Mitigating negative cant problems for taller bodies

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Stan Pope
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Re: Mitigating negative cant problems for taller bodies

Post by Stan Pope »

Here is a slightly different arrangement of ballast that provides room for rasping slope on the sides. Cost: loss of 1/16" distance from rear to axle.

Ballast: 1 row of pennies about 1.3" high epoxied across the back + side-by-side stacks of pennies vertically arranged and located according to CMx requirements. Axle in 1/4" diameter section tangent to rear penny stack.

Drilling the rear 0.75" dia probably needs waste wood and both waste and block clamped to drill press table.

Will need some careful planning as to location of the vertical stacks ... see tool at http://www.stanpope.net/cmcomput.htm.

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pwrd by tungsten
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Re: Mitigating negative cant problems for taller bodies

Post by pwrd by tungsten »

Key to relocating the COM is reducing the weight of the wood...

Cutting the car to 3/8" (I cut to 5/16" slab and drill holes but that may be a bit too tight for beginners).

Weight can be added on top of this to obtain any desired COM. (Ideally some of the weight would drop to the axle height. )

Stan has a car design that meets all of these goals and uses free weight... It is lead but pounded into shape... The lead is free from tire shops and ued to balance rims....
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Vitamin K
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Re: Mitigating negative cant problems for taller bodies

Post by Vitamin K »

So, I've been chewing on this some more.

I want to limit the bore holes to those done straight down into the top of the car. I feel that this would be simpler, and could be done with a hand drill (and a steady hand) as opposed to resorting to a press.

I also want to avoid external weights atop the car, so that they can paint a nice design on the finished car.

Here's my latest schematic:

Image

The second bore from the left would accommodate nickels and go all the way through. It would require epoxying two nickels into the bores, flush with the bottom of the car. Not a terribly hard task with a flat surface and some waxed paper, I think.

The bores in front are done with a smaller bit, 3/4", and are solely to drop weight. If they want to go further, it would not be difficult to notch out the section between the four bores with a coping saw.

I had initially worried about drilling two 7/8" holes into a 1.75" body, as there'd be literally no space on either side. However, I mic'd a few nickels and discovered that they were not .875", as specified on the US Treasury site, but actually a little less than .840". Armed with this knowledge, I can move the bore centers in a bit, and the nickels fit side by side, even with the overlap.

If I can steal some time on the workshop, I'll probably try to put together a proof-of-concept soon.
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pwrd by tungsten
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Re: Mitigating negative cant problems for taller bodies

Post by pwrd by tungsten »

Vitamin K wrote:So, I've been chewing on this some more.

I want to limit the bore holes to those done straight down into the top of the car. I feel that this would be simpler, and could be done with a hand drill (and a steady hand) as opposed to resorting to a press.

I also want to avoid external weights atop the car, so that they can paint a nice design on the finished car.

Here's my latest schematic:

Image

The second bore from the left would accommodate nickels and go all the way through. It would require epoxying two nickels into the bores, flush with the bottom of the car. Not a terribly hard task with a flat surface and some waxed paper, I think.

The bores in front are done with a smaller bit, 3/4", and are solely to drop weight. If they want to go further, it would not be difficult to notch out the section between the four bores with a coping saw.

I had initially worried about drilling two 7/8" holes into a 1.75" body, as there'd be literally no space on either side. However, I mic'd a few nickels and discovered that they were not .875", as specified on the US Treasury site, but actually a little less than .840". Armed with this knowledge, I can move the bore centers in a bit, and the nickels fit side by side, even with the overlap.

If I can steal some time on the workshop, I'll probably try to put together a proof-of-concept soon.
When I do a car like this (and it is often) I cover with plywood from the hobby store. I think it is 1/32" I use tungsten for weight though... But the plywood paints better than the pine...
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Vitamin K
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Re: Mitigating negative cant problems for taller bodies

Post by Vitamin K »

pwrd by tungsten wrote: When I do a car like this (and it is often) I cover with plywood from the hobby store. I think it is 1/32" I use tungsten for weight though... But the plywood paints better than the pine...
I think I'll be going with masking tape on the bottom and white vinyl (from a printable packing label) on the top.
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pwrd by tungsten
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Re: Mitigating negative cant problems for taller bodies

Post by pwrd by tungsten »

Vitamin K wrote:
pwrd by tungsten wrote: When I do a car like this (and it is often) I cover with plywood from the hobby store. I think it is 1/32" I use tungsten for weight though... But the plywood paints better than the pine...
I think I'll be going with masking tape on the bottom and white vinyl (from a printable packing label) on the top.
What is really slick on the bottom is the silver tape from Lowes or HD. Though I don't really know the name of it...
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Re: Mitigating negative cant problems for taller bodies

Post by ngyoung »

Just aluminum HVAC tape.
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pwrd by tungsten
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Re: Mitigating negative cant problems for taller bodies

Post by pwrd by tungsten »

ngyoung wrote:Just aluminum HVAC tape.
That is it! thanks!
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Vitamin K
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Re: Mitigating negative cant problems for taller bodies

Post by Vitamin K »

pwrd by tungsten wrote:
ngyoung wrote:Just aluminum HVAC tape.
That is it! thanks!
That stuff does look nice below the car...and is strong enough to hold tungsten cubes in place, if you use them. Just be warned that it is slick, and might send your car sliding a little further along the stop section. :)
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Re: Mitigating negative cant problems for taller bodies

Post by Speedster »

You could sand the car for clearance with the car I just completed. The car is designed for folks who struggle building a car. It is a basic wedge like the one shown in the pictures. Its highest point is 7/8". Pennies were used for ballast and have different weight through the years. A 1960 D penny weighs 0.110 oz. and a 2014 penny weighs 0.088 oz. which is 2.50 grams. I used all 2.50 gram pennies so the builder would know what to add at check in time if that became necessary. The car has a stock wheelbase and the block is left stock. After being cut the car weighs 1.396 oz. A 3/4" Forstner bit was used to drill the holes. The rear hole was drilled 1 3/8" deep and holds 19 pennies ( 1.67 oz ) with a bit of room for wood filler. The front hole was drilled 7/8" deep and holds 12 pennies (1.06 oz.). Total cost for ballast is 31 cents. Two 3/4" holes were drilled in the bottom if weight needed to be brought up. I do not have a weight amount for wood putty. The car, pennies, axles, wheels and allowing 3 grams for paint weighs 134.25 grams. The wood filler will bring that amount closer to 141.75 grams. Adding pennies and/or drilling a little wood should get them exactly 5 ounces. The car has a CM of 3/4" in front of rear axle slot. I drilled the holes to that depth to make it easy on the builder. There is plenty of wood left to drill the holes deeper which would allow the builder to taper the sides. The top of the car on one side would be removed but decal, body skin, duct tape would solve that problem. The builder might have to buy a 3/4" Forstner bit and take it along to a workshop, or to someone who would cut and drill the car, but that should be their only expense. If a struggling person follows the directions exactly they will not need a scale. We will have additional info on wheel prep. Folks will be directed to Derbytalk for The Physics of the Pinewood Derby and Troy Thornes book, Build a Winning Pinewood Derby Car. I hope this will help those who struggle.
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Re: Mitigating negative cant problems for taller bodies

Post by Vitamin K »

Speedster wrote:You could sand the car for clearance with the car I just completed. The car is designed for folks who struggle building a car. It is a basic wedge like the one shown in the pictures. Its highest point is 7/8". Pennies were used for ballast and have different weight through the years. A 1960 D penny weighs 0.110 oz. and a 2014 penny weighs 0.088 oz. which is 2.50 grams. I used all 2.50 gram pennies so the builder would know what to add at check in time if that became necessary. The car has a stock wheelbase and the block is left stock. After being cut the car weighs 1.396 oz. A 3/4" Forstner bit was used to drill the holes. The rear hole was drilled 1 3/8" deep and holds 19 pennies ( 1.67 oz ) with a bit of room for wood filler. The front hole was drilled 7/8" deep and holds 12 pennies (1.06 oz.). Total cost for ballast is 31 cents. Two 3/4" holes were drilled in the bottom if weight needed to be brought up. I do not have a weight amount for wood putty. The car, pennies, axles, wheels and allowing 3 grams for paint weighs 134.25 grams. The wood filler will bring that amount closer to 141.75 grams. Adding pennies and/or drilling a little wood should get them exactly 5 ounces. The car has a CM of 3/4" in front of rear axle slot. I drilled the holes to that depth to make it easy on the builder. There is plenty of wood left to drill the holes deeper which would allow the builder to taper the sides. The top of the car on one side would be removed but decal, body skin, duct tape would solve that problem. The builder might have to buy a 3/4" Forstner bit and take it along to a workshop, or to someone who would cut and drill the car, but that should be their only expense. If a struggling person follows the directions exactly they will not need a scale. We will have additional info on wheel prep. Folks will be directed to Derbytalk for The Physics of the Pinewood Derby and Troy Thornes book, Build a Winning Pinewood Derby Car. I hope this will help those who struggle.
Nice, sounds solid! Any pictures?

I hadn't known that the weight of pennies varied so much over the years. Definitely something to keep in mind when ballasting.
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Re: Mitigating negative cant problems for taller bodies

Post by Speedster »

The weight of the pennies wouldn't be a problem for a person with an accurate scale. This car was designed for a person that has absolutely no tools or maybe a person who just isn't interested in helping the scout. The total junk that came through the line for the last Pack I inspected broke my heart. I'm working with my brother about possibly publishing an ebook, or whatever it's called. I'm obviously not the one who will handle computer stuff. I'll try Tinypic again for a picture but it is a simple wedge. That last line should tell you how brilliant I am with a computer.
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Re: Mitigating negative cant problems for taller bodies

Post by davet »

Cut out a piece of 80 grit sandpaper and glue it to the backside of a junk wheel with a hole in the center for an axle to pass through the wheel. Trim the paper around the outside edge of the wheel. Now put an axle through the wheel and install on the car. Press the wheel against the body and spin it slowly. Should give you a nice groove that perfectly matches the diameter of the wheel and not really noticeable.
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