Center Of Gravity

[Black Fox]

Greetings Gents

My sons in the process of building his PWD car for Cub Scout.

Just need a little refresher on the CG. I know 5/8 to 3/4 in front of the rear axle on a standards wheel base is ideal. His is currently a little less that 5/8. I know if its too short your rear end will start to wiggle. So I'm just wonder if I'm pushing it with this CG.

By the way, the weights are 2.8 oz and were racing on a aluminum track. Some years it has been 32 ft and others it has been 28 ft..

Thanks

 

[W&K Creations]

I think if you have the ability to take that car and tune it on a track or the official track (even better) then you could set up for an aggressive COM and then tune other areas of the car to keep it stable but that's really only reliably done by running down a track, making adjustments, and running it again until you have it where you want. Since COM is just one area of importance, all other components will play in role the speed and stability....at 5/8 to 1/2 inch its my opinion that down there it does require some tuning to ensure there is no wobbling, whether that's adjusting wheel gaps or steer or moving weights a bit....

Maybe others who do run down there consistently and don't have a track to tune on can chime in with their opinion on what things they do to try and ensure a stable run...

If there is no track to tune on then 3/4 and even up to 1 inch is pretty safe and for most scout pack races is just fine and your car will still be fast and competitive with less worry about a unstable car due to COM (at least).

Make sure you don't have large gaps between the body and wheel either as that can add to the wiggles....

WK

 

[GravityX]

Is your car set up to be able to adjust steer? If not, you'll need to do this. Are you running 4 wheel or 3 wheel touching? If your COM gets too aggressive you'll need to be able to adjust car to handle it. At 5/8" COM I would be looking at 4" of steer over 4 foot to start with and adjust from there. As W&K said, if you can tune on the actual track that would be the best case scenario, if not a tuning board will do.

 

[Black Fox]

Unfortunately the only chance I will have time to tune is the night before the race.

Both my son and I are pretty knowledgable in building a car. In the last 3 years I have taken first place twice in the parents category. (Would have won 2 years ago but my wife dropped my car 2 days before the race