Center of gravity
11-29-2011, 04:22 PM
#11
The CG numbers on my website CG Height Calculator are purely fictional--I just made up numbers to plug in and verify the calculations were correct. I have never actually raised an end of the car 10+ inches and re-weighed it.
I used to have a note stating the sample numbers were fictional but I just looked at the calculator and the note is gone. I'll add the note back to the web page. Sorry if the fictional numbers created confusion.
Rob Robinette
I used to have a note stating the sample numbers were fictional but I just looked at the calculator and the note is gone. I'll add the note back to the web page. Sorry if the fictional numbers created confusion.
Rob Robinette
11-29-2011, 11:28 PM
#12
Sounds like the OP found my post in the Suspension forum. 
I got the 18.7" figure by backing it out from the NHTSA's Static Stability Factor.
http://www.nhtsa.gov/cars/rules/regr...ges/index.html
http://www.nhtsa.gov/cars/rules/regr...nd%20final.pdf
NHTSA data was: SSF = T/2H = 1.57 for the S2000, where T = track, H = CG height.
I used the average of the S2000's known F&R track widths, so T = 58.7", from which H = 18.7".
There's still some uncertainties about the conditions of the NHTSA's measurement -- e.g., I assume that they used full fuel, no driver, top up; and that they averaged the front and rear track widths. But overall It seems like the most reliable data point I've run across for a stock S2000...at least, the final number seems reasonable.
I got the 18.7" figure by backing it out from the NHTSA's Static Stability Factor.http://www.nhtsa.gov/cars/rules/regr...ges/index.html
http://www.nhtsa.gov/cars/rules/regr...nd%20final.pdf
NHTSA data was: SSF = T/2H = 1.57 for the S2000, where T = track, H = CG height.
I used the average of the S2000's known F&R track widths, so T = 58.7", from which H = 18.7".
There's still some uncertainties about the conditions of the NHTSA's measurement -- e.g., I assume that they used full fuel, no driver, top up; and that they averaged the front and rear track widths. But overall It seems like the most reliable data point I've run across for a stock S2000...at least, the final number seems reasonable.
11-30-2011, 02:42 AM
#13
Sounds like the OP found my post in the Suspension forum. I got the 18.7" figure by backing it out from the NHTSA's Static Stability Factor.
http://www.nhtsa.gov/cars/rules/regr...ges/index.html
http://www.nhtsa.gov/cars/rules/regr...nd%20final.pdf
NHTSA data was: SSF = T/2H = 1.57 for the S2000, where T = track, H = CG height.
I used the average of the S2000's known F&R track widths, so T = 58.7", from which H = 18.7".
There's still some uncertainties about the conditions of the NHTSA's measurement -- e.g., I assume that they used full fuel, no driver, top up; and that they averaged the front and rear track widths. But overall It seems like the most reliable data point I've run across for a stock S2000...at least, the final number seems reasonable.
I got the 18.7" figure by backing it out from the NHTSA's Static Stability Factor.http://www.nhtsa.gov/cars/rules/regr...ges/index.html
http://www.nhtsa.gov/cars/rules/regr...nd%20final.pdf
NHTSA data was: SSF = T/2H = 1.57 for the S2000, where T = track, H = CG height.
I used the average of the S2000's known F&R track widths, so T = 58.7", from which H = 18.7".
There's still some uncertainties about the conditions of the NHTSA's measurement -- e.g., I assume that they used full fuel, no driver, top up; and that they averaged the front and rear track widths. But overall It seems like the most reliable data point I've run across for a stock S2000...at least, the final number seems reasonable.
11-30-2011, 09:14 AM
#14
Former Moderator
I updated my website's CG height calculator and Excel spreadsheet so the fictional numbers give 18.3 inches of CG height. With 10 inches of rear axle lift that equals 10 more pounds per wheel on the front wheel scales. At only 10 inches of rear axle lift a 1lb difference (per wheel) on the front scales equals 0.66 inches of CG height change! So more lift will yield more accuracy.
11-30-2011, 02:23 PM
#15
lol...Getting *sprung* CG height is trickier - you might need to know 2+2.
First estimate unsprung wieght (using some weights people have found on stock wheels/tires, brakes, and suspension)...Assume susp. bits are half sprung/half unsprung, and assume unsprung CG height is basically at wheel centerline. Then back out what the sprung CG would be. I guesstimate 310lbs unsprung weight at 12in. height, so for a 2800lb car (stock, full fuel), sprung CG would be 19.5 in.
Adding a driver w/ estimated CG height of 24 in. raises both numbers by about 0.3 in. (total CG ~19 in., sprung CG ~19.8 in.).
First estimate unsprung wieght (using some weights people have found on stock wheels/tires, brakes, and suspension)...Assume susp. bits are half sprung/half unsprung, and assume unsprung CG height is basically at wheel centerline. Then back out what the sprung CG would be. I guesstimate 310lbs unsprung weight at 12in. height, so for a 2800lb car (stock, full fuel), sprung CG would be 19.5 in.
Adding a driver w/ estimated CG height of 24 in. raises both numbers by about 0.3 in. (total CG ~19 in., sprung CG ~19.8 in.).
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