Lowered Center Wheel for 6 wheel drive

[AdamHeard]

Originally Posted by Wayne C.

Gee Joe- Us Joke?

would we do that?

actually- everybody asks us how much we lower the center wheel and we always tell them that we dont lower it at all. But then we are told we NEED to so I guess our bots just dont work.

We don't lower the wheel. The robot overpowers lateral friction to make the turns. The wider the wheel base the better it works.

No matter- the Evil Machine base design is being revamped for 07 to be "a bit more powerful". There were a few bots this year that were able to move us when we had the brakes on so we want to have a little more "umph". We gave a bunch of teams the schematics through last season so we know there will be others like us out there this year. It is still a great basic drive design- just not strong enough for some applications (like pushing free of a corner goal).

WC

(PS- I have some kidnapped Moe Sticks sitting here in with a pile of FIRST mailboxes if you want them)

I can't wait to see it! It's amazing how you guys always find a way to improve.

[Rich Kressly]

look out ... they are all liars In 2007:
25 will go with five omni wheels
217 will become a proponent of tank treads
357 will get sick of Jester Drive and revert to the kit wheelchair wheels

It'll be chaos I tell you ...
but Dr. Joe's thoughts on progressively large has me thinking...

[Wayne C.]

Originally Posted by Rich Kressly

look out ... they are all liars In 2007:
25 will go with five omni wheels
It'll be chaos I tell you ...
but Dr. Joe's thoughts on progressively large has me thinking...

sssshhhh! darn- you said you wouldnt tell!

[Cody Carey]

We have a six-wheel drive setup with fairly soft six inch wheels, and we just used the standard kit-bot deviation... It worked like a charm. I believe that it is 1/8 of an inch, but if I'm wrong please correct me.

[Andrew Blair]

Ah, I see the problem with my thinking! The wheels are not linearly larger as you go back! Interesting, and Dr. Joe, I think you are completely right in the uN component arguement. Unfortunately, I would think you would wear tread quite quickly. You also sacrifice a bit a of anti-push resistance when you're moving, because the wheels that would normally be providing static frictional force to oppose movement are now only providing sliding frictional force. Still, food for thought...

You could also use 4 wheels of the same size, and two in the front (or back) that were smaller, and achieve the same effect. We did the same thing, now that I think about it, in 2005 to reduce a hopping problem.

[Qbranch]

Note very closely the placement of the middle track sprocket on our '06 shooter bot without the drop you cant really turn very well. We use a machined aluminum box frame so warpage was no issue.

-Q

[Joe Johnson]

Not everybody tells you to lower the wheels!

There are trade offs to every decision in a design. You pays your money and you takes your choice.

While I like 6WD with the lowered middle wheel, I have made 4WD robots with awful aspect ratios and with the wheels in the corners. As Wayne puts it, enough torque can just "overrpower the lateral friction" .

Now to the specific point of lowering the middle wheels: The decision to not to lower the middle wheels is not as bad as one may initially fear. In the worst case, it is still much better at turning than a 4WD robot with the wheels in the corners and in many cases it is as good at turning as a 6WD robot with the wheels lowered.

How so? In order to understand, you have to think about center of mass and free body diagrams.

Case 1:
CG very nearly over the middle wheels. Assume a rigid frame, flat floor, same size wheels, round wheels with axles in true center AND alligned axles.*

Then the weight on each wheel is W/6 (W=weight of the robot). It is the front and rear axles that will scrub (i.e. move laterally on the floor).

Compare this to the 4WD case where the front and rear wheels scrub exactly the same amount BUT in this case, the weigh on each wheel is W/4.

Bottom line, scrubbing effects are reduced by 33% ((1/4)-(1/6))/(1/4))

Case 2:
CG is not near the middle wheels (this is the more common case), same assumptions*.

The weight on the wheels that the CG is between share the full weight of the robot. The remaining wheels have no weight on them. For example if the weight is toward the back of the robot, the middle and rear wheels carry all the weight of the robot while the front wheels are unloaded.

Bottom line, this is the same performance as the 6WD case WITH lowered middle wheels.

Joe J.

*It is because none of these things are true that I recomend lowering the middle wheels. By lowering the wheels you allow things to work out well even with non perfect assumptions.

I was in a rush to get dinner when I posted the above. Since eating, the blood has returned to my brain. My analysis in Case 2 is just wrong. The case is a classic example of a "statically indeterminant" system. In such cases, you need resort to the stiffnesses of the members to determine the loading on each wheel. But... ...even so, in this case, to the extent that the middle wheels carry any load at all, it is better than the 4WD case with wheels in the corners. And with reasonable assumptions about the chassis it is not too hard to get the same answer as the one in the above message, even though I arrived at it in error.

Joe J.

[Veryser127]

Do teams ever adjust their middle wheel height automatically to accommodate turning. I swear I saw the triplets lowering theirs when they turned last year but I am baffled how.
Can someone point me to the right forum/ upload some pictures or CADs. I've searched around a bit and it seems to be nonexistent on the forums....probably the better question I should be asking myself is why arent I using a swerve drive.