Re: pic: High Traction Drivetrain Concept
 

It may be beneficial to consider what the normal force on each wheel would be and the behaviour of your chassis as it drives over a floor that is not perfectly flat. What happens if the four high-traction wheels in the middle have a slightly smaller diameter than the omni-wheels?

Re: pic: High Traction Drivetrain Concept
 

Today I saw an FTC robot on Twitter which basically looks a 6-wheel version of OP's model.

Re: pic: High Traction Drivetrain Concept
 

The resistance your robot is going to be able to give is related to the amount of friction your robot has to resist that force. When it comes to lateral force, it's obvious that an omni wheel is going to present negligible friction compared to a traction wheel. Even ignoring the wheel placement for the moment (the outboard wheels will have a greater lever arm to your center of mass for resisting the applied moment), you can determine the friction from your wheels with the equation f=μN. For our purposes here, the normal force is equivalent to the weight placed on each wheel.

When the quantity of wheels in contact with the ground is increased, the normal force on each wheel is decreased (less weight on each wheel). In the case of a 8WD with 4 "drop center" wheels, you'd have the weight of the robot on 4 wheels. 100% of the robots weight would then be placed onto those 4 traction wheels. In your corner omni 8WD, the weight of the robot is distributed among 4 traction wheels and 4 omni wheels. Assuming an even distribution of weight, you'd have 50% of the weight of the robot placed on wheels with a high coefficient of friction, and 50% of the weight of the robot placed on wheels with a low coefficient of friction. As a result, the total friction your robot generates to resist that lateral force would be less than a robot with all of its weight placed upon high traction wheels.

Now, we've made quite a few assumptions to reach this point, and many things will end up being far more complicated in reality than I've presented here. For instance, resisting a spinning moment is going to be very dependent on wheel placement, drop height, and frame interactions. But I wanted to illustrate a general point. To phrase that point differently, the advantages your design has in terms of turning itself easily also serve to make it easier for other outside forces to turn. None of this is to say your design is poor, just that it will behave differently than a drop center drive. In some cases, team's have taken advantage of ultra-low resistance to turning and incorporated it into how they wanted their robot to behave.

Re: pic: High Traction Drivetrain Concept
 

Thanks for sharing your design! CD needs more of this.

Some quick comments. Unless you adjust the spacers on the Ballshifter third stage I don't believe there is enough room for #35 chain. Remember you want to have clearance on both sides of the sprocket for the chain.

With the 8wd if you switch your chain routes it will allow you to sneak the outer chains in a little making the shafts shorter and save a little real estate in your bellypan.

It looks really solid. Nice work!

Re: pic: High Traction Drivetrain Concept
 

I've seen this done before, and it provides little to no real world advantage - ignoring the fact that this is illegal. Bumpers don't seem to be rigid enough to act as an effective wedge, which seems to lead to minimal weight transfer.

As others have noted before, low (literally, the bottom of the bumper zone) bumpers are pretty much all you need if you want to build a drivetrain that's effective at holding it's ground and/or moving objects.

If you really want to have fun, you can start playing with the center wheel spacing relative to the outer wheels, a larger center wheel base will lead to a drive train that has more of a tendency to drive straight while a shorter wheel base will lead to better agility at the expense of straight tracking.

I personally am an advocate for minimal drop on 8 wheel drives. The last two I had direct involvement with had .090" drop (2013) and .060" drop (2014). The difference in drop were due to different strategic objectives and robot configurations but each was somewhat optimized for the role. The 2013 drive train was optimized to sit on the back 6 wheels when in shooting position to provide a stable platform and while in transit position it rocked forward to "tip" the whole robot back for more stable driving and a slightly angled leading edge, helping to get "under" other robots. 2014 was designed to "Squat" when pushing, digging all 8 wheels into the carpet while also providing good stability at speed.

A couple things to remember when chasing high traction performance:

-Wheels sink into the carpet, sometimes as much as 1/8". A drive with minimal drop may not ever have it's wheels leave the carpet, nor does it truly rock, instead the force on each wheel changes depending on the conditions.

-Traction on carpet is a bit more complicated than F=CoF x Fn. There are a series of factors such as carpet wear, tread geometry and tread hardness to consider as some wheels actually "Dig" into the carpet, creating additional mechanical forces that appear to increase traction.

-When working with 6 CIM drive trains, you have to be extremely conscious of current draw, especially when pushing. Once you're pushing another robot, if it's weight begins to transfer to your machine, suddenly a traction limited machine may no longer be traction limited, which usually doesn't end well.

Re: pic: High Traction Drivetrain Concept
 

Just a heads up, those west coast drive clamping hubs are a nightmare to work with. They constantly slip if you use them to hold significant tension on a chain.

Re: pic: High Traction Drivetrain Concept
 

Have you been using the WCP Tensioning Cams?

http://www.vexrobotics.com/217-3431.html

Re: pic: High Traction Drivetrain Concept
 

The trick is you do it ever so slightly..... :rolleyes:

Re: pic: High Traction Drivetrain Concept
 

I think there is a fundamental issue with your approach. Many people have tried to solve this problem over the years and I think they're all solving the wrong problem.

What benefit does pushing another robot provide?

On offense, if you engage a pushing match you've done exactly what the defense wants you to do: waste your time. FRC isn't about what you can do, it's about how efficiently you can do it.

On defense, you've placed yourself in your least maneuverable orientation as a tank drive. A good offensive robot will avoid pushing matches and try to just go around to the side. It's hard to move sideways when you're facing forward.

As a defender you have the advantage when you sit perpendicular to an approaching offensive robot. By using your wide side against their narrow side you can be in the wrong spot by a bit and still get in their way. You can react to jukes by just driving straight while your opponent has to turn. It's also really easy to build a robot that's good for this, you just need traction wheels, quick acceleration, and a long wheelbase that's hard to spin. A great driver could play perfect defense without ever touching the other robot just by being in the right spot at the right time.

Now, that works great until they actually do get around you, in which case you still want to slow them down. The majority of robots are still susceptible to the T-bone pin, which almost any drivetrain can do. Just drive into the side of their robot and watch them drive helplessly in circles wishing they had some omni wheels or an angled frame.

Re: pic: High Traction Drivetrain Concept
 

In my experience, these sort of drivetrains are susceptible to massive friction loss in a pushing match due to the effects of front-end lifting. For a visible (though somewhat extreme) example of this effect, check out videos from the 2010 game, where bumpers were mounted high.

To reference Lil' Lavery's comments above, in a drive system like this, the weight is distributed evenly between the wheels, however, when you get into a pushing match with another robot (assuming bumpers are the same height), both robots have a tendency to push each other upwards when in any kind of pushing match. On an 8wd robot where the center wheels are lower than the outer ones, the robot simply tilts back onto the rear wheels and maintains traction, but for a no-drop drive system, this effect will result in all of the robot weight being shifted on to you rear wheels, which in this case, have the lowest friction. All other things being equal, this design will likely not stand up in a pushing match with the majority of drop-center 8+ wheel robots, IMO.

Re: pic: High Traction Drivetrain Concept
 

I see what you did there.

Re: pic: High Traction Drivetrain Concept
 

We have used this drive arrangement multiple times.

Re: pic: High Traction Drivetrain Concept
 

Care to share what has lead your team to choose this over a normal 8wd with all traction wheels or shrink it down to 6wd?

Do you keep all the wheels on the same plane or do you drop the center traction wheels slightly?

Re: pic: High Traction Drivetrain Concept
 

It seems like this issue could be easily remedied by mounting your drive motors/gearboxes (and battery if you have room) to the front of the robot to make it less resistant to shifting onto the back wheels. You could also build a suspension system into the wheels so that when the frame tilts, all the wheels stay on the ground (some might say that this would be too heavy, but you can make suspension systems fairly light :D ).

Re: pic: High Traction Drivetrain Concept
 

The weight solution is a good one for pushing matches, but it affects turning in most drives and only works in one direction (often I've found you need to be able to push from either side).
The suspension is also a good solution but far more complex to build.