Last Hurdle

[schmidy]

Hello Fellow Derby Enthusiasts,

I am a newbie to this forum, but have been a lifelong scouter with two of boys currently in scouts. I have been lucky enough to have taken all I read on the Internet and get my boys all top finishes in their Pack. My problem is getting past the three or four other faster cars at the District Level. We are close, but just not there.

Here is what we are running: a low profile wedge design, some with a big piece cut out of the middle of the block and others just smooth, top and bottom. All are exactly 7" and 5 oz. I have taken great care in smoothing the wheels and axles and used a fine graphite product I bought on-line. The wheel base is the max allowed at 4.5". COM is generally in the 1-1.5" area in front of the rear axle. I also feel that the car should be fairly wind resistant and i spray several coats of Crylon on it as a finishing step. I do not use the slots provided in the kit, instead I use the drilling tool to drill new holes. One thing I wonder is where is the best spot to put the rear wheels? I generally put them so they are even to back edge of the block and always try to put as much weight as possible toward the rear. Is there any preferred measurement from the rear of the car to place the rear axle holes to allow for the best position of the COM? Please let me know if you have any suggestions. Our Pinewood Derby is third weekend in Feb and the Boys and I are anxious to get started.

Schmidy

[Dice Crew Racing]

Schmidy,

We've always had good luck with 5/8" from the rear and 1/8" from the bottom for rear axle holes. This leaves room for 2 oz of tungsten or 1.24 oz of lead (roughly) behind the rear axle, if you are drilling 3/8" holes.

Hope that helps.

Dice

[Parrot Racing]

Welcome Schmidy! If you can keep the stability, I would move your com back to 1" max, even more depending on your track and alignment. You didn't mention alignment so I assume you are tuning for dead-on. I would read up on the railraiding techniques posted by the veterans in the forum, that should help squeeze a little more out of your cars. Also, make sure when you are wieghing in, to keep adding weight until you reach the 5.0. Our pack scale weighs to the 1/10th of an ounce, so we get our cars to 5.0999 on our scale at home. Every little bit helps. As far as rear axle, I also put them dead even with the rear of the car. Not sure what kind of weight you are using, but if possible, use tungsten. You can put more weight in a much smaller space than with lead or zinc. When adding your weight, I think it is also important to have it as low as possible, try to get it so as much of it is at or below the axle plane and still have ground clearance.
Hope this helps, and good luck.

Enrico

[Go Bubba Go]

We mount the rear wheels as far back as allowed. Don't have the exact dimensions, but here's how we do the rear wheels:

We mount the Pro Body tool on the block, then insert the drill bit backwards into one of the holes. We then take a spare wheel and slide it over the bit so that it is resting against the tool. We place another block sideways against the back of the block, and slide the tool back until the mounted wheel just baaaaaaarely touches the other block. We then back it off just a little to leave a little gap between the back of the block and the very rearmost portion of the wheel, basically just to protect against rough handling with the "length" check-in equipment.

Once we have the right location for the tool, we clamp it down to the block. We then turn it over, insert the bit backwards, spare wheel, etc. and ensure we have the same minimal clearance on the other side. If it checks out OK, we drill. If not, we move the tool forward a little until we have the right clearance on both sides, then clamp and drill.

I use this procedure with the Scouts that come over for help. It is easy for them to understand (i.e. "don't let the wheel rub against the back block" instead of "5/16 this, 3/8 that"). Once the tool is clamped in place, you can hand the block, cordless drill, and safety glasses to the boy and let them "have at it".

We use a similar procedure for the front wheels, only leave a bit more space between the front of the wheels and the front of the car to allow for rough stop sections (impact body instead of wheels) and last-minute length adjustments, although those should be minimal to non-existent.

As to your other specs, we are more agressive rearward with COM (3/4" to 7/8"). We use the 1/4" Tungsten cubes, just under half immediately behind the rear axle and the rest immediately in front of the rear axle. We also leave a little 1/4" square for filling with Tungsten Putty that can be filled and thinned out as needed for fine tuning of weight at check-in. Don't know your District, but chances are at the District level that the top few competitors are closer than 1 to 1.5" with their COM.

Our 2 cents.

Bubba

[ProQuest]

Hey Schmidy:

I'll second the other's thoughts and add a couple of my own. A BSA wheel after truing will be less than 1.2" in diameter. You can safely position the center of the rear wheels .6" from the rear end of the block. 5/8" is equal to .625" so there is only .025" difference between 5/8" and .6" but I use .6" so I thought I would mention it. Also, I use 3/16" from bottom. This will leave only a few 1/1000" clearance over the 3/8" requirement, but if it passes inspection (and it will), then it will have no problem on the track as the center rail is generally only 1/4" high.

The other suggestion I would add is get a little more aggressive with your center of gravity. I like 7/8". I know some really fast builders who go even more aggressive at 3/4". If you are running on a very smooth track and have good to excellent alignment, 3/4" might be better.
I can't really say since I've never gotten that aggressive.

I'm not the resident guru. Just my thoughts. Hope they help.

ProQuest/Steve H.

[Scott Schmid]

Thank everyone for the tips.

Schmidy

[Gravity Steve]

parrotracing said:

Welcome Schmidy! If you can keep the stability, I would move your com back to 1" max, even more depending on your track and alignment. You didn't mention alignment so I assume you are tuning for dead-on. I would read up on the railraiding techniques posted by the veterans in the forum, that should help squeeze a little more out of your cars. Also, make sure when you are wieghing in, to keep adding weight until you reach the 5.0. Our pack scale weighs to the 1/10th of an ounce, so we get our cars to 5.0999 on our scale at home. Every little bit helps. As far as rear axle, I also put them dead even with the rear of the car. Not sure what kind of weight you are using, but if possible, use tungsten. You can put more weight in a much smaller space than with lead or zinc. When adding your weight, I think it is also important to have it as low as possible, try to get it so as much of it is at or below the axle plane and still have ground clearance.
Hope this helps, and good luck.

Enrico

Huh...who'da thunk? I missed that one. I went and drew a force diagram, and sure as shootin', the lower the weight is in the car, the more difference in the potential energy, from top to bottom of the track. <runs to buy more tungsten, since there is even a little more to be gained>

I've picked up three speed tips in less than three hours. As a ROM, I'd say there's probably 10 milliseconds in the batch. <muttering, stumbles off to do the math>