Build a Fast Derby Car

BUILD A FAST

PINEWOOD DERBY CAR

Having built two first-place cars two years in a row, many people have been interested in the construction process that Joshua and I use. Being a woodworker, I enjoy showing Joshua how to use the tools in my shop. It is certainly not a requirement to use all these tools to build a pinewood derby car. I suggest tool alternatives below, for those without access to these woodworking tools.

Here's the finished car Joshua built for the January 2006 race:

In a nutshell, all winning pinewood derby cars have the following in common:

They weigh as much as possible. If your car is light, it will lose. 5 oz. is your target weight. 4.75 oz isn't close enough. Strive for five!
They minimize friction between the wheels and the other car parts. This means the axles are smooth, the inside of the wheel hubs are smooth, and the wheels don't rub aggressively against the car body.
They minimize friction between the car and the track. This means the car runs as straight down the track as possible (without bouncing against the track's rails). This also means the outside of the car's wheels are smooth, and the car's body never rubs against the track.

Here was our step-by-step process for building the car shown above:

MAKE THE BODY

The first step is to cut the thickness of the wood block in half. This is a difficult cut on the table saw because there's not much clearance between the fence and the blade, so I make this cut for Josh.
This cut could also be made with a jig saw or a coping saw.

The final body thickness is 3/8 ", so Joshua uses the planer to remove the extra thickness. I could have cut to final thickness at the table saw above, but using the planer gives a smoother surface, so we use the planer instead.
This step could also be accomplished with a coping saw, a jig saw, a hand planer, or a belt sander.

Joshua then uses a round-over bit at the router table to put a soft edge on the car's body.
This could also be easily accomplished by hand with a sanding block or a palm sander.

Joshua then sands out any imperfections left by the router.

To prepare for using the coping saw, Joshua uses the drill press to drill a hole at each corner of where he will cut the hole in the body.
The holes could also be drilled with a hand-held drill.

Next, Joshua uses the coping saw to cut the hole.
We remove this extra material so that more of the car's 5 ounces will come from the lead weights, which we position towards the rear of the car to maximize potential energy at the starting line. Josh's first car didn't have this hole, but his second car did (and it ran even better than the first car).

Here you see Joshua using the mortising machine to cut recesses into the bottom of the car for the weights.
This step could be accomplished with a hand drill, drill bits, and a chisel.

Joshua then uses a chisel to remove the extra wood left behind by the mortising machine.

Joshua uses tin snips to cut the lead weights to the length required to fit into the recesses. Notice the flat weights. These are the segmented lead weights sold by Maximum Velocity. We selected these weights so that they would fit into the thin-body design without protruding out of the bottom of the car (protrusions from the bottom of your car might cause it to drag on the track).

Next Joshua spreads some epoxy into the bottom of the recesses, then pushed the lead weights into position.

He then spreads more epoxy over the top of the weights to make the bottom of the car smoother, and to make sure there's no chance of the weights falling out of the car.

We decided to remove even more wood from the body this year than last year (so that more of the weight will come from lead positioned in the rear of the car). To accomplish this, Joshua cut a hole in front of the front axle slot.

We then brought the car body, wheels, axles, and extra lead weights to the post office so that we could get an accurate weight on the digital scale in the lobby. Without any extra weight, the car parts weighed 3.6 oz. We then added lead weights to the scale until it read 5 oz.

When we got home from the post office, Joshua epoxied the extra weights onto the rear end of the car (on the car's top side, not bottom!), just in front of the rear axle slot position. It's important to keep the car's balance point in front of the rear axles. If you put too much weight behind the rear axles, the car will become unstable.

We then got nervous that the small amount of wood remaining in the car's front end wouldn't trip the sensor at the track's finish line. There's little point in building a fast car if the track's sensor won't see the car until it's half way over the finish line—many races are decided by fractions of an inch!
To address this concern, Joshua epoxied a thin sheet of clear plastic over the car's front hole. When Joshua paints the car, the plastic sheet will become opaque and will block the finish line sensor. It may have worked fine without this plastic sheet, but we didn't want to take any chances!

Before painting the car, Joshua places small pieces of masking tape over the ends of the axle slots, so that there won't be any paint at the point where the wheel hub will rub against the body.

Next Joshua paints wood sealer onto the wood parts of the car. The sealer will prevent the spray paint from soaking into the wood, which wouldn't look good.

Once the wood sealer dries, Joshua spray paints the car. This year he selected gloss black paint.

After the paint dries, Joshua removes the masking tape from the axle slots.

PREPARE THE WHEELS

AND AXLES

Joshua mounts each wheel into the drill press (using the wheel mandrel we bought from Maximum Velocity) and lightly sands the outside of the wheel with wet sand paper. Keep the sand paper wet so that you don't overheat the wheel and melt it!
If using a hand-held drill, this step requires two people (one holding the drill while the other holds the sand paper).

Next Joshua polishes the inside of the wheel hub using the bore polisher and bore polish sold by Maximum Velocity.
This step is optional. We didn't polish the inside of Joshua's hubs on the first car he built (which won the race), but we did on the second car (which ran even better than the first car). This step could also be done using a hand-held drill (two-person job).

Here Joshua is straightening the axles using the axle press that we bought from Maximum Velocity. He also uses the press to make sure the head of the axle is perpendicular to the axle shaft.
This step is optional. We didn't straighten the axles on Josh's first car (which won the race), but we did on the second car (which ran even better than the first car).

Next Joshua files the ridges off of the inside of the axle head.
This step could also be done using a hand-held drill (two-person job).

Then Joshua uses five grades of wet sandpaper (180, 240, 400, 600, and 1200 grit) to polish the axle shafts and the inside of the axle heads.
This step could also be done using a hand-held drill (two-person job).

PUTTING IT ALL TOGETHER

When mounting the wheels, you want just enough clearance so that the wheel will rotate smoothly. Too tight and the wheel won't roll well. Too loose and the wheel will wobble too much. To solve this problem, Joshua and I made this wheel spacing tool out of the plastic lid from a coffee can.

While I hold the wheel spacing tool between the wheel and the car body (with the slot in the tool straddling the axle), Joshua hammers the wheel tight against the tool. When the tool is removed, the wheel spacing is just right.
NOTE: Joshua runs his cars in a three-wheel configuration by raising one of the front wheels off the ground. This reduces friction by 25%. Be aware that this is a risky configuration, especially if your race track is not very smooth and your car's balance point is not toward the rear of the car. If you haven't seen your track, ask someone if it's a smooth track. If it isn't, configure your car with all four wheels down.
If you decide to go with a three-wheel configuration, and you noticed during your wheel and axle preparations that one wheel and/or axle is inferior to the rest, use it in the position of the raised wheel. Put your best three wheels and axles in the down position!

Joshua then rolls the car next to a straight edge. If the car doesn't roll straight, we adjust the wheel alignment until it does.

Once we're happy with the alignment, Joshua squirts hot glue into the axle slots to hold the axles in place. You don't want your careful alignment to change before or during the race!

Joshua then squirts dry graphite into the inside and outside of each wheel hub.
He'll do this again on weigh-in night, before the car gets impounded!

We then brought the car back to the Post Office for a final weight check. It measured 5.1 oz. So we drilled a hole through one of the lead weights (but not all the way through the car body!) in the back of the car.

This brought the weight down to 5 oz. Perfect!

When we got home from the Post Office, Joshua used some rub-on stickers to decorate the car.

That's it, the car is ready to rumble. Should be a fun race!

RACE RESULTS AND CONCLUSIONS

This car did well, but not as well as Joshua's previous two cars. It came in 7th place out of 60 cars. Not bad.

We knew that the alignment on this car wasn't as good as it was on Joshua's previous cars. It was very good but not perfect, steering ever so slightly to the right. So now we know—a very good alignment (versus a perfect alignment) cost Joshua six positions in the final ranking.

This is a good lesson for a scout to learn: There are many levels of quality, and sometimes (depending on how important the result is) very good isn't good enough. If Joshua wants to do better than 7th place next year, he'll need to work even harder on the alignment.

Good luck building your winning car! If you have any questions, send me an email.