Rookie team questions on chassis and drive train

[RoboMentorY]

Originally Posted by KleinKid

Ok so i'm going to assume that we choose to do a custom frame would the 3/8" aluminum be best or is there a lighter or stronger option? Maybe i'll do a fast sketch of what we lined out.

My study suggests that you need to be about 1/16 or 0.064" sheet Aluminum. 1/8" is too much. 3/8" is a tank for a chassis with no ability to to add anything.

Do a weight study. At most you should have 40 pounds allocated to chassis, gearboxes, motors, chain, and wheels.

Depending on your approach above 8" above the floor, you need a lot of stuff, motors, actuators, hinges, etc. to make an arm or whatever work for you.

Any comments on designing for weight are welcome.

[EricH]

For the drivetrain, unless you're actually using sheet, 1/8" wall tube can take everything thrown at it. I don't quite trust 1/16" in drive chassis applications. Even with the bumpers, the drivetrain takes a massive beating. From before bumpers were even allowed outside of the box, 1/8" has yet to fail on any robot I've known about. Welds have failed, but the structure has at best bent. And this is with full-speed, metal-on-metal, defense all over defense. Heck, falling from the 10' high bar in 2004 hurt my team's robot less than running our lift down when it was all the way down already!

Superstructure, yeah, I can see (and have used) 1/16" wall. But we aren't talking superstructure yet.

40 lb for a chassis is kind of light. A 50-60 lb chassis (with some of the superstructure power) is more like it, though I'd shoot for 50 when driveable. Heavy chassis=low CG. Low CG=less chance of tipping. As we all know, tipping == bad.

Designing for weight is not about making everything as light as possible. It's about tracking weight--where is it going, how much is going there, stuff like that. If you don't have one already, get a scale--you'll want both a platform type and a fish (or hook) type. Before a part goes onto the robot, weigh it and note the weight in the sheet. Try to get an estimate of how much the robot will weigh--most CAD programs have a way to do that. Then you know where you can remove a lot of weight quickly.

[Gavmaster]

I have a question, and it fits this thread well. Me and one of the 3 other freshmen on the team see a problem, and no one believes us since we're "freshies"

Our team wants to use a two-wheel drive system , and the other person and I are the only people against it...
I really need some help on this one unless we're gonna have a robot snails call slow :/

[ks_mumupsi]

Originally Posted by Gavmaster

I have a question, and it fits this thread well. Me and one of the 3 other freshmen on the team see a problem, and no one believes us since we're "freshies"

Our team wants to use a two-wheel drive system , and the other person and I are the only people against it...
I really need some help on this one unless we're gonna have a robot snails call slow :/

The number of wheels does not affect your speed, the only thing it will affect is handling/maneuverability.. a 2 wheel drive system has been used by several teams in the past effectively. However, the recent standard has been to use a 6 or 8wd. The only disadvantage to a 2 wheel drive this year is not having enough power to push your way through, since I am assuming you will have casters or omni wheels on one end.

I would recommend one thing for you, the kit comes with a great basic 6 wheel drive base, if your team is not using the Kit of parts system, the three of you should spend 2 or 3 days building this and putting it together to show your point.

dont let your shorter FRC experience slow you.

[Al Skierkiewicz]

For what it is worth, rookies should take the kitbot chassis, assemble it without cutting for today and get it driving. You will find what works best and give yourselves some experience putting the parts together. But start driving. As you make decisions on what you want the robot to do, then start making parts. Please watch the Grant Imahara video on design. Unless you have a good veteran team helping you and have a lot of experienced mentors who might be able to bypass some of these design steps, you are letting time slip through your fingers.
If you have a welding shop capable of welding aluminum and if you have someone experienced enough to weld tubing, then 1" square tubing is a material that many teams use for the robot frame. It does require other specialties for robot building like knowing that square tubing will deform when bolts are passed through and hogged down tight.
Oh, one last item, the sizing box you need to fit into is unforgiving. You either fit inside or you go borrow a saws all and starting cutting. Plan your chassis 1/4"-1/2" under max dimension in every direction. (i.e. 27"x37"x59")

[joeweber]

This is a question regarding the rookie kit of parts. This Year the two teams we are mentoring only received two CIM motors in the KOP. I thought that the rookie teams use to get four CIM motors in the KOP. If not, this is an unfair advantage for the veteran teams because we already have ours.

[Al Skierkiewicz]

Joe,
Only two have been provided in the past. The optional two additional CIMs were purchased by teams.

[Jon Stratis]

Even as a veteran team (our 5th year in competition now), we're using the kitbot chassis. It's strong, light weight, easy to work with, and comes with mounting holes pre-drilled! We made a custom chassis in 2009... while we got everything working just fine, we ended up spending a lot of time on the chassis that we could have really used elsewhere. There is no underestimating the affect of any time saving activities.

I want to second what Al said about the sizing box, and to add to that weight. Both are unforgiving. Pay attention to them both throughout the build season, and actively design your plans with them in mind. Don't aim for a 120lb robot - you'll end up over. Don't aim for a robot that's 28" wide - you'll have bolt heads that stick out. One of the hardest things for a team to do is show up at competition and have to start drastic modifications to get within the height/weight restrictions. I've seen teams spend an entire day (or more!) drilling holes in their robot to try and lighten it enough to pass inspection. I've even seen teams that had to remove their entire manipulator in order to pass inspection.

We always build ours 27"x37" to ensure we fit in the sizing box. We always aim to be at least 10 lbs under weight - then bolt a steel plate onto the bottom of the robot at competition to bring us up to 119.5lbs, once we have the official measurement and know how much we need to add. Probably the worst feeling in the world was weighing in at our second competition last year and being almost 5lbs over... all the students started freaking out a bit. Then the inspector removed his foot from the scale, and we were fine.

One final note about number of motors in the drive train... 2 versus 4 is always an interesting argument, and it relies completely on what capability you want your robot to have. If you feel the need for extra pushing power, use 4. Otherwise, 2 will give you the same max speed and work great - just without that extra oomph of pushing power. If you want to use a CIM motor to power a manipulator of some sort, you'll have to only use 2 on the drive train - the rules only allow 4 maximum on the robot.

[Jeffy]

Another vote for kitbot here.

Our first year, we built the kit bot, bolted it together,and got it running. About week 3, we tried to weld it together. Removing all the bolts saved about 6 pounds. However, it left us with a warped and unusable frame. And we had to "copy" the kit bot out of the 1.5" square al tubing we had. It turned out fine for us, but it was a close call.

As per the welding, it can be done without warping, by an experienced welder, and with some braces. If you want to weld it, you can cut some 1" thick pieces of wood that fit inside the channels in the tubing and span across the "channel" that the drivetrain is ussualy located in. Clamp it all together, and you should have a solid base to weld together.

Good luck this year!

[KleinKid]

Well after i showed how easy it was to put together the KoP chassis and drive train we kinda went with it and have it halfway done. Currently we have the frame together with the wheels and are working on the gearboxes.

Most of the members however want to only use a 2 wheel drive powering just the middle of the 6 wheels. I had thought that the extra traction of using all 6 wheels would be best so the wheels don't slip on the carpet. Also we may switch the 4 corner wheels to omnis or some other type of wheel.

Also i kinda feel like the team lost direction and organization. We don't have a clear leadership chain of command or any way to come to a consensus other than having everyone vote voting. Unfortunately most member havn't looked into the manual or chief delphi much and i feel that as a team were making some very bad decisions. I don't want to make the mistake of trying to take command myself though but a leader is better than no leader.

[EricH]

Power all 6. If you decide to try other configurations, remove a chain. If it doesn't perform the way you want it to, put the chain back on.

If you do omnis, remove any drop. 6WD drop+omnis on the corners = spin city, because you effectively have a 2WD the entire match with the resistance of ball casters or less. Having omnis on one end, or having 4 omnis on the floor, provides slightly better traction.

Regarding the leadership vacuum:

If the team has no direction, somebody needs to step up and provide direction. That could be a mentor, could be a student. That person needs to have the final say on a design, but should see what the team thinks and why before making that design. If that person is you, step up and do it. If someone else is doing it, let them step up and offer to help them. Even if it's keeping the team focused on, "We need to do X, Y, and Z", someone needs to step up to leadership.

For next year: Team Handbook. Seriously, it'll save this type of situation happening.

[Bob Steele]

I would highly encourage you to use the kitbot. You have an unprecedented opportunity to use a well-designed six wheel drive system. In the past teams have been only been given a 4 wheel drive system... which could be altered and made into a six but did require additional work..
You HAVE a solid six wheel drive system in your hands....you only have to order the sprockets to make it a six wheel system (and purchase more chain and links...)

You can customize it to get the speed you want by purchasing whatever cogs you want to use on your wheels if you think these are not the speed you want.

Try it out now... see if you can handle the speed.

By all means ... try to power all wheels on the floor... you will receive maximum traction this way, which leads to better acceleration.

When I started doing this we had no kitbot... we had no transmissions in the kit... our first bots weren't bad but could have been much better in this critical robot component.

By all means make your own if you want... but this is an opportunity you should not pass up.

Remember your resources are limited... time being the most critical...
spend your time and effort on programming, a way to move and place tubes, the minibot and its deployment... the use of sensors and automation.

In the off season, study other robots and what they do do for drive trains... or design your own from scratch... work out the kinks...

I don't think you will find many veterans that will tell you that this drive train couldn't be on Einstein in April.... indeed historically, something over 80% of all Einstein robots have been six wheel drives.
This % goes up if you don't count the first few years by the way... recently six wheel drives dominate.

They may not be the "sexy" drive train but they sure work!!

Good luck... We hope to see you in St. Louis!!