Pinewood Derby Axles

[Jim]

Thank you to all of you who have provide tips on Pinewood Derby cars in response to a previous post. We would like to ask a couple of follow-up questions:

1. What is the best way to make sure that the axle slots are square to the body of the car? We have the clear plastic tool sold at the Scout shop that helps make the determination. However, we were not sure the besy way to "straighten" the slots and then get the nails/axles to follow the new line of the slot.

1. We used the dry lube sold by the Scout shop. Everyone has been mentioning using graphite. We are only allowed to use dry lubrication, but we can use graphite. Is that what is typically sold in the Scout shops? In other words, do we need to look specifically for dry graphite lube or is all of the dry lube graphite anyway?

2. How do you straighten the axles in order to make the car go straight? When we tried to do this last year after the axles were on the car, it seemed to be impossible to move them and have them stay in place. Is there a good process for doing this? What are some steps before and after the wheels are on the car?

Thanks again for your advice. We are looking forward to using all of these tips when we build our next car.

Jim

[Geo]

My answers are based on what me and my son did for his car (1st place his Webelos division). YMMV, but it worked for us.

Depending on the rules you can do a couple of things. Last year we didn't use the slots that were there but rather drilled holes in the side with a drill press. This year, we did something a little different. We filled in the slots with a filler and re-cut the slots ourselves. We simply used a square and laid out the first axle. Then, we very carefully measured that point ot the rear axle and laid out and cut that axle.

If it's sold by the scout shop you should be in good shape. One of the things we did was to polish the snot out of the axles. An additional step that someone recommended was to place your nail in a drill press with the wheel on. Hold the wheel as the nail is spinning and add lube to the axle while it's spinning. Apparently, that helps to further polish out the axle and embed lube in the wheel and axle. Seems to have worked for us.

Bruth force! Actually, if you lay it out carefully you will get a fairly straight riding car. Any adjustments would be simply grabbing the wheel and bend it by hand. It doesn't take much.

Just a couple other things that might make a difference. I've heard that having 3 wheelers makes a difference. Check your rules on that one. That is, one of your wheels simply doesn't touch the track. The friction of three wheels is less than four. Also, on the wheels themselves some people make the part of the wheel where the nail goes through convex. Again, less friction. And last, again with the wheels, people also sand back, at an angle, the portion of the wheel that could touch the car.

[info]

besure to see the pinewoodderby tips at the PinewoodDerbyDen.Com web site....

[Doug Miller]

Incorrect -- the total friction is unchanged, because the weight supported remains the same. With only three wheels touching the track, the wieght on each wheel, and consequently the friction force on each wheel, goes up by 33%.

Again, incorrect. Changing the size or shape of the contact area does not change the amount of friction.

[Bill in Detroit]

When I worked at Delta Research in Livonia, MI we sometimes used molybdenum powder (molybdenum di-sulfide? I'm not certain of the exact name -- nearly 15 years since I used it). I was told at the time that it was much more slippery than graphite because it is shaped like little teeny-tiny ball bearings.

And it's very, very dry.

Bill

[onoahimahi]

A three wheel car is faster because it takes less energy to spin three wheels than four.

Moving the friction point closer to the axle reduces the torque on the wheel.

[onoahimahi]

You can get graphite with molybdenum and it really makes a difference over plain graphite. We timed our wheels and they all spun at least 5 seconds longer with the molydbenum lube.

Bill

[Doug Miller]

Again, incorrect. The total energy is the same, because the total friction is the same.

If anything, a car riding on only three wheels is likely to be somewhat slower: since the friction force will be imbalanced left-to-right, the car will travel a slightly curved path, with the two wheels on the concave side.

And that makes a difference how?

[KERRY MONTGOMERY]

Regarding 3 wheels VS 4; 3 wheels will require less energy to accelerate than 4, won't they? Kerry

[Tom Watson]

Watson

tjwatson1ATcomcastDOTnet (real email)

[J. Clarke]

A little physics is a dangerous thing.

Doug really should perform some experiments with actual pinewood derby cars before he pontificates. He is likely to find that his knowledge of physics has some gaps.

[Doug Miller]

Go back a few posts and see where the 3 vs. 4 discussion started. The case being discussed is 4 wheels with only 3 in contact with the ground, vs. 4 wheels with all 4 in contact.

The friction forces are _exactly_the_same_.

[Doug Miller]

Care to point them out?

The friction forces are _exactly_the_same_.

[Doug Miller]

[KERRY MONTGOMERY]

Doug, Wasn't questioning the friction forces at all. It seems that accelerating 4 wheels to the same rotational speed will take more energy than accelerating

3 wheels to the same rotational speed. Not due to friction, but due to the rotational inertia of the wheels. Which would mean that the total energy would not be the same, even though the total friction is the same. Kerry

[Doug Miller]

Perhaps -- but the mass of one wheel is very small in relation to the mass of the body of the car, and the radius is only about 15mm, so the moment of inertia of the wheel is negligible. Any gain achieved in this manner is, I suspect, likely to be more than offset by the fact that the unbalanced geometry will cause the car to deviate from a straight line.

[onoahimahi]

Again incorrect - you said above "If anything, a car riding on only three wheels is likely to be somewhat slower" but this reference, as well as the comments of other posters who actually know how to help their sons build fast cars, says "raising one front wheel does increase speed considerably"

I can speak with authority on this subject because my son's last two cars have been superfast not only winning first place pack but going on to win first place district. Last year at the District race, he had three of the 5 fastest times of the day for all age groups. Further, we got to "Support our Troops" in a small way by helping a boy whose dad is deployed in Iraq. He came down to my shop and we helped him build his car and he won first place in his pack. Both cars had a lifted front wheel.

-Scott

[fredfighter]

It makes a difference in the angular acceleration of the wheel.

It's a real small difference, but if you win by half an inch, you win.

If you can lighten the wheels by removing mass nearest the hub that will help too.

[fredfighter]

This is true. Yet if you want to slide a heavy object, it helps to put it on skids, even if the object has a flat side and it's on a hard surface. I've never been able to figure out why.