Drive train wheels idea
 

So last night I was thinking of a way to improve my team's drive train, when I had an idea. We usually use a 6 wheeled tank drive, with 2, 6" plaction wheels in the center lowered 1/8", with 4 standard FIRST wheels on the outside, with each wheel connected by chain.

My idea was this: A similar drive train, using 6" PLaction wheels in the center, lowered 1/8", but with Andymark trick wheels on the outsides instead of the standard FIRST wheels.

Has anybody tried this? Anything to change? Should all wheels be connected via chain, or only the center wheels powered?

We're going to try a WCD next year, so will that affect anything?

Thanks!

Re: Drive train wheels idea
 

Many many teams already do this, you just don't lower your center wheel. All wheels are on the ground, all receiving power via chain.

Re: Drive train wheels idea
 

What are you trying to improve? What is the problem you are trying to solve?


From the AndyMark web site:

Re: Drive train wheels idea
 

Sorry, I'm new. There weren't many robots with this drive train at SVR.

Re: Drive train wheels idea
 

What are you trying to improve? What is the problem you are trying to solve?

We're trying to get better maneuverability.


From the AndyMark web site:


Are you suggesting we use different wheels? Any recommendations?

Re: Drive train wheels idea
 

If you implement a 6-wheel skid properly, you shouldn't need more maneuverability. Take a look at 254's drivetrain if you want a WCD; they are very maneuverable without sacrificing the vital side-to-side static friction that allows you to resist pushing. In other words, you should probably go with all traction wheels.

As for the wheels, basically pick any omni wheel that isn't an andymark trick wheel. Personally, I like the plastic ones. They aren't too expensive, they're robust, and I haven't heard of any common failures.

Re: Drive train wheels idea
 

Can you describe in a bit more detail what your existing drop-center 6WD chassis is not doing well enough?


Just what the AM website recommended in the quote I posted in previous message:

Re: Drive train wheels idea
 

[quote=Ether;1065896]Can you describe in a bit more detail what your existing drop-center 6WD chassis is not doing well enough?

Our current design is alright, but what we're trying to get is maximum friction on the center wheels with minimum friction on the outer wheels to assist with turning, since our current model can be a bit better at turning.


Just what the AM website recommended in the quote I posted in previous message.

Gotcha. Thanks!

Re: Drive train wheels idea
 

Thanks for the tip!

Re: Drive train wheels idea
 

Be careful. There is a point at which the robot becomes much too easy to turn, and you will start having issues with the simple things like driving straight or driving while being defended.

Also, make sure you experiment ahead of time. All 6" wheels are not exactly 6", and all 4" wheels are not exactly 4": they sit on the ground differently. We found this when we didn't pay attention to the 4" plaction and the 4" omnis from Andy Mark.

Re: Drive train wheels idea
 

Which were larger: The plaction wheels, or the omnis?

Re: Drive train wheels idea
 

we did exactly this, and i would recommend against it. 6 wheel drive with the outer wheels as omnis turns way to fast and is difficult to control without a lot of driver practice.

AM trick wheels are not what i would recommend, the plastic omnis are excellent in the 6 inch size, and they are also not quiet 6 inches, they are actually 5.94 if i remember right, which means you don't need to make your center wheel lower.

if you want to make a tunable robot, i would take a look ate 1986 team titanium's robot from this year, they used an 8 wheel tank drive, with omni wheels on the ends, and were amazing, because it still has 4 traction wheels, it doesnt turn too easy, but turns better than a standard tank drive.

you might try reducing your gear ratio some if you want to turn better, even just 5 percent will make all the difference in the world.

in summary;
if you want to use omni wheels on the ends, use 8 wheels total, 6 turns way to easy and its easy to get spun, and the plastic omni wheels are the best option as far as cheap, versitile, durrible, and lightweight.


Edit: here is a video showing 1986's drive train, if you want an drive with omni corners to learn from, theirs is very good.

http://www.youtube.com/watch?v=DdnYpgoClW4

Re: Drive train wheels idea
 

Andrew,
Contact me and let me know when and where your team is meeting. I should be able to help with this. Your mentors should have all of my contact info. If not, just PM me here and we will set something up.

Re: Drive train wheels idea
 

If you do opt for the omnis, you'll want to remove the drop from the center wheel.

The reason why: if you leave the drop in there, you shortened your wheelbase (the point of the drop), while simultaneously making it easier to turn the resulting 4WD (the point of the omnis is to make the 6WD easier to turn). It'll most likely try to act like a 2WD, which is really easy to turn, but might not be the best with traction.

What I might suggest instead would be to:
1) Increase the center drop slightly. This will pick the "up" set of wheels up just a little bit more, resulting in less scrub from them.

OR
2) Change the center wheels to pneumatic wheels. This will allow for slight variations in the drop without permanent modifications, and, depending on the wheels you use, it may increase your center traction. (It may also be fairly simple to test out during the offseason--make your modifications and drop them in.)

Re: Drive train wheels idea
 

If you're going for maneuverability, you can't ignore the other 2 fundamental concepts that help your maneuverability (i.e. helping you to turn). The Poofs (et al) understand these fundamentals so well that they keep implementing similar designs year after year that are tailored for them. Quite honestly, I don't know if there will ever be a skid-steer configuration superior to WCD since WCD plays so well into these concepts while maintaining a very simple, efficient approach to the design. *Note that the KOP is also setup for both concepts, yet no kit drive train can force you to put your center of mass in the right spot ;).

Concept 1: Wheel base is wider than it is longer.

• This is achieved with the 6WD drop center with since only 4 wheels ever touch the ground in the right configuration for our robot dimensions.
• In a 6WD corner-omnis configuration, this is somewhat achieved by the reduced lateral frictions of omni wheels.

Concept 2: The center of mass should not be close to your center of area.
For a 6WD drop-center there are 2 centers of area depending on which 4 wheels are on the ground -- it's either in the middle of one or the other set of wheels. For a 6WD with omni-corners, it is dead in the middle of the robot. To find the center of area overall, draw lines from wheel to wheel -- where they meet in the middle is the center of area.

• By placing the center of mass in the dead center of the robot with a 6WD drop center, the center of mass is never the same as the center of area. The center of mass is also far away from the sides of the robot, which minimizes tipping issues.
• With 6 wheels on the ground, special care has to be take to move the center of mass away from the center of the robot -- which then may cause tipping issues if moved too far. Moving the mass towards the corner omnis also puts more weight on them, meaning more lateral forces that play into Concept 1.

White Paper which shows the math and the conclusions at the end. Don't let its 'old' date fool you -- it's a gem full of good info.
http://www.chiefdelphi.com/media/papers/1443


You can get by with corner-omni placements and ignoring the 2 concepts. However, you will be much better off by at least understanding the concepts and somewhat adhering to them. Robots that turn easier are not only more agile at mid-high speeds, but also draw less current overall during a match when compared to non-optimal configurations (all else equal).

Re: Drive train wheels idea
 

Omnis on all four corners make things difficult--more difficult to control, and lack of ability to resist being pushed or turned.

Perhaps you could put omni/trick wheels only on either the front pair or back pair of wheels and remove the 1/8" center drop. This maintains all 6 powered wheels on the ground at all times, removing the minor rocking due to the drop center, but it increases the manueverability without loosing contol. Teams 48 and 3193 did this with great success this year, using custom 4" treaded wheels (modeled after the AM peformance wheel) with 4" omnis up front. The robot was very maneuverable, very stable, and had no issues being pushed/turned.

Two notes: we made the custom treaded wheels 1.5" wide instead of the standard 1", and we located our main mast/arm at the center of the 4 traction wheels.

Re: Drive train wheels idea
 

Factors that I'm surprised haven't been mentioned yet:

Motor selection
Gear ratios
Tread selection
Wheel placement

Re: Drive train wheels idea
 

Which have teams had better success with (From andymark) as far as traction wheels go? Rough top or Wedge?

Re: Drive train wheels idea
 

Here: http://www.andymark.com/product-p/am-0014.htm
it states that roughtop has slightly higher CoF. As far as tread wear goes, hopefuly someone with more experience with both treads can chime in.

Re: Drive train wheels idea
 

Tip of the day: Do not waste your time trying to improve drivetrain performance when the kitbot is so good!

My team ran a 6WD Kitbot with 4" Plaction Wheels and Supershifters from Andymark. It was the most reliable and effective drivetrain I have ever been involved with. If you preped the parts prior to build season, it literally takes a couple hours to put together and build (lead times for parts were the only reason we were just driving in week 2). I have seen lighter drivetrains, but the weight we had on it was only 2" off the ground, dropping CG of the robot.

Focusing on the mechanisms that get you points will make you more competitive than building what is essentially the kitbot, but lighter. Engineering is all about picking the most efficient areas to make gains.

Re: Drive train wheels idea
 

The kitbot this past year wasn't as good as it has been from 2010 back to when andymark started providing it. They stopped including toughboxes in favor of the lighter, cheaper CIMpleboxes, which is fine, but they still gave teams 8" wheels. The since there is less torque due to less reduction, the 8" wheels required the motors to work harder and at a less desirable point on the performance curve. This can cause power failures, especially during turns, since the voltage drops far enough for some parts of the system to brown out or otherwise lose power.

My tip of the day: match you wheels to your transmissions and regardless of whether or not you use the kitbot and/or its frame.

Re: Drive train wheels idea
 

I generally prefer roughtop to wedgetop for carpet driving, and haven't really giving much effort into determining whether or not that would change if we were driving on another surface (such as different carpet, aluminum, polycarbonate, PVC, or HDPE). Keep in mind that there are also alternative tread materials, in addition to the roughtop vs. wedgetop distinctions.

McMaster has numerous options. So do other places (Brecoflex, Gates, etc).

Re: Drive train wheels idea
 

Wedgetop tends to wear out much faster in my experience; on our 2010 bot, we had a 1/8" center drop, roughtop centers, wedgetop corners. Wedgetop tread had to be constantly replaced, about every 4-5 matches. The roughtop was placed in the center for traction; wedgetop on the corners to facilitate turning.

Re: Drive train wheels idea
 

I don't think the problem discussed above has to do with the normal tread wear properties. It probably has to do with your weight distribution. I'm going to guess you have most of your weight on the center wheel and rocked quite a bit during the match. And with a 1/8" drop the wedgetop on the corners would most defintely be scrubbing. Especially because wedgetop is a tiny bit thicker than roughtop (making the effective drop even less).

Let me know if I'm right.

Re: Drive train wheels idea
 

Maybe, but I do know that wedgetop seems to wear faster than roughtop.

Re: Drive train wheels idea
 

Wedgetop wheels have narrow, tall ridges which means less of the tread transmits the same force compared to roughtop. This seems to fit the anecdotal accounts of wedgetop wearing out faster.

If wheels were a dirt road, roughtop wheels would be a dirt road where the cars pass over all of the surface evenly resulting in even ware; Wedgetop, on the other hand, would be a road where cars only travel down the center resulting in ruts in the road.

Re: Drive train wheels idea
 

Here's a much better view of our 8-wheel in action.
http://www.youtube.com/watch?v=WPD82b-y6dc