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PostPosted: November 25, 2016, 2:33 pm 
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No. You just need to know how high you raised the axel.

https://www.google.com/url?sa=t&source= ... SVaaXlW95g

Maybe I need to replace the coil overs with solid parts? I assume this would make the cg worse.

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PostPosted: November 25, 2016, 4:27 pm 
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CG is dictated by the collective CGs of each part, with the heaviest and/or highest parts having the greatest effect. That would be the drivetrain, driver, and fuel tank in that order, followed by all the lesser parts. Just looking at your car you can guess where and how high it'll be, which is somewhere around 14-18", so something's wrong somewhere. I suggest looking to other sources for the equations. For my own car I used a spreadsheet of all the major parts, their location and weights, to come up with my own figure of 16". Your's should be lower because you aren't running a cage.

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PostPosted: November 25, 2016, 5:20 pm 
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Yes but the cylinder head is almost level with my head. The main tubes are pretty hight, the floor weighs nothing, and all 4 coil overs are about chest level. I wouldn't be surprised if it was that height. The unsprung weight in the front is 55lbs per side and 65lbs per side in the rear.

The only thing I can think of is that maybe the garage floor not being perfectly flat might cause an issue but then I wouldn't get the same height when testing by the front vs the rear

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PostPosted: November 26, 2016, 10:00 am 
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After jacking you're putting the weight back on the wheels? If you're using a different fulcrum farther back that would explain your problem.


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PostPosted: November 26, 2016, 4:31 pm 
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mkejim wrote:
After jacking you're putting the weight back on the wheels? If you're using a different fulcrum farther back that would explain your problem.


I'm not sure I am following. I am just jacking the car straight up so the rear wheels are in droop.

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PostPosted: November 26, 2016, 5:28 pm 
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Basically you have the weigh of the car twice - both times with all the weight on the wheels. If you were to pick up the car from the rear bumper there would be more weight transfer to the front.


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PostPosted: November 27, 2016, 4:40 pm 
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mkejim wrote:
Basically you have the weigh of the car twice - both times with all the weight on the wheels. If you were to pick up the car from the rear bumper there would be more weight transfer to the front.


OK so I took your advise and got something closer to 12" at a 10" raise. I still am not super confident with the numbers but it's way better. I ended up just putting wooden blocks between the scale and wheels when lifting the rear then setting the car back down on them. Each block set weighed 9lbs. The raised height is the pic with the higher total weight since the blocks were in there.


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PostPosted: November 29, 2016, 4:29 pm 
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Hello mjalaly,

When I punch your numbers in I get something closer to 20 inches. Or maybe I got a dimension wrong?

I looked at a couple of online calculators and they seem to assume the tire diameter is the same on both ends. Do you have different tire sizes front/rear? The attached spreadsheet lets you specify the tire sizes independently (if I got the math correct).

Playing around with this spreadsheet, it seems that small changes in measured axle weight have a big impact on the calculated CG height. With a 10" lift, each pound (total for an axle) moves the calculated CG almost 3/4". If you can do a 20" lift, the sensitivity drops to about 3/8" per pound.

Given that sensitivity, it makes me wonder what the effect of not using rods in place of your dampers might be. You could note the change in damper displacement before and after the car is raised to get an idea of how important that is. Depending on the amount of stiction in your suspension, you could maybe even bump the car up or down at both ends to try and achieve the same damper displacement with the car raised as when it is lowered.

-Kerry


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PostPosted: November 29, 2016, 4:38 pm 
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yeah the tires are roughly the same diameter even though they are different. 205/50/15 vs 225/45/15 which is a 23" diameter.

yeah i don't know if i will be able to find the right cg without jumping through a ton of hoops. I noticed the same thing as you in that if the scales arnt super consistent then there will be issues. If each block weighed 9lbs then the total weight when lifted should read 1205 not 1218 like it did.

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PostPosted: November 29, 2016, 5:55 pm 
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Plugging your numbers into the calculator you linked to gave me a height of around 20" (same as Kerry). However, I have my doubts about the result for two reasons:

a) Lifting the car 10" over a 94" wheelbase results in a very small pitch angle (about 6 degrees I think), which results in a relatively small weight change. Your scales seem to have a fairly significant inconsistency which may drown out the actual weight change you're trying to measure.

b) The calculator that you linked to does not take into account the longitudinal location of the CG in its calculations. The author must make an assumption about its location, maybe they assume 50:50 weight distribution. It is, however, an important variable. To illustrate, if your weight distribution is very close to the front axle then lifting the rear will result in a bigger front weight change than if it was all concentrated over the rear axle, despite both having the same CG height.

I think the best way to measure CG height is to lift the car with a single point hoist (i.e. an engine crane) at a point somewhere in front of the CG and again at a point somewhere behind the CG. The intersection of the gravity vectors from these two lifts will be your CG.

Alternatively, you could build a tilt table and tilt your car sideways until it's just a the point where the upper tires are starting to lift away from the tilt table (as it's done in FSAE).


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PostPosted: November 29, 2016, 8:01 pm 
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yeah i don't feel quite comfortable lifting it up 20" and resting it on blocks. Not really worth the hassle just to look at some weight transfer numbers.

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PostPosted: November 29, 2016, 9:35 pm 
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I was going to ask that, eventually you'll come up with a number that's believable - and then what? Feel good about it? Like I said, "I" know your CG is roughly 16-18" based upon what you've told me, just go with that and be happy. How much time are you going to dump into finding a squishy number of dubious value?

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PostPosted: November 30, 2016, 12:11 pm 
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Do you guys wanna see an engine build thread? Hummmm

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PostPosted: December 11, 2016, 2:41 pm 
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I guess I should have figured that things would be worse. According to the machine shop, the block line is to bad to use and they said the pistons are junk too because of the scoring around the ringlands. I don't know how to tell if that is correct or not so I think the best thing to do is to send them to aries and have them verify it. it would suck trash $500 worth of pistons that have like 8miles on them.

Maybe one of you guy can tell frome these pics and let me know what they are talking about


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PostPosted: December 12, 2016, 1:54 pm 
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with all of the engine headaches, i started back on finishing up the engine cover and scoop (never stop!!).

Not sure if I like it a whole lot as I think it could be "sexier". These will be the molds I plan on using and the two side pieces will be connected in the front and rear to make one uniform piece (i just need to weld that in). And i know the driver side rear needs to be trimmed like the passenger side and yes know the scoop needs to be way longer. open for suggestions.


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