6 wheel drop center drive train
 

Hey guys, we have a 6 pneumatic wheels drop center drive train but our robot shakes and jumps a lot when turning. I'm wondering how other teams prevented that from happening this year. I've heard teams wore down some of the front wheels or put duct tape over them to help prevent the amount of friction.

Re: 6 wheel drop center drive train
 

You can inflate your center wheels more than your outer wheels also. More drop built into the chassis will also help.

Re: 6 wheel drop center drive train
 

One way that 5811 avoided this issue this year was to build the widest possible kitbot drive train we could with the existing holes in the kitbot rails and the belts that came with the KOP drive train. This meant ditching the AM upgrade kit, but had benefits of a higher track width:length ratio (the wider your wheelbase is relative to its length, the easier/smoother it is going to turn in a skid steer configuration).

We wound up with a 28.3" long by 30.5" wide chassis, and although we did not purchase the AM upgrade kit, we were able to use many of the aspects of their design (longer axle bolts, spacers, gearbox output shaft design) to make it work with the existing belts and frame pieces that came in the KOP. We were able to make it cheaper, and frankly I believe better performing that the upgrade kit you could buy. This worked DESPITE the center drop for the holes we used for our axles only being 0.070" (that is a really small center drop...). We also ran with 25 psi in the center wheels and 15 psi on the corner wheels, like Matt mentioned. No hard evidence that the exact pressures made any difference, but it worked and we liked it, so we ran with it.

We arrived at this solution as a constrained optimization problem. We asked ourselves that if constrained to the belts already in the KOP (to save on costs), what was the maximum track width feasible? We relied on CAD and basic math to help us answer the "feasible" question. There ended up being several modifications to frame pieces that were all individually simple, but did take some time and thinking to figure out.

The drawbacks to this design approach this year included having less clearance going through the dividers for defense crossing and some strange geometry concerns that would make scaling the tower more challenging because the bar was beyond legal reach while resting on the batter. We discussed the former before making the decision to make a wide bot, and decided that drive practice would be the appropriate solution. The latter drawback did not come up until later in the season when we wanted to add a scaler. That was so far down on our initial priority list that this drawback did not have a significant impact on our success this season throughout regional play, and really wasn't the hurdle that held us back later in the season either. Our strategic priorities played directly into our design trade off decisions with our drive train, as was the case with the other systems of the robot.

Re: 6 wheel drop center drive train
 

This happened to our 2012 robot, we ran 200mm AM pneumatic wheels with a 1/8" drop. The robot had a lot of trouble turning one of the reasons was the 1/8" drop. If you are able to do some re-manufacturing, I would suggest a 1/4" or a 3/16" drop if you think the 1/4" is too much. Pneumatic wheels compress more under weight than normal wheels do. That's one of their great features when going over rough surfaces.

Over inflate the middle wheels if you still are having some turning issues. Make sure you inflate all the wheels to their appropriate PSI. Under inflated wheels will produce more rolling resistance which will make turning harder. I wouldn’t recommend letting air out of the corner tires to try and make the diameter smaller, all this will do is create more rolling resistance on the corner wheels.

Re: 6 wheel drop center drive train
 

Our Stronghold robot ate batteries for breakfast, lunch and dinner because it was a 6WD tank drive with 8" pneumatic wheels, but we designed it to be long and narrow in order to drive over half the ramparts without getting caught on the other half. (The worst possible geometry from a friction perspective) Strategy-wise this worked out well, but we did have to adjust the tire pressure periodically because as the season went on and the tires deflated, it was getting harder and harder to drive and turn.

Re: 6 wheel drop center drive train
 

If you are using pneumatic tires then the center drop should be 0.375"

Re: 6 wheel drop center drive train
 

We had success with both 1/4" and 3/8", but in the end we ran 3/8". This was mostly because our tires were extra-deflated so we could stay on the batter.

Re: 6 wheel drop center drive train
 

From what I understand, the longer the chassis' orientation is (where distance from center wheel to front or back wheel is < center left to center right wheel), the more severe the center wheel can be dropped without compromising stability, meaning the chassis will remain closer to level. The larger the center wheel is dropped, the less likely scrub will occur after other variables are accounted for.

I have never worked with pneumatic wheels so I am not to familiar with how they preform on a field, but as stated above, over inflating the center wheels seems to be a reasonable workaround to tire scrub.

Team 1114 Simbotics did a remarkable job explaining the physics at play behind every tank drivetrain in detail @
http://www.simbotics.org/files/pdf/drivetraindesign.pdf

I encourage everyone to give it a read when considering or designing any drive train :)

Re: 6 wheel drop center drive train
 

Back that statement up. I'll wait.




What I'm getting at is that each setup is different. Blanket statements like that one should be double-checked. I could see that big of a drop for large wheels, sure. But there are a variety of factors that could play in, like size of pneumatic tires, number of wheels, presence or absence of non-pneumatic tires, inflation of tires both absolute (gauge pressure) and relative (different gauge pressures between different tires), and wheelbase vs trackwidth. Even desired "rock" can play in.

So for any given setup, that statement may or may not be correct. Stating that "X is the way to go" is not a particularly good idea--but stating that "for this particular setup [key details like wheel size and drivebase size], X worked well" will provide a really good starting point.

For reference... I don't think the team I was on in high school used that much drop on a 6" all pneumatic 6WD drop center. That being said, after the first event two of the pneumatic wheels were swapped out for AndyMark 6" non-pneumatics due to bouncing that made controlling the robot "interesting". [edit--forgot earlier] As I recall, we were a 37" long 27" wide robot (this was before frame perimeter, and you had to fit in the box to compete--no bolt heads sticking out).

Re: 6 wheel drop center drive train
 

We did 1/4" with AndyMark 8" pneumatic wheels because the front of our drivetrain was disconnected (so it was not as rigid as a WCD with a single crossbar in the front). If it was a single piece across the front, we may have gone with 3/8" or so. Generally, the less rigid your drivetrain, the more drop you'll need.

Re: 6 wheel drop center drive train
 

The reason you would want to do this is because it prevents the friction of a standard 4 wheeled pneumatic as well as, a 6 wheeled drive that has little to no wheel drop. The amount of centripetal force the robot experiences while turning either on a dime, or steering (like a car) causes the robot to stabilize on the center wheels, allowing it to turn easier... on the other hand, if wheel drop is too small, then the robot will not be able to achieve the same results because it doesn't have enough clearance to get on it's center wheels, this in turn causes more strain on the motors as well as the battery level. So basically what I am getting at is, that all wheel elevation/drops will work fine for their intended purposes, but 0.375" has the upper hand in terms of maneuverability as well as motor strain.

Re: 6 wheel drop center drive train
 

The concept is kind of correct. Still, I think what EricH was getting at was that 0.375" is not the absolute number due to other factors. For example, a drivetrain 48" long should have a different dropped distance compared to a drivetrain 20" long. A drop of .375" might not have the "upper hand" in both situations.

Thinking about it, there must be a holy grail equation for the optimal wheel drop based on distance between wheels and the size of the wheels, which always yields the optimal angle of rocking. Has this been done before? Am I oversimplifying this, missing other variables that could affect the optimal angle of rocking? If it's just geometry it seems simple to just experimentally determine the optimal drop, which I would define as "able to turn while rocking as little as possible"

Re: 6 wheel drop center drive train
 

I believe Eric knows about the benefits of the dropped center wheel. What he's trying to say is that 3/8" may not always be correct, and making blanket statements is misleading.
Like I said, our practice bot ran 1/4" without issues for weeks. Factors like the length of the wheelbase, number of wheels, weight, and tire pressure are all things to be considered.

Re: 6 wheel drop center drive train
 

Factors that determine the optimal wheel drop:

• Wheelbase length
• Wheelbase width
• Chassis rigidity
• Deformation characteristics of wheels (pneumatic/compliant vs rigid, tire pressure, etc)
• Traction / CoF of wheels
• Compressibility of wheel tread
• Surface robot is driving on

A lot of these are very tangible things, but others are less tangible. For most non-pneumatic wheels, it seems 1/8" is a safe "drop" number that covers a wide variety of wheelbase lengths and widths, even if it's sometimes more than necessary. The rock caused by 1/8" drop has a very minor effect on most manipulators.

Pneumatic wheel drop is more complicated and I have less experience with it, so I don't know a "perfect" number for that. Experimentally determining this is probably your best option.

Re: 6 wheel drop center drive train
 

We tried 1/8" in 2014 and it was very noticeable. I've always thought that something between 1/16" ad 3/32" is a better drop for firm treaded wheels like blue nitrile.
What did people run in 2009, out of curiosity? Were drops even used?

As an unrelated side note, using omni wheels on the corners instead of treaded wheels will of course greatly reduce the required drop.

Re: 6 wheel drop center drive train
 

That is largely because the 1/8" "standard" was settled on back when frame perimeters were 38x28. In 2014, most wheelbases were 28x28, so they may have been able to turn with very little or even no drop depending on the wheel choice and setup.

Re: 6 wheel drop center drive train
 

For reference, we used 3/16" drop with the AM pneumatic wheels. Zero issues with turning. Our setup was not standard tho. We had 8WD but performed more like a wide 6WD; our front wheels were raised 2" off the ground. So for us 3/16" was plenty.

Re: 6 wheel drop center drive train
 

No need for a drop in 2009 because of the lower CoF, I think a lot of bots went with 4wd because of it. You see the same thing in battlebots too because of the steel floor.

Re: 6 wheel drop center drive train
 

While CoF was a factor, a much bigger factor was that nearly every competitive robot in 2009 already had a wheelbase wider than it was long, and didn't need a drop center to create this effect.

You can see this effect in games like 2012, teams who built wide robots did not need a drop center at all in order to turn.

Re: 6 wheel drop center drive train
 

It was also when teams were running 4 inch or smaller high friction wheels. The 1/8 drop has been further standardized by the advent of the versablock, which when applied in a certain way in wcd configuration create a 1/8 drop.

We historically have tried to minimize the drop for better or worse. This year we were able to pull off 3/32 on the 8in AM pneumatic wheels.

Re: 6 wheel drop center drive train
 

This is exactly what I was getting at.

@ ModMaster: I know the physics, at least in broad terms. You simply rehashed the broad physics without giving any numbers--even situational ones--to back up your assertion that "this dimension is the best". Just so you're aware, I was a student back when the WCD was introduced, in the area it was introduced in, and had the opportunity to learn about that drive, including the drop, from the teams that developed it.

If you're going to assert a blanket statement that "X is the best", this is a group of engineering types. You'll get an average of 1.33 counter-anecdotes per person on it not being optimal for a different configuration. (BTW, remember that 85% of statistics are made up on the spot--but in this post, it's 66%.)