My Pinewood Derby Story so far...and Now Pack Champion!

[DocJancsi]

Hello all,
I have been using this board as a reference for the past several years. I really appreciate the dedication and great information.

I have been building derby cars for about 7 years now. Three year ago, I managed to help my nephew build a car that took 2nd in the pack. We went on to race at districts and council.

The following year my son was determined to go to council like his cousin. So I seriously stepped up my game. The car we built last year was a hollow body, plywood veneer design with balsa fenders. That year we took third in the pack and after some tweaks before districts and council we placed eighth in the Great Lakes Field Service Council which serves the Detroit metro area. The large size of the council, and high density of “car guys” with access to machine tools leads me to believe that this has to be one of the most competitive councils around. We were only .0925s back in cumulative time across 4 heats.

This year I have to figure out how to gain .023s/heat.

Plan this year;
-build and race 2 cars
-join a local maker space for access to tools.
-create a <10g laser cut ladder body.
-use a 1.5” balsa “nosecone” forward of the front axle to allow for better control over the COM
-leave 0.5” of space behind the rear axle for 14 0.25”tungsten cubes.
-discontinue the practice of bending rear axles.
-Employ/develop a reliable method for drilling properly aligned axle holes
-add more aerodynamic leading and trailing fenders
-do not employ any building techniques which violate council rules. Specifically: no lathe turned or lightened wheels, and use graphite for lubricant.

We just turned in both cars for the pack race yesterday. Our pack races are conducted on a 42’ Best Track with an electronic timer started by the first car past a photo gate ~2”downhill of the starting pins. After tuning, both cars turned in times of 2.86 +-.03s.

 

[Rebel Racer]

Wow this sounds like a real aggressive car or cars your planning to build... Personally, not so sure if I would go with 14 cubes in the rear. It’s in my opinion a little to aggressive. If you look at some of the pro builds(some are exposed to view rear weight placement) you will find most are using pure tungsten bars. It takes up the space of 12 cubes but adds up to almost 13. This bar gives you some “wiggle”room to shift your weight side to side and balance the weight more evenly across your rear wheels. If your looking for a better alignment, silver bullet pro is the way to go! Very easy to use and will do tons of cars. I brought mine to our pack build day for all to use. It has hardened bushings so the holes keep true. Can’t go wrong...
Good luck,

 

[DocJancsi]

Wow this sounds like a real aggressive car or cars your planning to build... Personally, not so sure if I would go with 14 cubes in the rear. It’s in my opinion a little to aggressive. If you look at some of the pro builds(some are exposed to view rear weight placement) you will find most are using pure tungsten bars. It takes up the space of 12 cubes but adds up to almost 13. This bar gives you some “wiggle”room to shift your weight side to side and balance the weight more evenly across your rear wheels. If your looking for a better alignment, silver bullet pro is the way to go! Very easy to use and will do tons of cars. I brought mine to our pack build day for all to use. It has hardened bushings so the holes keep true. Can’t go wrong...
Good luck,

If it's too aggressive its too late now... the cars are built and turned in.

I do not have 3 scales (yet) so I was not able to perform any CoG/CoM calculations. Rough observation put the COM(x) around 5/8"-3/4" forward of the rear axles with CoM(y) aproximately on the centerline. (Rear axles are located 5/8" from the rear of the cars)

I did compensate for the aggressive CoM by applying some equally aggressive steer to run along the rail. The rear axle height was 5/32" keeping CoM(z) slightly below height of the axles. By eliminating as much weight at the front I was able to keep the car's overall moment of inertia relatively low. Hopefully this will limit the incidence of transition induced oscillations.

I was also able to tune the cars using the track they will be racing on prior to turn in. During the practice runs I was consistently getting 2.86 s (+-.03s) runs across all lanes (on a 42" Best Track. Slightly shortened due to the use of a laser starting gate). I was not able to observe any oscillations after the transition and both cars rain laser straight down the straightaway.

I hear you on the Silver Bullet Pro. They were out of stock by the time I started shopping for tools to improve my process. I will be keeping tabs on the status and pick one up as soon as they are available again.

I belong to a local Maker Space (MakerWorks in Ann Arbor, MI). Lacking a reliable drill jig. I was able to achieve pretty good results by cutting and squaring off a pair of 0.25"x 1.5"x 7" blocks using a band saw and planer. I clamped those into an angle vise (unfortunately not a sine vise) that I squared and calibrated to 3 degrees of tilt on a Bridgeport mill (MakerWorks does not normally permit tramming the mill head). I was then able to drill my holes fairly accurately using the mill's DRO.

I used a Jacob's Chuck in the Spindle and a #1 center drill to set the holes followed by a #44 drill bit in the chuck. Once the rear holes were done on both blocks. I put the blocks into the standard milling vise and followed the same procedure to set and drill the holes. Once completed,I did some basic checks with pin gauges/123 blocks on the surface plate to verify my accuracy.

I cut the body into a ladder body type form using an Epilog 40w laser. (I am NEVER using a coping saw again!)

Once the bodies were formed my boys and I worked together in my shop at home to fabricate the other parts and assemble them into a functional pinewood derby car.

The race is this Friday. I will post the results after the race.

 

[Thinkin'Bout Racin]

If it's too aggressive its too late now... the cars are built and turned in.

I do not have 3 scales (yet) so I was not able to perform any CoG/CoM calculations. Rough observation put the COM(x) around 5/8"-3/4" forward of the rear axles with CoM(y) aproximately on the centerline. (Rear axles are located 5/8" from the rear of the cars)

I did compensate for the aggressive CoM by applying some equally aggressive steer to run along the rail. The rear axle height was 5/32" keeping CoM(z) slightly below height of the axles. By eliminating as much weight at the front I was able to keep the car's overall moment of inertia relatively low. Hopefully this will limit the incidence of transition induced oscillations.

I was also able to tune the cars using the track they will be racing on prior to turn in. During the practice runs I was consistently getting 2.86 s (+-.03s) runs across all lanes (on a 42" Best Track. Slightly shortened due to the use of a laser starting gate). I was not able to observe any oscillations after the transition and both cars rain laser straight down the straightaway.

I hear you on the Silver Bullet Pro. They were out of stock by the time I started shopping for tools to improve my process. I will be keeping tabs on the status and pick one up as soon as they are available again.

I belong to a local Maker Space (MakerWorks in Ann Arbor, MI). Lacking a reliable drill jig. I was able to achieve pretty good results by cutting and squaring off a pair of 0.25"x 1.5"x 7" blocks using a band saw and planer. I clamped those into an angle vise (unfortunately not a sine vise) that I squared and calibrated to 3 degrees of tilt on a Bridgeport mill (MakerWorks does not normally permit tramming the mill head). I was then able to drill my holes fairly accurately using the mill's DRO.

I used a Jacob's Chuck in the Spindle and a #1 center drill to set the holes followed by a #44 drill bit in the chuck. Once the rear holes were done on both blocks. I put the blocks into the standard milling vise and followed the same procedure to set and drill the holes. Once completed,I did some basic checks with pin gauges/123 blocks on the surface plate to verify my accuracy.

I cut the body into a ladder body type form using an Epilog 40w laser. (I am NEVER using a coping saw again!)

Once the bodies were formed my boys and I worked together in my shop at home to fabricate the other parts and assemble them into a functional pinewood derby car.

The race is this Friday. I will post the results after the race.

Doc,

Sounds like you took an anyitcal approach to you build. As you already mentioned, the SB Pro would make life easier. I believe that you COG (balance point) would be less than 5/8" based on the 14 cubes and a balsa nose. What wheelbase did you build this on?

Maybe a looong WB and a biased weight placement to the FDW side moved enough weight forward to help keep the car stable.

One other thing going forward, buy some extra wheels and axles. Sort thru them and pick out the straightest axles and the wheels with the least runout for prepping. Take the extra wheels and practice prepping them with DD4H's Black Ice wheel prep system. It will make you better so when you get to your "choice" wheels, you will have it mastered.

Good luck on the race! ....make sure to let us know how it goes.

 

[DocJancsi]

The cars were built using a 4.75” wbheelbase. The Rear axles are 0.625 from the rear and I set the front wheels 1.625 back from the front of the block.

In addition to the 14 cubes behind the axle. To bring the car up to weight I placed 8 cubes 0.125 forward of the rear axle. One row of 6 +2 more placed to the outside. For council I may move them to the FDW side.

I used Revell axles and a set of mold matched #2 BSA wheels for each car. I started using Black Ice last year. I went through many wheels working on my technique between pack and districts last year. After prepping with Black Ice I divided the wheels into left and right then burnished the graphite into the bores to reflect the intended direction of travel of the wheels.

I was out of Johns “next level” axle polish, so I used Mother’s chrome polish, progressively polishing the axles on my Dremel from 1000 to 12000 grit micro mesh. I finished the axle polishing with Mother’s on a thin strip of 4 oz veg tanned leather. Between each stage I cleaned the axles with a damp cotton string. After polishing I sealed the axles with a quick spray of Sail Kote.

Then I mated the wheels to their axles and applied additional graphite and spin the wheels until I got a good 30s spin from each one. For consistency I spin each wheel in the direction of travel using a polishing pad mounted to a mandrel on my drill. I set the speed with a C-clamp on the trigger.

Wheels were mounted on the car gapped with the edge of my credit card, and I was able to tune the cars on the track.

 

[Thinkin'Bout Racin]

I really like your approach. The cars should fair well. One more comment...the wheel gaps might be a bit large. I have found with graphite they can be a little smaller. Not as tight as with oil, but less that the approximate 0.035 of a credit card.

Ok, two comments ...did you treat the sides of the body where the inside hubs will rub with anything other than graphite? If not, do a little more reading and there are a few options to use depending on your rule set.

 

[DocJancsi]

The gap question has always bothered me. I have used a .032 feeler gauge in the past but it disappeared a few years ago and I have not replaced it.

If anyone on here has data on proper gaps for graphite and oil it would be great to have.

I applied a thin coat of polyester resin around the axle holes. I wet sanded this down to 2000 grit with 3m sandpaper then used some Maguires ultimate rubbing compound to polish it to a near mirror finish. Then I burnished this surface with graphite.
I know I could use Delryn or Teflon washers or sheets or even polystyrene in a similar application. All of those approaches seem too close to bushings and might raise eyebrows at inspection. 2018 Great Lakes Field Service Council Pinewood Derby Rules

 

[Rebel Racer]

The gap question has always bothered me. I have used a .032 feeler gauge in the past but it disappeared a few years ago and I have not replaced it.

If anyone on here has data on proper gaps for graphite and oil it would be great to have.

I applied a thin coat of polyester resin around the axle holes. I wet sanded this down to 2000 grit with 3m sandpaper then used some Maguires ultimate rubbing compound to polish it to a near mirror finish. Then I burnished this surface with graphite.
I know I could use Delryn or Teflon washers or sheets or even polystyrene in a similar application. All of those approaches seem too close to bushings and might raise eyebrows at inspection. 2018 Great Lakes Field Service Council Pinewood Derby Rules

Doc I’m starting to find that there really is no set gap that works best. Each car is different, a gap that works for one car may not on another. Another builder on this forum stated “ wheel gaps should be as tight as possible without loss of speed “ and I stand behind this statement! Most of the cars I’ve built run best with gaps that range from .02-.01 . As far as oil vrs graphite not so sure. Dd4H has a nice gapping tool on his site that works well and is reasonably priced

 

[DocJancsi]

Well the results are in!
My oldest (in the middle) turned in the following times:
Den Races
2.8466
2.8849
2.8473
2.8546
Total 11.4334
Mean 2.85835
Std Dev 0.0180659

My youngest (On the right with the jacket)did very well also:
2.8926
2.8914
2.9097
2.9024
Total 11.5961
Mean 2.899025
Std Dev 0.00865578

For the finals, We did very well also. When it was all over, my oldest took first in his den, and was pack champion.
My youngest finished Second in his den (there was this one kid...) but held on to very consistent times and ended up beating that kid in the finals to take 3rd in the pack. The Second place winner has gone to the council races with us for the last several years and has always built very fast cars. This is the first year we beat him out for the pack race though.

 

[Peregrine Racing]

Sweet. Love it when the hard work pays off.

 

[Rebel Racer]

Congratulations! Glad all your time and effort paid off..The smile on the little guy’s face says it all!

 

[SuperSlow]

Great job guys, I always thought to do well at something like this says alot. I believe pwd is certainly one of those things that you get what you put in!

 

[jatofau1]

Congratulations. The cars look great.

 

[xanthrum]

That's awesome .... you should step up your game .... besides learning a lot ... not to mention those are smoking fast times ... you should come be a part of this year's Mid America Pinewood Derby... you can send cards in via proxy or come in person .....its an amazing day ... you will be exposed to everything you could ever want to know about pwd

 

[Skywalker Racing]

Congrats!

 

[DocJancsi]

That's awesome .... you should step up your game .... besides learning a lot ... not to mention those are smoking fast times ... you should come be a part of this year's Mid America Pinewood Derby... you can send cards in via proxy or come in person .....its an amazing day ... you will be exposed to everything you could ever want to know about pwd

I should point out that those times are probably not as blazing as you think. The track uses a laser to start the timer. I noted considerable variation in heat times. A lot depends on the design and speed of the cars in each heat. The timer is started by the first part of any car to break the laser beam ~ 6” downhill of the starting pin. 6” of travel at the start of the race equates to a lot of time reducing the accuracy of the time measured. Variation in wheelbase, nose shape and speed out of the gate for the cars in the heat reduce the precision.

 

[Reecedad]

Congrats!