Author 
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altereddoug

Posted: Mon Aug 13, 2007 9:00 pm 

Joined: Wed May 17, 2006 3:19 pm Posts: 234 Location: il

I've been watching all the talk about 4 links and ladder bar setups. My question is how do you determine the height of center of gravity on a solid suspension altered? Since there are no 4 link bars or ladder bars. I can get the center of gravity by the old fashion way of jack stands.
Does anyone know of a book or website that I can find some answers on tunning the chassis besides just running it down the track.





gdmii

Posted: Mon Aug 13, 2007 9:11 pm 


Site Admin 

Joined: Tue Apr 18, 2006 7:28 pm Posts: 1780 Location: Newport News, VA

Finding CG height is a multi step process. Here's a link to a good tech article and calculator. It explains the process much better than I would and saves me a s**t load of typing. =;
Find CG Height
Jerry Bickle and Dave Morgan both have great books out on chassis setup and theory.
George





gearhead1011

Posted: Mon Aug 13, 2007 10:14 pm 

Joined: Wed Apr 19, 2006 4:11 pm Posts: 2749 Location: Shepherdsville, KY

While were on this subject...
I used a 4 link calculator program to map my car out. In the program they give you the same method for determining the CGH. Only difference is you plug numbers into the program. None of these guys specify where to lift the car and where to measure it. Unless you jack it up EXACTLY where the axle centerline is you don't get an accurate measurement. What I did and would assume to be the correct method is used blocks of a specific height under the wheels of the end of the car I jacked up. This is the only way I found repeatable results.
_________________ Chuck Woloch Chuck's Automotive Full line Computech dealer chucksautomotive@yahoo.com





Bubstr

Posted: Mon Aug 13, 2007 10:32 pm 

Joined: Tue Apr 18, 2006 9:54 pm Posts: 1619 Location: Western Illinois

If you have scales you can find it. Billy Shoppe gave me an equation to find it. It's posted at Speed talk. Trouble is,I've done nothing with my math skills but balance a check book for 40 years. here's a link. I used to have this in a simpler form. Kinda algebra for dummy's thing. Just can't find it.
http://www.speedtalk.com/forum/viewtopic.php?t=4295
_________________ Older I get the less I know for sure Dennis
Last edited by Bubstr on Mon Aug 13, 2007 10:40 pm, edited 1 time in total.





gdmii

Posted: Mon Aug 13, 2007 10:35 pm 


Site Admin 

Joined: Tue Apr 18, 2006 7:28 pm Posts: 1780 Location: Newport News, VA

Sounds like you have the same 4 link program I have. To be honest I haven't done the calcs on mine to determine the actual CGH. I'm sure you're right though on where your jack point is as the closer you jack to the front the less weight will be on the front and the farther from the front you jack the more weight will be on the front. How much variance in the actual CGH did you find by changing the jacking point? I always "assumed" that if I jacked the rear differential I would be close. More so anyways than just using the camshaft height.
George





gearhead1011

Posted: Tue Aug 14, 2007 12:43 am 

Joined: Wed Apr 19, 2006 4:11 pm Posts: 2749 Location: Shepherdsville, KY

gdmii wrote: How much variance in the actual CGH did you find by changing the jacking point? I always "assumed" that if I jacked the rear differential I would be close. More so anyways than just using the camshaft height.
George
I raised the front of mine for the calculations. If I put the jack at the front of the chassis the weight was so far off it the CGH wasn't even close to the car. I didn't try by the differential, it seems that I read somewhere that raising the front was the best way to do it.
_________________ Chuck Woloch Chuck's Automotive Full line Computech dealer chucksautomotive@yahoo.com





Bubstr

Posted: Tue Aug 14, 2007 10:24 am 

Joined: Tue Apr 18, 2006 9:54 pm Posts: 1619 Location: Western Illinois

The basics of it is mesuring accurately, as far as wheel base,scale weights, and how much you raise one end from first scale reading.
First there are two ways to find COG. first is scale it, add front two readings and add rear two reading. then turn that into a percentage, say front weighed 75 lbs and rear weighed 25 lbs that would give you a75% front weight. 50 lbs and 50 lbs would be 50%, ect. then take measurement between front and rear axis point of wheel and figure what % is in inches. For easy math the 50% of a 100 inch WB would be 50 inches, the 75% would be 75 inches, measured from the rear. That gives you COG but not hight.
Which end to raise? logic would say the front, because the fluids would be in a similar spot as a launch or accelerating. But unless your going to run long enough to burn off a bunch of fuel, it don't make much difference. How high should you jack it? that also don't make a lot of difference, if measurements are dead on, but higher will make the calculation closer. To make the scale reading more accurate put high end of car on tires or at least lower ball joint center. The weight on the lower end will go up, and the angle relative to car at rest will to the jacked car will be different. therefor if we had a 50% starting weight and marked the frame there at the balancing point. After we jacked it in the front, the balancing point would move to the rear. Remember the rear is now heavier. The math basically uses these two balancing points and the old and new angle to find how high by where these angles intersect.
Now all we need is a math wizard to use Mr Shoppes equation, and explain it in simple terms to us that are math challenged.
COG hight=L(nsubrNsubr)/(WT)+r
L is wheelbase. nsubr is weight of wheels before lifting, Nsubr is weight after lifting, W is total car weight, r is the tire radius, and T is determined by T=R/ (square root of (1resquared))
Anyone here have math skills?
_________________ Older I get the less I know for sure Dennis





