The photo above shows two important things to consider when you use weights on the bottom of the car. The back right rear wheel has been removed to show how the weights will now clear the track. The red rectangle shows what would drag if you did not sand the back of the car down.
- put the axle holes down close to the bottom of the car. If you can not do that very well, then cut or sand off the bottom to make the axle holes close to the bottom.
- If you have to keep the back axle about 1″ from the back (NOT very common, read those rules again. Are you sure you can not remove some wood and then glue it back on? OK, for some reason you are keeping the out of the box axle location from the back of the car. Then you must sand or cut a bit of wood off the back of the car at an angle to allow the first weight to be placed where it will not rub on the track in the curve. Remove the last 1/2″ to make a trangle 1/4″x1/2″ on the sides.
The first thing to consider about your car design is this question- Can I move the back axle back 1/4″ closer to the back of the car?
Read the rules over many times. Here is an example that is confusing.
“You must use the original slots for your axle location. You must not change the distance between the slots.”
OK, here are two cars, do they both meet the rules?
If you think the lower drawing is not allowed, ask these questions,
Am I allowed to remove any wood from my car?
If I want to, can I glue some wood back on my car to make it a full 7″ long?
If the answer is yes to both these questions, you can move the wood from the back of the car to the front. If you have a saw, you can make your job easier by cutting the block down close to the final height and cutting 1/4″ off the back and gluing to the front.
The car above has a number of problems. If you really want good speed, it is still to high. You also want to move that nose up 1/4″ so the car will not have lift up or down and that gives the least drag. Now when the axle is close to the back of the car, there is little danger of a weight rubbing. But it can happen with a 1″ distance to the back axle as the car goes through the curve..
Now if the rules say the back axle can not be closer than 7/8″ from the back of the car (very rare rule), then you must leave it where it is. We will now look at a few speed tips for a car with about 1″ of wood behind the back axle. First, why does it make a difference what axle location we choose for the back of the car? Well, we want the center of gravity to be as close as possible to the back of the car and still have enough weight on the nose of the car to not get the wiggles when it runs. In practice, that will mean don’t get the center of gravity closer than 1/2″ to the back axle., so it follows that anything you can do to move the axle back ( but not so far you can’t put some weights in the car or have your wheel sticking out the back)
You will have a much faster car if you never drag anything on the track. You can also increase speed by reducing drag at the nose by lifting the nose by 1/4″ from flat. You also get faster if you remove wood and make a lower profile (shown in purple compared to the yellow starting design. The last thing to think about is if you have to keep that back axle near 1″ away from the back of the car, it might drag on the track so remove the blue area shown below to make room for a bottom plate that will not rub on the track. This can be cut off or sanded without much work and makes a big difference in your speed. The drawing below shows the curve of a typical Aluminum track in blue, and the 1/2″ high rails in red. The light blue zone is what you should sand off to allow a weight on the bottom that will not rub on the track.