Drive Base Help

[Monochron]

If your concerns are that you don't yet have the experience for a lot of design work (but you do have some money to spend) then PURCHASE A DRIVETRAIN, instead of designing one. VersaChassis is great but you still have to machine it together, make sure all your distances are correct, source the correct length belt/chain. Using the AndyMark pneumatic wheel upgrade kit as a guide for what to buy is a great way to go (or just buying it if it comes back in stock). We were very successful crossing defenses using that kit by simply slapping an angled plate on the front of the robot. The 1/8" drop center distance was perfectly adequate after we under-inflated our outside tires.

If cost is really an issue you could try to reuse an old LOP drivetrain (or even this year's if you have it!).

As you learned this past season, the drivetrain is the most important part of the robot, without it you can't do all that much. Focus on putting great things on top of the robot rather than mucking around with the most important part. Next year you will likely be able to use the KOP again, and once you have stronger design skills, you can start building your own drivetrain.

[IKE]

My biggest piece of advice:

While asking for guidance and rules of thumb are a good way to start, learning how to test and tune a chassis quickly is where you will get a lot of benefit.

Center "drop" is there so that there is more normal force on the inner wheels than the outer. This reduce scrub (lateral friction which counteracts steering and turning). Too much drop will actually have the wheels off of the ground, and can cause tipping during starting and stopping depending on Center of gravity height and relative position.

Reduction in scrub can make a world of difference in smoothness and power draw when turning. Too little scrub can make a chassis feel "squirrely" or unstable. A good testing program will have you adjust things until you no longer like the performance (a different negative attribute shows up like pitching or difficulty driving straight), and then backing off that parameter to find a happy medium.

Make sure you run your tests at weight and on similar carpet. Testing on smooth concrete and only 50 lbs will lead to very different results than testing at 120-150 lbs on carpet. This is especially true for pneumatic tires. With hard wheels, the primarily "spring" element is the pile depth of the carpet, thus teams tend to run lower drops (1/16" to 3/16"). With pneumatic tires, the tires will also spring down a bit which varies a lot depending on tire pressure.

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Lastly, with regards to 6" wheels, you are at a disadvantage to impacting a 4" beam with the 6 inch wheels. In order to go over it, you will need some sort of "leading edge" or guide. Even with COTS chassis, this is important to consider. That is not to say a 6" tire will not go over, You just have to be a bit more clever. I saw a team this year using 4" wheels and it would go over the Rock wall just fine, but they were very clever with other chassis elements to help them maneuver (494/70).

Do some searching, and you will find good testing programs other teams have conducted. I know 234 has some nice white papers out, and there are several other threads discussion chassis and mobility.

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Lastly,
Buy extra sprockets to play around with gear ratios a bit. Use the JVN calculator, but then compare your results with the calculator, and try other ratios (say plus and minus 20%) to see what you think. Several years I have seen teams "go faster" by gearing slower... For many FRC games, robots are doing short sprints of about 12 to 16 feet. Sometimes sacrificing top speed will help improve time to distance.

[carpedav000]

Originally Posted by roboruler

Omni-wheels sort of destroy the point of having pneumatic wheels, the main reason for having pneumatic wheels is for the impact absorption, especially on the front and rear wheels. If you were going to have to use omnis it'd be better to just make a conventional chassis with non-pneumatic wheels, like hi-grips, versa-wheels or colson wheels

I should've worded that better I was talking about using omnis with conventional wheels (like 1986 in 2013)

[Chris is me]

One option you could do to get effective drop would be to run 8" colsons with a press-in hub for the inner wheels and 8" pneumatic wheels on the outer, using VersaBlocks. The 1/8" drop of the VersaBlocks will be exaggerated by the diameter difference in the wheels - the 8" pneumatics are actually 7.65" diameter - and the Colsons won't sink into the ground as much as other wheels would. The inner and outer wheels are generally the ones that have to take impacts anyway.

[Billfred]

A couple questions:

1) What is your current robot running? Without a team number, we can't exactly pull up pictures. (And there is no shame in sharing your team number. We had this beast and still didn't make the show at either of our regionals.)
2) Are you going to be running this at an off-season tournament somewhere? (This shifts the priorities of "good experience" vs. "can compete with it".)
3) You mention your in-season drivetrain fell apart--what failed? Depending on what you have and the failure modes, it may just be something was installed improperly or a matter of beefing up a part.

[ctt956]

Originally Posted by redfancy

Wheel-wise, we want pneumatic wheels for sure. We have 8-inch wheels from our original bot, but because we want to upgrade from a 6WD to an 8WD (moat), we want to switch to 6-inch wheels for frame perimeter reasons. Our major concern is being able to get over drive obstacles-have any teams run into issues with 6-inch wheels on obstacles?

I've driven a basic robot with six 6" pnuematic wheels over wooden defenses. It was built on an AndyMark frame from 2012. For motors and gearboxes, it had two CIMs and a ToughBox Mini on each side. There were no bumpers, but it had no problem crossing defenses, even the rock wall. However, I don't think that frame would fit eight 8" wheels; there wasn't a lot of free space between the 6", so I think six 8" would be a very tight fit, if they fit at all. I haven't seen the AM chassis this year though, so it might be different.

[carpedav000]

It seems like using this with pneumatic wheels would be very easy:

http://www.team221.com/viewproduct.php?id=130

[redfancy]

Wow, so much help! You guys are great!

[redfancy]

Ok, to answer some questions:

We are planning on taking the new bot to an offseason event, probably Calgames. Our old bot ran a KoP 6WD with 8-inch pneumatics. We had major issues with our gearboxes, which happened to be the exact same 3-CIM ones suggested before. Since the only thing we could do was defense, our gearboxes took a lot of stress and it was enough to fracture and completely shred our aluminum gears. We were advised to just use 2-CIM systems in the future and also decent steel gears.
Also, we didn't secure our gearboxes very well, and that combined with the hole we had in the frame for intake resulted in the bot bowing inwards and twisting the wheels/axles out of alignment. The wheels were actually hitting the metal of the base. This was probably exacerbated by turning scrub, since all wheels were on the same level. So, we took the bot apart multiple times at competition, which was a pain with the KoP base.
We aren't trying to rebuild the same bot though-we're entirely redesigning.

Also, with 6 8-inch wheels, it was a really really tight fit. We had to trim down the bars perpendicular to the wheels so that they wouldn't hit the metal. Also, does the KoP base have pre-drilled holes that would let us drop wheels a good distance?

The AM2494 and 221's chassis seem simple and stable, but would we be able to drop wheels with them?

After reading through, we're leaning towards the VersaChassis. Also, using four Colsons and four pneumatics sounds good. When teams go over obstacles like the cheval and moat, they tend to fly over and then land. Is stress upon landing an issue with the Colsons?

Thanks so much for the help, everyone!

[carpedav000]

Originally Posted by redfancy

Ok, to answer some questions:

We are planning on taking the new bot to an offseason event, probably Calgames. Our old bot ran a KoP 6WD with 8-inch pneumatics. We had major issues with our gearboxes, which happened to be the exact same 3-CIM ones suggested before. Since the only thing we could do was defense, our gearboxes took a lot of stress and it was enough to fracture and completely shred our aluminum gears. We were advised to just use 2-CIM systems in the future and also decent steel gears.
Also, we didn't secure our gearboxes very well, and that combined with the hole we had in the frame for intake resulted in the bot bowing inwards and twisting the wheels/axles out of alignment. The wheels were actually hitting the metal of the base. This was probably exacerbated by turning scrub, since all wheels were on the same level. So, we took the bot apart multiple times at competition, which was a pain with the KoP base.
We aren't trying to rebuild the same bot though-we're entirely redesigning.

Also, with 6 8-inch wheels, it was a really really tight fit. We had to trim down the bars perpendicular to the wheels so that they wouldn't hit the metal. Also, does the KoP base have pre-drilled holes that would let us drop wheels a good distance?

The AM2494 and 221's chassis seem simple and stable, but would we be able to drop wheels with them?

After reading through, we're leaning towards the VersaChassis. Also, using four Colsons and four pneumatics sounds good. When teams go over obstacles like the cheval and moat, they tend to fly over and then land. Is stress upon landing an issue with the Colsons?

Thanks so much for the help, everyone!

The 221 system does not have a center drop, but you can easily remedy that by using smaller wheels for the outside wheels

[troy_dietz]

Originally Posted by carpedav000

The 221 system does not have a center drop, but you can easily remedy that by using smaller wheels for the outside wheels

If you're using pneumatic tires, you can just inflate the center wheels slightly more than the outer wheels, or use a belt sander to take off some of the tread on the outer wheels. (or both) Not only will it create a drop center, you'll have less issues with turning scrub.

[BetaHelix]

Do you have access to a press brake? If so you could build something similar to our drive this year (pictures below). We used 8 8" wheels with 15mm belts and 4 toughbox micros at 12.57:1 (I think). We used 1/2in of drop on the centre wheels. The side plates are .090 5052 aluminium, laser cut, but could be handmade.

[asid61]

Originally Posted by troy_dietz

If you're using pneumatic tires, you can just inflate the center wheels slightly more than the outer wheels, or use a belt sander to take off some of the tread on the outer wheels. (or both) Not only will it create a drop center, you'll have less issues with turning scrub.

Inflation alone does not work, from our experience this year. Sanding down the tires may help but in the end we ran a 3/8" center drop (although 1/4" was working ok). It would be hard to get enough drop without actively doing something to add it.

[messer5740]

I would be careful with using the angled plate of you plan to have an opening in the middle.

[OccamzRazor]

Originally Posted by asid61

Inflation alone does not work, from our experience this year. Sanding down the tires may help but in the end we ran a 3/8" center drop (although 1/4" was working ok). It would be hard to get enough drop without actively doing something to add it.

The 3/8 drop works well with pneumatic wheels, you are right in that inflation barely does anything at all on these specific AM wheels. I saw a team next to us really struggling at Champs to get their robot to turn without brownout issues because they neglected to include the drop. I told them to try wrapping the center wheel with layers of duct tape and that actually fixed their issues aside from the duct tape appearance.