I was cadding up our last years frame up and a thought hit me. How many pounds of force actually hits your bot? Weak hits, medium hits and the hardest hit possible. So i need some help finding out the forces.
Thanks
CD
I would like to think so… and hard, harder, and hardest
mike d
The speed at which the collision occurs, as well as the angles and location of the hit, and how effective the bumpers are, etc will play a big part. For weight, figure 150 lbs to make it easy.
Some materials absorb impacts better than others…for example, fiberglass tends to bounce back, while some aluminum shapes get bent easily.
Thatz what i was figuring, but i wasn’t sure. What do you think composite sides
I don’t know the forces involved, but there are dents on the front of our robot that would be tough to make with a hammer.
Our hardest hits this year were when hybrid went awry. 15 ft/s into the wall is tough. Turns out the wall is a flexible member. Other than that, bumpers help a lot. We have some pretty vicious scars from years past.
As a little bit of how bad can it really get, I recently saw a vehicle go to 10 G’s during a 5mph impact with a wall. 15 ft/s is almost 10mph. For a little math let’s assume that your robot collides with a stiff wall at 3 m/s (approximately 10 ft/s). bumpers are about 3 inches thick or rough .1m Let’s assume the bumpers are perfectly balanced and do an ideal deceleration.
Therefore d=Vt-1/2at^2 which is really d=Vt-1/2Vt therefore d=1/2Vt
or t=2d/V=.2/3=1/15 second.
V=at therefore a=45m/s^2 or about 4.5 G’s. This would be if the bumpers had perfect crush causing constant deceleration. If you assumed a linear rate of decel from 0 then the peak will get closer to 10G’s. You can continue playing this game and second guessing it quite a bit. Overall having it survive a 10 G impact is probably pretty reasonable. You shouldn’t see a lot of those, but you may likely see a couple a season.
I agree, those robots can take a beating! But, no matter how many numbers you run, your probably never come close to those freak incidents. So, always throw in a hefty safety factor.
We didn’t use bumpers our 07 season and all the chassis aluminum is bent in.
One thing to keep in mind is that dents and dings aren’t always a bad thing. They absorb impact energy that could go elsewhere. If your robot is still fuctional consider it good energy management. If your robot was incapacitated. Make it stronger.
we did our robot with an 80/20 frame this year… the same chassis lasted from NJ to Chesapeake to atlanta. our hardest hit was in atlanta first elimination match our autonomous goes crazy and we hit the wall soo hard the entire wall is moved… the bumpers arent there our robot steering mechanism is toast… turns out the force angled one of the beams so the connecting point to the main chassis went from 90 degrees to say 65-70… neways our robot weighed 130 with bumpers and top speed which is what it hit the wall at was 25fps.
other than that the 80/20 frame has held up very well.
Squirrel from 1726 can attest to the toughness if 1662’s frame. We were pretty busted up in 05 & 06. We totally redesigned the frame. We have ran it into a wall at full speed to test it. It passed. Pic is on CD
I’m not sure about force, but at the Pittsburgh regional hitting another robot caused our 1/4 in plexi to shatter into 5 pieces.
oooh, any pics?
Was the “plexi” acrylic or polycabonate (Lexan)?
The reason I ask is that while they look similar, acrylic can shatter like glass, whereas polycarbonate is much stronger and normally it should never shatter (unless you spill Loctite on it). The only “downside” to polycarbonate is that it is more expensive than acrylic.
On both the teams that I mentor, acrylic is almost never used on a FIRST robot with only a few very limited exceptions, as polycarbonate is the preferred “plexi” material.
For the good of the order (since I can’t think of any such cases), what are those limited exceptions where acrylic is used? (1618 used it as side panels its rookie year, but wised up quickly following that.)
We used acrylic last year to cover up our electrical board. Lexan would probably have been better. The plexiglass cracked due to improper hole drilling. It worked though. If you saw last year’s robot you would understand. 
-Vivek
EDIT: Our frame has always been plenty strong for us. I don’t care what anyone says, the kitbot frame is a beast.
We used the kitbot frame this year and it held up well. The frame suffered virtually no damage, but our bumpers sure got ripped up at the Kettering Rookie Competition (the judges at Buckeye were very surprised by how much damage we had from it; it was brutal, and we were tipped a few times with no penalty called).
However, I know it can withstand a good blow without bumpers since I was test driving it after we/I got the drive code running, and I went in full reverse . . . into a metal column in our school lobby. The battery casement came off (secured with one screw at that time, but that was fixed), but the pole suffered a chip and the frame had no damage.
On 467 we used a unibody design this season that used the skin to support the internal components without a frame. it worked pretty well as it was light weight and durable. It took the force of being flipped twice at the Connecticut
regional without any dents or damage, our bot was so light that we had to bolt
a 14.1 pound steel plate to the bottom to stabilize it.
Frame, pre-construction:
http://www.team467.org/gallery/main.php?g2_itemId=512177
Final Bot:
http://www.team467.org/gallery/main.php?g2_itemId=512365
I’m pretty sure it was polycarbonate but i’m not sure. It didn’t shatter exactly but it cracked apart.
Good way to distinguish between polycarbonate and plexiglass:
Place a corner of it on the end of a table, clamp down the other side, and have a good whack at it with a hammer (hard). If it didn’t break, it was polycarbonate.
-Vivek