2-wheel versus 4-wheel drive

[Ben Mitchell]

Is there EVER a case in which two wheel drive is superior to 4 wheel drive??

It has been my impression that 4wd (or higher) is the way to go, rather than 2 wheel drive.

Any advantages/disadvantages to either system??


Personally, I think 4 wheel drive is superior, (at least for this years game.) 4 driven wheels gives you that little extra kick if things get messy on the field.

Anyone beg to differ?

--Ben Mitchell

[Chris Hibner]

The most important thing to think about is traction to the floor. It is important that whatever comes into contact with the floor be driven by a motor.

Let's say you design your robot to be two wheel drive and the coefficient of friction between your wheels and the floor is 1. If you distribute the weight of your robot equally, the most pushing force you can have is 65 lb (130 / 2 since half of your weight is being supported by wheels that aren't driven).

If you begin to push something such that the two driven wheels are away from the object your pushing, you might end up with all 130 lb on the two drive wheels giving you 130 lb of pushing force.

However, if you push an object such that the two driven wheels are toward the object you're pushing, you might take all of the weight off of the driven wheels giving you no pushing force at all.

Now let's say that all of your wheels are driven by motors (if you have 4, 6, 8 wheels - it doesn't matter). Then, no matter what your weight distribution is, you always get the 130 lb of your robot being supported by driven wheels, which means you'll always have 130 lb of pushing force.

So, to answer you original question, 4 is better than 2 from a standpoint of pushing force. However, the real answer is that whatever touches the ground should be powered by a motor.

-Chris

[jasoni]

If your 4-wheel drive has wheels at 4 corners, then a 2-wheel drive with the wheels in the middle and casters in the corners will turn MUCH easier.

Our machine for the past two years was the standard 4-wheel drive. In a straight line, it was a beast for power and speed, but when we turned, but was pretty gut-wrenching. You could hear the whole drive train getting stressed. 2 years ago, we actually broke our drive trains a few times during turns. Last year we made everything beefy enough to hold up, but if the battery wasn't at peak power or the motors were warm from use, we couldn't turn well at all.

This year we're going for speed, so we went with 2 center-wheels. Taking out 2 wheels, the chain, and a lot of the weight in our gearbox should have a noticeable effect on the interia of the system. We're hoping we gain back a significant amount of power in inertia reduction alone.

The key to 4-wheel drive is getting just the right amount of friction so you have the power for movement but enough slip to turn well.

Also remember, 4 wheel drive means more parts and mechanisms that can go wrong.

If 4-wheel drive was the be-all-and-end-all of motion systems, every car on the road would have it.

[Joe Johnson]

If you do a lot of turning and you have 4 wheels in the corners of a square robot, you will have only a fraction of the turning torque you have with a single set of drive wheels with all you weight on those wheels (i.e. the CG of the machine is on the line between two wheels). This means you will have less torque to turn a goal for example.

More than this, you are going to pull more current and your motors are going to run hotter because a significant amount of energy is going to go into scrubbing your wheels accross the carpt.

This is not to say that 4WD is not worth it, but that this is something that goes onto the negative side of the balance sheet when considering 4WD vs. 2WD.

Complexity and weight and design freedom are other issues that are potential negatives, but these will have to wait for another day...

Joe J.

[team222badbrad]

IN 2000 we had 2 casters in the front, and 2 8 inch wheelchair wheels in the back

It worked out great for that year!


but in only having two wheels you lose traction because you have to have something to balance it.

The only way around it is if you build a segway bot.

[Ben Mitchell]

How about in terms of pushing or turning a goal??

What about the ability to hold position when rammed or hit??

I'm leaning definitly towards 4wd, but other members of my team are voting for 2wd.

What do you guys think??

--Ben Mitchell

[team222badbrad]

We considered both and tested both by lifting the front wheels off the ground. We did not have enough traction in 2 wheel drive.

A pic is here: www.tahsroboticsteam.org

[Jeremy J]

Last year, my team implimented a two wheel to four wheel shifting system. We had a plate powered by the shortest pneumatic piston that lowered 2 casters down onto the playing field, enabling us to turn much faster. At nationals we took the casters off, as it was uncontrollable, and put a plastic skid plate type thingie there. this allowed us to turn easily, and shift back into four wheel shove ev erything out of the way mode. Either way has it's advantages, as Chris Hibner said. It depends on what you want your bot to do in the competition.

[Pat Roche]

It really depends on the setup and the task. Two wheel is very effective with tracks (only power two of the wheels in the tracks) and wide wheels. They are good at manuevering and can be extremely good at pushing and whatnot. However, two wheel drive is very difficult to handle with larger diameter and narrower wheels. For that situation four wheel drive is extremely powerful and offers good agility.

Summarizing what i just said....depends on the situation....

[Tristan Lall]

For future reference, this is an old thread! (But the points, especially by Joe and Chris are well-taken.)

2 wheels or four, that is the question. with 4 you get more traction but with 2 you can turn better. so you can clearly see the trade off. But could you have both? Last year and the year before, my team used 6 wheels with the 2 middle wheels a little lower than the rest. by doing this the robot is 4 wheels when pushing and pulling. and on 2 wheels when turning. This worked very well, in the games we played the last 2 years. We will just have to see about next year.

[Joe Johnson]

I am a BIG fan of 3 wheels on a side with the middle wheels lower (1/4 to 1/2 inch depending on a number of factors including the flex in the tires you are using).

This gives a good mix of the best of 2 wheel drive w/casters and 4 wheel drive: the ability to turn without large losses + the ability to climb rough floors, ramps, etc. and the ability to push a ton even if you get rocked back on your wheels.

I like it a lot.

Unless you need swerve (a.k.a. crab drive)... ...which is a whole different subject...


Joe J.

[Rpifirst]

In 2k1 and 2k2 we used what we called wonder wheels which give you the benefits of both2 wheel and 4 wheel drive. The wonder wheel is simple a set of rollers that runn perpendicular to the wheel. So in other words omniwheels.

[Joe Ross]

Originally posted by Joe Johnson
I am a BIG fan of 3 wheels on a side with the middle wheels lower (1/4 to 1/2 inch depending on a number of factors including the flex in the tires you are using).

So you're still a fan after finally trying it? After I saw your first post in this thread, I was going to ask. I remember your post in 2k1 recomending it to Anton and the Rambots.

[Andrew]

As to the comment that four driven wheels gives more things to fail and hence is less reliable than a two driven wheel system, it depends on how you do it.

If you drive all wheels (and nobody has talked about six driven wheels on the floor!), then you have a more reliable system. If one drive motor fails, you still have a redundant motor giving you some drivability.

If you have six driven wheels on the floor, you have a double redundancy.

(Note: you can only effectively have six wheels on the floor if you use a suspension.)

Depending on the game, you may want to have low side slip, such as a four driven wheel system gives you. In a pushing contest, an easy to turn robot (two driven wheels + casters) can be turned easily by an opponent. Two wheels + casters also has a harder time maneuvering on a hill once it gets turned a little sideways.

A third alternative is two driven rear wheels + steerable front wheels. (Actually, last year, team 16 had two driven front wheels and a steerable back wheel, same principle.) This gives the advantage of steerability with the advantage of hard to rotate against your will. However, it requires a steering design + control of the steering system.

There are so many options and trade-offs in drive system design that the best approach is to leave everything on the table and figure out what is best for the game.

That having been said, if pushing is a significant part of the game, then six driven wheels with a suspension and the ability to redirect weight will win everytime.