Pinewood Derby 2025

[LCHIEN]

I found an experimental result of effect of wheel weight on car performance.

They took lathed wheels which normally weigh 2.6 grams. lathed down to 1.0 grams.
Then they added O-rings inside the wheels to weight them back up to 1.9 and 2.9 grams without removing or otherwise messing with the cars.
Made a consistent difference of 25 and 45 ms..
I only lost by 6 ms on the last race.

For those just tuning in, wheel weight makes a theoretical difference because it robs energy to spin the mass of the wheels up to the running speed, that would otherwise go into making the car higher linear velocity.
PotentialEnergy (start) = Linear kinetic energy + Rotational Kinetic energy + Energy lost to friction

PE = mgh (mass times height x gravity)
LKE = 1/2 mV^2
RKE = f(mass of wheel, rpm, diameter)

so if PE is fixed (ramp height and mass of car) and friction is fixed,
then LKE can be increased if RKE is reduced, which gives bigger V.
The Diameter is prescribed and the RPM is basically based on the speed of the car, so only the mass is under control of the user to lower the RKE

[leehljp]

"Where can I get some fine lathing tools for trimming saft plastic at low speeds?"

Pen turning chisels would be best in my opinion - or at least would be the tools of my choice. Pen turning chisels are mostly just short lathe chisels but the same basic size handles and short shafts.

Look at these: https://www.amazon.com/s?k=hand+carving+tools&crid=H1FJNP060VMR&sprefix=H and+Carving+%2Caps%2C161&ref=nb_sb_ss_mvt-t7-ranker_1_13
I would NOT use these (above) because the different size carving knives do not have enough mass and hand holding material or gripping area.

Pen Turning chisels (shorter than normal bowl turning chisels) still have enough mass and enough hand hold material that the trimming at slow speed will not jerk it out of your hand.
https://www.amazon.com/s?k=pen+turning+tools&crid=3EX6Q2Y85KKR6&sprefix=p en+turning+%2Caps%2C209&ref=nb_sb_ss_mvt-t7-ranker_3_12

I personally am used to HSS chisels and sharpening them to razor sharpness, however carbide insert tools/chisels will do almost as well.

[LCHIEN]

I chuck up the nails in an old 1960's K-mart drill with a variable speed Variac I have mounted (usually used for winding fishing line onto my numerous fishing reels) and we polish the nails with varying (usually 400, 600 and two higher grits we used to use for polishing the ends of fiber optic connectors).
I also take a triangular file to the underside of the nail head. There will be various burrs and flashing and I try and make it a bit conical to reduce the friction area in contact with the wheel hub.
For all purposes this is the same as a small lathe.

I also have a wheel mandrel for mounting the wheels in the chuck. Typically I try and deburr the wheels but the molds are pretty clean and round so I just run some 400 grit gently over them.


Where can I get some fine lathing tools for trimming saft plastic at low speeds?

[leehljp]

about 4 or 5 years ago, I was looking for a good "protectant" polish (polish isn't the right word) for our cars to help keep bugs off. I ran into a compound that was developed by a military supply company for aircraft to do similar. it was expensive (about $40 for 16 oz. I think). I ordered it and used it about twice and then it came up missing. It did super and it was super slick. The closest I can find now is "graphene" spray in Amazon: https://www.amazon.com/s?k=graphene+coating&crid=3CJS0Y5NB7SWB&sprefix=gr aphene+%2Caps%2C137&ref=nb_sb_ss_ts-doa-p_2_9

I was curious if that or something similar could be applied to the axle and inside the hub of the wheels. Are the inside the wheel hub burnished? BTW, I remember what you said about your wife preventing a build up of new tools after retirement, but a lathe with the right accessories sure is handy in many situations. I use my wood lathe for drilling often and on rare occasions tapping a precision hole. that lathe is good for burnishing and of course removing the extra weight inside the wheels.

[LCHIEN]

Here were the "family" entries, you could view them:


But they were more or less sequestered after checkin (weighing and checking dimensions) which makes sense so that they can't be altered or tampered with.
So no, I didn't handle or inspect the other cars.
There are rules but except for the weight and dimensions, I don't think they were strictly checked.
Specifically, concerning wheels and axles, you are supposed to use the provided wheels and nails from the BSA kits. With no modifications other than mild sanding smoothing and polishing.
no bearings. Only dry lube like graphite.
a lot of people sneak in prepolished grooved axles but as I noted of three I tried, all three looked much worse than the ones I polished in years before. A speed trick is to lathe the official wheels taking off material on the inside leaving the outside unattended but losing half the rotating mass. Expensive to buy. I haven't tried that. Then there's magical lubes, I haven't tried. So someone is beating my legal cars i don't know what they are using.

[leehljp]

Are you able to look at other people cars and inspect them - to see if there are clues as to the difference in speed?

Are you allowed to add ceramic or other wax coatings to the axle? or graphite or similar?

You got my curiosity up.

[LCHIEN]

delayed results... they ran out of time on race day so I got the 3rd place award family div, today.
My time was 3.12 sec (average of 6 runs)
Second place was 3.11 sec (apparently 6 ms faster)
First place was 3.08 sec.

[leehljp]

That is brilliant! Thanks for the explanation! I appreciate you taking the time to draw up the picts with the written details.

Thank you!

[LCHIEN]

Congratulations on all the great work you put in and the results that were garnered.

Loring, A question for you that I have mulled over and over the past two days: When the bend the nail as in post #7, 3rd picture - how or where do you put this nail and what is the orientation to make the car not hug the rail? I can understand it being an axle and changing the orientation of one wheel, but that doesn't appear to be an axle. I haven't figured that one out. Thanks.

For all the wheels we use a 2.5 degree camber. With the top of the wheel tilted toward the car. That makes the wheel run to the outside and rubs against the polished axlehead and not the sticky painted body - less friction. Also only the edge of the wheel touches the track to reduce rolling friction. So I build a jig to tilt the body 2.5 degrees on the drill press while holding the body against a fence to drill repeated distance from the bottom of the car and perpendicular in the other direction. More than about 2.5° will cause the upper edge of the wheel to rub against the body of the car.



One secret is to really make the car ride on three wheels - a tricycle. So I put the right front hole about .030 higher than the others and weight the car to the back so the right front wheel is off the ground. That allows the wheel to help steer the car with the center rail if need be, but the rolling friction and the energy to spin the right wheel are saved.



FInally, if your wheels are perfectly perpendicular to the direction of travel and all parallel, then it will roll straight down the track with no rubbing against the guide rail. But nothing is perfect and they have a tendency to wander either left or right over 40 feet. So this is where the bent axle comes in. I bend the axle a couple of degrees.
If the bend is up then the wheel is essentially parallel to the other wheels
If the bend is forward (as shown below) then the wheel has a 2 degree turn to the right
if the bend is backwards then the wheel has a 2 degree turn to the left.
Using the slot i milled in the head of the axle, I can rotate the axle to get a range of +/-2 degrees steering.
By running down a test track I can make the car run straight as possible to minimize contact with the center rail and the friction/speed loss it causes.
I say ~2° - I have no easy way to measure that but its about the smallest angle you can see by eye. I figure if I drilled the axles all perpendicular to the length of the car then the cumulative steering correction needed should be under a degree.

[leehljp]

Congratulations on all the great work you put in and the results that were garnered.

Loring, A question for you that I have mulled over and over the past two days: When the bend the nail as in post #7, 3rd picture - how or where do you put this nail and what is the orientation to make the car not hug the rail? I can understand it being an axle and changing the orientation of one wheel, but that doesn't appear to be an axle. I haven't figured that one out. Thanks.

[LCHIEN]

Results.
Grandson won third in his age class.
They didn't announce overall winners for Family group but here's three captures of a race with both of my cars (Right two lanes) finishing 1-2

[LCHIEN]

why did I make 2 cars?
Because I have multiple ideas and probably too much time being retired.
The Money one is faster apparently but the Midnight Lights one lights up which I think is unique.
I apparently can only enter one in the Family division. Last time I entered one under my name and one under my son-in-laws name and they were not happy.

Not sure which I should enter. Go for speed prize or design prize?

[LCHIEN]

This car deserves a video, called Midnight Lights

[LCHIEN]

cars tuned and weighted and finished

[LCHIEN]

How I put a screwdriver slot in a nail head.
Mount the dremel.
install a thin abrasive disc
chuck the nail into the drill press without turning it on
Lower the nailhead onto the spinning abrasive disc until the slot is cut




The nail is bent a degree or two. Turning it allows me to adjust the front end alignment so it runs straight to counteract any tendency to go right or left rubbing against the guide rail.