Welcome to Pack 760's speed hints page. This is by no means a comprehensive listing of everything that you can do to speed up your car. We will leave the wind tunnel testing for others to complete. What we have attempted to do is to provide some practical hints that can enable Scouts to build competitive cars. If you have any ideas or suggestions send them to the Pinewood Derby Committee.
1 - Designing
There are two things to think about went you begin designing a Pinewood Derby car. The first thing to think about are the dimension restrictions in the rules. The second is planning ahead for adding weights to the car. For more help with designing your car take a look at our Design Hints page.
2 - Rules
Understand and follow the rules. The fastest car can't win if it never gets a chance to race.
3 - Weight
You may or may not remember your high school physics (or if you are a Scout you can look forward to them) but the principals of Potential Energy and Kinetic Energy were described. The bottom line is that the heavier the car the faster it may be able to go the track (depending on friction). One of the most significant things you can do to ensure that your car is competitive is to ensure that it comes in at or very close to 5 oz. To get the average car up to 5 oz. you will likely need to add 2 or 3 ounces of weight to the car. But where on the car do you add the weight?
Plan ahead where to place the weights. Be certain to mill, carve out, or drill holes in the car to set aside a place to easily add weights. It is much to easier to drill or mill out space when the block is square but almost impossible after painting and wheel assembly. In addition, failure to plan space for weights at the in the design process has often lead to last minute damage at the registration weigh-in.
Anything can be used to add weight to a car so long as the weight is fixed and securely attached to the car. However there are some preferred methods the "Pros" use to bring the car up to maximum weight.
Pinecar Weight Blocks: these are perfectly legal and can be purchased at local hobby shops. They are made of a relatively safe lead alloy and can be screwed into a pre-formed pocket on the bottom of the car. The method is to show up at check in with the car slightly over 5 oz. After the first weight check, parts of the weight block can be broken off to bring the car to legal weight.
Lead Tape Weights: Available at local hobby shops. These are generic lead weights with sticky foam tape on the bottom. The mass of these weights is greater then the Pinecar Weight Blocks so they can take up less space. They can be stuck to the car as is or with hot melt glue. The method is to show up at check in with the car slightly under 5 oz. After the first weight check, additional pieces of weight block can be added bring the car to legal weight. These weights should be handled with appropriate care as they do contain lead.
Lead Fishing Bullet Weights: Available at local sporting goods stores. These weights come in various sizes and weights (typically 1/16 oz and 1/32 oz). They can be used to fine tune an underweight car to the maximum allowable. The method is to show up at check in with the car slightly under 5 oz. After the first weight check, the bullet weights can be pressed into predrilled holes in the bottom (or side) of the car. A touch of hot melt glue secures them in place. Pre-drilling means putting the holes in before the car is painted...adding the holes at check in is almost impossible with out damaging the car. These weights should be handled with appropriate care as they do contain lead.
In regards to where you should locate the weights, you achieve the highest potential energy by adding weight towards the rear of the car as high as practical. It won't make a huge difference but then every little bit helps. The theory is that the higher weight pushes the car for a longer period of time.
Please keep in mind this can be overdone, but a simple test to make sure the car is properly weighted is to balance the finished car on a pencil. The car should balance somewhere in front of the rear axle. Otherwise, the car will to tend to do wheelies and won't necessarily go straight down the track. The track section joints can provide a significant jolt to the car. This can make the car's front wheels jump up, maybe even enough to derail it from the track.
4 - Axles
The axles provided (nails) in the official BSA kit must be used. The "nail" axle out of the box may look to be in good shape. Do not let the fact that it may be bright and shiny deceive you. If you look carefully you will see at least two metal flashing "webs" between the nail head and the shaft. Sanding the axles to remove the die flashing material that are a result of the manufacturing process is recommended. These small metal "webs" can cause a lot of friction if they are not addressed.
Mount the axle in a drill motor or drill press and to perform file and polish these webs away. Use a file and/or sandpaper with water or light oil to polish them. Use 600 grit sandpaper or finer. Polishing the axles with toothpaste or jewelers' polishing compound is good idea. Axles should be polished smooth to make certain that the wheels can roll with minimal friction.
Chroming, or any other type of special technical process is not allowed. To reduce the diameter of the head of the nail, in order to minimize friction is allowed - within limits. To reduce the size of the head of the nail by more than 25% is considered extreme and is not permitted. A properly finished axle should look like this:
Significant beveling, rounding, tapering, thinning, perforating, or any other significant altering of the shape or performance of the axles or axles heads is prohibited.
In addition, keep in mid that the axles must be mounted into wood.
5 - Wheels
Imperfections in your wheels cause friction in many areas. Sanding your wheels, however, can be tricky. If the wheels are not precision sanded or lathed, you can actually make matters worse!
A wheel sanding mandrel can be used to help sand the injection burr off the wheel. They can be found at local hobby shops. The mandrel is mounted in a drill motor or Dremel rotary tool. The mandrel can check for a wildly malformed or warped wheel. BE CAREFUL!! It is incredibly easy to melt the surface of the wheel when high speeds are combined with a little pressure. Many a wheel have been destroyed with these mandrels. Use 600 grit paper at a slow speed to start.
A properly polished wheel should look like the following:
Insure your wheels roll smoothly and are not binding. A good test is to spin each of your wheels with your finger and let them run to a stop. It should take 20-30 seconds for well tuned and lubricated wheels to stop completely. You will want to rub some dry graphite on the surface of the car body where the inner wheel hub touches. You can't use washers so minimize the friction of these rubbing surfaces. Another trick is to rub the entire surface of the wheel with graphite too. Messy, but it doesn't hurt.
Wheel bearings, washers, or bushings are prohibited. This includes "wheel covers" which serve to keep the wheel hubs from contacting the car body. The district also prohibits "hub cap" decals or anything else that might serve as a lubrication "well" or "tap".
Examples of ILLEGAL WHEELS:
Examples of finished wheels that are legal for use:
6 - Straight
Insure that your car is tracking straight. A car that bumps the lane guide more often gets slowed down more often. A "front-end alignment" may be necessary. The slot is fine for installing the wheels but ensure your cuts are absolutely square to the car body. You can also drilled holes into the body (flipping the block upside down) but make sure they are also drilled at the same distance from the bottom of the car (impacting ground clearance) and that you do not modify the distance between the wheels (see rules for the specific tolerances).
One method to assist in getting the wheels inserted straight is to sacrifice an axle, cut off the head and use a drill press to insert 80% into the car. This significantly helps in getting the axles in square.
Always check the car by rolling it along a smooth clean surface. If it veers to the right or left too much, the axles are crooked. Just like steering a car, you need to adjust the steering on your car. To do this, you must adjust one or more of the axles so the car rolls straight. You can do this by re-drilling the axle holes and re-inserting the axles.
There are a few different alignment procedures, and they vary depending on the use of axle slots or holes. The best procedure is to use shims made from wax paper to adjust the alignment. This can be a long, time consuming and often frustrating part of the build - set aside the time to get it right.
7 - Lubrication
Use a good dry lubricant. Dry graphite seems to work better than the white Teflon compounds now available.
A dry lubricant such as graphite, or the BSA white dry lubricant (powered Teflon) is allowed. Some oils are caustic to the plastic wheels, causing damage. Therefore, all "wet" lubricants are not allowed.
The lubricant should be applied to the wheel hubs, axles (nails) shafts/heads, and all other contact points where the wheel touches the body of the car.
Dry graphite is a messy material. Please do not bring this material into the building. When spilled, it is impossible to remove from the floor, and is a potentially dangerous substance, if ingested.
Please apply the lubricant at home, under adult supervision, or outside in the parking lot. It is helpful to "work in" the dry lubricant, by spinning the wheels, often. A car can only be lubricated prior to confiscation. Cars may not be lubricated while competing in a set of heats.
8 - What not to do
Never roll your competition pinewood car on the ground or concrete. These rough and dirty surfaces can ruin the car's wheels, axles and alignment.
Don't waste a lot of money on all the elaborate tools you can by for pinewood derby cars. Basic household tools are more than adequate. Ask a Den Leader or contact the Pinewood Derby Committee if you need help or do not have adequate tools.
Don't ruin a good thing. If you build an adequate car that can roll straight don't spend too much time making it "better". The final cars are often quite fragile. If you spend too much time attempting to make it "perfect" you can damage it. Changes can sometimes work in the reverse of the best intentions.