COM tips wanted

[Noskills]

I will be placing my tungston cubes in the back of my car soon. I have read some folks say that you should place 2/3 behind the axel and 1/3 infront. I realize this is just a rule of thumb as the exact placement should be based on the COM you want.

But practically speaking, what if I glue 2/3 of the cubes behind the axel, let the glue dry, then tape the other cubes in various places while I aim for my desired COM. I have read that you can just place the cubes on top of the car while you do this, but this only works for a flat car.

What are your suggestions to temporarily hold my cubes in place while I refine my COM.

Noskills

[sporty]

Taping them on, is what many of us do.

Sporty

[Noskills]

So I just glued 2/3 of my total weight behind the axel which was 12 cubes. Waited for epoxy to set. Then I put the remaining 5 right in front of the axel. This gave me a COM of 5/8th. I was aiming for 1 in as I run on a bumpy wooden track. So I had to move my row of five cubes forward 1/2 in to get the one inch COM. Now I don't have a nice concentric mass of mass. I am guessing this is suboptimal. Perhaps I should have experimented more while taping them in place. Ah live and learn.
Noskills

[macd]

I also run on a wooden track.

Do your rules allow you to shift the wheel base, for example by drilling your own axle holes further back on the block?
I found that shifting the wheel base further back allowed us to have a more stability while having the COM further from the nose of the car. The car that beat us at districts this year had a wheel base shifted so far back the wheels extended past the end of the car.

[FatSebastian]

I am guessing this is suboptimal.

Well, if it makes you feel better, Seth, the mass concentration is not something that our family tends to worry over with Scout racing. (Have you seen MaxV's controlled experiment?)

[drathbun]

The car that beat us at districts this year had a wheel base shifted so far back the wheels extended past the end of the car.

Did the car still fit in the "staging box" meaning was it still 7" long, nose to back of rear wheels?

[macd]

Did the car still fit in the "staging box" meaning was it still 7" long, nose to back of rear wheels?

Yes.

[LightninBoy]

Just a thought ...

Instead of moving 1/3 of a weight forward as a contiquous block of weight, why not take a small part of that (a 3-5 grams?) and put that weight close to the DFW?

Because of the lever effect, that small amount of weight far forward will have a large impact on your COM, and it may result that you don't have to place the rest of the weight as far forward from the rear axle.

[Noskills]

Lightning,
That would make sense. But having read the data on the link FS sent above I am not going to sweat it.
Thanks all,
Noskills

[j.m.]

I can tell you that not having the weight concentrated as close to the balance point, or as far back as the car will run stable, cost us the race at districts. Our car had way too much weight spread out with the body shape we picked. Even though it looked cool as heck, and stole the show as far as design and looks, we just didn't have enough weight in the back to give us the needed momentum out of the transition. Wheather the difference you are talking about is anything to sweat about, that's up to you, but our race was determined by .002 sec.

[pwrd by tungsten]

Concentrate the added weight. You have too much wood in your body. You want at least 3.6 ounces of added weight in the car. I like to obtain my COM by never adding more weight behind the axle then in front. You do not want to spread the weight out across the car. That includes using too much wood.

[arrell]

You have too much wood in your body. You want at least 3.6 ounces of added weight in the car.

Please explain this theory of yours to me? How can you have too much wood in your car? Why does it matter where your weight is if you COM is where you want it?

If your COM is where you want it, and you are at 5 ounces total weight, why does it matter whether your mass comes from wood, tungsten, putty, a lego man, etc? I truly understand using tungsten to make up 5 ounces because I don't have to find so many places to hide weight, but why must you add at least 3.6 ounces? My opinion is that you work towards a COM you want and you make sure that you have enough wood to hide the additional weight (if you want to hide it). I would actually prefer to not have to buy as much tungsten for my sons cars each year, but I hate tearing into our old cars to get the weight out.

Our method is:
1. come up with a car design we prefer
2. layout the car design to detemine it's weight/COM (I use CAD programs and excel to figure the proposed weight and COM of our proposed cars before the first cut is made).
3. using cad/ excel to then figure out where to place the additional weight that gives us the COM we are looking for.
4. cut the design out of the wood and then double check the weight placement (using a engineers scale as a fulcrum point and sometimes some tape to hold the weight in place)

We have created many different designs with varying amounts of wood/tungsten/zinc/lead/lego men that have all been fast (usually top three every year). I don't understand how placement of weight, what type of weight, what type of material you use, matters as long as your COM is where you want it?

[Stan Pope]

You have too much wood in your body. You want at least 3.6 ounces of added weight in the car.

Please explain this theory of yours to me? How can you have too much wood in your car? Why does it matter where your weight is if you COM is where you want it?

With respect to the car's potential energy and its optimum dfw load, your analysis is correct ... it doesn't matter. However, any car motion which changes the direction of the car involves energy usage. That usage, which reduces the energy available for speed, is a function of the car's moment of inertia aka rotational inertia. The greater the car's moment, the more energy is used. The more rapid the change in direction, the more energy is used.

A car with the weight distributed further from the CM will have a higher moment of inertia than one whose weight is compact, even though the CM locations are the same. So it will use up more energy when its direction changes.

Different track designs impose different rate of change in the car's direction. The sharp short curve of a Best track extracts more energy than the gradual curve of the Freedom or Piantedosi tracks. Similarly, any bumps or undulations in the track's surface or rail extract some energy.

HTH!

[arrell]

Stan thanks for your reply. How much energy, relative to all energy losses, is lost due to variations in moments of inertia?

I would think that if you normally place your center of mass at a certain point, your mass is normally placed in the same general location anyways. I would then guess that the difference in the moment of inertia between two very similar center of mass cars would have to be relatively small. Taking these two relatively close moments of inertia to figure the loss due to rotational kinetic energy would have to then produce a relatively close rotational kinetic energy. If this energy loss was in the first place rather small, compared to other losses, how much impact does it have overall? It is a factor, but how much?

Do we all need to be calculating the moment of inertia of our cars to make them faster? Will we notice the improvements when we are already concentrating the majority of our weight to get the center of mass just in front of the back axle? Thanks for your time and I appreciate learning more.

[Stan Pope]

Now that you know the source, you can compute the impacts. Keep in mind the differences in the tracks on which you might compete.

None-the-less, you can tell by the time over which the direction of motion changes that the loss will be pretty small as compared to the major losses that affect a car.

The question of how important those losses are depends on your opposition. If you and your opponent do everything equally except you give away that small advantage, he beats you. Can you concentrate your ballast well enough to minimize those losses easily? If so, it is "free speed." On the other hand, if you are running 0.010 seconds behind the opponent, then this isn't what you are doing most poorly!