6WD: Which wheels to power?

We are very tempted to use the 6WD base that came in the kit of parts (simplicity, cost, agility, it’s here now, etc.). The KoP frame, however, only powers the center wheel. I understand that a bot built this way can work (since the center wheel is hopefully always touching the carpet), but we’re wondering about powering all the wheels.

Is working the details of chaining all six wheels worth the effort? Will powering only the center wheels put us at a disadvantage (pushing, for example)?

Most teams using a 6WD base will link the wheels together with chains, gears, belts, or some other linking system I can’t think of off the top of my head. Yes. I would highly recommend doing this.

Yes you will want to run chains to all six wheels. Unpowered wheels are dead weight.

Ideally, you want every wheel that is touching the ground to be contributing to your motive force. Since your traction is intimately related to the friction on your driven wheels, and since friction depends on the weight applied through those wheels, every non-driven wheel with any weight on it robs you of traction.

The 2 most common ways I know how to power them is West Coast Style: you direct drive the middle wheel then belt/chain drive the other 2. The other way is to put the motor&gearbox between the middle and back wheel and then chain/belt to the back wheel and to the middle wheel and then the middle wheel sends chain/belt to the front wheel.(this way is more for redundancy, because you can lose your back wheels and still have 4 wheels that are driven and you can lose 4 wheels and still be driven by the back 2)(when i say lose i mean that they are no longer spinning, not that they have fallen off the robot lol)

Unless you do a drop center to help with manueverability.

FWIW, it’s not terribly difficult to chain all 6 wheels together, unless you’re trying to do it with a single length of chain! 6 sprockets (1 motor, 3 middle wheel, 1 for each of the outer wheels) and some carefully cut spacers to make sure everything lines up properly, and your set!

Stogi’s pretty much right on the money here.

Every 6WD we’ve ever build has had each wheel powered, usually using two chains on each side. The front and center wheels are chained together and the center and rear are chained together - we use this method to add a certain amount of redundancy to our drive so that we know we can throw one chain on each side and still have some degree of effective maneuverability.

Always always power every wheel.

Yup. In a perfect world, your weight (or normal force) is perfectly divided between each contact point with the floor. So a 6 wheeled robot has 25 pounds of downforce one each wheel. You only convert that normal force into tractive pushing force if that wheel is powered – otherwise you might as well just stick a caster there.

(Imagine you have a really heavy piano. If you put it on unpowered wheels, it is easy to move. :))

Ie, a 6 wheeled robot (in a perfect world) with 4 unpowered wheels gets 1/3 the pushing of a robot with 6 powered wheels. If you do the dropped center wheel, then its likely 1/2, since typically only 4 wheels are on the ground at any given time.

All of them!

QFT

Remember that around the towers is a 1/2 plywood extending out under the carpet which could bring the center wheels off the ground if the front or rear ones hit it which would in turn loose traction.

When using 6 wheel drive, we chain to each wheel for redundancy. You can lose 2 of the 3 chains on a side, and still drive. Worth the extra effort IMO.

In addition, try to center the CG over the center wheel. :slight_smile: :slight_smile:

That’s on the “team” field, not the official field, right? The official fields will have 3/16" aluminum plate under the carpet?

But yeah, ground clearance is a good thing!

As people have said: All.

The kit-bot axle spacers are designed so that there are two positions for the wheels on the axle. They position the wheels to line up the inside sprockets with the double sprocket on gearbox. You can attach sprockets on both sides of one pair of wheels to bring power to the third set of wheels.

Just make sure you match the ratios of the sprockets, so all wheels are turning at the same rate.

Good luck rookie teams - get the kit-bot working fast and quickly. Concentrate on your strategy, and what to do with the game pieces.