Motor Power to Wheels

So for this build season our team split up into design groups. One of the designs that we came up with was a 6-wheel drive base. Our group designed the base to have 4 motors and connect to every wheel by chain. My mentor said “is it really necessary to have chain going to every wheel, you lose motor efficiency by adding more chain” I reply something like “if not every motor in the six wheel drive is being powered then the wheels that are not being powered are dead weight”. So is my mentor right, should we change the drive so that the motors are only powering one wheel? or should have a better explanation on why to have every wheel should be powered, and keep with the design we have now? (we are using omni’s on the outside and a high traction wheel in the middle).:confused:

You should have motors powering the omnis. The center wheels are not dead-weight, they serve a purpose - preventing you from sliding sideways. As for chaining them, are your omnis spinning in place when you drive? Do you need the extra traction? If the answer to both the questions is no, then there is no point in chaining the center wheels. It will just add unnecessary complication and friction (loss of efficiency).

I think there are certain applications where you would want the extra traction, but for most situations in vex your fine only powering 1 wheel on each side. Thats my opinion anyway. I could be wrong.

Every unpowered wheel is tractive force you’re wasting.

+1 on this one.

  • Sunny G.

So what exactly is tractive force?:confused:

If it is touching the ground and it isn’t providing traction, then it is providing drag.


  • Dean

Unless your robot has a very bizarre wheel arraignment and/or very bizarre weight distribution, I would never leave any wheels unpowered. As Chris said, unpowered wheels are a big waste of tractive force.

To understand this, draw a Free Body Diagram of your robot. Since the robot is not accelerating vertically, the sum of forces in the vertical direction are zero. In this vertical direction, it is pushed downwards by the force of gravity, and upwards by the normal force at each wheel. These normal forces precisely cancel out the gravitational force, because the sum of forces is 0. The precise value of each of these normal forces will depend on how many wheels you have, and how your weight is distributed. But for this example, we can assume that it’s roughly equal in each wheel. So for each wheel of your six wheel drive, the normal force generated in each wheel is equal to (robot-weight)/6

The normal force is also important in calculating friction (F=uN). N in each wheel is (robotweight)/6. Tractive force, or just traction, is a name given to frictional forces, when they are used to transfer power, like in a powered wheel. Tractive force is equal to this frictional value. If all 6 of your wheels are powered and providing tractive force, you will have tractive force equal to 6u(robotweight/6).

But if you leave wheels unpowered, you’ll have a problem. The normal force in each wheel is still equal to (robotweight/6), because each wheel still supports the same percentage of the robot’s weight. But now, you only have four wheels providing tractive force. So your tractive force is now equal to 4u(robotweight/6). 1/3rd less pushing force than with all 6 wheels powered!

Yes, there are slight losses in efficiency from chain. But although I don’t have any numbers in front of me on VEX chain’s efficiency, I cannot imagine that a single run would rob you of anything close to this much force. Dead weight from wheels, likewise, is pretty insignificant when compared to the loss of tractive force. 1/3rd your traction is huge, and I cannot conceive of a reason to voluntarily ditch that much pushing force.

Hope this helps!

Aside from normal force arguments, it’s also really important to remember that the VRC playing field is chaotic, and that your robot may not always be sitting flat on the floor. If you are only powering one end of the robot and that end isn’t on the ground, you are in big trouble. This can be a problem even if you are just pushing against another robot, a goal, a game object or field structures and one end is lifted slightly and that end contains your driving wheels. With all-wheel drive as long as ANY wheel is touching the floor you have a chance to have some control.

For what it’s worth, adding two more wheels to your robot adds at least 3/4 to 1 pound of extra weight that you simply won’t need if your robot is in the “normal” 8-15 pound range. Only heavy robots of 20 pounds or more sink so far into the foam that the extra two wheels come into play by reducing the “sink” and improving maneuverability. I’ve personally never seen a robot with normal weight that had a problem that couldn’t be solved more efficiently by better placement of its four wheels rather than adding two more.

The simple answer: Your weight is distributed among all contact points the robot has with the floor. The amount of traction / pushing power your robot has is a function of your wheel tread and the normal force it exerts on the ground. If your robot’s weight is supported by unpowered wheels, then each unpowered wheel is normal force that isn’t being converted to traction.

What the hell is with you guys and traction? 4-wheel drive robots with 6 wheels do not lose traction on tiles.

What you guys have been saying has been very misleading. Traction (calculated F=uN, where N is normal force on powered wheels) gives a value for static friction which is the **maximum **force the wheels can exert before they slip. It isn’t the force actually exerted.

If you have a legal motor arrangement, some weight over your driven wheels and a reasonable gear ratio for an average robot (usually 1:1.5 - 1:1.8 for speed) your motors should stall before your wheels can slip. Traction should not be a concern for most robots.

(It isn’t very instructive from a physics point of view, but it’s also worth remembering that medium wheels with high traction tyres - the smooth tyres - have a smaller diameter than omniwheels. This means that on a 6-wheeled robot your middle traction wheels won’t actually be taking their full 1/6 share of the load).

This is the real reason to power multiple pairs of wheels.

You can increase the diameter of standard 4" wheels by inserting cut pieces of the smaller wire ties between the ti(y)res and the wheels. Take a flat blade screwdriver and gently pry the edge of the tire up and slip the section of wire tie into the groove in the wheel. Repeat this for every groove around the whole perimeter of the wheel, which should only take a week or so. This will then make the overall diameter of the standard wheel JUST enough bigger to be bigger than the omni wheels. The robot that starts in the back left corner in this video used this trick on its 6WD drive system.

I agree that I should have been clearer that tractive force as I used it is a maximum, rather than an absolute. However, I respectfully disagree with the ideas that stall torque is typically the limiting condition, and that changing the number of powered wheels doesn’t affect this.

I did the math for what I would consider to be a “typical” VRC robot these days, using data from this thread for coefficients of friction.

Drive motors: Two 269 (8.6 in-lb torque), and two 393, geared for torque (13.5 in-lbs torque)
Gear ratio: 5:3
Wheels: Six 4" omni wheels (u = 0.9325)
Weight: 15 lbs.

Force generated by motors at stall: (44.2 in-lb total torque)(3/5 from gear ratio) / (2" lever arm from wheel) = 13.26 pounds at stall

Maximum tractive force per wheel: (15 lbs)*(0.9325)/(6 wheels) = 2.33125 pounds per driven wheel

Maximum tractive force with 6 wheels driven: 13.9875 pounds, just barely more than the force generated at stall. In this case, stall torque is the limiting condition (just barely).

Maximum tractive force with 4 wheels driven: 9.325 pounds, signifigantly less than the drive’s stall torque. Traction is the limiting condition. You loose almost 4 pounds of pushing power by leaving two wheels unpowered.

Using other common, more powerful motor configurations, like four 393 motors (54 in-lbs) or six 269 motors (51.6 in-lbs), it is quite possible that your wheels will slip before stall, even if all wheels are powered. Many people will argue that this is a desirable condition, since it prevents your motors from stalling and overheating, while still giving you the most traction you can get without making your robot heavier.

Now go build a robot and test your numbers. I am curious to see what you find, although I suspect you will discover some other factor that makes what you report less valid in practice than in theory (power availability in the motor controllers, friction, motor power curves, the phases of the moon, sheer mechanical perversity, etc.) It’s why engineers test even after doing the calculations.

btw rocketperson, is that your real vex number?
thats an awesome number :slight_smile:

0000? No, I’m just not part of any team right now (:()

And I’m not from Green Eggs either…44 was my number in little league baseball, and I started attaching it to my username for just about everything when I discovered the internet. On a related note, is it possible to change your username on these forums?

I agree that a test would be interesting to do. However, because I’m not currently with a team, I don’t have any VEX parts immediately avaliable to me. If someone else is curious enough to try this out, I’d love to hear your results.

just look at Mr. “TYler” above :wink:

Making some of those wheels unpowered is a very quick way to make your wheels slip.

If you’re not traction-limited or close to it, then you’re probably putting a fair bit of stress on those motors.

Even if you don’t slip your wheels, traction is important for when you get pushed yourself.

Which is generally a good thing; every year many teams stall their drive. If your wheels just slip when yours motors would otherwise stall, you can save your motors from stalling. I think more important than how many wheels you are powering is which wheels you power.

This is very true; i feel that more teams should pay attention to this! Teams that use only omni wheels on a “tank drive” are very easily pushed sideways, even if their opponents have very little power themselves.

This is the very issue that high-friction center wheels attempt to address. But powering those wheels will not make them do that job better.

As many other people have said, you waste available traction be leaving those wheels unpowerd. But I don’t know if that much traction is necessary for most VEX teams. If you’re going in a head to head pushing mach, then yes, but otherwise, I really don’t think so.