Pinewood Derby time

[Rivical]

Like Regal-Nut says, there's a number on the back face of the wheel. That is the mold# it was pulled from, and the #4's seem to be the most concentric of all of them. Our rules state that only the casting nub(on the tread surface) may be lightly sanded so you want the truest possible wheel as we can't modifly them SUPPOSEDLY! But I've found that by chucking them in my cordless drill and turning them over hand held sandpaper, you not only get the nub but dish out the center of the wheel slightly which also reduces friction points while helping to maintain a steady tracking wheel. Of course the sanding scratches have to be polished out too then! You'll feel how outa round the wheels are when you're sanding them like that too. Reducing friction is more important than reducing drag since they never get up to speeds where the forces of aero really come into serious consideration. Check out my "King" superbird, not particularly aerodynamic compared to a wedge. I even tried to handicap it with the steep w-shld and the nose smile brings it down to the 3/8" min height, making it slow offa the peg. But it smoked several wedges that dad's had made to come for my blood anyways! I put the lead as high up in the back as possible cuz we have a 50-50 track and it's the best compromise. The engineering behind that is that nose wt. brings you dn the hill, tail wt across the flat. By high mounting the wt., the center of gravity moves forward when the car is on the incline and shifts rearward on the flat w/o moving and causing wobble. You can establish where you should have your weight per what kind of track you're running on. Rear wt behind a tailight can be added or subtracted to bring it into weigh-in spec. and helps offset for the lifted wheel on opposite side front. By running the cars on an incline that's level side-to-side I make sure they track straight first and even weigh dn the nose to use both fronts and adjust them in case the lifted one ever touches. I've never used one of the metal axle alignment jigs avail on the net. Also, say your car pulls to the right. That could be as much fixed or a cause of left side wheels needing to be bent back as bending the right side whls forward. Make sure you're aligning the proper location. Several times down your incline while watching the wheel's position on the axles will show which one is pushing in or pulling out to the end of the nail.

"Pinning it down" is that I'll be using shortened straight pins to secure the upper and lower parts of my car as "No moving or loose parts" is in our rules. I can pull them out later to display it opened up then!

This year my body is cut from a single piece of 2x4. 2-5/8" wd.(cannot max out @ over 2-3/4" finished and painted) I cut the middle out 1-13/16"wd on the table saw and will Dremel it to as thin as possible. It fits over the 1-3/4"wd. scout block that will begin to become my chassis. Previously, as in last year's const. pics I've used 1/2" wd side blocks like the other guys have mentioned too, and pinned them to the main body with cut coathanger pieces for pre-glue alingment. Like was said, trace the cut out center section onto the side pieces then cut them idividually, mount em, then fine tune for fitment! Be sure you've Dremelled out enough wheel room before you get the epoxy out.

Lookin forward to enthusiastically talking with you guys more, but agreed that I wouldn't touch my car any more till Feb. Our derby isn't till end of April, boys haven't got their cars yet and it's not really fair to others to have 4months to perfect mine. So it'll be a while before I show progress pics.

 

[Rivical]

Race Jase I mistakenly veiwed you guy's car at first. Looked like you vortex several holes into that tail. I have SOOO wondered if that would increase speed like an aero undertray does on GT cars? Had actually considered making a Fighter Jet themed car but wondered if the speed achieved would make the idea practical or if even just slow it down by trapping air? Still curious about the merits of that idea!

 

[turbojimmy]

Thanks UNGN and Rivical for the tips!

Jim

 

[GNVenom]

Here are a few tricks to turbocharge your ride. First, fill in the factory grooves that were saw-cut into the block and, using a bridgeport mill, cut two new, perfectly parallel grooves into the block. This will assure perfect alignment of the axles. Next, using a 4-40 tap, run it thru the axle holes of each wheel. This will help maintain the graphite powder into the axles. Then, once you set up each wheel, take one of the rear axles and lift it upward to keep the wheel off of the pavement ever so slightly. This move will reduce the rolling friction by 25%. You can crown the wheels slightly without "shaving". It is critical that the car rolls perfectly straight. Also, the wedge design of the car is best aerodynamically (yes, aerodynamics play into the top speed). Be sure all axles are polished with crocus cloth and all ridges are filed off.

Back when I, er, my son, built my, er, his car, I used a chamber inverted diagonally into the body of the car filled with a glass mercury switch. The idea was that the mercury would roll to the rear of the car during launch, and slingshot to the front when the car got towards the halfway point of the track. It worked beautifully but was illegal since it was a "moving weight". I had to revert back to lead filling.

Well at least I let him paint the car. Actually, no, I didn't.

 

[Little6pack]

Tomorrow is my son's Cub Scout Pinewood Derby.

Because our cars always are covered in graphite, we figured we would just paint the car black this year.

The wheels were turning, so we decided to make a GN:

Dam that is nicer than my T!

 

[aminga]

Here are a few tricks to turbocharge your ride. First, fill in the factory grooves that were saw-cut into the block and, using a bridgeport mill, cut two new, perfectly parallel grooves into the block. This will assure perfect alignment of the axles. Next, using a 4-40 tap, run it thru the axle holes of each wheel. This will help maintain the graphite powder into the axles. Then, once you set up each wheel, take one of the rear axles and lift it upward to keep the wheel off of the pavement ever so slightly. This move will reduce the rolling friction by 25%. You can crown the wheels slightly without "shaving". It is critical that the car rolls perfectly straight. Also, the wedge design of the car is best aerodynamically (yes, aerodynamics play into the top speed). Be sure all axles are polished with crocus cloth and all ridges are filed off.

Back when I, er, my son, built my, er, his car, I used a chamber inverted diagonally into the body of the car filled with a glass mercury switch. The idea was that the mercury would roll to the rear of the car during launch, and slingshot to the front when the car got towards the halfway point of the track. It worked beautifully but was illegal since it was a "moving weight". I had to revert back to lead filling.

Well at least I let him paint the car. Actually, no, I didn't.

Wow, that's hardcore.

 

[Rivical]

Yeah, I was gonna say that too! :biggrin: I just eyeball our axle grooves, but then I have certified microvision:wink: !!!! I like the idea of tapping the wheels, even tho it's illegal for our pack. Except that you'd need a left-hand tap on the left side. Reason being that you want the wheels to be stable or move outward on the axles. Not good if they touch the body as the lefts would eventually riding on the thread points.

Just wait till this thread turns to axle prep and graphite application! That's where it'll get interesting and divergent in commitment levels!!!:biggrin: :biggrin:

 

[UNGN]

Also, the wedge design of the car is best aerodynamically (yes, aerodynamics play into the top speed).

All good tips, but aerodynamics in a pinewood car is a myth.

The ideal shape, if you can get away with it is to have the nose 1" or higher above the track. Because of the action of the starting pin, the higher the nose, the quicker the car will be, no matter the shape.

I build a test car with a 1.75" high nose and tested it against our normal cars (I have the pack's track set up in my upstairs hallway) and the high nosed car put 2 car lengths on all the other cars coming down the slope, even though the crap wheels and axles I put on it didn't have enough speed to make it to the finish.

No way aerodynamics will get back 2 car lengths lost at the start.

Our local pack discourages high nosed car, and runs them backwards, but build a car with a high nose and higher tail and play dumb, and they're going to let it run.

 

[GNVenom]

The aerodynamics don't play into the design until you're at the end of the track where the high speed of the car definitely makes a difference. I was able to prove it by setting up a PC on the track. We used a small proximity switch on each lane at the end which got triggered by the car hitting a tiny hinge, and a start switch off the gate at the beginning so we could very precisely time each run. The timer would compare all three lanes in 0.01ms increments so it was about as accurate as you could get (somewhere I still have that system laying around). By making subtle changes to the car, we could pick up precious hundredths of a second using trial and error variables. The wedge design always wound up fastest at the end, and, oddly, sloppy wheels made the car faster at the start. This was contrary to my original thought which led me to believe that wheels need to be tight and true...testing proved otherwise. By and far, the most important factor was that the car rolls straight. The test was to roll the car on a vinyl tile floor, making sure the car follows the tile lines throughout the pass. Also, placement of the weights seem to factor in as well - you want to position the weight so that there is a bit more towards the end of the car that both tires are contacting the pavement, and a bit less on the other end of the car (where you lifted the one corner). Lifting one corner off the pavement has a very significant effect on the speed of the car as well.

Seems there is some weird science involved in these little blocks of wood.

 

[UNGN]

I have a computerized 30' track set up in my upstairs hallway with a timer that reads to the .001 of a second.

For car design, a high nosed car kills all others. Its not even close.

If a 1.75" high nosed car that so much wheel friction that it can't roll 30 feet without stopping can put 2 car lengths on a car that got 5th place in the pack going down the hill, that trumps every other design.

The higher the nose, the quicker the car will be, regardless of what the rest of the car looks like, especially if the person running the starting gate is slow. The slower the starter, the bigger the difference will be.

For weight distribution, the rear wheels should be moved back as far as possible and the CG put as high up and as far back as possible so it starts with the most potential energy which converts into more kinetic energy

My high nosed car also has tungsten in the back and a center of gravity almost 5.5" from the nose. I will try to put a real set of wheels on it this week see if I can beat our tracks record time.

Without a set of cheater wheels, it will be hard to do, but I guess I could make a set of those, too.

This is why we don't try to win the races. The winners carry with them the stigma of being a cheater by the rest of the pack, so we make a car that will run in the top 10 and look cool enough to win a trophy.

If they tear ours down (not that anyone would in our pack, since we all know the winners are "cheaters", no need to embarrass them), it has an unmodified set of wheels and uses the axles and block of wood that came in the box.

 

[Rivical]

I admire you UNGN. You realized the stigma I'm trying to disolve and I just can't(or won't allow myself to 'throw the game') make a car uncompetitive enough to get the others to catch. I swear, I know they're cheating when I smoke em by lengths and after they come up the loser's bracket 7-8 runs later and have dropped out the graphite they were obliged to apply at weigh-in they nearly catch me. We've never cheated and never will. It's aggrevating to have never been beaten. I'm hoping to give help to a few dad's I like this year to make it competitive and show that it can be done within the rules. We can't high-nose and must oblige alla the same rules the boys compete with. I don't really care that I'm aware of the superior aero of wedge's either. Realistically, while the Scale speed is high, aero resistance increases at the square of Actual speed which you don't get That much of in these parameter's. So I'm not letting any speed guru's dictate that I not have fun with design. And regardless of how fast those wedge's are going at the end, if the aero actually matter's, they just can't catch a car with a cg that's pendulum shifting from a point an inch higher than their whole car! I run a single front wheel touching since the shift occurs rearward as speed has increased at the hill base. I've even tapered my bottoms from the 1/2" std. in front to the 3/8"min. at rear and use rear upper downforce to pull weight offa the nose wheel. It's all just physic's isn't it? Da-a ng. You'd think I had an ejucashon readin that!
I still believe these thing's speeds are more a function of friction elimination than aerodynamics tho.
Regardless I still wonder, if the aero really does play a part, would vortexing air into decreasing taper body channels, thereby increasing air velocity which spits that air out faster at the tail than the car speed...would you actually boost the car's speed like having little jets? Or slow it by the bottling of air w/o horsepower to assist that forward motion? Weird science with little blocks of wood indeed!!!

 

[UNGN]

Speaking as an Aerospace engineer that has actually tested stuff in a wind tunnel... If aerodynamics did actually matter to you, the air going under the car would actually cause more drag than all of the air going over the car, no matter what the shape of the body and the wheels would have more drag than the entire body (no matter what the shape)... which is why aerodynamics don't matter.

 

[UNGN]

Here are some pictures of the "Mach 5"

It's not as fast as it could be, but we aren't about speed. It beat one of our old cars by a car length or so, so it won't finish last and might win a race or two.

 

[turbojimmy]

Here are some pictures of the "Mach 5"

It's not as fast as it could be, but we aren't about speed. It beat one of our old cars by a car length or so, so it won't finish last and might win a race or two.

Sweet! How wide is it? Ours can only be 2 3/4" wide, so I can add 1/2" to each side of it. I bought some extra kits so I was going to cut 1/2" strips off of one of them. Our rules state that you have to use 'official' kits, so I don't want to use other lumber for it (they'd never know, but thems the rules).

Thanks,
Jim

 

[Rivical]

Niiiiiice !!!!!

 

[regal-nut]

love the look. wish we could see more picks from the other guys that help thier kids.

 

[UNGN]

Sweet! How wide is it? Ours can only be 2 3/4" wide, so I can add 1/2" to each side of it. I bought some extra kits so I was going to cut 1/2" strips off of one of them. Our rules state that you have to use 'official' kits, so I don't want to use other lumber for it (they'd never know, but thems the rules).

Thanks,
Jim

It is right at 2.75" wide and with paint, maybe 2.76" but I could have made it 2.70" wide and have the same look. The center block is the official block that came with the kit, as are the wheels and axles.

The tires are about .415 wide so the body can actually be slightly wider than the tires (Like a real Buick, you'll have to roll the fender lips carefully for clearance). I may try to make a car with completely enclosed wheels (with screw in chassis). It will be tough, but I think it can be done.

For buick GN, I've used a 1-by for the sides and cut it in 1/2 so I have two matched half that are less than 1/2" wide. I didn't like the wheel gaps on that car so we went for the tucked in look for this one

For this car, I cut the sides out of the 1-by-4, then used a sabre saw to free hand out the coke bottle shape, the roughed it with a dremel and sanding block. Do all the hogging on the inside of the sides and center block BEFORE you glue it all together.

Wil's car ended up having the 5th fastest average (of 34 cars) and the 3rd fastest individual run of his Pack (out of 150 timed car runs), but his best friends car rear ended it after the first race and a large chuck of fender broke off and it was just slowed down enough to be out of the trophies (His Webelos I were the fastest 4 cars in the Pack).

After the first race when he covered the field by a car length, I went "uh oh, I made it too fast", but thankfully the crash slowed it down. Since I was running the track computer, I didn't want anyone to think I was trying to let my son win.

 

[turbojimmy]

It is right at 2.75" wide and with paint, maybe 2.76" but I could have made it 2.70" wide and have the same look. The center block is the official block that came with the kit, as are the wheels and axles.

The tires are about .415 wide so the body can actually be slightly wider than the tires (Like a real Buick, you'll have to roll the fender lips carefully for clearance). I may try to make a car with completely enclosed wheels (with screw in chassis). It will be tough, but I think it can be done.

Sorry to hear about the damage. You've got the skills to repair it though.

Our derby isn't until 3/27. My son wants to do an Audi R8. I have a plastic model and the wheelbase is perfect, and if I add sides to it the width is pretty close. The block is just a little short height-wise so I'll need to add a 'roof' to it. It's a fairly easy shape to duplicate.

Jim

 

[Rivical]

I actually used the plastic kit sides and narrowed tail on my Ford GT. What a headache! But it was accurate if not slightly chopped!

Oh hey, if anybody likes Lambo pearl orange!?! You can duplicate it with Testor's Competition Orange sprayed till it's finished. Then mist their Boyd's Alumacoupe Yellow Pearl over to your liking while the last coat is still wet!

 

[Little6pack]

Ha my son just came home with a kit for his cup scout derby.

I may copy your design of the GN. Is it trademarked