LocostUSA.com

I'm not sure I'm smart enough for this....I downloaded a 14 day demo of OptimumKinematics to test drive and see what it can do. I set up a suspension I had done in Vsusp to do some simulations and I had the great idea of downloading another OptimumG product, OptimumLap, a free lap sim software, to use the track data in OptimumLap for simulation data in OptimumK. Well one can't use the data directly in OptimumK for simulation but after an email to support I got this response
Long story short I tried to work my way through the formulas to get coherent answers but I'm not up on physics and it's been several decades since I've had to do any real mathematics...I'm lost
I made it through the spring rate formulas and added them to a spreadsheet after struggling to understand the difference between mass and weight and how it affects the spring rate, wheel rate, ride frequency, etc. The next step was to try to understand how to calculate roll rates and roll gradients.
I'm hoping someone here who's a bit smarter than me can take a look at the spreadsheet and see if my formulas are even close to being right and the same as the formulas here http://www.optimumg.com/docs/Springs%26Dampers_Tech_Tip_2.pdf and more important if the data out is even useable, particularly lines 21 - 32

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http://www.lightningbugcars.com/

First thing I noticed is that it looks like you're mixing units. You converted from lb to kg at the top, just need to be consistent throughout.

Line 23 is in inches, but all the calculations are in metric.

F=MA. So, Force(N) = Mass(KG) * Gravity(G), G=9.8 m/s^2 if you're working in metric. You did the correct lb to kg conversion at the top. To get the weight in N, multiply the mass by G. If each corner is 170kg, the total weight is 6664N. Now to confuse the point. We measure weight in Imperial units, not mass. The only time I remember using lbm is when doing heat transfer or fluid dynamics calculations. The gravitational constant in Imperial is 32.2 ft/sec^2, so divide your weight by 32.2 and you'll get lbm. That said, I tend to use SI units when doing physics because that's what I'm more comfortable with; there's a lower chance for error.

Haven't worked through the formulas yet.

Yeah I caught that after I posted the file. The numbers for the roll gradient seem a little wonky compared what I'm getting out of Suspension Analyzer, of course I basically have no idea what I'm looking at, but I'm learning ...right?

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http://www.lightningbugcars.com/

What's with the bronze colored car on your Lightning Bug Cars site? Is this just a concept? Or?

http://www.lightningbugcars.com/index.html


Roy

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Build log http://www.locostusa.com/forums/viewtopic.php?t=16510

It is a concept that I'm building over the next 2 years. I've bought a cnc router to cut the body in foam, I'm building a cnc tube cutter, and right now I'm trying to get the suspension nailed down so I can start cutting tubes. The body is designed to fit a 442 chassis with a 98" wheel base and a Miata track. The chassis is designed to fit anything from a Kubota to a LS7.

Here are some photos from a few years back of a 3d print I did of one the early iterations.
Thanks to Kreb for hosting them.

http://www471.pair.com/stalkerv/gallery2/main.php?g2_itemId=2583

http://www471.pair.com/stalkerv/gallery2/main.php?g2_itemId=2585

http://www471.pair.com/stalkerv/gallery2/main.php?g2_itemId=2580

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http://www.lightningbugcars.com/

The only errors I saw are for the vehicle weight calculation. Do: =9.81*2*(F4+G4) .
And you dropped a negative sign when calculating the roll gradient.

That said, I still get a wacky number for the roll gradient. I haven't tried following the units to see if the equations make sense for the expected product, or breaking out Race Car Vehicle Dynamics to cross reference the calculations.

Scratch that, got it. Your wheel rate is N/mm, you need N/M for the subsequent calculations. Ditch the /1000 in those two fields.

I made the changes that you pointed out and the solutions seem to be closer what I was expecting. I misinterpreted the vehicle weight and was putting in the full vehicle weight not the sprung vehicle weight. I still end up with a negative roll gradient, looking through OptimumK tech pages I should be shooting for a roll gradient of somewhere between 1-1.8 deg/g. The lower the # the stiffer the roll gradient and I'm coming up with negative #s...leaning into the turn?!? Ah well...I'm learning.
Thank you so much for taking the time to look at this and setting me straight.

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http://www.lightningbugcars.com/

I've got another equation/formula question I would like to be able to calculate the moment arm or Cg to roll axis dist. I found this equation
MA=CGH-(B*RCF+A*RCR/WB)
using my inputs:
-194.901=0.457-((3983.76*0.0536+4315.74*.0632)/2.4892)
sprung weight=6668.42N
48/52 f/r weight split
MA= Moment Arm
CGH= Center of Gravity Height=0.457
WB= Wheelbase=2.4892m
RCf= Roll Center Height Front=.0536m
RCR=Roll Center Height Rear=.0632m
A= WB*rear weight percentage
B= WB*rear weight percentage
When I put this into an excel formula I got something completely different than what I modeled in Solidworks. This equation I found was using inches not meters but I got an equally unlikely answer using inches. Does anyone have an equation to work this out and maybe can explain it so that i may understand it?

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http://www.lightningbugcars.com/

This is long winded, and very stream of consciousness. TL:DR = Can you link the equation source?

Use the percentage for weight, not the actual weight.

0.457-((.48*0.0536+.52*.0632)/2.4892)

What's the source for the equation? The formula and the equation with values substituted in aren't the same. Watch your parentheses when dividing the wheelbase.

Lastly, I think the equation is inherently wrong/flawed. Why are you multiplying the front roll center height by the rear weight bias? Is that a typo? And, the wheel base terms cancel out, the way it's written now. I suspect that is supposed to be the distance from the center of gravity to the axle. The objective is to essentially connect the two roll centers with a line, and have that line slope weighted by the weight distribution of the car.

As it's written now (and as I understand it):
MA=CGH-(B*RCf+A*RCr)/wb. That turns into: MA=CGH-(wb*R%*RCr+wb*F%*RCr)/wb. The wb cancels out.

Now, this is what I think it might be:
a=distance between CG and front axle
b=distance between CG and rear axle. Also equals wb-a.
MA=CGH-(a*F%*RCf+b*R%*RCr)/wb
a/wb should equal the front weight percentage, no? I need to think about it some more. I know what you're trying to do, but need to think about how to calculate it.

I tend to doubletap with my responses. More stream of consciousness.

We need to calculate the vertical distance between the roll axis and the center of gravity. Looking at the car from the side, x axis being along the length of the car, and Y is vertical. X=0 is at the front axle. Y=0 is the ground. CG location is (wb*%front,CG_height).

Now, draw the two roll center points. Front = (0, RCf). Rear = (wb, RCr). Connect the two with a line. Equation for a line is y(x)=mx+b. M=rise/run. M=(RCr-RCf)/wb. b= the y intercept, which is equal to the front roll center height. Putting it all together:
y(x)= (RCr-RCf)/wb*X+RCf.

Now, the distance between the CG and the roll axis need to be calculated. We know the X component where we want to measure this distance since that's the CG location, we calculated that above, wb*%front.
So, substitute that into the equation above. Now you have the roll center axis height at the location of the CG. Note, the wheel base cancels out.
Y(CG)=(RCr-RCf)*%front+RCf.
The moment arm is now:
EDIT: Fixed a sloppy mistake
MA=CGh-((RCr-RCf)*%front+RCf).

The process makes sense to me. Does that get a more reasonable answer? I got 0.398 M.
-Andy (I enjoy this stuff)

Here is the source:

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CCsQFjAC&url=http%3A%2F%2Fwww.uky.edu%2F~mfi223%2FNASCAR_files%2FCenter%2520of%2520Gravity%2520and%2520Moment%2520Arm.docx&ei=3vr0VNj_IoOuogS64YHYBA&usg=AFQjCNFzYQFFQx4EgfDHsNS4APPxkjx39g&sig2=8nMfa53TuJnQD2ZUQMKD4g&bvm=bv.87269000,d.cGU
I probably wrote the formula wrong.
Your answer was pretty close to what I got drawing it out in Solidworks 0.347m to your 0.398m
MA =CGh- ((RCr-RCf) *%front+RCf)
.398792=0.457-((0.0632-0.0536)*0.48 +0.0536)

Thanks
EDIT: I did write the equation wrong but I fixed and came up with the same answer as you from you equation

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http://www.lightningbugcars.com/

I just drew it in SW and got .398 meters.

That said, I did the calculation wrong the first time, but got the right answer. The CG X coordinate is not wheelbase*%front. If that were true, then if the front weight bias is 75%, the CG would be closer to the front than rear. So, CG X would be wheelbase*%rear.

MA=CGh-((RCr-RCf)*%rear+RCf).

Now to understand why they used a different method.

Thank you so much for the help, let me know if I'm coming to the well too many times...but one more time

In this equation I should be shooting for a roll gradient of somewhere between 1-1.8 deg/g but how do I get there if I'm dividing by a negative number, (-W*H)/KF+KR
A negative number would mean I'm rolling into a turn as opposed to rolling out, right?

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http://www.lightningbugcars.com/

You need to get this crap off the Locost forum. Me having to get a new brain cause the old one exploded is definitely not Low Cost.

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mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.