John H

My son is building a pinewood derby car for cub scouts. We have the chassis down to 2.5 oz (includes wheels and axles but not paint). So we are in a three way argument about where is the best place to put the ballast to raise to the maximum weight of 5.0 oz.

We made a wedge shape type formula car so the front end is thin. I planned on helping him drill a hole from the back as far toward the front as possible to add bb's or something that we can fiddle with depending on the scales. My son wants to drill down from the top (bigger hole) to put a driver's head figurine on it near the middle and put the weight under the driver. My wife thinks the weight should be as far forward as possible (skiing analogy).

I think it is better to spread the weight out so the friction on each axle is the same rather than weight one end where friction will be greater on the front for example. I hate to make anymore drag by having a head sticking up out of the car either. Did I mention this is my son's cub scout project?

Anyway, I eagerly await this board's input. Maybe I should call the Tappet brothers. I am sure they'd have a field day with it.

BC

This is an age old consideration. There is no need for traction bias, so the weight, I believe, is only going to technically impede the initial acceleration per the down hill slope. But the weight is also used to make sure the car falls (rides) down the slope.
Its a compromise. The 5oz could simply be a box with wheels, but you have the aero consideration, and the friction consideration.

You have the aero down, and the friction consideration may not be changeable.

If Anything, the weight could possibly be centered so as to reduce the per-wheel weight of say, a front or rear ballast - thereby reducing the average friction on each wheel.

pu911rsr

John,
Been a few years since I built one of these but it seems that weight placement is more of and issue with straight line stability more than anything else. Google the subject and you'll get some good answers.
Good Luck,
Phil

BC

Less friction on each wheel, aggregate. So put it in the middle.

ritzblitz

That's a great question. I would probably end up trying to keep the weight centered. Although, I'm not sure if it would make a difference if it were 50% placed on the very front and 50% placed in the very back or 100% between the axles.

John H

pics (please keep in mind he's 9 and I'm useless with saws and dremels)

ervtx

I can't believe no one has gotten this right!

John, go look at your car. Where's the engine? Where's the weight bias?

Nuff said. Put the weight in the back!

Jimbo951

Put the weight toward the rear. The race starts with the car pointing down a ramp. By putting the weight in the rear, you're starting with more gravitational potential energy. Once the car is at the bottom of the ramp and on the flat part of the track, the extra potential energy will be turned into extra kinetic energy.

ervtx

Agreed. Just don't put so much in the rear that it pops a wheelie on the flat section. As the picture I posted earlier illustrates, there's plenty of wiggle room for your son to add figures and still get the majority of the weight in the rear (behind the rear wheels).

BTW, I don't think the whole osmosis thing you've got going on there with the latest issue of Panorama will help. Nice try

rlm328

As mentioned rolling friction is a large concern on these cars. When my son was involved in building them the fastest cars at the events were running on 3 wheels not 4. They were balanced in such a way that one of the front wheels (the axle was also pinned a bit higher on one side) was not touching. Don't know what kind of tech inspection they go through now a days.

ervtx

Now that's taking a page from Porsche's racing success of old! Know the rules and exploit them to your advantage.

onefastviking

I'm going to take a different approach with this one.
The customer is always right, build it the way your son wants. Remember while it is about learning, if he doesn't have fun then he won't learn, it's his car let him build it his way.
He will see at the races which design may or may not be faster and you can discuss it then. There is always next year to build a faster car.

Veloce Raptor

+ Juan +

John H

I was thinking about the "longer" run for most of the weight as being a factor but wasn't sure if that 4 inches or so would offset the wheel/axle friction co-efficent. I am 100% positive we will sit down this evening and discuss all of this. It was the first thing he said to me this morning when he got out of bed.

I agree with the posts above, the decision is his. We've had a great time bouncing ideas off each other.

John H

ERVTX

Love the flames. I'm going to show Ethan that pic this evening.