971's chassis

[highlander]

Originally Posted by Bryce2471

If you don't mind, I've got a few questions.

1. How much does the assembly weigh?
2. How much is the drop center?
3. What is the gear ratios on the drive gearbox?
4. What is the diameter of you wheels?
5. What is the thickness of your sheet metal?

Thanks in advance!

Take a look at previous replies:

-We use mostly 060 on the robot, however the drivebase was half 060 and half 090. Any sheet metal that could be probably impacted by other robots (bellypan, outer rail, and maybe one or two others that I can't remember) were 090. The rest was 060 to save weight. This year was really brutal and both chassis still looks like they are new (probably helps that the bumpers also held up very well).

[AustinSchuh]

I'll take the easy ones. One of the students can get the harder ones. We have weekly Wednesday meetings, so I'll have someone work up some answers then if nobody does before.

Originally Posted by Bryce2471

2. How much is the drop center?

3/16"

Originally Posted by Bryce2471

4. What is the diameter of you wheels?

3.5" pitch diameter. We had one of the students roll a wheel a lot of revolutions and see how far it went to figure out the pitch thickness (is that even a real term?) of the tread.

[switte]

Originally Posted by Bryce2471

If you don't mind, I've got a few questions.

1. How much does the assembly weigh?
2. How much is the drop center?
3. What is the gear ratios on the drive gearbox?
4. What is the diameter of you wheels?
5. What is the thickness of your sheet metal?

Thanks in advance!

Based upon our CAD, our drivetrain is at 66.5lbs with battery, electronics, transmissions, etc., 39.3lbs without electronics but with transmissions, and 13.6lbs with only the sheetmetal (belly pan, battery box, and the two siderails).

We have a theoretical free speed of 16.4fps high gear and 8.8 fps low gear.

[roystur44]

Originally Posted by AustinSchuh

We use countersunk rivets. There are punches on the CNC turret punch which will make a .129 hole that is countersunk to 120 degrees, all in 1 hit. This makes it so the bottom, front, and sides of our bot are perfectly smooth and can't catch on anything.

The tool actually makes two hits. A prepunch hole and the tool that forms the csk. Metal is malleable. Here is a link to show some cool things you can do on a punch press. Take a look at the stiffening rib tool and the Zip Tech.



http://www.wilsontool.com/ProductSub...ng_p_spcl.aspx

[Thad House]

Why do you guys run 3/16 drop? I've heard 254 does this as well. With the smaller chassis, we've found that you can completely remove the drop and still have just as great of turning. Our's was actually built for a 0.1 inch drop, but because of the wear on the center wheels after a few matches our drop was basically non existent, and it seemed like our turning and pushing actually improved when this happened. Looking at Chris Hibner's old paper on turning physics, it actually makes sense because with shorter chassis the effective wheelbase is almost identical to the dropped wheelbase on the old size.

[AlecMataloni]

Originally Posted by Thad House

Why do you guys run 3/16 drop? I've heard 254 does this as well. With the smaller chassis, we've found that you can completely remove the drop and still have just as great of turning. Our's was actually built for a 0.1 inch drop, but because of the wear on the center wheels after a few matches our drop was basically non existent, and it seemed like our turning and pushing actually improved when this happened. Looking at Chris Hibner's old paper on turning physics, it actually makes sense because with shorter chassis the effective wheelbase is almost identical to the dropped wheelbase on the old size.

From my understanding, Blue Nitrile conveyor tread from mcmaster is very "grippy" and doesnt wear fast. Depending on their wheelbases AND tread width, teams like 1323/254/971 etc use 3/16 drop to minimize wheel scrub. This is because of their previous experiences designing drivetrains utilizing the material.

[AustinSchuh]

Originally Posted by Thad House

Why do you guys run 3/16 drop?

To be honest, most of it is because we've always done it that way and it hasn't caused us any problems. We ran less drop a number of years ago (2008 ish), and found that because both the front and back wheels could catch the carpet at the same time, the robot didn't handle as consistently. It works well enough, so we haven't messed with it. Nothing is worse than a poor handling robot or a robot that can't turn well.

[Thad House]

Originally Posted by AustinSchuh

To be honest, most of it is because we've always done it that way and it hasn't caused us any problems. We ran less drop a number of years ago (2008 ish), and found that because both the front and back wheels could catch the carpet at the same time, the robot didn't handle as consistently. It works well enough, so we haven't messed with it. Nothing is worse than a poor handling robot or a robot that can't turn well.

Ah OK. I was just wondering if you had tried changing with the smaller chassis. We plan on doing alot more testing with this over the summer, because if we can get rid of the rocking and still turn just as well we might just have to do that.

And I remember back in the old days where if you didn't have the drop or too little that think would hop like there was no tomorrow. Both the 2011 and 2012 robots I helped build didn't have enough and they didn't turn well at all.

[Ingar]

I feel like I'm missing some huge detail; I can't seem to figure out the purpose of the second hole on the tensioner. Would anybody happen to know what the non-bushing containing hole is for?

[James Kuszmaul]

Originally Posted by Ingar

I feel like I'm missing some huge detail; I can't seem to figure out the purpose of the second hole on the tensioner. Would anybody happen to know what the non-bushing containing hole is for?

The hole itself doesn't serve a particular purpose (other than lightening, which is always good). But, by doing it this way, the tensioner blocks on both sides of the wheels are identical, thereby reducing the total number of unique parts in the robot. And minimizing the total number if unique parts we have (even if it doesn't reduce the total number of parts) is important to us, especially when we are having to send them out to our sponsors. Our sponsors have generally cited the number of unique parts as a more important factor than quantity when we talk to them about how many parts they can make for us.