Building Demo Cars for Workshop

[ironband]

Hi all,

I'm doing a workshop in November for our Cub Scout Pack to teach the parents and scouts how to build better cars and improve the level of competition. Because a lot of the new scouts are not familiar with pinewood derby racing, the Cubmaster suggested that we set up our track and make the presentation interactive, where we have some cars pre-built which have certain design features and race them as the presentation goes to show the effects of various improvements.

I'm planning to do ten cars, with the first one being a block of wood with untouched whels and axles from the kit in the stock slots, and the last one being a carefully crafted speedster.

Where I am stumbling is deciding what the middle 8 cars look like.

Our rules are essentially that the kit parts have to be used (extra wheels and axles can be purchased), 5 oz, no aftermarket wheels or axles, no machining of the wheels such that substantial mass is removed (yes, it's vague). Graphite only, no oil. Otherwise it's pretty lax - raised wheel, canted axles, extended wheelbase are ok. Unsure if fenders would be permitted due to the 3/8" clearance rule underneath the body, I'll probably ask, though I think fenders are a little too in depth for the level of folks I'm teaching.

My initial draft of my lineup was:

1. Brick (slots)
2. Brick at 5 oz (slots)
3. Brick at 5 oz (slots) with Graphite
4. Brick at 5 oz (straight drilled) with Graphite
5. Wedge at 5 oz (straight drilled) with Graphite
6. Wedge at 5 oz (straight drilled) with polished axles and Graphite
7. Wedge at 5 oz (straight drilled) with polished axles, polished bores, and Graphite
8. Wedge at 5 oz (canted axle rail runner) with polished axles, polished bores, and Graphite
9. Plank at 5 oz (canted axle rail runner) with polished axles, polished bores, and Graphite
10. Plank at 5 oz (canted axle rail runner) with polished axles, polished bores, more aggressive COM, and Graphite


The idea is that we show:
More weight is better
Lube is better
Straight drilled better than stock slots
Wedge better than brick
Polished axles better than not polished axles
Polished bores better than not polished bores
Rail Running better than straight drilled
Plank better than Wedge
COM tradeoffs.

But I'm struggling with it. I'm not sure I'm putting the improvements in the correct order for the best demonstration. I'm also wondering if it would be better to show different levels of axle polish, effects of matched wheels chosen for minimum runout, various COM settings, wheelbases, etc. I'm also struggling with ranking in terms of speed improvement or in terms of time commitment.

What do you think? How would you do it?

 

[bracketracer]

I think your Brick and Brick at 5oz might be redundant? An uncut block is pretty much right on 5oz with the wheels and axles installed depending on it's moisture content.

For our pack, I'd have to build a sample car with wet tempera paint on the wheels and between the wheels and the body to show how much gooey paint slows a car!

I'd probably do just one with horrific axle alignment and 1/8"+ wheel gaps to show why details matter?

I'd like to see a typical scout car, something fairly un-aerodynamic so the scout can weigh whether they want to go fast or aim for a design trophy? We still get very few planks in our pack race but they dominate the district finals.

 

[Thinkin'Bout Racin]

Hi all,

I'm doing a workshop in November for our Cub Scout Pack to teach the parents and scouts how to build better cars and improve the level of competition. Because a lot of the new scouts are not familiar with pinewood derby racing, the Cubmaster suggested that we set up our track and make the presentation interactive, where we have some cars pre-built which have certain design features and race them as the presentation goes to show the effects of various improvements.

I'm planning to do ten cars, with the first one being a block of wood with untouched whels and axles from the kit in the stock slots, and the last one being a carefully crafted speedster.

Where I am stumbling is deciding what the middle 8 cars look like.

Our rules are essentially that the kit parts have to be used (extra wheels and axles can be purchased), 5 oz, no aftermarket wheels or axles, no machining of the wheels such that substantial mass is removed (yes, it's vague). Graphite only, no oil. Otherwise it's pretty lax - raised wheel, canted axles, extended wheelbase are ok. Unsure if fenders would be permitted due to the 3/8" clearance rule underneath the body, I'll probably ask, though I think fenders are a little too in depth for the level of folks I'm teaching.

My initial draft of my lineup was:

1. Brick (slots)
2. Brick at 5 oz (slots)
3. Brick at 5 oz (slots) with Graphite
4. Brick at 5 oz (straight drilled) with Graphite
5. Wedge at 5 oz (straight drilled) with Graphite
6. Wedge at 5 oz (straight drilled) with polished axles and Graphite
7. Wedge at 5 oz (straight drilled) with polished axles, polished bores, and Graphite
8. Wedge at 5 oz (canted axle rail runner) with polished axles, polished bores, and Graphite
9. Plank at 5 oz (canted axle rail runner) with polished axles, polished bores, and Graphite
10. Plank at 5 oz (canted axle rail runner) with polished axles, polished bores, more aggressive COM, and Graphite


The idea is that we show:
More weight is better
Lube is better
Straight drilled better than stock slots
Wedge better than brick
Polished axles better than not polished axles
Polished bores better than not polished bores
Rail Running better than straight drilled
Plank better than Wedge
COM tradeoffs.

But I'm struggling with it. I'm not sure I'm putting the improvements in the correct order for the best demonstration. I'm also wondering if it would be better to show different levels of axle polish, effects of matched wheels chosen for minimum runout, various COM settings, wheelbases, etc. I'm also struggling with ranking in terms of speed improvement or in terms of time commitment.

What do you think? How would you do it?

I like your thought process ....allowing the kids to visually see the changes should help them understand what is happening.

A couple of thoughts. Maybe you could build the wedge at about 2 oz. Then have 3 oz of weight that could be taped to the top. First run it with the weight forward, then move the weight rearward as far as possible. Re-tape the weight so as to minimally change the aero . This should help them understand the potential energy change by having the weight in the front or towards the rear. By doing this on the same car, with no other changes, it may have a greater impact on the dad's?

Second, along the same lines, you could have a car with two wheel bases drilled. This might be a bit harder to show the differences and may be a bit above their level? You would need to determine your audience and decide if this would be worthwhile or not. I would drill one at 5 1/4" and a second set at 4 1/2". By moving the same wheels and axels to the "other" wheel base you can show the difference between the wheel base length. I would be sure to discuss the advantages and disadvantages of each.

I agree with BR in that the block and the 5 oz block is redundant. During our workshops, I let the kids know that the goal is to have fun. They can choose to have a creative design and go down that path or they can pursue the path to build a fast car. They are not completely mutually exclusive, but there are some limits of design if you really want a fast car. I tell them it is their choice and it is ther car, so have fun either way! Basically, I tell them if they want to build a school bus, because of the tall car/truck, that they will be limited to how fast it can go. But I also tell them that a school bus or a "Mator tow truck" would be really cool!

I know time and energy spent is always a consideration. If you have the time, build the various steps and test them. By doing this, you will see which steps (and in which order) you should present. You may find that you really only need 6-8 steps. I would stick with things that show a noticeable difference unless you audience is already pinewood Derby educated.

 

[ironband]

Thanks bracketracer and Thinkin' for that input and insight.

I really like the idea of having cars that I can improve incrementally on the fly. Now that I've built a solenoid gate for our track, the timer should be more consistent, so I can use that to demonstrate the improvements from run to run and I don't necessarily need to race the cars head to head.

I also totally hear the point that I may be going too far for some of the folks coming in. It may be a bit much to talk about wheelbases and stability vs. speed in the transition, aero beyond basics, etc. I like the idea of promoting cool designs as well - my older son always built for design and not for speed, so I can pull those out of storage to show.

I figure I can get away with 3 cars which can be tweaked while I talk - one brick, one wedge with a straight drill, and one wedge with a canted drill set up for rail riding. I'll do most of my tweaks on the middle one, which I can put weight pockets in to show the effect of moving the COM backwards, as well as swap out axles and wheels. Of course I'll present all the theory as we go, and for the rail rider I'll demo the tuning board.

That should save me quite a bit of physical work. I like it.

Still interested in other ideas too.

 

[txchemist]

You might consider that the individual Scouts almost always fall into two groups- the minority will want to learn all about speed, the majority just want to race, so make the demo short and sweet for everyone, and invite interested Scouts & parents to come back to run experiments to learn about speed. When all are present, the few kids asking really good questions need some one on one to understand what's going on and not get distracted by all the uninterested kids just messing around. Usually, some clever lad will grab a car and send it back up the track to create a crash, and that is the end of enlightenment as all now want to see who can make the most violent crash.