Drive Base Help

[redfancy]

A bit of background: We had a very very rough season this year with a bot that fell apart multiple times and could barely drive, let alone play the game. In our rookie year two years ago we did very well, but everyone that was on the team that year has since graduated, taking any technical/mechanical experience with them. We have no stable mentorship (working on that, though) and team culture virtually does not exist. Basically, we're a rookie team with no rookie benefits, little commitment, and minimal guidance. So, three of us have decided that we want to get our stuff together and are leading a summer journey to start from scratch and design/build a new bot for offseason, learning tons of stuff along the way. Except, we're very inexperienced (explain like we're five). Plz halp!

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?

Drive base-wise, here are our ideas:
1. We want the wheels to stick out the front so that drive base height doesn't matter
2. 8-wheel drive
3. two center wheels dropped (also need advice on how much to drop wheels proportional to wheel size)

We have entirely ruled out a custom base and are now choosing between the KoP AM14U3 and VEX's VersaChassis. The VersaChassis looks like it will allow us to fulfill requirements 1 and 2, but we're not positive on whether we can drop the center wheels a significant enough distance to matter on the field. Which chassis would be a better fit for the bot we're trying to make?

Also, got any other miscellaneous tips?

Any and all input is appreciated!

[asid61]

If you have the drive, all power to you to make your team better!
Keep in mind that bumper rules are a thing, so sticking your wheels out the front will only work if you have a bumper support ahead of that (maybe higher than the wheels?).

Versachassis stuff can only drop you 1/8", which is not enough for pneumatics most likely. Many teams had luck with colsons this year or a combinations of colsons and pneumatics; maybe 6" colsons in the center, dropped, and 6" pneumatics on the outside? That could solve your problem.

Versachassis is expensive once you total up bearing blocks, gearboxes, etc. KOP chassis with 6 8" pneumatic wheels is my suggestion purely from a cost standpoint. Custom will run you a bit more, and versachassis the most, mainly because of the gearbox setup. Unless you want to mill your own mounting holes for the gearbox you are practically forced to use the $300 3-cim ballshifter, so although the build quality will be really good you will be spending some money.
That being said, while working with versachassis I may have missed something major on gearbox integration. The advertisements say "any gearbox" so there might be a method of mounting that I've missed.

If you CAD everything, you can solve some of your beaching issues and stuff there. Just draw your drivetrain and the moat, and set it up in different positions. Just a simple 2D sketch side view is enough.

[SoftwareBug2.0]

Originally Posted by redfancy

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.

An 8WD w/ 8" wheels shouldn't cause a frame perimeter issue:254 971 1425

[carpedav000]

Originally Posted by redfancy

.
3. two center wheels dropped (also need advice on how much to drop wheels proportional to wheel size)
D

For pneumatic wheels I would reccomend a drop of 1/4" and for any other wheels I would reccomend a 1/16" drop (of course if you really want 8wd but can't do a center drop you could have omnis as your outer wheels)

[roboruler]

Good COTS parts like those offered by Andymark, VEXpro and West Coast Products do cost money, but they are also high-quality and reusable. It is important to consider COTS parts as investments, investments that when used well will not only assist you in gaining success but also something that you can reuse many times, in different seasons if you take care of them. You listed the two options you are currently considering, the Andymark KOP chassis and the Versa-chassis. You said that you have ruled out a custom chassis, the great thing about the Versa-chassis is that it is custom. By using bearings blocks, sprockets and chain and the versa-frame tubing you can build a high quality custom chassis easily and quickly. The Versa-chassis is a West Coast Drive (WCD) arrangement, and is undoubtedly one of the best FRC drivetrain designs. Building a WCD chassis with the parts available from VEXpro and West Coast Products will put you in a great position for future seasons, as you will learn a great deal and you can use the Versa-chassis design every season (254 has used a Versa-chassis style arrangement (WCD) with great success every season, its principles can be applied to any game).

The off-season is the perfect opportunity to experiment with the Versa-chassis and the skills you learn will be of great benefit.

[roboruler]

Originally Posted by asid61

Versachassis is expensive once you total up bearing blocks, gearboxes, etc. KOP chassis with 6 8" pneumatic wheels is my suggestion purely from a cost standpoint. Custom will run you a bit more, and versachassis the most, mainly because of the gearbox setup. Unless you want to mill your own mounting holes for the gearbox you are practically forced to use the $300 3-cim ballshifter, so although the build quality will be really good you will be spending some money.

Have you looked at the WCP SS gearboxes? They are about $120 for a 3CIM gearbox,that is high quality, lightweight and natively Versa-chassis compatible:

http://www.vexrobotics.com/vexpro/mo...es/wcp-ss.html

I disagree with you about cost, the VERSA-chassis will be about $780 with everything minus wheels for 8 wheels. If you are wanting to use the Andymark chassis with 8 wheels, you will need to purchase the upgrade kit which costs $300 and allows you to use 8 6inch higrip wheels. This means the total cost for the Andymark chassis is $899( Keep in mind the $599 KOP bit)

In comparison, you can make a Versa-chassis with 8 6 inch traction wheels for about $860, so roughly the same cost. But the Versa-chassis is way more versatile and you can use the same parts next year, and the year after, which you can't do at all with the KOP chassis.

If you learn how to make a WCD chassis in the off-season you can opt-out of the KOP, receive a $450 PDV from Andymark, which you can buy anything from Andymark with and be able to build a really good chassis next year, which will give you so many more options when designing your robot next.

[roboruler]

Originally Posted by carpedav000

For pneumatic wheels I would reccomend a drop of 1/4" and for any other wheels I would reccomend a 1/16" drop (of course if you really want 8wd but can't do a center drop you could have monism as your outer wheels)

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

[DaveL]

Quote: Our major concern is being able to get over drive obstacles-have any teams run into issues with 6-inch wheels on obstacles?

We first considered 6" wheels on the current KoP frame and found the wheel center was too low to get over the rock wall. Thus we went for 8" pneumatic wheels. Even then, our frame hit the rock wall. To help us get over, we cut the side rails to hit the rock wall with an inclined face of the frame.

Another option we considered was a custom lower axle location. Due to our lack of machining tools and expected build time, we opted out of this option.

Last summer we built some robots out of 3/4" plywood. We used wood blocks in the corners to add strength. It worked great and was very cheap.

I like the idea of using 8 wheels with a diameter of 8". A number of top teams used this approach. If you have a drill press, you can clamp all the frame rails together and drill one axle at a time, so the axle holes line up. If you use chain the axle hole positions won't be as critical as if you were using belts.

Let us know how it goes.

Dave

[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