cg Height

[Joseph]

We just finished measuring our cg height and found it to be 10 inches above ground. We were suprised to get such a low value, but then again, what is considered low for FSAE cars.

What kind of measured values do other theam have ?

Joseph
University of Oklahoma

[Joseph]

We just finished measuring our cg height and found it to be 10 inches above ground. We were suprised to get such a low value, but then again, what is considered low for FSAE cars.

What kind of measured values do other theam have ?

Joseph
University of Oklahoma

[jack]

just wondering if you guys have wet or dry sump oil?

jack @ WWU
http://www.etec.wwu.edu/

[Denny Trimble]

Our '03 car was around 11 to 11.5 inches with driver (lower with the little guys, higher with the big guys). If you're at 10 with driver, that's pretty good. I think UMC's '03 car was around 9, which is very low.

University of Washington Formula SAE ('98, '99, '03, '04)

[Joseph]

We are not using a dry sump, but the engine is mounted as low as possible. We also measured the cg with a smaller person, so we may try it again with someone bigger and see how much of a difference it makes.

Joseph
University of Oklahoma

[V2 - Italy]

Our CG is 265 mm height without driver.

Daniele

Firenze Race Team V2
http://www.firenzerace.too.it
DUCATI POWER at the UniversitÃ* di Firenze

[Chris Allbee]

i'm not THAT small...

[IsheeM]

I was wondering, when you are designing a completly new chassis and dont know where the engine or anything else for that matter will be placed. How do you find or estimate the cg height so that you may properly design the suspension geometry.

[ATOhoosier]

what method do you guys use to measure your CG? Do you use the method where you simply jack up the back end, get weight and distance measurements and chug through the math? Anyone have any ideas oh how you'd measure the vertical CG on something larger like a big rig or something?

[Denny Trimble]

The lateral tilt test gives better results than jacking up the front or rear of the vehicle and using scales.

Make sure you put lots of air in your tires, and use an angle iron or something like that as the pivot point (on the edge of the tires on the ground). Use dummy shocks to prevent suspension movement, and make sure to keep the driver in the car. Put a digital level on the frame, lift it up, and find the balance point. Do some trig, and off you go.

[jack]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by IsheeM:
I was wondering, when you are designing a completly new chassis and dont know where the engine or anything else for that matter will be placed. How do you find or estimate the cg height so that you may properly design the suspension geometry. </div></BLOCKQUOTE>

you have asked the forbidden question...

[MikeWaggoner at UW]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by ATOhoosier:
what method do you guys use to measure your CG? Do you use the method where you simply jack up the back end, get weight and distance measurements and chug through the math? Anyone have any ideas oh how you'd measure the vertical CG on something larger like a big rig or something? </div></BLOCKQUOTE>

Take a guess where your engine will go, model it (even if only approximately; the engine can be a solid lump), and your cad software can probably give it to you.

[D J Yates]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jack:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by IsheeM:
I was wondering, when you are designing a completly new chassis and dont know where the engine or anything else for that matter will be placed. How do you find or estimate the cg height so that you may properly design the suspension geometry. </div></BLOCKQUOTE>

you have asked the forbidden question... </div></BLOCKQUOTE>

lol. I've been having that exact problem today. I figured the best way is to start with something simple and reiterate as more information becomes available. Time consuming and annoying for the rest of the team when you want to change something.

[Denny Trimble]

You can start with 11 to 12 inches with driver as a good estimate of CG height, based on past FSAE cars. Longitudinal weight distribution takes more work to estimate, but a few rules of thumb:
-with the front wheels near the driver's knees, it will be 50/50 F/R
-with the front wheels near the driver's ankles, it will be 40/60 F/R

And yes, the more parts you get in CAD (including the driver), the more accurately you can estimate weight distribution. We were within 0.5% last year.

[Courtney Waters]

Besides modelling everything in CAD, you could plug the info into a spreadsheet. That way you don't have to have everything modelled to figure out the CG. You can throw guesstimate numbers in there and update as you go along. As you get more of your car modelled you will have a second method of computing the CG for accuracy. We made a "weight budget" spreadsheet to keep track of that stuff, including target weight, actual weight, CG location (x,y,z), etc.

One of the pitfalls of CAD is forgetting (neglecting?) to set up the material properties for each part which can screw things up, or mock-up modelling stuff like tubes that aren't hollow (but should be) and forgetting to fix it later. Just happened to me today http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Frank]

longitudinal weight split can be detrermined with surprising accuracy by knowing the mass and cg of the:

driver
engine
and diff

unsprung mass should be nearly equal front and rear, so that cancels, and the rest of the stuff is fairly evenly distributed anyhow

[GTmule]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Frank:
longitudinal weight split can be detrermined with surprising accuracy by knowing the mass and cg of the:

driver
engine
and diff

unsprung mass should be nearly equal front and rear, so that cancels, and the rest of the stuff is fairly evenly distributed anyhow </div></BLOCKQUOTE>

yup, but I'd add axles and fuel to that list.

[Frank]

i just said (axles & cv's) /2 were unsprung

and unsprung front = unsprung rear

splitting hairs.. but if you think about whats happeneing you'll get it right

you almost need a "prior design" to work it accurately, but without prior design, you shouldn't miss by more than 5%


using this same method for height usually gives you a "high value"... the sneaky extra bits are usually quite low on the car

we measure 275mm by sideways tilt, but i wouldn't bet money on this being better than +/- 25mm

[mario_rbr]

11" ,with driver.We mesured it this week, we use a wet sump and followed Milliken instrutions about this procedure (lift up the rear wheels).

What angles have you inclined your fsae to mesure cg height accurely?

[Nick McNaughton]

Our 05 car is 237mm with suited and helmeted driver, which has taken some doing. Little bits everywhere...