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?

[Ekcrbe]

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.

[Adrian Clark]

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.

[Garrett.d.w]

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.

[drwisley]

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.

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!

[DampRobot]

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.

[Chris is me]

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?

[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.

[Gregor]

Originally Posted by drwisley

The inspection sheets also check for 1" clearance, but there isn't an explicit robot rule for minimum design 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.

[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?

[Dkt01]

Originally Posted by Gregor

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.

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.