus you're adding a large design constraint in that all your heavy mechanisms must be in a column over your driven wheels to maximize force.

I understand your idea (though I did gloss that over in my original post, sorry) and it is definitely the most optimal way to implement a 2WD system, I just think that _any_ 2WD system is far from optimal this year.

well from concept everything is in the front anyway, but in this situation pushing power doesn't matter. I you want control you want pulling power. The same reason drift cars are rear wheel drive is the same principal rear wheel drive results in loss of control, and 4wd is going to work too hard to steer sicne the wheels would rather slip inline than transverse (which makes skid steer out of the question) i will admit that 4wd is a valid point, but all drive systems of this year have MAJOR tradeoffs, thus making not one type (except for rear wheel drive) the best, it will be interesting to see at competition....

[Nuttyman54]

Originally Posted by Betty_Krocker

I am going to have to respectfully disagree. Having lived in snow/ice conditions for most my life, the best option is FWD with Rear wheel steering.

Agreed. My comments were was based upon the assumption of a standard kit style frame with no steering and the choice being between 2 or 4 wheel drive. Most steered mechanisms will be better than that, but if you are limited to a standard kit drive system, it is my experience with the kit base and attached trailer that the more rearward your pivot is, the better able you are to turn.

[Betty_Krocker]

Originally Posted by Nuttyman54

Agreed. My comments were was based upon the assumption of a standard kit style frame with no steering and the choice being between 2 or 4 wheel drive. Most steered mechanisms will be better than that, but if you are limited to a standard kit drive system, it is my experience with the kit base and attached trailer that the more rearward your pivot is, the better able you are to turn.

i see, the introduction of a powered steering system changes things quite a bit...