Weight Placement

Gents

I came across some information in regards to weight displacement on another forum and would like to get your thoughts on it.

In the first picture is the weight configuration I was planning to use. My understanding from reading other people comments is that for the best results I should have my weights as low as possible with as much weight in the back while still maintaining a COG of around 3/4 ~ 5/8 (which is what I have here). My assumption is by having as much weight as possible in the back, you get that little extra push coming off the down slope.

Now I also assume that once you're flat and level this advantage becomes neutral.



Now this one gentlemen stated that if you compress all your weights with in the diameter of your rear wheels, as I have done in the picture below, you will have better results. (By the way, I was able to achieve the same COG this way as in the photo above.) In order to do this I had to stack my weights higher.

This is the first time I've heard of this type of weight distribution and I'm just having a little trouble understanding how his method is better than the way I had it set up above. If anyone can explain his concept better, I would appreciate it.

 

it doesnt look like you have enough weight behind the rear axles to me..how much is back there?

 

There are 5 1/4 inch tungsten cubes. I tried putting more but I can't achieve a 3/4 COG if I do. The only way I can do it is if I put some weight under the front wheel.

Right now I have 0.85 oz behind the axle and 3.05 in front of it.

 

I played around with it a bit more and was able to get 1.15 behind the rear axle.

Would it be worth putting a little weight under the front wheels if it means I can get more weight behind the rear wheel?

 

You want the weight on the hill as long as possible, more time to fall. If you put all the weight in the rear and then used some tungsten puddy up front you could move it around till you found your desired COM.

 

Many of the league cars run twelve cubes behind the rear axle. A 3/4" COM is pretty tame IMO. But I don't know what your track looks like so take that with a grain of salt. /images/boards/smilies/smile.gif

Not that you asked, but I would suggest that you need to cover that open hole in the middle of the body or you'll be cutting the air twice and costing yourself some speed.

 

Here are pictures of the whole car below, top and bottom.

Now I played with the weights a bit and I now I have 7 cubes in the back as shown in the picture. This gives me a COG of 5/8. I tried putting 10 1/4 cubes in the back but that gives me a COG of a 1/2. My concern with this COG is that it might cause the rear to be unstable and swing around. If I had a track to test it on then I wouldn't worry but I don't. So, I thought I would be better off with a safer COG of no less than 5/8.

Now I could put a 0.20 oz plate behind the front wheel which would allow me to add more weight behind the rear axle. But is it worth the trade off? The impression I'm getting from Quicktimederby comment is that it is. Let me know.

By the way Bracket Tracer, our pack has a new aluminum track. Also, I do plan to cover that whole once we decided how we are going to place our weights.

A little added info, the rear axles were drilled with the Jewkes block. So they are pretty stable. The front right is the steering wheel and the front left is a floater. Also, this is my son's PWD car. He has actually done about 90% of the work and I'm trying hard to keep my hands off of it as much as I can. (Not easy though So at this point, we are just trying to figure out the best weight placement. This is his last year as a cub scout. We hope he can get one more 1st place trophy before he goes.

 

i would go with 5/8 to 3/4 without a test track

 

Are you messuring com with wheels and axles in?

 

Keep your weight as tightly bunched together as possible around the rear axle. Don't split the weight up and put some out front, I don't think that's what Quick Time was saying. I don't know how your getting the COG. I can put ten cubes behind the rear axle on my razor wheeled cars and still be OK and my cars are half the thickness of yours. Are you getting the COG with the wheels on? If not that could be the where the confusion lies. What track are you running on? I hope you just threw the wheels on there for the pics. They really need to be installed with the proper gaps and your DFW needs to be set to the proper steer.

 

That's what I was saying. It's a interesting test. The things that make you go hummmmm.....

 

Well I stand corrected. /images/boards/smilies/wink.gif

 

I have one I'm working on now that has 2.5 ounces behind the rear axle.

 

Kinser

Thanks for the clarification about whether if I should space the weight apart or together.

For my COG, I'm using the Derby Monkey Center of Gravity Stand (See below) and I am doing it with with the wheels on. And I did throw the wheels on just for the picture. I'll put them on properly when we are closer to testing the car.

Our pack has a new aluminum track that is about 5 or 6 years old.

 

By the way, some of you are using the term COM (Center of Mass), are you using this as the same as meaning as COG?

 

Put 10 or more cubes behind the rear axle. The rest in front, to bring it up to weight (you could even put 12 by opening up the rear space).
Forget about the COM. After the car is finished, time it on the track by changing the steer.
Start at about 13 inches (over 8 feet) then test by increasing/decreasing the steer, until you get the best times.

Incidentally, based upon the above picture, you COM is almost 1.25 (3/4" is from the rear axle forward.)

 

Just for clarification, the red car is not mine. It's just a picture I took from Derby Monkey web site to show others what I was using to determine my COG.

 

COG /COM=same

 

COG and COM are the exact same thing.
It is a point somewhere in space where the car will be perfectly balanced.

The equations of motion used to calculate the potential energy of the car use the COG and how high above the bottom of the track it sits.

Because your car is at an angle at the start of the race, the closer the COG is to the back of the car body, the more potential energy it has.

Because over time, some of us have built cars that had too much weight close to the back of the car, and it either did a flip or ran very unstable, a common rule of thumb for pros is that the COG should be 1/2 to 1 inch in front of the back axle. Some can push that, but most are OK with about 3/4". The monkey is talking about Cub cars having it 1 to 2" in front of back axle.

This is an example where common usage has become sloppy and imprecise. In the pros, you can put the axle wherever you want. To say 1/2 " in front of the back axle only works if we know exactly where the back axle is.