pic: 8WD CONCEPT DRIVETRAIN



PWNAGE TEAM 2451 revisited our 2013 WCD 8WD Drivetrain and made it lighter, simplier, lower cost while implementing ideas inspired by other teams. Refining our 2013 design we retained large bore bearings and wheels with large integrated axles to reduce drivetrain losses. External chain allows for easy repairs. WCP tensioning cams were added to improve tensioning reliability. Hex bolts for bearing block fastening on the outer side replace allen head bolts on both sides. Custom chain sprockets were replaced with COTS Vex components. Weight of wheels were reduced. Latched bumper mounts are inspired by Team 254. 3 CIM custom gearbox direct drives central lowered wheels so loss of chain does not disable robot. Front wheels are raised .25" allowing six wheels to contact carpet for superior grip and stability during high speed maneuvers.

I am curious to what advantage was found by raising the front wheels 1/4". Most 8 wheel drives seem to have the center 4 wheels dropped between 1/8" to 1/4"

Is it just me, or are you using chain for an encoder on your gearbox?

If so, could you shed some light on why this was chosen, and how it will work?

Thanks

If I remember correctly from talking to Nick Coussins, it’s plastic VEX chain.

If it does use chain, plastic or metal, then I strongly feel gears would be far more reliable and easier to work with

Or a mounting the encoder to one of the wheel axles like most other WCD teams.

It makes turning easier because then the wheels don’t drag

True, but traditionally 8WD setups have raised both the front and rear set of wheels to shorten the wheelbase length to promote turning.

With the shorter robots, it’s possible to drive 6wd without a drop center fairly smoothly. With the further shortened wheelbase provided by the front wheel being raised, I’d imagine it’s pretty easy to drive the other 6 wheels… Although I’m still not sure exactly why they chose 8wd with only a single set of wheels raised.

Well, I would just 3D print some gears for the encoder myself. Maybe they have some other reason to go with chain.

Depending on where your center of gravity ends up on the final robot, you can adjust which wheels you raise/drop and by how much, because your frictional/scrub force will be greater in the areas close to the CoG. All about the weight distribution.

My guess is it has more to do with the tilt/COM change that robots undergo when they are changing velocity and direction, rather than weight distribution and wheel friction.

It seems they were planning more for higher speeds and accelerations rather than maneuverability. I feel that having only the front wheels raised gives the robot better tracking. However, I still think an 8WD with the 4 center wheels dropped is better because you are still going to get 6 wheels of contact (when moving or acceleration or whatever) and also get the maneuverability

You only get 4 wheels touching the ground at any given time with an 8wd

Two topics:

  1. Encoder Vex Chain
    I don’t really like this myself but we haven’t had time to change it. It should work but could be a pain. Any slop would result in inaccurate positioning for low speed maneuvers. I personally would like to see gears, thanks for the input.

  2. 8WD Chassis Wheel Lift
    We ran the same configuration in 2013 and the thing was a beast. Where your center of gravity is positioned does matter. You would ideally want it to be located central to the six wheels touching the ground, so between the gearboxes. If you made the front and back wheels both raised your robot would be tipping forward and backwards every time you accelerated or decelerated. It would also make your robot unstable during dynamic moves. It would make your contact patch width to length ratio way to high and your robot would be extremely unstable and twitchy at speed. Your width would be about 28" and length would be around 9" so your ratio would be 3:1. Way too high. We modified our 2013 practice robot to test the ideal ratio by making the side rails slide in and out on extrusion and I can tell you the current ratio is about ideal for high speed stability and low speed maneuverability. There was no perfect ratio but too long and it took too much motor energy to overcome the scrubbing wheels, too wide and it became too twitchy. I would like to know what Team 67 had on their wide 2012 robot. From their tech notes it looks like none of the wheels were raised but that’s from looking at the pixelated AutoCad drawing. I can tell you from actual testing that with six wheels touching we can go full speed, throw one stick the opposite direction and pinwheel to go the opposite direction, at full speed. I see no issues with maneuverability, low speed scrub is minimal and it doesn’t take much power at all to pinwheel with no forward motion. The wheel contact patch is still wider than it is long so you get the best of both worlds.

https://drive.google.com/file/d/0B67Uv_S_WWjpVHBMTHNRczAyM0U/view?usp=sharing

CAD can be found here if anyone’s interested.

That’s what I was thinking, so the raised set is just there to prevent the robot tipping.

I like the idea of using larger bearings and shafts to minimize losses, but I’m curious how much of a difference it would actually make.

I could be wrong, but it looks like the rational behind this design is higher efficiency by eliminating any losses from the standard hex shaft approach.

On a standard WCD, there is no space between the wheel and the bearing. So the bending of the shaft is almost nonexistent. Because of this I think your bearing setup will make very negligible difference.

Last season during a meeting in which we discussed what we wanted to accomplish with our off season drive train design, a very respected mentor offered a great piece of advice. That advice was to go after low hanging fruit. This means to go after the small problems that can be easily solved, and as many as you can. I say this because I think you’re tackling a very small and possibly nonexistent problem with a solution that demands a lot of resources. I think those resources could be better spent, a lot could be accomplished in the time it takes to design and fabricate your bearing configuration. My suggestion is to identify that low hanging fruit, and to allocate your resources accordingly. In my eyes it’s better to tackle several small problems that fix one with a complex solution.

This is just my 2 cents. All that being said, kudos to all involved in this design. You guys identified a problem and came up with a very elegant solution.

Best of luck rumbling in 2015’s ultimate assist :wink:

-Adrian