I did some testing last year, and listened to all who gave advice as to where to place the weight. I was using an unraced basic car as my test car, and building one for my son. The center of gravity on the basic car was 1.5" in front of the rear axle. (This was without wheels and axles) I read everyone saying that the weight has to be towards the rear, so I did that on the new car. The C of G was 1.25" in front of the rear axle. The new car also had the nose extended by .5". I believe that by extending the nose it pushes the car further up the ramp to get more momentum. The basic car was always faster than the new car. I even switched wheels and axles and the results were still the same. I even see some suggest to put the weight over or behind the rear axle. I'm not sure that works for every situation. I have proved, in my opinion, that it wouldn't work for me. I don't believe the extended nose hurt anything so I can only come to the conclusion that the weight was to far back. The track we use is a slope tapering off to a flat. If anyone has any information or ideas as to what I did wrong, please let me know. THANKS!!!
I see you move two variables. First put your weight back to 1.5 see if the time was better or worse. Keep on adjusting the weight placement.Looking foward to see what happened. I think you will speed up.
I would also be concerned with checking the alignment of the wheels on the new car. Poor alignment can more than make up for improved weight placement.
Did you use a drill press to ensure the axle slots or holes were straight?
You stated: "The track we use is a slope tapering off to a flat." This sounds like the typical pwd track geometry. If alignment isn't the cause of the new car being slower, than I would suspect that the track isn't very smooth and that a center of gravity/mass more forwards would be the way to a fast pwd car. Before changing anything and if you can test both pwd cars again, run them both but watch the front ends of the cars. If the front end of either car wiggles when it is on the flat, than you need to move some weight forwards. NOTE: if you race on a smoother track, than you may need to relocate some of this weight backwards again.
By the way, when you re-run the cars, if either seems to stick or bump the center or side rails excessively, than alignment may be your problem.
Thanks for the input. As far as alignment, the car went down the track smooth and straight, so that was not the problem. My son had an old car that was nearly identical to the basic car and both of them(basic & old) ran very close to the same times. Keep in mind, the car was still fast but not fast enough for me. It did not finish in the top 5 as I thought it would. The car that did win was similar to the basic car. My point was to point out that putting the weight in the back is not a fix all. We can not race old cars in my son's group. We must build a new one every year. This year I plan on extending the nose more, maybe 1", up from 1/2", and moving the weight front so the C of G is 1.5" in front of the rear axle. Last years car was always slow off the line, but would catch up toward the end of the track. Maybe by extending the nose that is what made it slow off the line? Are there any drawbacks to extending the nose? Maybe it was the rear weight? Anyone have any ideas about being slow off the line but catching up at the end? Confound these little blocks of wood!!!
All I can tell you is that every extended nose car that I've built has been slower than my other cars. If you raise the nose, you have to raise it very slightly. As for the center of gravity, that's something that you determine after the wheels are on. Back is absolutely better. My son just won his District Championship with a low wedge with a center of gravity of 1 inch in front of the rear axle. Very aerodynic. I believe that you said your car had the COG 1.5 inches in front of the rear axle with the wheels off. This is too far forwards; unless the track is really rough.
Feb 8, 2004 - 9:58AM
Re: Re: Re: Re: Re: Re: weight placement
I can't answer as to why your slowed with an extended nose. In "theory", the extended nose should be faster because you are pushing the car up the ramp further but also using the same starting point. This gives the car more ramp to build more speed. Try taking it to an extreme. Put 1 car at the normal starting position, and another 25% of the way down the ramp. Who would win?
Here are the results from a test that I ran. This was all done with the same car in the same lane all by itself and adding graphite about every 3rd run. It was a legal car that weighed the required weight of 5 oz. I used a 3/4" nut as weight that I could move around to get results.( obviously that made it heavier) I also would tape a straw to the front to extend the nose 1/2" when the test required it.
plain car-2.49 seconds
w/extended nose 1/2"-2.45 seconds
w/weight in middle-2.50 seconds
w/weight in back-2.46 seconds
w/weight in back & extended nose- 2.43 seconds