COG question

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46u

COG question

Post by 46u »

What affect on the car does changing the COG have. I know most run between 1/2 to 1 inch in front of the rear axle. But what affect does 1/2 have over 1 inch and so on?
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Jeff
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FatSebastian
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Re: COG question

Post by FatSebastian »

46u wrote:What affect on the car does changing the COG have?
Jeff, your question is very broad. Quantifying the effect of CoM changes is dependent on many variables, including the type of track. FWIW, here's one nutshell response however.
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Re: COG question

Post by Speedster »

Hi Jeff,
I'm new to this forum but I'll take a shot at answering your question. The farther you move the COG toward the rear the greater velocity you will have when the car reaches the horizontal run if you are running on a circular arc track. You will also gain an advantage involving the "Front End Extension" effect. However, the farther you go back with the COG the more unstable the car becomes.
46u

Re: COG question

Post by 46u »

What do you recommend on a aluminum track straight runner 4 wheels touching no camber. Thanks all for the input and link. I have a better understanding now. Will be a while but this is the reason I want a practice track with a timing system then I can experiment with different setups.
Thanks
Jeff
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Re: COG question

Post by Gameday »

46U,

Let me give you some sort of anecdotal evidence. In our Pack derby yesterday we used one of my old cars as the Dummy car. This Dummy car was very fast and was running times of 2.38 on a 32' track. Late in the day we ran the dummy car backwards and it ran 2.59. This meant that the COG moved from about 3/4 of an inch from the back of the car to about 3/4 of an inch from the front of the car, or about 4.5" in total. You can see what a dramatic difference that made, and you can extrapolate the results of moving it along that plane.
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Re: COG question

Post by BallBoy »

46u wrote:what affect does 1/2 have over 1 inch and so on?
Like FS said above, the track is one of the variables on configuring your COG. Based on your post here you anticipate running on a BestTrack. The BestTrack has a fairly agressive transition from slope to flat, while the Freedom aluminum and most wood tracks have a gradual transition from slope to flat. If you have the COG too far back then your car is likely to pop a wheelie, or perform some other undesirable maneuver, coming out an aggresive transition. On a gradual transition, which varies from track to track, you may have a little bit more lattitude on COG. After the transition a car that doesn't have sufficient weight on the front wheel(s) can be prone to bouncing off of the guide rail.
46u

Re: COG question

Post by 46u »

So would 3/4 of a inch on a best track be OK? Thanks all for the education as I understand much better now.

The reason I am trying to figure this out is I do not want to get in a jam like I got into this year and have 3 days to do the built. ;)
Thanks
Jeff
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Re: COG question

Post by sporty »

46u,


On a best track I have ran a 3/8 balance point on a exstended wheel base car and on a stock wheel base car ran 5/8th's balance point. All with good success.

So yes, 3/4 would be fine.

There however is always factor that may not be fine. To much rear loaded weight, stil giving you that balance point, to light of a front end. Not having the ability to tune the car well.

Sporty
46u

Re: COG question

Post by 46u »

Do to having a short build time this year I had to go with just a hair over one inch which I had to route out the front a lot just to get it to that point. Just from my one build seems like it is easier to increase the COG then decrease. I have at least a 1/2 inch deep on the middle routing job and on the front one I did go clear though in one small spot. :O
Thanks
Jeff

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sporty
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Re: COG question

Post by sporty »

Here is what I think is wrong with the picture you posted.


Blue- weight.

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46u

Re: COG question

Post by 46u »

I drilled 3 holes in the back and put weight there. You can kind of see them in this picture.
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sporty
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Re: COG question

Post by sporty »

K, thanks for the clarification.

You would have to remove allot more wood and have the weight lower in the car.
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Re: COG question

Post by macd »

Are you measuring your COG with wheels and axles? I think most folks here do.
My son likes interesting shaped cars. We have not been able to get our finished COG under 1".
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Re: COG question

Post by Scubersteve »

macd wrote:Are you measuring your COG with wheels and axles? I think most folks here do.
My son likes interesting shaped cars. We have not been able to get our finished COG under 1".

Well, you let the kids pick the design, then you do what you can to help make it fast...
:bigups:
If you don't take your fun seriously, who will?
46u

Re: COG question

Post by 46u »

macd wrote:Are you measuring your COG with wheels and axles? I think most folks here do.
My son likes interesting shaped cars. We have not been able to get our finished COG under 1".
Yes that is with the wheels and axles installed. Only thing that is not done when I do the COG is paint because I might have to drill or route some more.

I am new to PWD but I have been doing machine work on motorcycles for 40 years which include but not limited to balancing the flywheel assemblies.
Thanks
Jeff
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