Ground clearance
 

I'm designing a drivetrain for the offseason, but after doing some math the ground clearance looks pretty unnerving.

The robot is a 6WD, center wheel drop 1/8", with 5" Colsons. The math shows that the outer wheels will have .25" of ground clearance and the middle pair of wheels will have .375" of clearance. I have no experience with what clearance is good or bad, so I need some help.

What would you suggest as the lowest ground clearance for a drivetrain? Are my numbers decent for clearance, or are they too small?

Re: Ground clearance
 

How far are the front and rear axles from the extreme ends of the chassis, and what is the total robot length? I'm not much for design tolerances, but I can run the numbers on how tall of an obstacle you could get over, if ya care.

Re: Ground clearance
 

A render would be really helpful to understand what your're saying. If you only have .25/.375" ground clearance with 5" wheels you must have either a really thick bellypan or you're mounting the wheels really high up. I wouldn't suggest having under 1" of ground clearance for safety, but if you post a render, or at least a screenshot I'm sure there's some CDers who can give you some suggestions on how to tweak your design for better clearance.

Re: Ground clearance
 

In a "normal" FRC setting, ground clearance won't matter because the fields are pretty flat. The carpet used is also not "perky", so the wheels don't sink into the surface all that much.

I do not know what the purpose of your drivetrain is, but for a flat surface, it will work great.

To give you an idea, our robot clearance for 2011 was about 1/4 (Swerve drive) and our 2012 robot was about the same (6 wheel WCD measured from the lowest point). No complaints from any of our drivers and we never bottomed out.

Re: Ground clearance
 

The frame should have at least 1" of clearance which allows fasteners to protrude below the frame 1/4", resulting in 3/4" minimum clearance to the floor. This allows the wheels to climb 3/4" plywood which a lot of field elements are mounted to. The plywood doesn't always show up in the field drawings, however, FIRST adds a design note tolerance to cover this. An example is the 2010 bumps. The inspection sheets also check for 1" clearance, but there isn't an explicit robot rule for minimum design clearance.

Re: Ground clearance
 

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!

Re: Ground clearance
 

To be clear, it sounds like you are only planning on supporting the bearing blocks on one side, and to leave them above the frame inside C-channel. I really wouldn't recommend this.

For starters, there's going to be a lot of stress on the bolts (I presume you're planning on using bolts) that connect the wheel blocks to the frame. If the robot weight is equally distributed over the 4 wheels in contact with the ground, then there will be approximately 37lbs/wheel. Assuming 1 inch by X inch C-Channel and that the wheels are 1 1/2" from the frame and two bolts per wheel truck, then there will be at least 37*(1 1/2)/(1/2)*1/2=55lbs per bolt. In my opinion, this has a lot of potential to either damage the tapped hole in the wheel truck or to bend the C-channel. It would be best to make the wheel block a tight fit in the C-Channel.

However, the internal dimensions of C-Channel has much lower tolerances than the outside, which would make it difficult to machine a slip fit wheel truck even with CNC machinery. Also, there is usually a rather large radius inside of the channel.

Before you decide to just bolt some of the bearing blocks that bear the whole weight of the hole to some tubing, think about the stresses and tolerances involved. You may have thought these things through, but as I see it there is a little advantages of this system over a more conventional wheel block system.

Re: Ground clearance
 

We ran a frighteningly low ground clearance in 2011 (roughly .5 inch in the center) and had surprisingly few problems. When you put the robot down, you'd inevitably pinch your fingers though.

I would only be comfortable with a ground clearance like that for a flat field. Are you sure you have to cut it that close?

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

Re: Ground clearance
 

I cannot find anything relating to ground clearance on the inspection checklist, and I certainly can't find anything on the inspection checklist that doesn't have a rule to back it up.

Re: Ground clearance
 

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.

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.

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?

Re: Ground clearance
 

I think he's referring to the fifth check under mechanical: no risk of damage to field <R08>. The inspector did mention ground clearance issues to 1756 in 2010 because of that.

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

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...

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

Re: Ground clearance
 

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.

Re: Ground clearance
 

I concur with IKE

FIRST has a tendency to put plywood and metal plates as field element bases under the carpet, and most of the time that results in about a 1/2" lip in the carpet. I've seen many robots over many years get hung up or spun around when they hit one at the wrong angle, or get pushed over it. Not worth risking when it's easily avoided.

Re: Ground clearance
 

It's worth pointing out that ground clearance between the wheels, and ground clearance in front of the frontmost wheel (ditto on the back) have different needs/affects.

Re: Ground clearance
 

Thanks everyone! I've redesigned my chassis so that there will be 7/8" ground clearance. Makes a lot more sense, and the redesign has allowed me to loose some weight. (I'll try and get a CAD up once the CAD team starts meeting)

Re: Ground clearance
 

Another thing about the inconsistent turning axis is that it can be used to your advantage with the right drivers and driving practice. Watch the 2011 Einstein finals(both matches) and watch 254 very closely. You will see that they do this. One example of the right way to use the inconsistent turning axis is when they come from picking up a tube in the feeder station. They transverse the whole field backwards and then right before they get to the racks, they brake, shifting their turning axis and causing them to use the turning axis closer to the racks, allowing them to turn and immediately score instead of having to turn and then drive closer to the racks.

Re: Ground clearance
 

Darn, i just got the math ready. But yeah, the low clearance wouldn't have been enough to go over the bump or even get on the bridge. :eek: (you could climb over a 2.13" barrier or drive onto a 3.6 degree incline)

Re: Ground clearance
 

Very interesting observation. I'm stoked to see what you guys come up with for 2013 btw.

Re: Ground clearance
 

Didn't think about the symmetry of the parts. That is a good point, especially at the manufacturing stage.

And yes, it really is a trivial difference.

This is a good ground clearance. As mentioned earlier, the sub-floors and methods of placing field elements can cause issues at .5". Then there is always random debris from robots that fall off. Yes, you will never be able to completely design for this, but the design should be capable of fasteners, small plates, chain, etc.

Hmmm. So you can control the teeter and use it your advantage. This is something I didn't think could be done. Poofs always teach me something new (in this case with the help of Michael).