Design for Speed

If you want to make a really fast Grand Prix car, you need to know which design factors increase your car's available energy and which ones decrease it. You must start from your initial design to get the most in speed. Some races judged by electronic timers have been decided by one thousandth of a second (that's less than a quarter of an inch at the finish line).

There is probably NOT a single BEST design. Given the competing nature of some of the performance factors and track shapes, there IS room for difference in design. Though cars shaped like high performance grand prix racers may win, a car designed by following these procedures to the letter will tend to look more like a dragster. These cars should win more often given the wide range of Grand Prix race tracks and judging.

Factors That Increase Energy Available for Speed

1. Length of your car

2. Location of your car's balance point

3. Total weight of your car

Factors That Decrease Energy Available for Speed

1. Rubbing between axles and wheels

2. Resistance of your wheels to spin

3. Wheels that bob up and down (your car is not flat on the track)

4. Wheels that are not aligned straight

5. Rubbing between the air and your car

6. Rubbing between inner wheel hubs and your car body

7. Rubbing between axle heads and outer wheel hubs

Each of these factors must be dealt with in some way in the design itself because they affect each other. For example, in order to place the balance point where it should be, you will make your car body as light as possible. This allows most of your car's weight to be free weight that can be precisely positioned in the rear of your car. However, the nose of your car may become so light it could hop off a bumpy track!

If you make the body too slight, how will you secure the free weight to the car or keep it from breaking when it hits the pillow or blanket at the end of the track? So there are real trade-offs with real consequences to be judged and weighed in the design of your car. How much of a risk-taker you are may determine how you apply the procedures in this manual.

Grand Prix Racing -	How To Make A Fast Pinewood Car
Copyright © 1997, 2000, 2001 by Michael Lastufka, All rights reserved worldwide.

In order to polish or grind your axles:

1. Insert the end of the axle into a drill chuck and tighten. Do not put the hub end in a drill chuck.

2. Put the abrasive material in a jig to hold it, or wear leather gloves and hold it in one hand. It helps to bond the material to a rigid holder for best results.

3. Apply some of the polishing compound to the axle or abrasive. If no polishing compound is used, set the direction of the drill appropriately for a left or right axle. (See Definitions below)

4. Trigger the drill and carefully press the stem of the axle on to the abrasive medium, lightly. Care must be taken so it does not race off the medium onto your hand or heat the medium so it burns you. It can be tricky to keep the hub away from the medium so it doesn't cause a jump or excess grinding of the hub.

5. Move the axle back and forth along its axis on the medium to help even out any ridges that may form around it. A polishing compound helps prevent this.

6. Polish with successively finer abrasives until your axle is bright and shiny or the desired diameter has been obtained. For a final mirror polish, polish with a lintless cloth and polishing compound.

As long as even pressure is applied along the stem from the hub and out about 3/8 inch from the hub, a good result can be obtained.

If you slope the back of the hub with a small file, don't over do it. Excessive "coning" can act as a cork in the borehole of the wheel and increase friction. On the other hand, a flat back on the hub can be bad if the wheel has roughness in the hub area.

Definitions

The right side of the car is the right side of the car when the car is facing you front first.

Looking at the axle with the hub end pointed away from you,

• counter-clockwise grinding is used for a right side axle
• clockwise grinding is used for a left side axle

Prepare the Wheels

Wheels from different kits have a few important differences. For the most part, they can be prepared in the same way. However, the different types of plastic and the molding processes used dictate that some kit wheels need more work than others. For best results, get access to an electric drill and have some fine sand paper (220 grit) and lintless paper or cloth ready. A hobby knife will be used first.

Remove Flash

Kit wheels most often come with little pieces of plastic still attached from the molding process. Carefully cut this molding flash off, using a hobby knife. Try to get a straight clean cut that is as flat as the rest of the surface. You can carefully plane off slight ridges in the plastic by cutting at a grazing angle. Do not apply much force when cutting at an angle or the knife could slip and cut you. Instead, rock the blade slowly along its edge to move it forward.

Round-up Tread

Now, mount each wheel on an axle or mandrel drill attachment as described below. Our purpose is two-fold. First, we want to check the roundness of the wheel and correct gross deviations. Second, it is good to sand the edges of the tread round if they are sharp. The inner edge of the tread will be rubbing the lane median and the outer edge will follow wood grain in the track if left sharp.

In order to sand and polish a wheel's tread:

Get some help in mounting and sanding the wheels with the drill. An improperly mounted axle or mandrel attachment can be dangerous.

1. If you have a mandrel attachment, mount the wheel using it. Be sure the wheel is centered. If you can't center it, the wheel may be grossly out of round. If you don't have one, wrap a small triangular piece of masking tape around an axle - the point of the tape should extend away from the hub so the wheel can slip on more easily.

2. Put the wheel on the axle snuggly. The wheel may slip on the tape, so be prepared to re-tape a couple of times until you discover just how much is needed.

3. Mount in the drill chuck.

4. If you can, anchor the drill in position with a vise.

5. Secure 150 to 220 grit sand paper to a flat surface.

6. Rotate the wheel - the direction won't matter if you polish it as described in the last step.

7. Carefully press the tread flat on the sand paper. If the wheel slips in the taped axle, try re-taping.

8. Sand over the outer edge of the tread to round it off.

9. Sand around the inner edge of the tread to round it off.

10. Polish the tread and inner edge of the wheel with the lintless cloth or paper. This works especially well with AWANA wheels since the irregularities left from sanding melt back into the soft plastic matrix without deforming the wheel. This melting is highly localized so the wheel is not distorted.

Polish Wheel Bores

For some kit wheels this step is the difference between fast cars and very fast cars. Inspect the wheel bore (the hole the axle goes in) with a high power hand lens (like 10X).

This procedure can leave a mirror polish. Dimples in the bore won't be helped. If there are plastic filaments in dimples, raise that wheel to reduce its impact on speed. Try to get new ones if this happens on more than one or two wheels.

To polish (more accurately - burnish) your wheel bores, you'll need a polished, thin metal rod, awl, or needle-nose tweezer. A polished axle can work if you can get a good enough grip on the two ends with a wheel between. With the right implement, a child can easily do this.

1. Insert the metal implement (burnisher) into the wheel bore.

2. Get a good grip on both ends of the burnisher.

3. Push down and roll the wheel on a flat surface at the same time. The pressure from pushing down and rolling melts the irregularities back into the plastic, even if you don't roll fast or push particularly hard. BSA wheels are harder, so more action is necessary.

4. Checking with a strong hand lens is recommended, but not required - the bore will be better.

Detailing Lettered Wheels

Some clubs' kit wheels have raised lettering molded on them. This is the time to paint that lettering for a professional racing look. All you need to do is dab a toothpick in a bit of acrylic paint, and carefully touch the top of the lettering with it. Don't let the paint wet the sides of the letters or it will smear. But, if paint goes astray, just tweak it off with the dull edge of a hobby knife.

Some Grand Prix "drivers" prefer to sand off the lettering altogether. Do this if your organization's Grand Prix track does NOT allow enough space between lanes. Wheels from neighboring cars CAN rub together! Aerodynamic drag, however, is NOT a good reason to sand off the lettering. It's effect has yet to be measured!

How To Narrow Your Wheels

Often times, this procedure is disallowed by Grand Prix rules. The reason may be related to safety since many aspiring auto builders may employ dangerous methods to cut the plastic wheels. A slip of wheel under the pressure of a hobby knife could be very painful, possibly with permanent consequences.

You need to exercise care following this procedure. If done properly, your wheels will roll more easily. It will be as if your car only had two wheels! You will need a drill, small saw (coping, hack, drywall, etc.), and a vice. Don't do this procedure unless you use a vice or other holder to keep the saw from slipping. Get your parent or leader to supervise.

1. Wrap a small triangular piece of masking tape around an axle - the point of the tape should extend away from the hub so the wheel can slip on more easily

2. Fit a wheel on the axle so it is snug but not permanently stuck on the tape. You may need to remove some tape.

3. Insert the end of the axle into a drill chuck and tighten. Do not put the hub end in a drill chuck.

4. If possible, clamp the drill in a vice to hold it steady.

5. Mount the small saw in the grip of the vice or other holder teeth up. Clamp it tight so the saw cannot move. Be careful moving around the saw. If you forget it's there, you can get hurt. I've done that!

6. Trigger the drill, holding it very steady over the teeth of the saw. The axis of the wheel should be at right angles to the axis of the saw blade. The goal is to make a groove in the tread that can be deepened gradually. Lightly press the wheel into the teeth of the saw at the place you want the tread cut. Care must be taken so it does not race off the blade. Hold it steady if not clamped down!

7. Stop before you cut all the way through. You don't want the cut plastic to fly and hit someone in the eye or cause you to jump so your hand gets scratched on the blade.

8. Separate the cut plastic by penetrating the groove with a hobby knife and running it around. It should separate easily. In any event, NEVER force a cut. Make many light passes with the blade instead.

9. Sand off burrs and polish with fine grit (150 - 200) sand paper as described above.

Lifting Wheels

For the serious competitor

Lifting a front wheel a fraction of an inch is an advantage if you can make your car roll straight without bobbing. If it bobs, it will be worse than if it was not lifted. Bobbing will likely happen if your weight is not back far enough and there are bumps where your club's track sections join. With a long wheel base and weight near the rear axle, bobbing is virtually impossible.

In order to screw an axle into an axle groove:

1. Hold an axle by the hub with a pair of pliers.

2. Place a wheel on the axle.

3. Make sure you put the axle in a groove on the side of the car it was ground and polished for. Holding the axle next to the side of the car, begin to push it into the axle groove until it stops. When it stops, give it a twist and push at the same time. It should continue to twist in.

4. Don't twist it in all the way. Leave a little room for the wheel to move back and forth. If the wheel is plastered against the hub and body, it will defeat the polished axles and lubrication.

5. If a wheel vibrates when you spin it with your fingers, you need to relubricate that wheel.

6. Be sure to check the median clearance. You need at least 3/8ths of an inch on an AWANA Grand Prix track.

Canting Wheels

Your wheels are slotted into your car. If you want to cant your wheels up or down, do so now. Canting may reduce wheel/track friction if you are not allowed to cut your wheels. Canting is simply angling your axle up or down so only the outer or inner edge of the wheel touches the track.

Canting wheels up (the axles actually point up a bit) may help if your wheels have little "beads" on the outer edge (as all three types of Scout kit wheels do). It is best to remove the beads and smooth the wheels, but if that's not allowed, canting may help. Some experiments indicate canting all axles up that way may actually detract from speed.

The wheel bore/body interface should be lubricated, or painted with a Teflon coating when canting wheels down since body friction is substantially increased. Bushings between the wheel and body are usually not allowed.

A Simple Wheel Alignment Check

Before gluing your axles in place, follow this procedure to make sure your car rolls straight. You will roll your car gently on a glass or polished surface.

1. Draw a straight line on a flat surface or large piece of paper on a flat surface.

2. Roll your car forward, gently along this line and note the amount of deviation.

3. If it wobbles while rolling, either the surface is too rough for this test or not all the wheels are touching the surface.

4. If the car rolls away from you to the left, see if a wheel on the right is loosing contact with the surface. If so, push its axle down a bit for better contact. If it rolls to the right, check the left side wheels for contact.

5. If the car is still not rolling straight, either the surface is unevenly smooth or oily, or an axle groove may not be straight.

6. If the car has a lifted wheel and rolls away from that wheel to the other side, tow the axle of the opposite wheel to the front slightly with a sliver of wood (or while the hot glue is cooling) in the groove. Tow it out a bit otherwise.

7. When your axle/wheel configuration is set, burn it in with a glue gun. Don't over do the glue! If it runs over, it can glue your wheel hub to the car body. Just a few well-placed drops near the middle and end of the axle in the groove does fine. Using a glue gun is the best way to secure your axles because it:

o does not require you to move them while gluing

o holds them tight when cool

o can be melted so you can reposition your axles (heat the glue by touching the axle over the glue with the tip of the gun; wait a minute; pull the axle straight out with pliers)