off center 6WD help

so my team is letting me start my own drive train project and i decided to try to build a 6 wheel off center drive. can anyone post some specs, tips and how to’s so everything goes more smoothly?

6WD “drop center” or “rocker” isn’t too hard. The main thing is to get the drop right and the CG right.

The drop shouldn’t be very much. 1/16" -1/4" is sort of the “standard” range, with the majority being around 1/8"-3/16". Occasionally, someone has more.

CG is kind of important. You don’t really want it dead center, surprisingly enough. This will aggravate the rock, causing undesirable results. Somewhat forward/aft seems to work much better.

Other than that, try different combinations of wheels. Sometimes, that can help performance.

By ‘off center’ do you mean the center wheels further outboard/inboard or do you mean the center wheels further forward/back?

EricH is exactly right but I should point out that when he states ‘dead center’ that really means the CG being directly over the lowered wheels (assuming the lowered wheels are not in the center, ‘forward/back’). The closer (forward/back) to your lowered wheels and the higher the CG is the more it will affect the rocking action of the robot.

by off center i mean drop the center as said by Eric.

I have a question related to 6 Wheel Drive.
It is generic for all 6 wheels.
What are the possible ways to drive the wheels through the shaft?

Live axle? Key and hex are pretty much your options.

Key: cut a slot in the axle. Cut the same size slot in the wheel hub. Put a small piece of metal (the key) into the slot in the axle and slide the wheel on.

Hex: Cut the axle so it’s hexagonal. Cut (technically, broach) the inside of the wheel hub into a matching hex pattern and slide them together. [Edit] This could work with other shapes, though hex is the most common.[/Edit]

Secure the one you use so it won’t slide along the axle.

There is also dead axle, where the axle doesn’t move relative to the wheel. That has its own tricks.

A drop between 1/8th and 1/10th of an inch will do you plenty of good, assuming you take a bit of time to see that your superstructure is remotely balanced. Otherwise, you might be better off to shift the center wheel underneath the center of mass for your robot (if you need to find this, CAD is your best bet).

Oh, also: Post pictures once you make it! We all like to ogle them shiny robots.

As Eric mentioned, this is actually NOT what you typically want to do. If the CG is directly above the center wheels the robot will tend to rock far more frequently than if the CG is located in front of or behind the center pair.

Weight distribution is like Eggs for Breakfast: Some people pay more attention to detail than others. I personally like to have my weight centered when I build a 6 wheel, and will shift the middle wheels to match. If your robot is actually “rocking” to the point where it’s affecting your steering, you may have dropped your center wheel too much. It really doesn’t take much, if ANY, drop to allow a 6 wheel to turn without bouncing.

EDIT: Example of drop not being entirely necessary if you do your design work right: Team 25. Those crazy folks haven’t dropped a wheel in years, and they don’t bounce…

I think it’s insulting to say Eric’s experience with a 6 wheel had no attention to detail. He learned his 6 wheels with 330, which has had some pretty nice ones the last few years.

I’ll second his recommendation not to put the weight over the center wheels, you’ll get a much better performing 6 wheel.

As for team 25, yes their bases work, and yes they don’t drop the center wheel. But I’m pretty sure I’ve seen them bounce like crazy while turning before.

Think of it this way:

Your center of gravity is right above your middle “dropped” wheel. (I don’t much care how far up it is; that’s your choice.) So far, so good, right? Now, you accelerate forwards. Your CG is a little bit slower than the drive frame to respond (law of inertia–it takes a little bit to get the motion up there), and you rock onto your back wheels. You’ll stay there until you stop. When you stop (accelerating backwards), the CG again takes a bit to figure out, “Oh, I need to stop!” and goes into the front section. You rock onto your front wheels. The rebound sends you onto your back wheels. Repeat until the CG stops moving. But…

You may be moving again already, or you may be trying to acquire a game piece. This prevents the CG from settling. Worse, the CG bouncing around could potentially throw off your attempts to pick up the piece. In a typical game where speed in acquiring and unloading is important, this can adversely affect scoring potential.

Having the CG on one end or the other WILL NOT stop the rock. It will, however, make it settle much faster.

It’s like a seesaw-- sure you can balance it on the middle. Give it a light tap, and it leaves the middle. So you have to work to return it to level. Add a little more board to one side to keep one end down, and you don’t have to work as hard to return it.

Oh, and you can easily have a 6WD that turns without bouncing or drop. Stick omni wheels on one end.:stuck_out_tongue:

i second that motion,

Meant that in no way to be insulting, I’m truly sorry if it came across that way. It was more meant as a statement towards myself: I’ve always been a little bit lackadaisical when it came to mechanical principals, physics, math, and all that (hence why I’m a sociology major! gasp!), but center of mass has been one of the few that I actually spend time on.

Though hey, the idea of Omni’s is also an awesome one. No need for drop, no need for rocking, and you can put your weight in the middle. It’s like getting cake, and eating it too!

But while eating your cake, you’re more easily spun.

It can be desirable in some cases, but I wouldn’t do it.

the spining factor is why i dont want omnis. our team has had omnis almost every year ( excluing rookie year and 2008 * sliders and mechs*)

Check out the .step files on my site near the bottom of the page.

Universal Chassis

This chassis has many of the features you’re interested in. Just looking at the cad models may give you some inspiration for how to design your own custom version. Good luck.

Eric mentioned omni wheels. Just thinking aloud here…
Should omni wheels be on the end where CG is greater or less than the other end? The end that has a greater CG would remain on the ground for most of time where as the end with less CG. So the end with omni wheels will help in turning and thus should be on the end with a larger CG.

When using omniwheels, you’d typically eliminate the “rock” from your 6WD – and so both ends are on the ground at all times.

Assuming that you have four traction wheels and two omniwheels, you’ll want for your center of gravity to live above the traction wheels, not the omniwheels. Increasing the total percentage of the robot’s weight acting through the traction wheels increases your tractive force compared to placing it over typical omniwheels because the omniwheels have a lower coefficient of friction.

in 2006 my team used 6wheel drive with no omni and no lowered center wheel and we learned that when turning our robot bounced around too much. in 2007 we again used a 6wheel with no omni but we did lower the center wheel. in that robot our weight was dispersed outwards horizontally and our robot didnt have any rocking or bouncing problem. however, it turned slowly because the weight was too far from the center of gravity. in 2008 we used the same setup as 2007 but this time we put all our weight as close to the center as possible. we still didnt bounce when turning and we turned much faster, but we did have issues with too much rocking. this year we plan to use 6wheels that are all the same elevation but this time we will use omni for the front 2. i saw someones post that said this will make us too easily turnable. with a 6 wheel drive with a lowered center and no omni the force of static friction is only present on 4 wheels at a time because 2 wheels will always be slightly off the ground. that means we should have the same static friction ratio with a level elevation wheel setup and 2 omni wheels in the front.

On Team 254’s 2008 robot, there was almost no weight forward of the center wheels, due to the open front on our frame. It only really went on the front wheels at all when rapidly stopping or rapidly reversing. We had good performance, and almost never had to change tread on the front wheels.