COM Calculations

GravityX

GravityX

0
Pro Racer
Feb 25, 2012
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Tornado Alley
Trying something new here and I'm needing a little help, my brain is locked up right now. I'm trying to calculate COM with weight measurements from the corners of the car. Weights are measured, calculated and percentages are figured, now what? What I need to know is would I take this percentage of weight fore and aft, as an example 10% front and 90% rear, then use the actual wheelbase as measured in inches, i.e. 4.750", and multiple it by .90 (90%) and then subtract that value (4.275") from the 4.750" to get COM location forward of the rear axle? The example would give me a COM of .475" forward of rear axle? Then use the same idea to locate left to right COM. Then where ever this lines intersect would be the point at which you could balance your car on the tip of a pencil, or not.

Is my thinking correct? Any holes? Does this make sense? I'm trying to locate the actual point of COM fore and aft, and left to right. If I had to guess an ideal COM position is located somewhere just forward of rear axle and right of the centerline of the car, for a right DFW car.

What methods are you guys using? I know some of you guys do not measure this and just roll'em down the track. I'm looking for some consistency and building a database with my car information. Logging wheelbase, corner weight, and anything else I can think of. Why go to these lengths? To understand what makes a these cars tick. I'll then know what works and what doesn't and go from there.

Thanks in advance.

GX
 
Probably a question for txchemist, but I'll take a shot. I didn't understand exactly your explanation but here's how I do it (I'll probably make a mistake typing this up, so someone correct me if I make a mistake).

Let's say you are looking down at your car from above and it looks like this:
___
_| |_
|_| |_|
| |
| |
_| |_
|_|___|_|

Assign the bottom left corner of the body an X and Y of (0, 0) and the top right corner (1.75, 7). Now say your car is 1 3/4" wide, has back wheels at 5/8", the wheel base is 5" and the DFW is on the right side. For simplicity sake, say your wheels touch directly below the edge of the body and give them these coordinates:
Left back wheel (LBW) = (0, 0.625)
Right back wheel (RBW) = (1.75, 0.625)
DFW = (1.75, 5.625)

Let's say you weigh your car on 3 scales. The weights are (just making this up):
LBW = 2.25oz
RBW = 2.25oz
DFW = 0.5oz
TOTAL = 5.0oz

In general, the equation for COM is:
COM = (Weight1*Distance1 + Weight2*Distance2) / (Weight1 + Weight2)

In the X dimension the calculation for COM would be:

COM_X = ((2.25*0) + (2.25*1.75) + (0.5*1.75)) / 5.0
this comes to 0.9625", or 0.0875" to the right of the center

In the Y dimension the calculation for COM would be:

COM_Y = ((2.25*0.625) + (2.25*0.625) + (0.5*5.625)) / 5.0
this comes to 1.125", or 0.5" in front of the back axle.

Edit: added an awesome picture and some colors for X,Y, and weight
 
blowup
I'll need a little more time this evening when I get home to digest that, after I recollect my head this morning.

Thanks for the input!
 
With GravityX as the beta tester, I put this together to help visualize the COM on a car:

http://jsfiddle.net/minionsracing/UUnT5/embedded/result/

You can manipulate various measurements as well as click on a value to change it directly. Feel free to try it out and PM me with any issues/bugs or improvements you'd like to see. No guarantee that it's accurate at all! But I would be interested to know how close you found it to be after comparing with values from one of your cars.
 
This is cooler than a fan minions... I will try to get my friend Quadad to test this out...I think this would be just the thing for him...Nice job, hope it works!!!
Minions Racing said:
With GravityX as the beta tester, I put this together to help visualize the COM on a car:

http://jsfiddle.net/minionsracing/UUnT5/embedded/result/

You can manipulate various measurements as well as click on a value to change it directly. Feel free to try it out and PM me with any issues/bugs or improvements you'd like to see. No guarantee that it's accurate at all! But I would be interested to know how close you found it to be after comparing with values from one of your cars.
Click to expand...
 
That's way too much thought for myself.. I usually just put the weight in the same way in each car, sometimes I might move 1-2 pieces around and that's it.. I do check the weight on each wheel .. But it will be different on each car.. Once I have a body ready, with axles prep , and wheel prep I might spend 30 mi . On a car tops... Beside letting the wheel do there thing... I personally think it has more to do with wheel and axle prep.. Basically ur steps.. And how u master it. I have never checked the COM .. And never will.. I think the only thing COM affects is ur weighting and amount of steer u have to run.. But I have a steer# I like to run in each class. Some takes more , some takes less. But weight placement does affect the steer for sure. Like I have told everyone I usually put my cars together At my desk, set my gaps and eye ball my steer. Then I go to my tuning board and set the steer.. 8 out of 10 times my faster time is 1-2 pass. If I have to spend more than 5-10 min on tuning a car to fine speed then I reprep.. Cause something's wrong ..
 
Well last night I went home and got out a car that was run at the first MOTM race. I got my scales out and did some calculations on weight and center of mass from the pictures that you posted ( Gravity X) once I determine what the COM was and moved weight around I gain (6 thousands ) on a 4 month old prep.. So I tore it apart and and reprep . I will re run tonight and let u know the results. But I will tell you it's 4 thousands faster than anything I ran last month which will be in the 57-58 times. Thanks for all the info and input. Maybe I need to be paying a little more attention to the board and post and all the free info u guys are giving it and no one putting to use... Thanks Again Gravity X!

FYI:: I never checked COM before, but all my cars will be checked and changed now!!!!
 
Alright just what we needed JBD picking up time. Well I guess I better check my COM maybe I can pick up the times JBD is running now. JBD if you never checked your COM and you have been tearing it up lately looks like you might end up MOTM. The info that flies around here I need to start coping and pasting and making a word document folder. As for JBD this is going to make him almost unbeatable. I think QT might have his hands full this time around.
 
You are most welcome, but I cannot take all the credit here. Minions Racing and I collaborated on this at the onset and this is what came of it. Glad to hear that you have found some additional speed. (Well... maybe not.... /images/boards/smilies/smile.gif JK) Lookout QT! JBD is coming.... Sorry guys! LOL

~JBD RACING~ said:
Well last night I went home and got out a car that was run at the first MOTM race. I got my scales out and did some calculations on weight and center of mass from the pictures that you posted ( Gravity X) once I determine what the COM was and moved weight around I gain (6 thousands ) on a 4 month old prep.. So I tore it apart and and reprep . I will re run tonight and let u know the results. But I will tell you it's 4 thousands faster than anything I ran last month which will be in the 57-58 times. Thanks for all the info and input. Maybe I need to be paying a little more attention to the board and post and all the free info u guys are giving it and no one putting to use... Thanks Again Gravity X! FYI:: I never checked COM before, but all my cars will be checked and changed now!!!!
Click to expand...
 
Whats this COM stuff?
Well played around with mine and well .006 was found on the first adjust!!! Ended with .012 of more speed!! Oct.-Nov race is going to be fun!!!
 
Dang did I say .012, sorry got my decimal point in the wrong place. It was suppose to be .0012.
blah
I never measure to COM I could not tell you what it is on any of my cars. I'm however building a new SS that I am hoping will take it to the next level. Not for sure if it will race this race or wait till the finials race.
hmmm
idea


Quicktimederby said:
Whats this COM stuff?
Well played around with mine and well .006 was found on the first adjust!!! Ended with .012 of more speed!! Oct.-Nov race is going to be fun!!!
Click to expand...
 
I am suprised because I think it would be hard to find big speed gains once you are as fast as Quick Time.
 
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