Gravity Vehicle C

[iwonder]

Here's a question that I hope is pretty simple... I got my shafting in today, I've had the bearings for a while, I mic'd the shaft, it's round and .1250" in diameter, I got the bearings from a pretty large manufacturer of bearings(sdp/si), they're abec rated, etc, so I trust that they're .125 id, but for some reason the shaft won't fit into the bearing... I've chamfered the ends, and the shaft has no burrs, but the bearing doesn't slide on the shaft like I would expect... any thoughts?

The problem is that both are extremely close to 0.1250. It can not fit, not even a press fit. Machine shop basics are the shaft needs to be 0.003" smaller than the bearing for a press fit if you have a good method of doing it so that you do not ruin the bearing. 0.004 gets closer to reality for most, me included. This is from memory, most large bearing Mfg's have a help section for better guidance.

Yep, put it in a drill press and ran some emery cloth on it, fit's perfectly now, thanks!

[Balsa Man]

by chalker on Tue Oct 30, 2012 8:47 am
XJcwolfyX wrote: Chalker, do you know why the height was weighted so heavily?

In order to make it significantly different from last year and to increase the challenge for the top end teams, while not increasing the difficulty for the majority of the rest of the teams.

bearasauras on Wed Oct 31, 2012 4:09 pm
Lalaloopsy wrote:
iwonder wrote:Ahh... Oh, and I did the math for ramp height myself, I got the same results as everyone else it seems(I included friction in there) that an insanely low ramp yields a much lower score... for example, a ramp height of .8m(the lowest height I calculated that would make the distance) and distance of 10m would give a score of about 190, whereas a 2m ramp and 10m distance would yield 822

I don't know how the lowest height to go 10m was .8m tall since I tested our car from last year and it was successful in going 10m from a height of only approx. .3m and our car had a lot of rolling friction before braking.

Yes, but how was the score based on the time? As you start at a lower height, you travel slower and so the time is longer. I think this is one where your actual testing will be really important

=====
Some very interesting things, and a number of them, going on here. Some very interesting implications.

Significantly different than last year, for sure. Challenge increased, absolutely. Multiple new factors that need to be considered and optimized if you’re seriously going after best possible scoring. Actual testing needed to figure out an….optimal solution, yup. Not increasing the difficulty for the majority of the rest of the teams…..yes and no…

Love this stage of the game- figuring out what’s going on; what are the underlying physics; what are the variables; what’s their relative magnitude; how do they interact; what are the trade-offs; what does the ideal/optimal solution look like; what would it take to create/build it; what are the…..available resources (money, and time); what (competitive) level you want to, and are able to play at.

First analysis looked pretty simple- the guys who did gravity vehicle last year are back this year, so we don’t have to start from scratch. Very competitive results last year; vehicles pretty much good to go; ramp work needed to deal with the new rules. A little saw work to trim ramp width down- cake. A little testing to get to a…..significantly lower launch height that optimizes height score. Put together a spreadsheet that captures time (T) vs (target) distance (D) (from last year)- at a starting elevation (H) just under 1m (actually 98cm), put in a set of lower ramp heights, with quick and dirty time adjustment for slower Ts for lower Hs (first cut on T adjustment, simple linear- ½ the height, twice the time….set it up to calculate total score-T score + H score, + distance (off target point) score (in last year’s range), + predicted vs actual (in last year’s range).

What that shows, is that the optimal height (lowest combined points) is different for different target distances. As you increment H down (reducing your H score), T score increases (at 50 pts/sec), and there is a “cross-over point”, where the increased T score ….overwhelms the decreasing H score; with lowering Hs, total score goes down, but you reach an H where it starts to go back up. That point, in terms of H is different for different Ds. In terms of H, cross-over point is higher (H) for longer distances. There also is, of course a boundary condition- the, let’s call it overall energy appetite [ a(e) ]; a combo of friction and energy loss factors (rolling resistance, bearing friction, brake system drag, etc); a minimum H at which the vehicle is able to get to a given D. The implication is that the launch height needs to be adjustable to optimize score. The numbers will depend on your vehicle’s a(e).

So, same as last year, minimizing a(e) is a critical requirement for good scoring. Where testing is needed, of course, is to nail the time, at various distances, at various launch heights; don’t think its truly linear.

In addition, the lower max mass (m) this year means momentum at whatever velocity (v) is lower, so whatever you’re a(e) is will eat into/degrade v faster; at a given H, T to a given D will be longer

Second order implication is in type of braking system. With a ~1m H (last year), with a decent a(e), and the momentum of 2.5kg, the scoring effect of the drag of a wingnut braking system was relatively small; with less H, therefore less V, and less m (so less momentum), the scoring effect of the drag of a wingnut braking system will be significantly bigger- lower drag braking system solutions will “pay off.”

There’s also an implication for the “gravity supercharger” we implemented last year (if anyone's thinking about throwing that in) – a moving mass that starts at H, slides down into the vehicle, w/ vehicle released to roll just as it …joins the vehicle. At an H of 1m, it picks up/adds the kinetic energy of the mass falling ~8cm; with an overall H ~1m, ~8%. With H at, say 0.5m, the percent of the energy of that 8cm fall (vs the kinetic energy of the system- the vehicle) roughly doubles; as H goes lower, contribution/gain from the moving mass increases. Lower max m for vehicles this year means m of the moving mass will be less, and hence it’s energy will be less, but it’s still…..free additional energy into the system. It’s a pain to implement, but will definitely “pay off” at Hs <0.5m.

So, hats off to “the boys in the back room at home office”: this year; challenges to the top teams have indeed been increased!

[wlsguy]

.....There’s also an implication for the “gravity supercharger” we implemented last year (if anyone's thinking about throwing that in) – a moving mass that starts at H, slides down into the vehicle, w/ vehicle released to roll just as it …joins the vehicle. At an H of 1m, it picks up/adds the kinetic energy of the mass falling ~8cm; with an overall H ~1m, ~8%. With H at, say 0.5m, the percent of the energy of that 8cm fall (vs the kinetic energy of the system- the vehicle) roughly doubles; as H goes lower, contribution/gain from the moving mass increases. Lower max m for vehicles this year means m of the moving mass will be less, and hence it’s energy will be less, but it’s still…..free additional energy into the system. It’s a pain to implement, but will definitely “pay off” at Hs <0.5m. .....

I'm not a rules expert but I'm not sure if this would be legal this year. The addition of "Entire" vehicle to 3b could be viewed to prevent vehicle and weight systems that split in 2 pieces. Also, since the vehicle must be powered only by the gravitational energy of the vehicle (3b), the highest point of the mass would be likely used for the height measurement since it is / will be part of the vehicle.

In any case, if you plan to use such a system, I recommend submitting a clarification..

[Balsa Man]

.....There’s also an implication for the “gravity supercharger” we implemented last year (if anyone's thinking about throwing that in) – a moving mass that starts at H, slides down into the vehicle, w/ vehicle released to roll just as it …joins the vehicle. At an H of 1m, it picks up/adds the kinetic energy of the mass falling ~8cm; with an overall H ~1m, ~8%. With H at, say 0.5m, the percent of the energy of that 8cm fall (vs the kinetic energy of the system- the vehicle) roughly doubles; as H goes lower, contribution/gain from the moving mass increases. Lower max m for vehicles this year means m of the moving mass will be less, and hence it’s energy will be less, but it’s still…..free additional energy into the system. It’s a pain to implement, but will definitely “pay off” at Hs <0.5m. .....

I'm not a rules expert but I'm not sure if this would be legal this year. The addition of "Entire" vehicle to 3b could be viewed to prevent vehicle and weight systems that split in 2 pieces. Also, since the vehicle must be powered only by the gravitational energy of the vehicle (3b), the highest point of the mass would be likely used for the height measurement since it is / will be part of the vehicle.

In any case, if you plan to use such a system, I recommend submitting a clarification..

With all due respect- truely;
a) All students and coaches certainly have an.....obligation to carefully read and understand the rules for any event they’re involved with. Don't know where that transitions to "expert." if you're serious about being competitive, you design, build, and perform as well as the rules allow.
b) I understand your uncertainty. But, of course, you only have a limited description on how this works to go on- the very brief summary I included, a few more detail pick-ups below; don’t know if you’ve checked out the bit more detailed description in posts last year; but even there I haven’t provided, and for obvious reasons am not going to provide, detailed drawings, or clear photos....

c) I also totally agree with Chalker’s earlier advice about pushing the limits, and getting official clarification “unless you are 100% certain whatever you are doing is legal because of either a rule or something official in writing”

d) In this case, I am actually comfortably at that 100% level, in the way we’ve implemented it. There may well be ways using a moving mass could push/cross the limits (of the rules- 3b, and all others), but we’re not doing that. If what anyone else may be doing/considering leaves them uncertain, I would also recommend appropriate formal clarification.

With that said, if someone can see a hole in the following analysis; a basis for filing a protest based on a specific violation of any rule, I’d love to hear it…

Running through the rules that are, or may be potentially relevant:

There are “Construction Parameters” (rules 3a through 3j – how the vehicle has to be built/designed. There are then “The Competition” rules (rules 5a through 5p) – how the competition is to be done. Last, there are “Scoring” rules (rules 6a through 6g)

First, there is no general prohibition – anywhere, against moving parts per se on the vehicle, as long as they don’t fall off during the run (5e). Obviously, all vehicles have some moving parts, and some level of relaative motion between certain parts,

Second, there are no restrictions/limitations/or objective or subjective constraints on the absolute or relative movement of parts (that are attached to/ are part) of the vehicle in the Construction Parameters (rules 3a through 3j

The context of ‘the entire vehicle” in 3b is simply and only that the entire vehicle- implicitly all it’s parts- must start from a non-horizontal position, on the ramp. The wheels must be up on an inclined ramp. The term “entire vehicle’ is not used anywhere else in the rules. I suppose one could seek clarification on, “does the entire vehicle mean all it’s parts?”, but I’m comfortable with the obvious answer to that.

Also in 3b, and certainly important to look at, we have the construction parameter/rule, “all energy used to propel the vehicle must come from the gravitational potential energy derived from the mass of the vehicle.” The weight is (a securely attached) part of the vehicle; it’s mass is part of the mass of the vehicle; the energy it contributes to the vehicle is clearly (and only) “gravitational potential energy”, and it is clearly derived from the mass of the vehicle. The position of the mass before starting simply means that the center of mass of the vehicle is higher off the ground than it would be if the (moving) mass were down in the chassis (where it ends up); higher starting center of mass (and lower center of mass when it rolls off the ramp) simply maximizes the gravitational potential energy available to be harnessed from the start position.

Last, there is a requirement in 3b for “a release mechanism to hold the vehicle in place…”. Beyond fitting within dimensional limits, holding the vehicle in place (prior to it starting to move down the run), and that it be activated by a pencil, there are no further constraints on the “mechanism.” No limits on simplicity, or complexity; number of parts, number/nature of energy transfers between the process of moving the pencil, and the release of the vehicle to start moving. Obviously, for any release mechanism to work, it has to have moving parts. So, we have a multi-stage release mechanism. Pencil moves a piece on a bracket mounted to the ramp; that frees the mass to start sliding down. Vehicle is held in place on the ramp by a moveable piece (a lever, attached to the vehicle) on the underside of the vehicle. Vehicle doesn’t start to move until the moving mass engages – whacks into) the chassis; when the vehicle starts to move, the mass is down in the chassis, no longer moving around relative to the rest of the car. Technically, there’s a maybe a couple thousandths of an inch movement of the mass left/happening at the time movement of the vehicle is just starting. If there is a prohibition against ANY relative motion of any parts at these kinds of tolerances, then all vehicles would have to get DQd – axles sliding in bushings/bearings sliding on axles being the obvious. There is, of course, no such prohibition – for obvious reasons - anywhere in the rules, so, it’s a non-issue.

No ‘transfer of gravitational potential energy into an elastic devices’ (3c), is going on.

So, that’s all the rules that speak to design/construction. Nothing in violation, nothing pushing any rule limits; everything fits to what the rules say.

Let’s go on to the Competition rules. The obvious potentially relevant rule is 5e-“all parts of the vehicle must move as a whole.”
Given that there is some level of (even though small to very small) relative motion between a number of parts on ALL vehicles, the fact of such relative motion is obviously not intended to ….go outside “all parts moving as a whole.” Some level of relative motion is obviously within the intent of the rule. With our implementation, once the vehicle starts to move (when the release mechanism has…..gone through it’s movements and energy transfers, and the mass is snuggly into the receiving bay), there is no relative motion (within the intent of the rule) between the mass and vehicle- all parts do, in fact move as a whole.

Rule 5m says "vehicle height is defined as the highest point of the vehicle in the ready to launch position', 6c says vehicle height is used for H scoring, so the take that H for scoring would be the high side of the weight, before it is released, is indeed correct.

So, defense rests. Any rebuttal testimony from the prosecution??

[wlsguy]

I have no argument with your latest explaination.

The original statement included "down into the vehicle, w/ vehicle released to roll just as it …joins the vehicle".
This implys the mass is NOT part of the vehicle and would be prohibited (by 3b "entire vehicle" and all energy must come from the mass of the vehicle)

Now, if your intent is to start the mass at the highest point within the vehicle and allow it to fall (within the vehicle) to a desired point in the vehicle in order to gain the maximum amount of energy from the fall, then this is likely not a rules violation (nothing says the mass cannot move within the vehicle).

[Balsa Man]

I have no argument with your latest explaination.

The original statement included "down into the vehicle, w/ vehicle released to roll just as it …joins the vehicle".
This implys the mass is NOT part of the vehicle and would be prohibited (by 3b "entire vehicle" and all energy must come from the mass of the vehicle)

Now, if your intent is to start the mass at the highest point within the vehicle and allow it to fall (within the vehicle) to a desired point in the vehicle in order to gain the maximum amount of energy from the fall, then this is likely not a rules violation (nothing says the mass cannot move within the vehicle).

Cool, we're good- do understand you raising your questions. The mass is very much an integral, completely attached at all times, part of the vehicle- it's just free to move, but only "within" the vehicle, in a.....helpful manner. Like i said in initial msg, the benefit is relatively small at H =1m; but when launch height goes down, its proportional contribution to total energy increases.

[bearasauras]

Some very interesting things, and a number of them, going on here. Some very interesting implications.

Significantly different than last year, for sure. Challenge increased, absolutely. Multiple new factors that need to be considered and optimized if you’re seriously going after best possible scoring. Actual testing needed to figure out an….optimal solution, yup. Not increasing the difficulty for the majority of the rest of the teams…..yes and no…

Love this stage of the game- figuring out what’s going on; what are the underlying physics; what are the variables; what’s their relative magnitude; how do they interact; what are the trade-offs; what does the ideal/optimal solution look like; what would it take to create/build it; what are the…..available resources (money, and time); what (competitive) level you want to, and are able to play at.

First analysis looked pretty simple- the guys who did gravity vehicle last year are back this year, so we don’t have to start from scratch. Very competitive results last year; vehicles pretty much good to go; ramp work needed to deal with the new rules. A little saw work to trim ramp width down- cake. A little testing to get to a…..significantly lower launch height that optimizes height score. Put together a spreadsheet that captures time (T) vs (target) distance (D) (from last year)- at a starting elevation (H) just under 1m (actually 98cm), put in a set of lower ramp heights, with quick and dirty time adjustment for slower Ts for lower Hs (first cut on T adjustment, simple linear- ½ the height, twice the time….set it up to calculate total score-T score + H score, + distance (off target point) score (in last year’s range), + predicted vs actual (in last year’s range).

What that shows, is that the optimal height (lowest combined points) is different for different target distances. As you increment H down (reducing your H score), T score increases (at 50 pts/sec), and there is a “cross-over point”, where the increased T score ….overwhelms the decreasing H score; with lowering Hs, total score goes down, but you reach an H where it starts to go back up. That point, in terms of H is different for different Ds. In terms of H, cross-over point is higher (H) for longer distances. There also is, of course a boundary condition- the, let’s call it overall energy appetite [ a(e) ]; a combo of friction and energy loss factors (rolling resistance, bearing friction, brake system drag, etc); a minimum H at which the vehicle is able to get to a given D. The implication is that the launch height needs to be adjustable to optimize score. The numbers will depend on your vehicle’s a(e).

So, same as last year, minimizing a(e) is a critical requirement for good scoring. Where testing is needed, of course, is to nail the time, at various distances, at various launch heights; don’t think its truly linear.

In addition, the lower max mass (m) this year means momentum at whatever velocity (v) is lower, so whatever you’re a(e) is will eat into/degrade v faster; at a given H, T to a given D will be longer

Second order implication is in type of braking system. With a ~1m H (last year), with a decent a(e), and the momentum of 2.5kg, the scoring effect of the drag of a wingnut braking system was relatively small; with less H, therefore less V, and less m (so less momentum), the scoring effect of the drag of a wingnut braking system will be significantly bigger- lower drag braking system solutions will “pay off.”

There’s also an implication for the “gravity supercharger” we implemented last year (if anyone's thinking about throwing that in) – a moving mass that starts at H, slides down into the vehicle, w/ vehicle released to roll just as it …joins the vehicle. At an H of 1m, it picks up/adds the kinetic energy of the mass falling ~8cm; with an overall H ~1m, ~8%. With H at, say 0.5m, the percent of the energy of that 8cm fall (vs the kinetic energy of the system- the vehicle) roughly doubles; as H goes lower, contribution/gain from the moving mass increases. Lower max m for vehicles this year means m of the moving mass will be less, and hence it’s energy will be less, but it’s still…..free additional energy into the system. It’s a pain to implement, but will definitely “pay off” at Hs <0.5m.

So, hats off to “the boys in the back room at home office”: this year; challenges to the top teams have indeed been increased!

Excellent summary! I can't agree with you more on the challenge this year and how it will be crucial to find that optimal height for each target distance.

[Balsa Man]

Excellent summary! I can't agree with you more on the challenge this year and how it will be crucial to find that optimal height for each target distance.

Thank you, sir. An interesting set of things to think through.

Small but important update/correction to something I said yesterday. Doesn't change the need to find optimal height for each target distance, just changes the ...range of variability.
I was thinking yesterday after I threw the spreadsheet together there was something wrong in how I treated the time (T) adjustment for lower ramp height (H) - (1/2H = 2xT...). There was.
Velocity(which drives T) off the ramp (V) is a function of the square root of gH. So, 1/2H = 1.414T.

With correct (theoretical) T adjustment, the cross-over points for Hs for varying distances are still there, but the range of Hs over which the cross-overs occur are narrower. Also added a look at slower Ts (because with lower mass this year, Ts are going to go up some).

For instance, using a time of 3.1 sec to 8m - at a 48cm H, score is 207 (using a dist off of 20mm, and 0 for estimated time off); at 38cm, score is 202.67; at 33cm, it is 203.21- somewhere between 38cm and 33cm, score starts to increase....

One great thing about this year's changes - slower velocity/longer run times mean the wild card factor of hand timing (from which your time accuracy score factor comes) will be proportionately smaller. That is a good thing. Still a wild card, unfortunately, but.....tamed a bit.

[chalker7]

One great thing about this year's changes - slower velocity/longer run times mean the wild card factor of hand timing (from which your time accuracy score factor comes) will be proportionately smaller. That is a good thing. Still a wild card, unfortunately, but.....tamed a bit.

Just to let you know, we are well aware of the hand timing issue. In fact, it is a highly contentious and long-standing disagreement among several of us in the whole rules process. There are two major reasons electronic timing isn't in the majority of the events.
The first is that electronic timing systems are relatively expensive and even though many schools have them for physics classes, many do not and we do not want to exclude anyone from participating or being able to run an event.
The second is entirely technical. While we could easily time the teams that travel the full distance perfectly straight, it is nearly impossible to time the teams that do not travel the complete distance or travel on a curve. This is why MagLev, with a track, encourages the use of electronic timers. Everyone will go in one direction (unless you hook up your batteries backwards!) and most teams will travel the length of the timed distance.

[Balsa Man]

One great thing about this year's changes - slower velocity/longer run times mean the wild card factor of hand timing (from which your time accuracy score factor comes) will be proportionately smaller. That is a good thing. Still a wild card, unfortunately, but.....tamed a bit.

Just to let you know, we are well aware of the hand timing issue. In fact, it is a highly contentious and long-standing disagreement among several of us in the whole rules process. There are two major reasons electronic timing isn't in the majority of the events.
The first is that electronic timing systems are relatively expensive and even though many schools have them for physics classes, many do not and we do not want to exclude anyone from participating or being able to run an event.
The second is entirely technical. While we could easily time the teams that travel the full distance perfectly straight, it is nearly impossible to time the teams that do not travel the complete distance or travel on a curve. This is why MagLev, with a track, encourages the use of electronic timers. Everyone will go in one direction (unless you hook up your batteries backwards!) and most teams will travel the length of the timed distance.

Not sure if you're talking to me, or the broader audience. Having been in the middle of the (Scioly board side of) those discussions, I'm quite aware. TOTALLY agree with, and understand the absence of electronic timing. Good that everyone understand the issue here. The adjustment of rules to slow things down was a great, realistic, solution. As I said, hats off!
When you say runs could be easily timed (electronically) if they ran straight - gate timing IS relative cake- catching a vehicle moving across two lines (like MagLev), but from start, to (even w/ a straight run) varying (by quite a bit) end points, how would you do that w/photogates?? Another technology/approach in mind?