Ground clearance

[Adrian Clark]

Originally Posted by SuperNerd256

Thanks for the replies all. I don't have a render, I was just designing in my head. it would be 5" colson wheels mounted by bearing blocks (7/8"), the outer wheels would have a washer under the bearing blocks (1/8"), and then the C-channel (1.25"). It would only be used on a flat surface. The goal was to get the frame as low as possible. The alternative is to mount the wheel+bearing block+washer combination onto the 3/4" tubing we used this year, resulting in the outer wheels having .75" of ground clearance and the inner wheels having .875" of clearance. The wheelbase is about 15" (end wheels are 3.5" away from chassis).

Thanks all for the help!

Thanks for the clarification, I should of realized that you were using that type of bearing block but my mind is on WCD mode right now.

Some things to consider:
-Why use such a large wheel when you have such low ground clearance?
-Why put washers under the bearing blocks? The same effect can be achieved by drilling the bearing/axle hole 1/16" off center and flip the block over on the middle/outside wheels to create the drop.
-Have you considered what gearbox you are using and how the wheels will be driven? Along with obstacle clearance gearing plays the most important role in determining wheel size.

Personally I would suggest having the smallest wheels you can (4" is advisable) while keeping your ground clearance within range. Having smaller wheel means less gear reduction, less weight, less weight in rotating components, and a better looking and simpler robot. It sounds like you're designing your robot around your wheel size and desired frame style. This being an off season project, I wound encourage you to take a step back from your design and think about attributes you want it robot to have and its overall conceptual design before going into specifics like wheel size. Don't design around the frame style you want, design the frame to serve your needs and you'll find yourself with a simpler design. I don't mean to bash on your 3/4" tubing, but in this case in seems like its limiting you from mounting the wheel axles on the same plane as the frame which wound let you run smaller wheel with better ground clearance. Try doing something new and crazy with your frame construction, even if it doesn't work you'll learn loads more than you would from repeating your traditional construction methods.

Just my 2 cents.

[protoserge]

Additionally, I would suggest against a drop center 6WD layout. Instead, raise the front and place your center of gravity (CG) within the rear 4 wheels. This will allow 4WD maneuverability while having capability of 6WD without the teter-totter effect. The 2012 RoboBees robot "Yow Sting" used a 0.5" raised front and could rotate in place on the back 4 wheels.

As far as ground clearance, the carpet is not necessarily "flat" once you've examined it closely. Often times, there are subfloor variations, carpet kinks from unrolling, and other non-uniformities that may add as much as 0.25" variation.

[Adrian Clark]

Originally Posted by stinglikeabee

Additionally, I would suggest against a drop center 6WD layout. Instead, raise the front and place your center of gravity (CG) within the rear 4 wheels. This will allow 4WD maneuverability while having capability of 6WD without the teter-totter effect. The 2012 RoboBees robot "Yow Sting" used a 0.5" raised front and could rotate in place on the back 4 wheels.

How do you think this drivetrain compares against a 6wd drop center in terms of maneuverability?

Typically the "teter-totter effect" isn't detrimental to a robots performance unless the 6wd drop center is poorly constructed or the driver is unnecessarily jerky. When you introduce any more than two wheels on a drivetrain you also introduce turning scrub, where the wheel has to move laterally when the robot turns. To much scrub can be the death of some 4wd bots as when turning the robot has to supply the force to drag the wheel laterally. A large part of the philosophy behind the 6wd drop center is that when you turn the robot does not pivot around either set of four wheels, instead it pivots around the center wheels while "teter-tottering" between the two sets of wheels. This makes it so the robot never fully rests on one set of the four wheels thus making the turning scrub close to non-existent giving you much faster steering and a surprising amount of control.

[protoserge]

Perhaps I should state that this was based on last year's game and our derived requirements. Obviously different games will drive requirements; however, I find the raised front with appopriately placed CG works exceptionally well in maneuverability and stability. For instance, we were able to rotate in place while balanced on the bridge in Rebound Rumble.

I might be mistaken with the CG of the OP's drop center, as I was expecting a CG placed over the center wheels for optimum rotation. As you are aware, this is a huge assumption and changes the entire dynamics of the robot. Correct my assumption if this is not the case. Moving forward with this assumption, would the robot not require a forward vector to pivot to the rear 4 wheels when turning, thus negating the ability to turn in place?

I was also under the impression that teter-tottering was a design deficiency, but I now see how you could use it as an advantage.

[Adrian Clark]

Originally Posted by stinglikeabee

I find the raised front with appopriately placed CG works exceptionally well in maneuverability and stability. For instance, we were able to rotate in place while balanced on the bridge in Rebound Rumble.

I don't doubt your maneuverability at all, it sounds like the set of four wheels you pivot around has a pretty short wheelbase, meaning low scrub and good maneuverability.

Originally Posted by stinglikeabee

I might be mistaken with the CG of the OP's drop center, as I was expecting a CG placed over the center wheels for optimum rotation.

Typically with a drop center you don't want to have CG perfectly centered, this only adds unnecessary teter-tottering and reduces your outside wheels contact with the ground.

Originally Posted by stinglikeabee

Moving forward with this assumption, would the robot not require a forward vector to pivot to the rear 4 wheels when turning, thus negating the ability to turn in place?

Not necessarily, even if the CG is perfectly centered the robot will always be resting on either set of four wheels. I'm not too sure I understand your logic here, why would the robot not be able to turn in place in your given scenario?

[tim-tim]

Originally Posted by Adrian Clark

Not necessarily, even if the CG is perfectly centered the robot will always be resting on either set of four wheels. I'm not too sure I understand your logic here, why would the robot not be able to turn in place in your given scenario?

I think what he was trying to say is that when turning we (The RoboBees) were always on the same set of wheels. We knew exactly what axis the robot would turn about.

However, if the CG of the robot is close enough to center that enables the teetering you know longer have this luxury. The acceleration at the beginning of the turn and throughout could switch which axis you rotate about. Meaning it could be between the front and center wheels, or the rear and center wheels. We knew that every turn would be about a central point with almost no variation.

It is like have a 4WD robot with a wheelie bar with powered wheels.

Again, the application should drive the design. This is just another option to keep in mind.

[AdamHeard]

Originally Posted by tim-tim

I think what he was trying to say is that when turning we (The RoboBees) were always on the same set of wheels. We knew exactly what axis the robot would turn about.

However, if the CG of the robot is close enough to center that enables the teetering you know longer have this luxury. The acceleration at the beginning of the turn and throughout could switch which axis you rotate about. Meaning it could be between the front and center wheels, or the rear and center wheels. We knew that every turn would be about a central point with almost no variation.

It is like have a 4WD robot with a wheelie bar with powered wheels.

Again, the application should drive the design. This is just another option to keep in mind.

This different method of raising a wheel results in the exact same setup as a regular 6wd. You still have two 4wd bases, and the cg location (+ accleration) determines rock.

[tim-tim]

Originally Posted by AdamHeard

This different method of raising a wheel results in the exact same setup as a regular 6wd. You still have two 4wd bases, and the cg location (+ accleration) determines rock.

Except that we didn't have the rock due to the location of our CG. The only times we used the front wheels (raised front) were to start the process of crossing the bump and when entering/exiting the ramp.

But I understand what you are saying though. But why would you want a robot with a CG + acceleration that would allow rocking in normal situations? Is there too much variance in when it does or does not teeter to be able to predict what axis the robot will turn about?

We chose this design for stability reasons. We always sit on our rear wheels. Likewise we always turned about the same point.

I guess it is just design intent.

[AdamHeard]

Originally Posted by tim-tim

Except that we didn't have the rock due to the location of our CG. The only times we used the front wheels (raised front) were to start the process of crossing the bump and when entering/exiting the ramp.

But I understand what you are saying though. But why would you want a robot with a CG + acceleration that would allow rocking in normal situations? Is there too much variance in when it does or does not teeter to be able to predict what axis the robot will turn about?

We chose this design for stability reasons. We always sit on our rear wheels. Likewise we always turned about the same point.

I guess it is just design intent.

I'm saying the reason you didn't rock has nothing to do with your different wheel configuration, and everything to do with CG. Your drive is pretty much a drop center 6wd. You still have two 4wd bases, but primarily drive one one. This is true for most 6wd's. We bias weight on all of ours to get a more consistent turning center.

I'm not saying your decision was a bad one, we did the same thing in 2009; Just clarifying for the lesser informed people reading that there isn't some holy grail difference here.

[tim-tim]

Originally Posted by AdamHeard

I'm saying the reason you didn't rock has nothing to do with your different wheel configuration, and everything to do with CG. Your drive is pretty much a drop center 6wd. You still have two 4wd bases, but primarily drive one one. This is true for most 6wd's. We bias weight on all of ours to get a more consistent turning center.

I'm not saying your decision was a bad one, we did the same thing in 2009; Just clarifying for the lesser informed people reading that there isn't some holy grail difference here.

Yes I agree, there is no holy grail answer.

Can I pose the question: if you know are going (significantly) bias the CG to one side of the robot, why drop the center as opposed to raising the front? Not trying to argue the poi t just want a different designers perspective.

[AdamHeard]

Originally Posted by tim-tim

Yes I agree, there is no holy grail answer.

Can I pose the question: if you know are going (significantly) bias the CG to one side of the robot, why drop the center as opposed to raising the front? Not trying to argue the poi t just want a different designers perspective.

Partly because even with a CG bias, you won't ever prevent the robot from rocking onto those other wheels (a 14-15" wheelbase just isn't very stable for any fast robot).

A bigger factor for us is we don't like giving up symmetry on our parts. If we can maintain it, it makes fab and assembly easier/less confusing.

It's a really trivial difference though, between implementations.

[IKE]

I would strongly caution about going less than 0.5" of ground clearance, here is why:
In 2008, it was a flat game in the rules.... except for th 5" plates supporting the poles for the center divider. That a lot of teams got stuf on.

2010 was flat between the bumps, except it wasn't as they added some plywood at teh entrance/exit ramp surface of the bumps so that teams didn't damage the flooring.

2005 was flat, except for the 3/16" plastic triangles on the floor to designate the loading. And the 1.5" tubes you might get shoved over while scoring a Tetra.

You can go less than 0.5" (I have seen many successful bots do this), but I would caution going much below that.

[Adrian Clark]

Originally Posted by tim-tim

I think what he was trying to say is that when turning we (The RoboBees) were always on the same set of wheels. We knew exactly what axis the robot would turn about.

However, if the CG of the robot is close enough to center that enables the teetering you know longer have this luxury. The acceleration at the beginning of the turn and throughout could switch which axis you rotate about. Meaning it could be between the front and center wheels, or the rear and center wheels. We knew that every turn would be about a central point with almost no variation.

It is like have a 4WD robot with a wheelie bar with powered wheels.

Again, the application should drive the design. This is just another option to keep in mind.

I might be misunderstanding the conversation, but I'm pretty sure your teammate was talking about supernerds(op) drivetrain. But as for your design, if I understand it correctly, having the CG in the back of the robot is critical since your drivetrain is designed to not teter-totter and be able to interact with the field in ways a standard 6wd drop center can't. I commend you for trying something different, I think your drivetrain has pros and cons when compared to a traditional 6wd drop center, and I love that you were able to boost your ability to traverse field elements with out the standard approach of just making your wheels bigger.

[protoserge]

Originally Posted by AdamHeard

This different method of raising a wheel results in the exact same setup as a regular 6wd. You still have two 4wd bases, and the cg location (+ accleration) determines rock.

The raised front creates a single 4wd base (for most play) with the ability to traverse obstacles with the front wheels. When in full reverse, there is potential for the acceleration to cause a shift to the [robot] front wheels.

Originally Posted by Adrian Clark

But as for your design, if I understand it correctly, having the CG in the back of the robot is critical since your drivetrain is designed to not teter-totter and be able to interact with the field in ways a standard 6wd drop center can't.

That is correct

Again, thanks for the info on tetering as a method of control. Definitely a new bit of info for me. Ever since stack attack, I've been wary of tippy robots :lol:

OP, I hope we haven't gotten too far off your original question...