Pinewood Derby Wheel Modifications

Wheel Modifications

There are many ways to turn that $0.02 wheel into a proven winner. We will discuss the steps in depth. Just a brief description of how PWD BSA wheels are made. Although thermoplastic vacuum is not a complicated process, understanding why wheels that come out of the same mold can vary from piece to piece is an even more complicated process with variables that most don’t care to understand. The mold can be too hot, too cold, the plastic can be slightly thicker than specs or too thin.

The plastic can warm for too long or not long enough. You could have a top sheet of plastic that has more water vapor than the sheet below it, etc, etc, etc. Since these wheels are mass produced, quality control is not German engineering quality. At 600,000 cub scouters per year with 4 wheels per scout you have at least 2.4 Million wheels produced each year.

With 16 molds, it takes about 150,000 sheets of plastic if you did it perfectly every time. 150,000 sheets at 260 work days a year = 578 sheets a day. On a 12 hour work day it is approximately 49 sheets an hour or roughly a sheet every 75 seconds without a break! It’s about numbers! Not quality!

Because of this, you could have 4 Mold Match wheels that roll different ways. This simple reason you could rob yourself out of a win. Smoke and mirrors. Just because you bought 4 match wheels doesn’t mean they are exactly the same.

With that said, one has to understand that no matter how many modifications are done to the wheel, it will never be perfect. It was never designed to be perfect. It is not manufactured to be perfect. It will never be perfect. What we try to do is make the best lemonade that we can. Before reading further, take a break. Think about this paragraph.

To increase the performance out of your wheel the following steps are how we bring performance to those factory wheels.

Step #1 – Inspect each wheel for the true center. The #4 mold is the worst wheel out of the current mold. Simple visual inspection with the naked eye one can see it is off center. The wheel basically travels in an elliptical rotation. A customer made us aware of this flaw in manufacturing in Jan ’12. Since this information was brought to our attention, we no longer provide wheel sets with this mold. We simply throw it away. NO #4 molds from us. Occasionally a few other wheels are off center for unknown reasons, we simply throw them away, as well.

Step #2 – Ream (or simpler terms drill) the inner diameter to a consistent diameter. A reamer is a type of fancy drill bit used to make exact dimensions out of pre-drilled holes. There are multiple cutting sides to a reamer so that it will leave a consistent smooth finish. At Derby Dust we have determined that the best possible inner diameter is, of course, what the BSA engineers designed it to be.

That is the reamer size we use. This is the first modification that we do. This is to ensure that there is no binding or friction caused by smaller than designed inner wheel diameters. This is the key to ensure you are getting the best possible speed out of your wheels. With most molds, there is no material removed with this ream, however there are a few that need it done.

Step #3 – (optional) Removing material from the inner portion of the wheel. If allowed by your pack, you can choose this modification that will lighten the wheel. Using a dedicated computer controlled lathe, we take away a small portion of inner wall thickness to lighten the wheel.

This modification decreases the overall mass of the wheel, thus allowing for quicker starts. The scientific theory behind this has to do with rotational coefficient of friction. We leave the outer edge thickness as is, so that this modification only be seen to the experienced inspector. This process reduces the weight of each wheel by xxxx. Increase in speed is usually about xxxx on our test tracks when compared to factory wheels.

Step #4 – The next step (optional) if chosen by you, would be to remove the outer step of the wheel hub. Once the outer step is removed, the head of it is shaped similar to a dome. By decreasing the surface area where the axle and wheel touch, you decrease friction.

This modification instantly increases speeds. The tapered head of the speed axle rides on the smooth surface of the domed head of the wheel. Our data shows this decreases overall times usually between .01-.015 seconds when paired with our speed axles. Most packs allow this modification. PLEASE read your rules. YOU be the judge. It is your child’s race.

Step #5 – Once the head is shaped, then the wheel is chucked into a mandrel. The outer surface of the wheel is made to a consistent shape using another dedicated computer controlled lathe (we have many of these). Our dedicated machine does not change in the z axis, so our wheels theoretically should always be the same diameter.

Approximately 0.005” is removed from the outer diameter of the wheel. We find this to be the necessary depth to remove not only the high spots on the wheel, but also the low spots. With the computer controlled lathe, we are able to control the rate at which the machine removes material. Because of this we can produce a wheel that is appears to be completely smooth at first inspection. It will feel smooth to the fingernail test. No noticeable ridges unless you use a magnify glass. Then if you choose, we will wet sand the wheel to 1200 grit smooth finish. If you pack does not allow lathed wheels these are the wheel set that will be under the radar.

We do not do any work at this time to the inner hub facing. At this time we have not found that there is much benefit to machine work at this location. Removing material at this location will also require removing material on the width of the wheel. This violates most pack rules. We offer wheels from Derby Worx that have these modifications. There is no need to reinvent the wheel. We continue to gather data and with hundreds of completed cars each season, we are able to compile information that improves our process and our results.

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