Examples of when it is not advantageous to be lightweight

Aside from the obvious loss of friction due to a decreased normal force affecting pushing ability, I cannot think of any specific examples in FRC history when it has been disadvantageous to be as light as possible. Is there anything I am missing, and if so, could you cite a specific match that shows this weakness due to weight? Also, is there a limit with weight where, like adding motors, you reach a point where it becomes less and less advantageous to become lighter?

I appreciate all input.

[madhav]

2012 Bridge Balancing issues?

I'm tired, I'll get back to you tomorrow

[Travis Schuh]

I think your implicit assumption is that being lighter gives you a more maneuverable robot by decreasing acceleration time.

If your goal is to make your robot more maneuverable, then I could see taking weight out of your robot at the cost of raising your CG above an acceptable height resulting in a net decrease in maneuverability. If there is no option to lower CG through re-arranging components, then it may make sense to ballast the robot. It also helps if the CG is closer to the center of the robot for best handling. I think for these reasons contributed to 254 ballasting their robot this year.

There are plenty of matches where teams either outright tipped or had to drive cautiously because they were tippy (you asked for specific matches, I would say watch some of 973's 2013 matches). I bet many of these teams would have added ballast if they had weight.

Originally Posted by Travis Schuh

I think your implicit assumption is that being lighter gives you a more maneuverable robot by decreasing acceleration time.

If your goal is to make your robot more maneuverable, then I could see taking weight out of your robot at the cost of raising your CG above an acceptable height resulting in a net decrease in maneuverability. If there is no option to lower CG through re-arranging components, then it may make sense to ballast the robot. It also helps if the CG is closer to the center of the robot for best handling. I think for these reasons contributed to 254 ballasting their robot this year.

There are plenty of matches where teams either outright tipped or had to drive cautiously because they were tippy (you asked for specific matches, I would say watch some of 973's 2013 matches). I bet many of these teams would have added ballast if they had weight.

Part is maneuverability, though the main reason is I just can't think of a reason to be heavy. 1323's resources make machining for weight easy enough, and I think it would be a useful engineering goal for the kids to shoot for in season to design for a lightweight yet strong robot if the game allows for a lightweight robot as a viable strategy. Also it would be a good way to allocate weight lower into the drivetrain to make a lower CG (which has been a clear problem for us this year).

One of the first things many mentors I have learned from tend to tell me is how to lighten a robot and that "lighter is better" (not always true, but it's a point that has been stressed enough to me in my education that I started this thread because of it, though further learning could prove differently) and while I understand the potential advantages of a lower weight, I cannot think of many reasons for increased weight. More mass in a robot just makes it harder to move, and I don't see any advantages to that, and want to learn what I may be missing.

[Kevin Sheridan]

When I was on 766 we made our robots as heavy as possible in 2006 and 2007 so we would not tip going up/down the ramps. I saw a lot of robots tip in 2006 especially because they were too top heavy. A common tactic that year was to shove top heavy robots up your own ramp on defense so they would risk tipping trying to come down the ramp during teleop.

[nathannfm]

Originally Posted by Andrew Lawrence

One of the first things many mentors I have learned from tend to tell me is how to lighten a robot and that "lighter is better" (not always true, but it's a point that has been stressed enough to me in my education that I started this thread because of it

I think this is always true of subsystems, including the drive, until you start to sacrifice structural integrity, but not true for the robot as a whole. When I say "lighter is better" it is usually because if it's not stressed you get to week 5 with a 150lb robot and a major sub system has to be removed because there is no time to redesign all of them to be lighter. On MOE we always have 120.00lb robots because we usually shoot for maximum functionality (do all the things!) and this usually requires more weight than a robot specialized to do a specific task. We embrace this so much that if we get to week 4 or 5 and realize we have 4lb to spare we try to think of a way to use that 4lb to make our job easier, more reliable, or faster.

[Michael Hill]

Originally Posted by nathannfm

I think this is always true of subsystems, including the drive, until you start to sacrifice structural integrity, but not true for the robot as a whole. When I say "lighter is better" it is usually because if it's not stressed you get to week 5 with a 150lb robot and a major sub system has to be removed because there is no time to redesign all of them to be lighter. On MOE we always have 120.00lb robots because we usually shoot for maximum functionality (do all the things!) and this usually requires more weight than a robot specialized to do a specific task. We embrace this so much that if we get to week 4 or 5 and realize we have 4lb to spare we try to think of a way to use that 4lb to make our job easier, more reliable, or faster.

Definitely agree with the structural integrity comment. I would much rather use a thinner material before I use lightening holes. Remember the saying "only as strong as the weakest link"? That applies to structures. Your robot is only as strong as the smallest cross-sectional area. When you have lightening holes, especially ones that are close together, you create a distinct point of failure. If you just design it with thinner material from the start, you can likely avoid lightening holes altogether and maintain a larger cross-section than you would have with a lightened structure.

[MichaelBick]

Originally Posted by Michael Hill

Your robot is only as strong as the smallest cross-sectional area. When you have lightening holes, especially ones that are close together, you create a distinct point of failure.

In compression, strength is correlated with geometry not cross-sectional area.

[Michael Hill]

Originally Posted by MichaelBick

In compression, strength is correlated with geometry not cross-sectional area.

I'm not sure I follow the logic here. I'm saying use the same shape, just a thinner material. Also, I would also say that compressive strength IS related to cross sectional area. It uses the same equations as in tension. Also, in terms of buckling, the point at which something will buckle is related to the cross-sectional area.

[cadandcookies]

I would distiguish between designing the robot to be as light as possible and actually being as light as possible.

Designing to be light* is always a good idea, for the simple reason of it being significantly easier to add weight than remove it.

Whether actually being light is a benefit depends on the game and your team's strategy in that game.

*To paraphrase "As light as possible but no lighter."

[JamesCH95]

Originally Posted by EricH

[snip]
That said... maybe one of the REAL old-timers on here can give us a rundown of the classic award, "Flyweight in the Finals"!

Ugh, I'm not a REAL old-timer... but I do remember "Featherweight in the Finals" that our iconic robot, Grace Hopper, won in 2000. If memory serves (and it might not) Featherweight in the Finals was awarded to the lightest robot in either the final bracket or all of the eliminations.

Originally Posted by Michael Hill

I'm not sure I follow the logic here. I'm saying use the same shape, just a thinner material. Also, I would also say that compressive strength IS related to cross sectional area. It uses the same equations as in tension. Also, in terms of buckling, the point at which something will buckle is related to the cross-sectional area.

Sort of... it's related to the cross-section inertia of the beam, strictly speaking, not the area. Compressive stress IS related to the XC area, but generally speaking this is very rarely the limiting factor.

[yash101]

i want to do pathfinding this year, and I've figured out that it would be extremely important for the robot to by capable of changing directions quickly and accelerating quickly, so that the robot will be able to follow the path and change paths with almost no smoothing and any overhead of acceleration time.

I think that this means that the robot will need:
-Tons of power in the drivetrain
-Lightweight
-COG: Center, at the bottom!

[JamesCH95]

Originally Posted by yash101

i want to do pathfinding this year, and I've figured out that it would be extremely important for the robot to by capable of changing directions quickly and accelerating quickly, so that the robot will be able to follow the path and change paths with almost no smoothing and any overhead of acceleration time.

I think that this means that the robot will need:
-Tons of power in the drivetrain
-Lightweight
-COG: Center, at the bottom!

What you have here is a great recipe for spinning wheels. Would that make accurate path-finding more or less difficult to implement successfully?

Bonus question: how does wheel tread selection play into your goals?

[MichaelBick]

Originally Posted by Andrew Lawrence

Also it would be a good way to allocate weight lower into the drivetrain to make a lower CG (which has been a clear problem for us this year).

Drive size has a lot more to do with this than your drive weight.

[scaryone]

We added weight to the base of this year's bot to avoid tipping over. (top-heavy)