Some of my friends were discussing possible beaching on platform corners, which apparently happened often in 2015. I’ve heard some discussion relating to not having 4" wheels this year. Would it be bad to use a 4" wheel on the drivetrain? What kind of information can my team take into account when deciding wheel size? What other info can we use to avoid beaching?
Beaching on platform corners in RR? never saw it personally in two competitions. Interesting if other comepitions had this issue.
4" wheels aren’t necessarily bad… it’s all going to come down to dealing with keeping the frame from beaching. Running 6 4" wheels, you will beach. I haven’t done the math on running 8 wheels yet. Or I’m sure there are other cleaver way to keep the frame from getting stuck. It’s probably easier to run larger wheels…
Yes it did happen in 2015.
Best way to avoid this is adequate ground clearance. Just enough to ensure you won’t catch in the worst case scenario to maintain an appropriately low CG.
You could make 4" wheels work in theory, but your belly pan/board would have to be higher than the bottom of your drive rails. Also, If you’re driving a long or square chassis, 8WD minimum so you’re less likely get the corner stuck on the bottom of the drive rails. I think 6" wheels would be a better bet, but that’s just my gut feeling.
What kind of math are we talking about here? I’d love to know how to work this out.
Could you clarify why the shape of chassis would make a difference?
This year’s platforms are a little steeper than those of 2015, IIRC. I remember seeing a few robots high-centered on the corners of the platforms that year. I also remember watching some excellent robots with 3" or 4"wheels navigate the game successfully. I guess it most depends on implementation and drive practice. There are always tradeoffs.
I’m not really seeing where a 6 wheel drive will beach or high center. It looks like it should be fine to me.
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In 2015, we used 4" wheels with a full-length AM14U2 chassis, and could not drive onto the scoring platform (indeed, we had “curb feeler” sensors to help us align to the platform). This year is a bit steeper. There are also 3/4" high cable runs from the SCALE sown the middle of each NULL ZONE to each side rail, and along the center line of the field from the SCALE each SWITCH. I haven’t done the math properly yet, but 4" or smaller wheels don’t seem right. We’re looking at 6" at this point, though we may jump to 8".
Did you mean 4"? I assume you didn’t mean 14.
Yup, though we did consider 14" in 2016. Fixed.
Ok, so my rough math on how I calculated for 4" wheels…
Assuming a 28" long chassis, with 4" wheels, you’d need to be about 3.5" from the front edge to keep from hitting the ramp on your way up. And this would require bumpers be higher then the bottom of the chassis… and with the middle wheel right in the middle, wheels would be about 10.5" apart. The 15.35 degree up angle, over 10.5", you have about 2.75" rise. 4" wheels only give you 1.125" clearance, at most. So you’ll beach…
Jumping up to 6" wheels, you get an extra ince of clearance. And you can move the wheels closer together since you’ll be able to get on the ramp easier. To get 2.125" of clearance, the wheels would have to be 7.75" away from each other. Which would put you 6.25" from the front, which I think will work. You could be up to 7.75" away from the front and still not hit the ramp when climbing. How would it affect your turning with the wheels that much closer to the middle? Not sure… but it looks like you’d be able to get on the ramp that way.
And as a disclaimer, I’m not an engineer by any means 
So take my calculations with a grain of salt… I’m probably wrong on this. I’d love to hear if I’m anywhere near close on this or not
The platform is 3.5 in high. If the floor of your chassis is 3.5 in or less you can beach the chassis dependent on the corner depending on cetrain conditions. They are:
Height of chassis floor
Wheel dia
Wheel base length (wheel dia effects this)
Slope angle of the ramp (which effects slope length)
Typically when you go up a ramp like this and approach it normally, as long as the wheels touch the bottom of the slope before the bumper touches the ramp, in most cases you’ll be fine. But not all. It becomes a problem if the wheel base length is too short, the bumper will touch the ramp before the wheel and beach.
At the corner, the problem is magnified by the fact that even if your wheel base is as long as you can make it, the corner of the ramp will reach the floor of your chassis before the wheels reach the ramp. Again, slope angle, wheel base, chassis floor height are what matter. You want to be wheels to the ramp and going up the ramp before the chassis can touch it to reduce the potential. A corner has the ability to protrude deeply into the area below the chassis space before the wheels touch.
If your wheel base width is shorter, you reduce how much corner is relevant before the wheels touch the ramp. Remember, when the wheels reach the ramp the chassis starts to rise. There is always the chance depending upon slope angle and length of wheel base you can dig at the top if the chassis floor is really low but wheels still touch the ramp in time. That’s because a long wheel base length “rotates” in pitch less than a short wheel base. In other words, a wheel base length with the same length as the ramp length will pitch the maximum at about the same pitch angle as the ramp angle and rotate faster. A long wheel base will take longer and rotate pitch slower and pitch less making it more likely to beach if wheels make contact first.
The faster it rotates in pitch relational to the speed it goes up the ramp the less chance of a beach because it will clear faster. But remember that a wheel base much shorter than chassis length makes the problem significantly worse because that increases the speed the chassis can come in contact before the wheels do. You always want your wheel contact point distance as long as the chassis will permit. Smaller wheels help there.
Only two ways to absolutely avoid it.
1.Set your chassis floor higher than the corners height.
2.Set lower but test or simulate it. Coming up with a simple math solution is difficult because there are too many possible apoach angles (45 degree range of possibilities) to consider and too many approach offsets.
I did see this in 15. It’s much more common than you might think
While “beaching” typically occurs when the angle of the ramp/platform touches the bottom of the robot in between wheels, it’s also undesirable to be scraping your robot on the platform when you’re trying to get up it as well. If we assume that bumpers are at max height off the ground (2"), that leading edge would determine how far from the front of your robot you would need to mount a wheel to avoid scraping:
Note that an 8" wheel gives you just slightly more than an inch of clearance between the 8" wheel and the back of the bumper.
A good way to test this in CAD Is to make the wheels of your drive base tangent to the different surfaces of the platform. You can even “simulate” how your wheels would contact as you are driving onto the platform. 4" wheels can work but don’t leave you much room to play with. Also be sure to check out the cable protectors at midfield.
Sure. Most of the beaching I saw was due to the corner of the platform hanging up on the center of the belly pan due to insufficient ground clearance. As a result the the high side would either be off the ground entirely or be barely touching to the point of having little traction, resulting in a robot that can rotate around the corner but not drive off of it without a push from a partner. In one bad case, I saw somebody stuck completely sideways, with the edge of the platform on the belly pan and the edge of the down side’s drive rail on the ground (not a WCD obviously), with no wheels getting enough traction to drag themselves free. Keeping the pan high(er) means the corner or edge can’t prop up one side of the drivetrain off the ground.
As for the long/square frame vs. wide frame, that’s more to do with wheel spacing. Wide chassis have shorter drive rails and thus smaller gaps between wheels, so fewer are needed for a given wheel spacing.