Pinewood Derby cars - GRM style

imirk wrote: Does the track flatten out at the end? If it does you can make a lead/steel spoiler to get the weight as high as possible for maximum potential energy.

I'd worry about it wheelieing and dragging ass at the bottom of the line.

Depends on the rules, and the transition for how heavy you could make it. Put the rear wheels as far back as possible to maximumimize the forward levering effect and you could have greater than 50% of your weight back there.

The track does flatten at the end, and depending on whether its an old wooden track or a nice extruded aluminum one, the transition can be fairly abrupt. My son's car last year had it's COG too near the rear axle, and did in fact pop a wheelie, although it never left the track.

So I picked up a rotary tool from HF, $10 on clearance. Forgot to get a coping saw, but will pick one up.

Asked my son what his idea for the car. Even though he's only 7, he's a Richard Petty fan... he thought the STP designs were cool, and he liked "The King" in Cars movie. So he wants to make the car resemble Richard Petty's Superbird. At least it's not a super complicated shape...

A trick. The car should be unbalanced. That is, only 3 wheels should touch the track at any given time. Less friction.

Back in the day, my son made a bobsled, complete with lego men.

I've helped build over 200 of them.

I open my woodshop to the local pack every year and use it as an opportunity for them to earn a badge in woodworking. Done it for 20 years.

I'll second what others have said. First off, remember the reason- quality time with your kid.

Secondly- wheels and axles matter.

2 things that have not been mentioned- tracking and wheelbase.

Work EXTREMELY hard to get the thing to track perfectly, then train your son in how to set it carefully on the track centered and pointed straight (most rules mandate that only the boy sets it on the track). The more times the wheels bump the center divider, the more friction. All boys are impatient and careless when they set them.

Also, don't use the pre-cut slots for the nails. They won't hold the nail axles firmly. Drill your own locations, and stretch the wheelbase to the maximum allowed in the rules (usually requires that it fit inside a box of a particular dimension). Longer wheelbase means stabler smoother ride, which translates to faster. Make sure to drill them and get them right BEFORE cutting the body into an unusual shape that makes it harder to drill straight.

Have fun, and expect to loose. All of my student's cars are very fast, but the store bought ones or the ones built by a machinist Dad always win. It is VERY annoying.

Most packs have an opportunity for most creative, best looking, or funniest. These are categories worth pursuing.

I work hard with my boys at managing expectations, and having fun. Sometimes loosing is a good thing. If you think the competitiveness is absurd at the local level, you should see the regionals! No fun at all!

Enjoy!

Our pack outlawed store bought cars and mandated uncut wheels and OE axle locations. We are allowed to run the wood in either direction and as long as it weighs 5oz we are free to use any design.

I like that.

It is probably not the same as the rules at the regional level (which will make your pack's entries uncompetitive).

PTFE Powder on the axles.

My dad and I used a drill press to precisely locate the nail starter hole. The nails were blue and the rules said at the time that if the blue was missing the car wasn't legal, that was to keep people from substituting all different kinds of nails. You couldn't get replacements at the hardware store, if you boogered one up you had to get another complete kit.

So we chucked the nails in the drill press and smoothed the back side of the head (that couldn't be seen) with a small file, then did the same to the shank, just enough to make it smooth and shiny.

The wheels were these skinny things, nothing like the ones I see in the kits today. The nail heads are painted in this picture, we weren't allowed to do that.

The holes in the wheels were carefully chamfered to get rid of the sharp edges, otherwise the center holes were pretty good so we left them alone. PTFE wasn't around at the time so we used graphite powder on the wheels/nails. The nails were set dead straight in the predrilled holes as the very last assembly stage.

Just before the cars ran, we'd rub a pencil around the inside edge of the wheel. The graphite would act as a lubricant if the wheels touched the guide rail.

Looks like everyone covered it for the most part. My kids have built a bunch of them over the years. Best finish was second, and would have won it if the front wheel hadn't fallen off. Make sure the axles are in tight. The slots on that car were a little on the loose side. We should have glued them. I even got to teach my middle son how to work the lathe on that car.

The only other thing I do that hasn't been mentioned, is build it loose. All the fasted cars at the local level have a little wiggle running down the track. I've got another one sitting on the bench now to help the youngest son build. He wants to build it to look like the RX-7

In reply to Curmudgeon:

Chamfering the wheels is almost always specifically ruled out in the rules these days.

When I did it, you had to use the specific kit. My dad hepled me. He was in to it, and we had a great time. We finished second two years running.

Oh well, we enjoyed it.

just don't dress up like this and steal some secret metal from the LHC so you don't have to worry about the car reaching warp speed and alerting the rest of the galaxy that we are a spacefaring nation..

actually, if you haven't seen it, watch that episode of South Park to see exactly what you are not supposed to do..

SVreX wrote: Secondly- wheels and axles matter. 2 things that have not been mentioned- tracking and wheelbase. Work EXTREMELY hard to get the thing to track perfectly, then train your son in how to set it carefully on the track centered and pointed straight (most rules mandate that only the boy sets it on the track). The more times the wheels bump the center divider, the more friction. All boys are impatient and careless when they set them. Also, don't use the pre-cut slots for the nails. They won't hold the nail axles firmly. Drill your own locations, and stretch the wheelbase to the maximum allowed in the rules (usually requires that it fit inside a box of a particular dimension). Longer wheelbase means stabler smoother ride, which translates to faster. Make sure to drill them and get them right BEFORE cutting the body into an unusual shape that makes it harder to drill straight. Have fun, and expect to loose. All of my student's cars are very fast, but the store bought ones or the ones built by a machinist Dad always win. It is VERY annoying.

Forgive my complete ignorance about these things. As I said, I'm a total novice in woodworking and have never built one of these. I'm guessing that you work on the tracking by making very slight adjustments to how the axle is inserted into the body? We'll try to put one of the front wheels slightly higher than the other, and put the weight in the back in the right spot to make sure the lower front wheel stays planted. What kind of glue were you using to hold the axles in?

110% agree with just having fun and not expecting to win. I've had the conversation with my son several times. We'll try hard to make a fast car, but at the end of the day the only thing that matters is that we had fun and he tried his best. I'm sure this isn't much different than a lot of other things parents get their kids involved in. They get competitive to the point where it sucks out all the fun. Hell, adults do it for their own stuff. I've been to autox and HPDE days where I've seen some guys get so wrapped up in it that I wonder if they're even having any fun.

You can't chamfer the wheel holes? Dang. I hope the plastic moldings are better now than they were. They would usually have some casting 'flash' that made it slightly difficult to even insert the nail. It would go in but it wouldn't turn freely. We used a small tapered Dremel type stone but turned it by hand, did it just enough to get rid of the casting flash and add a very small chamfer, then the wheel spun freely on the nail. The nails had these two 'ridges' on the back of the head, I guess they were stamped out? and had the same ridgeson the shank. We removed only the ridges so the wheel wouldn't catch and made sure it was slick and smooth. OBTW: if you go too far down the nail shank it won't fit snugly in the axle groove, particularly if you had to drill it out to get the nail straight in all planes.

Tracking: the centerline of both axles need to be parallel. The nail grooves are the centerline of the axles and they are not necessarily straight across the block or centered properly. So it's quite possible to have the grooves alighned but the blocks look misaligned. The easiest way to do that is to use two 6" long straight pieces of stiff wire, like coat hanger wire, put one in each axle block groove and then measure at the ends. When the measurement is the same on both sides the axles are aligned, then glue in. Of course you need to be able to pull the wire out after you are done, so choose the diameter carefully. The axles also need to be centered side to side in the body as viewed from above. We did this by measuring the body width, marking the center line, then measuring the axle block width and marking their centerline as well. Line all the marks up, done.

Curmudgeon, I think you're dating yourself a bit here. I remember the kits you are talking about that had the wooden axles that you glue into a groove and then add your wheels. By the time I built mine, the "axles" were just a slot cut into the bottom of the car that the nails went into. You could carve around it if you chose to, but you couldn't do much about the alignment, unless you used new holes.

Of course, I'm sure that the kit has been through several more evolutions in the past 35 years.

Here's an earlier kit:

The early kit is exactly what we used! Notice the blue nails? Damn I'm getting old and curmudgeonly. You kids get off my lawn!

My son's kit has a rectangular block of wood which has two slits cut across the bottom. The slits are pretty narrow.

The pack/den (I dunno) leader did tell them that store bought kits wouldn't be allowed to enter the official races, only home made ones from the kits they were given. I'm glad to hear that.

The slits need to be parallel or the car will dog track and create friction. The wires I mentioned can be used for alignment checks but short of recutting the slits I'm not sure how you would fix them if they aren't aligned. Long shot: narrow kerf saw blade in a sliding miter saw? Or maybe a band saw?

Speed:Sport:Life did a piece on pinewood derby competition. I think he had some tips for building them in there as well. It's a couple years old, but I don't think the technology has changed much since then, hah.

http://www.speedsportlife.com/2009/12/06/racer-boy-pinewood-derby-your-first-taste-of-victory-or-defeat/

The 'adult' version: Valve cover racing. Notice there is no guide block, so alignment is really critical.

Klayfish wrote: My son's kit has a rectangular block of wood which has two slits cut across the bottom. The slits are pretty narrow. The pack/den (I dunno) leader did tell them that store bought kits wouldn't be allowed to enter the official races, only home made ones from the kits they were given. I'm glad to hear that.

I've heard that every year for over twenty years from 4 different packs in 2 different states.

Never seen it enforced. That's the frustration.

What will happen is that someone will show up on the day of the race with a kit they bought from an "official BSA site", but it won't be identical. The leadership will not have the huevos to disqualify the kid because "it wouldn't be fair to the boy", or "It's just for fun anyway". This car will win, and then no one can do anything about it.

I'm not trying to be "that guy", just trying to prepare you for what VERY FREQUENTLY happens.

Klayfish wrote: Forgive my complete ignorance about these things. As I said, I'm a total novice in woodworking and have never built one of these. I'm guessing that you work on the tracking by making very slight adjustments to how the axle is inserted into the body? We'll try to put one of the front wheels slightly higher than the other, and put the weight in the back in the right spot to make sure the lower front wheel stays planted. What kind of glue were you using to hold the axles in?

I do this on a drill press BEFORE making any body modifications. That way the drill press drills a clean hole perpendicular to the center line of the car. It's harder to be accurate once the body has been shaped.

I ignore the pre-cut grooves in the block, and drill my own holes. Sometimes I turn the block upside down to accomplish this. I make the holes snug, and generally don't need glue. I roll the block a few times to check the tracking, then remove the wheels/ axles, and let the boys go crazy with saws, sanders, etc.

I've tried the 3 wheel thing without a lot of luck. The rolling resistance is pretty minimal. Friction hitting the center block is a bigger issue. Plus, our local rules say all 4 wheels must touch. If I do it, I have to pretend that it is bad workmanship. Doesn't feel right.

I've heard a lot about weight distribution. I've seen cars win running backwards when all their weight was in the rear (front). That's not the magic all by itself.

There are armchair engineers who all have the "formula" for these things. Don't get fooled- it's a lot of fun, and not nearly as much science as it seems.

Some of these armchair engineers will always start talking about aero. Come on. At 1.3 mph? One of the fastest cars I ever saw was an unmodified block with wheels on it.