I will definitely try to get video, and share it- it's pretty cool to watch; State is tomorrow.
Cost?- let’s see. If you had to go out and buy everything, right at $200- carbon fiber- $35, Bearings- $30, wheels & tires- $25, titanium front axle- $10, Epoxy- $15, Rubberized CA glue- $7, Lexan & Plexi- $10 (bought as scrap pieces fm local plastics company), Balsa- $10, nylon 3/8 threaded rod- $10, gears- $10, T6061-T6 aluminum- $10 (w/ a lot left over), rod & tube for gravity supercharger (stainless steel rods, brass tube that slip-fits over rod)-$10, miscellaneous hardware (screws, nuts, bushings)- $15, a (2+kg) chunk of lead- $5 (fm a metal recycler). Actual cost to us was more like $150, because we had things around from other events/previous years.
Bringing this together was one of the neater/better “design and implement exercises” I’ve been involved in over the years- the guys learned a lot. Good up-front analysis of what scored more points, the factors going in maximizing speed & precision, analysis of materials & properties, constructability analysis (how to get needed level of precision w/ tools available), and adjustability analysis- what things needed to be designed/built to be adjustable, and how to do that. Some really good proof-of-concept/prototyping work early-on, with good design changes and evolution from that, so that when the actual build was done, it…..worked- it was linear, and they didn’t have to circle back and make significant changes.
As one example, the gravity supercharger went through a very interesting evolution. First thing last fall, we put together a half scale, simple test chassis; ¼” plywood chassis plate, disc wheels on bushings. Used some chunks of steel bar for getting the weight up. Basic physics analysis said we wanted as much of the mass starting as high above the floor as possible. Found that with the mass stacked over the back axle, it didn’t want to roll straight at all- major wandering; with it in the middle, though, it did roll straight.
So, I asked the question, “can you think of any way to have the best of both worlds?”- have the mass starts high/to the back, and end up in the middle??. The first answer was a “trapeze.” Two vertical bars up from midpoint with a rod between them at the top; two arms down from that rod, with the steel bar (with a length a bit less than chassis width) hanging on them- pull the steel bar back/up, let it go, would swing down to the middle- put a stop block in to stop it at the middle. Realized two things; that going from around 200gr of moving mass to around 2kg (in final/actual vehicle) would present all sorts of problems in how to get the trapeze strong/stiff enough (without being really heavy), and having the weight end up above the chassis plate did not maximize the downward end of the distance the center of mass could fall.
So, cut a “bay” out of the back half of the chassis plate- so front half was solid, back half was “legs” extending back, with open space between them. Extended the trapeze arms so the weight ended up swinging in at the bottom edge/side of the chassis plate. Did some brain-storming on "how else can we get the weight to move from high/over the back to low in the middle"? First iteration was a plate- a piece of 3/32nds plexi- low end at the front of the bay, upper end above/over the real axle; steel bar weight would slide down it, but would rotate unpredictably on the way down.
From work on our robot arm, we’d found out about how well tubes sliding on rods worked (with tube i.d. just slightly bigger than rod o.d). That led to the configuration we ended up with. Rods (1/8” stainless steel, highly polished) are anchored in lexan blocks mounted on the underside of the chassis plate at the edge of the “bay”; they angle back (at about 25 degrees from horizontal) to a bit behind the rear axle; lexan bracket pieces glued to the inside of the chassis plate “legs” hold/mount the upper/back ends of the rods. With the vehicle on the ramp, the rods are at about 85 degrees from horizontal, so the lead weight falls almost vertically. 2kg falling 10-ish cm is a pretty good shot of energy. The lead weight is held in a framework of ¼” thick lexan pieces, and that framework holds 5/32nds” brass tube pieces (i.d. just a hair bigger than 1/8th”). Getting the rods parallel, and in a plane, and the tubes parallel and aligned with the rods was a challenging exercise in precision, and for this system to work, it has to be precise. But when you do that, even with the vehicle sitting horizontal on the floor, the weight slides freely down the rods. Front face of the weight assembly is shaped/angled so that it gets full/close contact with the edge of the bay. 1/16th thick lexan plate on the edge of the bay; ¼” thick lexan plate on the front edge of the weight provides a good elastic collision when weight meets bay edge (i.e., maximum energy transfer - of the 2kg falling ~10cm -from the collision). The carbon fiber/balsa core sandwich of the chassis plate acts as a very effective sounding board; the “whack” of the weight hitting home really resonates. With the energy shot, the vehicle launches much faster than it does from just a rolling start.
So, especially when you add in the complexities/challenges of the braking system, it is in one sense certainly…..a bit over the top; a lot of time & effort, and a significant cost. The Team 2 vehicle went down a significantly simpler pathway, and ended up with performance pretty close to T1. A very good demonstration of an important concept; once you’re at a “good” level, the incremental “cost” of improvement (time/energy, and $s) tends to really ramp up. Each year, in coaching the building events, I try to end up with a mix of “intensity” put into the various events; for some, the ‘game” is how to get a decent device at minimal time and cost; for some, its “let’s go for it- do something really well.” Depends on who wants to really invest time, and who is fighting the time demands of things other than Science-O. This year, my two Team 1 guys really wanted to go for it on GV from the beginning, and so we did. They ended up with a way cool machine, and they learned a heck of a lot along the way. As a coach, what more can you ask for….. it’s been a lot of fun.
Couple last thoughts, then, on the incremental improvement in speed that the Team 1 guys were able to produce – and the nature of this event.
The improvement – the speed difference between the two vehicles – is absolutely real; enough carefully timed runs on both vehicles now to know for sure there is a 0.2 (at 5m) to 0.4 (at 10m) second difference. Scoring-wise, that’s the same value as 2 to 4cm off-target. But, given the fact of hand timing, the difficulty the folk timing being able to clearly see both start of movement and end of movement, reaction time variations in different people – the unavoidable human factor, a difference of this magnitude is at a high risk of getting lost in actual competition timing and scoring. I’m quite certain that at many competitions across the country, the time scoring component scores/places were different than “reality.”
But, as I’ve said before, there is no practical way to get precise, actual times. So, there is an inherent, unavoidable “roll of the dice factor” in the event, and where it comes into play the most is at the “top end”- those little differences that could/should be ‘the winning edge.” Even given this, though, I have to say,I think it’s a neat event.
Last thought is on what might be done next year. Precise timing being impractical, how else might you more precisely measure/score the “speed” factor? It comes from and is dictated by how well you do two things; a) maximize the velocity off the ramp, and b) minimize the friction loss rate through the run. Those same two factors happen to determine a precisely measurable value – how far the vehicle can roll. Out of curiosity, we did this test on our vehicles last weekend- T1 goes a bit over 30m; T2 is a bit over 20m. If (for space practicality), the ramp height were reduced a bit, and maybe the weight reduced some (less momentum = less distance capability), a ‘total distance capability’ factor could be used as a practical and precise scoring factor. Just a thought….
Fort Collins, CO