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 Post Posted: Sun Apr 05, 2009 10:44 pm 
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There was an old thread about calculating the height of the C/G with a formula and a couple links to other sites with calculators etc. The old thread is here http://www.motorsportsvillage.com/forum/viewtopic.php?f=8&t=2649.

I scaled my car today and scaled with the front up 17" and then I set front down and raised the back (17") and weighed to come up with weights to use in the calculators. When I scale it lifting the back I get a C/G height of 15.8" and when I lift the front I get 20.4". After studying the formula I am confused as to why you only input one tire radius? The end not being raised (normally the front) is what has been used. If the front and rear tires are the same size, it wouldn't matter and I think it would be correct. When there is a large difference between the front and rear tires it seems that BOTH tire diameters should be considered (somehow) in the calculation. My rear tires are 33" and my fronts are 22".

I am working on an exaggerated example to prove/disprove my thinking. I think the original weighing needs to be done with both tire centers at the same height then raising one end up and using the calculators or different tire diameters will throw off the calculation. Then correcting that for the different original height with both tire centers at the same height.

Comments?

Rick

PS: Raising a car 17" in the air on one end and putting scales under it isn't easy. How many have actually done this?


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 Post Posted: Mon Apr 06, 2009 2:42 am 
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I have done the CGH measurement for race cars and i have done it with a forklift to get a lot of height. Its not easy is it. But i worked out my own maths/trig for calculating the CGH. I dont know about these calculators. But my maths and measurements work out to be exactly what the cars do and it agrees with data logger measurements etc of suspension movements etc. The commercial four link programmes get it wrong they say our car has 300 lbs on the front wheels but it doesnt, it has 2" of lift like my programme says.


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 Post Posted: Mon Apr 06, 2009 6:54 am 
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shrinker wrote:
I have done the CGH measurement for race cars and i have done it with a forklift to get a lot of height. Its not easy is it. But i worked out my own maths/trig for calculating the CGH. I dont know about these calculators. But my maths and measurements work out to be exactly what the cars do and it agrees with data logger measurements etc of suspension movements etc. The commercial four link programmes get it wrong they say our car has 300 lbs on the front wheels but it doesnt, it has 2" of lift like my programme says.




So if I was to do this with my door car does it need to be lifted 17" ??? Just not going to get it that high with a floor jack #-o

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 Post Posted: Mon Apr 06, 2009 7:09 am 
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The higher you can lift it the more accurate is the result 17" is not high for a low CGH car like a race car so the weight change on the other end is minimal and thats why your getting wrong answers. Low built cars need to be lifted much higher than 17" Its also important to have high tyre pressures too, on the end thats on the ground.


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 Post Posted: Mon Apr 06, 2009 6:08 pm 
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I have also read some info on this, and what was mentioned was, the same size tire used on the rear of the vehicle should be replaced with the same diameter size, and, have full air pressure in it to not flex, the slick sidewalls will flex too much and thus take more out of the exactness of the C/G you are looking for. And Barry, yank them wheelie bars off, jack the front end up about 1 foot, then jam jacks underneath it, then add some blocks of wood to the jack and do it again, real easy to get 17" then, even 24", but block them rear tires or make dbl sure your in park, PLEASE!!!!
I will be having this done also to my Elky to see where it is at.

I also read, that when you do this measurment, that the suspension needs to be locked into place at ride heigth! huh you say? well, what do you think the suspension is going to do while taking the measurment???? flex ](*,) and, the have the weight of (YOU) in the seat for the proper weight of the car race ready, and this is if youraise the front end, if you raise the back end, keep the scales under the front tires, and go higher than 10" and the higher the more accurate(already mentioned)
You only need 2 bits of info this way and then the math calculation, the 2 bits are the front weight and, the angle of your chassis once stable, then the math starts...

It's a lot of work, but this seperates the men from the boys and there toys, well, from what I've been told anyway =;

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 Post Posted: Mon Apr 06, 2009 6:59 pm 
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Beretta wrote:
So if I was to do this with my door car does it need to be lifted 17" ??? Just not going to get it that high with a floor jack #-o


At 17" raised, each pound of change in front weight makes almost a 1/2" difference in C/G height on my car. So every bit of height will make each pound of change less difference in C/G height while making the change in weight more. (ie more accurate). My front weight only changed by 13lbs at 17". I think it has a low C/G.

I built some blocks of wood to set scales/car on to help elevate it. One of the things that makes it hard to do is the jack(s) also have to be on something higher each step to jack it up the next step higher, unless you have a jack with a very tall lift.

I am almost sure that the formula and calculators that I have seen that only use one tire size CAN'T be correct. I haven't had a chance to rewrite the formula, but I will try to do it this week after I figure out the trig for myself.

Rick


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 Post Posted: Tue Apr 07, 2009 3:39 pm 
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Let it be known, I am no math guy at all. If you don't use it, you loose it. Here is the but. Couldn't you figure this out with simple geometry? We know the COG longitudinal point by balancing car at level. If we raised the front a certain amount of degrees and fixed a clamp to frame rail and found balancing point, it should make a triangle and the top point should be the COG height from the frame rail. Just by drawing lines vertical from both balancing points. It seems this was along the lines of the formula I used to have. Does that make any sense to anyone?

You would know one angle was 90 degrees and the other the opposite of the angle of the chassis raise and the inches between balancing spots.

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 Post Posted: Mon Apr 13, 2009 10:40 pm 
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Bubstr wrote:
Let it be known, I am no math guy at all. If you don't use it, you loose it. Here is the but. Couldn't you figure this out with simple geometry? We know the COG longitudinal point by balancing car at level. If we raised the front a certain amount of degrees and fixed a clamp to frame rail and found balancing point, it should make a triangle and the top point should be the COG height from the frame rail. Just by drawing lines vertical from both balancing points. It seems this was along the lines of the formula I used to have. Does that make any sense to anyone?

You would know one angle was 90 degrees and the other the opposite of the angle of the chassis raise and the inches between balancing spots.

Yes, it makes perfect sense and you apparently understand math without knowing the mechanics of it.

I used that basic premise to calculate those things from the weights when lifting each end. Calculating the vertical position of the CoG in each case and where it crosses the line between wheel centers. These two lines and the line bewteen wheel centers create a triangle. Since I also know the angle of each side of the triangle that creates and the distance between points I can calculate the distance above the line between wheel centers where the two sides connect/cross.

On my roadster with me sitting in it the CoG height calculated to 18" using weights when lifting each end 17" to calculate with.

Both wheel diameters MUST be known to do this correctly. The horizontal wheelbase changes a different amount as you tilt either direction depending on the actual length between the wheel centers. When the tires are different height/diameter the actual distance between the wheel centers is not the same as the level wheelbase. If the tire diameters are the same front-back the calculators/formula online will give accurate results. If the formula or calculator doesn't include BOTH tire diameters it CAN'T be right when front and rear tire diameters are different.

I will try to get my spreadsheet double checked, cleaned up, simplified and polished up a little and make it available to anyone wanting to check it out. The formulas are kinda involved and a lot of calculating, so I don't know if I'll post the math here. The spreadsheet helps things a bunch. Lifting the front only and weighing then lift rear only and weighing will give pretty good results I think.

Rick


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 Post Posted: Mon Apr 13, 2009 10:48 pm 
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shrinker wrote:
The higher you can lift it the more accurate is the result 17" is not high for a low CGH car like a race car so the weight change on the other end is minimal and thats why your getting wrong answers. Low built cars need to be lifted much higher than 17" Its also important to have high tyre pressures too, on the end thats on the ground.


Why would it be more important for the tires that are on the ground to have high pressure? One pair of tires increasing weight (not a lot) and the other end (lifted end) decreasing weight. Seems like the effect would be similar, but opposite, on either end.

The higher you can lift it the more accurate the results. Tilting in each direction helps too. 17" up in front, then 17" up in back changed the weights 23# on each end TOTAL .... Not much.

Rick


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 Post Posted: Wed Apr 15, 2009 6:03 pm 
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rick360 wrote:
Why would it be more important for the tires that are on the ground to have high pressure?

There are two main things going on that decrease accuracy, the rear slicks squash and alter the axle height when you lift it very high to get a lot of transfer. So I pump them up to 25 lbs the recommended max for most slicks. Leaving them down at 6 or 7 is no good. The other thing I have noted is the electronic scales usually only read in whole pound increments, this is not a good enough resolution if you only able to lift the car 2 feet or so. I lifted our car with a forklift when we did it, right up in the air about 6'. That makes sure you get a good transfer and I didn't use electronic scales. I used mechanical ones that could read between the lines.


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 Post Posted: Wed Apr 15, 2009 10:46 pm 
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So, shrinker what you are saying is the tire diameter is NOT important just the distance from the scales to the center of the rear axle should be the same when level and when the front is jacked up? What about the rear suspension compression when the front is raised? How would this compression change the weight reading?

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 Post Posted: Thu Apr 16, 2009 5:41 am 
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If the rear spring compress say 2" then most cars will get the CGH wrong by 1". Whatever the springs move the CG does relative. It depends on its position in the middle of the car but that is a general guide. Soft springs are definitely no good on a high lift. Low tyre pressure allows the tyre to squash down too much causing the same inaccuracy. Its not hard to pump the tyres up and whack a bit of wood on the shocks.


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 Post Posted: Fri Apr 17, 2009 5:13 pm 
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So, wouldn't it be best to lock the suspension at ride heigth and do the measurements that way???? From what I have red, this is an important step to eliminate suspension movement which causes bad data!!!! so by locking all suspension movement, you will get the best results and not get bad data, but accurate data. :-k :-k :-k and only one angle is needed and one end of the chassis raised.

John

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 Post Posted: Sat Apr 18, 2009 4:19 pm 
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rick360 wrote:
Bubstr wrote:
Let it be known, I am no math guy at all. If you don't use it, you loose it. Here is the but. Couldn't you figure this out with simple geometry? We know the COG longitudinal point by balancing car at level. If we raised the front a certain amount of degrees and fixed a clamp to frame rail and found balancing point, it should make a triangle and the top point should be the COG height from the frame rail. Just by drawing lines vertical from both balancing points. It seems this was along the lines of the formula I used to have. Does that make any sense to anyone?

You would know one angle was 90 degrees and the other the opposite of the angle of the chassis raise and the inches between balancing spots.

Yes, it makes perfect sense and you apparently understand math without knowing the mechanics of it.
Rick


Let me clarify this. After reading my comment it may be mis-interpreted. I should have phrased it this way.
"Yes, it makes perfect sense and you apparently understand math without knowing the mechanics of the mathematics.

meaning, yes you understand the math concept very well, even if you can't do the actual calculations.

Rick


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 Post Posted: Sat Apr 18, 2009 5:53 pm 
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shrinker wrote:
rick360 wrote:
Why would it be more important for the tires that are on the ground to have high pressure?

There are two main things going on that decrease accuracy, the rear slicks squash and alter the axle height when you lift it very high to get a lot of transfer. So I pump them up to 25 lbs the recommended max for most slicks. Leaving them down at 6 or 7 is no good. The other thing I have noted is the electronic scales usually only read in whole pound increments, this is not a good enough resolution if you only able to lift the car 2 feet or so. I lifted our car with a forklift when we did it, right up in the air about 6'. That makes sure you get a good transfer and I didn't use electronic scales. I used mechanical ones that could read between the lines.


My point was that both tires are equally important, whether they stay on the ground or raised. Less weight will cause the tire center on the raised end to rise, more weight will lower it on the end on the ground.

For ultimate precision locking the suspension and airing tires way up will help, but you'll need to adjust your calculated CoG for the change in axle center height with high psi vs normal psi. I'm not sure the change of 20-30 lbs when tipped will squish the tire radius enough to worry about. If you're raising it 5-6' high then everything would be more likely to change and I would want to do both, but the rest of the accuracy improves a lot when raised high.

I am working on a couple of drawing to try to explain why both tire radius MUST be known when weighing with one end raised to determine CoG. My spreadsheet is pretty much done and I'll have it available to anyone wanting a copy in the next day or two. Send me a PM with a email address and I'll send it out. I'll save the explanation for a separate post to go along with the drawings.

Rick


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