leperboy

Potential energy is directly correlated with mass and height. The more height, the more potential energy. The farther back int he car, the higher the weight to start, so more potential energy. The car has a long straight stretch after the drop, and it needs to convert the potential energy into energy and momentum to keep moving to the finish line. It is extremely critical to get your car to exactly 5.000 ounces.

Also, pack your axles with powder graphite. No wet lubes, just powder graphite and spray it in while spinning the wheel until it is packed.

Matt

Rob Edwards

My son and I just did his first Pinewood derby car, and there being no 'Dad' division, I struggled with the balance of having him do as much as possible and Cheating Bastard tactics. The troop had a build day with a band saw, a belt sander, palm sanders, and paint. He did a great job, it ended up looking a little like a 1970 Superbird. After shaping and paint, we brought it home and Dec got a lesson in the use of the drill press, the router, emery cloth, and powdered graphite.

None of this mattered one bit, because they've changed the rules on how the Derby is run. There were three 'events', only one of which was two or three heats down the track for times. Next the boys had to push their cars down a straight track and get it to stop in a particular colored zone for points. And last was another inclined track with a large jump, and you got points for having your car stop close to the end of the track without touching it. And of course, all my (oops, our) careful work aligning the axles was blown to hell by the car crashing down on its wheels Dukes of Hazzard style.

That said, my (our) carefully prepped 4.98 oz rear-weighted shiny axle car was slowest of six cars down the track in both heats.

So find out the local Derby rules before spending too much effort on a car that might end up getting trashed.

Jerry Feather

Good point Robob: Actually I think it is 49/51 isn't it?

jleidel

+1 to Matt's graphite powder suggestion. The liquid lubes are garbage. One other suggestion, relocate the axle points for an elongated stance. This, with a heavy rear weight bias for maximize the potential energy delivery over the rear wheels. The ultimate point of front/rear balance should be the rear axle. We would use precision measurement gear to pull the weight of the car within ~0.2 ounces of the max weight. Be careful with going any closer than 2/10s. The scales in our pack were always off by ~0.05-0.10 ounces. [our scale was accurate to 0.001ounces].

Have some fun doing it!! I can't wait until my boy is old enough for some serious pinewood engineering fun.

mark kibort

+1 for the graphite powders, BUT all falling objects fall at the same rate, regardless of weight. a 5 lb 928 of the same size will go as fast as a 5oz 928. the difference is the friction. . you are confusing kinetic energy and potential energy with gravity.

a 100lb shot put, and a 1000lb shot put, dropped from 10,000 feet will fall at the same rate. However, the damage the heavier ball will do on impact will be a lot more.

mk

Steve Martin

If you hand carve this 928, you will at the end, appreciate it not only for what it became but for how it became. Others will notice it aswell. the 928 should be an easy carve, with all the roundness and no really sharp angles. the only tricky part to carve would be the rear spoiler(GET A DREMIL TOOL FOR THAT AREA). Spend the day with your boy on the porch( or in the garage) just looking at the car and creating your car. in the end, no matter how good or shabby it turns out, you'll have a masterpiece that you and your boy carved together and that is the time worth $1,000,000,000,000,000,000,000.....

Also, think 30 years into the future, when your now grown boy, shows up with a hand carved car out of a chunk of pine that he and your grandson carved one sunday afternoon. upon closer inspection of that car you notice the resemblence to the 928..........

you got tears yet?

dont cheapen the experience for your boy.
You can go anytime and have a piece of good hardwood CNC'd into a nice 928 for your mantle. this is the bond time for father and son.....


Maybe I'm missing the whole point of this being a kid thing.
building not just a car thing but the bonding and memories that come with it. I think that if the two of you build a nice(maybe shabby nice, plainly evident that it was hand made) car you and your boy will have a deeper conection. whereas if you go all out and take this thing and monsterize the build process, your boy will have only gained ;no bonding or pride needed lesson, just get the win at any and all cost attitude.
This is a scout troop tradition that is becoming a vastly commercailized "who can go to the extreme extent without actually cheating event(rules and the bonds that last lifetimes)."

I take would rather have pride in a good hand crafted build.

jleidel

If one considers:



* H is the height of the drawbridge.
* m is the mass of the car.
* v is the velocity of the car once it reaches the bottom of the ramp.
* x is the distance along the length of the car from the front of the car to the car's center of mass.
* y is the distance along the height of the car from the bottom of the car to the car's center of mass.

The initial energy of the car is its potential energy relative to the end of the track.

Eo = mg(H + x sin(theta) + y cos(theta));

The end energy of the car is equal to its potential energy relative to the end of the track, plus it's kinetic energy.

Ef = mgy + 0.5 mv^2

The initial energy of the system equals the final energy of the system:

Eo = Ef

mg(H + x sin(theta) + y cos(theta)) = mgy + 1/2 mv^2

mg(H + x sin(theta) - y (1-cos(theta))) = 1/2 mv^2

v = (2g(H + x sin(theta) - y (1 - cos(theta))))^1/2

We have now solved for the final velocity of the car based on the properties of the ramp, and the properties of the car. It is important to note that this equation does not depend on the mass of the car, but it does depend on the location of the center of mass of the car relative to the dimensions of the car.

Given that the physical properties of the ramp, such as it's height and angle of inclination, are the same for all cars in the race, there is no advantage to be gained by altering the ramp (never mind the fact that the pack leader would never allow it). This leaves us to consider the changes we can make in our own car, that is, the location of the center of mass.

Our equation can be simplified by writing it as:
v = (C0 + C1 * x - C2 * y)^1/2
where:
C0 = 2gH
C1 = 2g sin(theta)
C2 = 2g (1 - cos(theta))

If we look at this equation, we can see that v gets larger as x gets larger, but that v gets smaller as y gets larger. The place in the car where the x is large and the y is small is the bottom back corner of the car.

Bottom line: Centering the weight of your car as much as possible toward the bottom, back corner of your car will make it faster during the race.

From: http://www-cs-students.stanford.edu/...ood_derby.html

mark kibort

simple a=F/m

G ends up equaling a

mark kibort

cool! things to consider when downhill racing.

I forgot about the runnout part.

mk

leperboy

No, I'm not. Please read more carefully where I said you need energy and momentum for the runout.

Matt

Richgreenster

You need to decide what you want. The look of a 928 which is cool but will finish probably dead last.
Or
A car that will finish 1st. If you would like some pointers and help on this just shoot me a pm and I will hook you up.

Jerry Feather

I would defer to the engineering types except my own intuitive engineering tells me that putting the weight so far back will tend to unload the front wheels which will allow the front of the car to wander more which will cause more friction against the sides of the track, inside or outside. I think putting the weight right over the rear axle is still the best compromise.

I would defer to the grafite guys too. My only problem with that is that when I did this years ago, even before my own son, but when my wife was the den mother for the neighbor hood cub scout pack I was working at the Molybdenum mine and had access to some neat little aerosol containers of spray Moly that I believe was a better lubricant that graphite. We used it in the wheels and axles, but found that it did not perform very well until we put a tiny bit of oil with it. I actually never tried the graphite. They might be right.

In regard to the design and fabrication of the car itself, I think you are concentrating on the most important aspect of this event. What you are going to find is that some poor little kid from a broken home is going to make his own car all by himself and is going to do something with it that nobody willl ever understand, but that will make it the fastest car there. It will look like crap, but he will win hands down.

What you will be left with is a cool 928 replica that placed (_____th) fill in the blank, but which you son will cherish forever. Even if you did most of it, if he is there when you build it he will always remenber that he built it with a little bit of your help.

Your original request was for help in carving the car and I hope what I and some of the others have suggested will be helpful. I wish I was there to show you exactly what I mean by some of my suggestions, but you will just have to do your best with the carving. One additional thing I have learned in doing this kind of carving, and I did a lot of it in building gun stocks during my gunsmithing phase, and that is to approach the curves with a series of flats. In other words rather that trying to round a corner off, first try to take the corner off with a flat surface, at least flat in one direction and then curving the other as required. Then take those resulting corners off with additional flats. When the flats get so small then you simply blend them together with the final rounding you desire.

As I suggested before with the cutting the block in side and rear/front profiles, do the same from the top. That really gives you a good start and keeps the shape exact for the final rounding.

I'm not sure how much detail you hope to incorporate in the windows and such, but I would leave out the mirrors and rear spoiler. The 928 actually looks its best without the spoiler. You might figure some way to put the windows on with something like stickers rather that painting them. Be creative with that. Let you son paint them after you show somehow where the lines are. If he screw them up help him do it over. If it is still too bad, wait until he goes to bed and do them over yourself. If you want the spoiler, use an S4 style and make it out of something else and glue it on last.

Jerry Feather

pcar928fan

One thing that we did back 35 years ago was to polish the axles (Nails) to a VERY HIGH FINISH! We took all the burrs off of them and used varying grades of sandpaper all the way to rubbing compound. We also made sure the car rolled PERFECTLY straight and the weight balance was perfect too.

I think the axles are different these days and I have seen folks take a triangle file and put small grooves in the part of the axle where the wheels roll. That should decrease frictional resistance as well as giving a nice place for the graphite to pack in for lubrication!

robot808

I friggin' love that Kibort is up in this.

inactiveuser1

Undercut the nails so the graphite stays in the axle as less friction on the axle with undercut.
Also radius the tires so the resistance is only on a small area in the middle.
The front nose angled so it rolls out faster it they are using a gate.