CG Height

[fpaccoret]

Hello everyone,

I have been following the f1000 forum for a good while now, and I was just curious to know an average cg height for a f1000. It would be nice to have some real world numbers to work with.

thanks,

Fabrice Paccoret

[provamo]

eleven inches?

[fpaccoret]

Thats actually really close to what most FSAE cars measure out to be. Didn't expect it to be around that height, but ill take it!
thanks,
Fabrice

[provamo]

like i said just a really rough guess....................i was suprised that this query languished without a reply for so long..............i would have thought that at least one of the designers or analysts who frequent ApexSpeed forums would have answered your question.................i mean what is the big deal? it is not as though that one number is going to win or lose a race

[David Ferguson]

If you want to know, then you have to measure it (a fun way to spend an hour or so). I've measured a few sports racers and gotten values in the 11.75 - 12.25" range.

I would use 11.5 - 12.0" if I were just looking for a "number".

[RobLav]

Has anyone calculated the anti-roll gyroscopic effects of the high revolution rotating assemblies in these engines?

[Purple Frog]

I would think a FB would be lower than say a FC (especially a pinto). FCs have a lot of iron in the engine higher than 11".

I always liked the Pare/Lathrop method of measuring. Put a bunch of air in the tires and lift one side of the car until it is balanced. Then imagine a vertical line from the tire patch on the floor going up through the car. Where it intersects the centerline of the car is close to the CG height. I did it one time.... just because.

[provamo]

I would think a FB would be lower than say a FC (especially a pinto). FCs have a lot of iron in the engine higher than 11".

I always liked the Pare/Lathrop method of measuring. Put a bunch of air in the tires and lift one side of the car until it is balanced. Then imagine a vertical line from the tire patch on the floor going up through the car. Where it intersects the centerline of the car is close to the CG height. I did it one time.... just because.

interesting except that you would have TWO tire patches in contact with the ground so i guess you could extrapolate along a line defined by those points but that does not locate the cg along the (longitudinal) vehicle centerline?..........i need more coffee..................pardon me...............i was confusing cg HEIGHT with CG ....................LOL

[Dick R.]

I would think a FB would be lower than say a FC (especially a pinto). FCs have a lot of iron in the engine higher than 11".

I always liked the Pare/Lathrop method of measuring. Put a bunch of air in the tires and lift one side of the car until it is balanced. Then imagine a vertical line from the tire patch on the floor going up through the car. Where it intersects the centerline of the car is close to the CG height. I did it one time.... just because.

What did you use to lift the car on one side? I just have floor jacks and my guess is that the high side would have to be rather high.

Thanks!

Dick

[fpaccoret]

I would probably used an air chain hoist for a formula b if you can get it around the roll hoop alright.
When my fsae team measured cg we did it a pretty quick way by lifting the car to the balancing point (where the cg height is perpendicular to ground plane) and then took the angle and put it into this formula:

tan(90 - angle of balancing) * (track width+.5*tire width+.5*tire width)/2 = cg height

the only thing that is missing in this formula is the thickness of the angle iron that you tilt the car on.

[Purple Frog]

Chain hoist. One of the most used tools in the shop.

[racerdad2]

Thankfully, CG calcs in airplanes don't require sky hooks more great info ! Thx !

[romoman]

I thought there was a pretty easy way of calculating the CG height with the front wheels on a scales and jacking the back up?

[Dick R.]

I thought there was a pretty easy way of calculating the CG height with the front wheels on a scales and jacking the back up?

https://www.intercomp-racing.com/Pro...SYSTEM_940.cfm

These scales will do it according to the manual which came with mine. I haven't done it yet. The instructions show lifting the rear of a stock car so I assume with a formula car the idea would be to lift the front since it is lighter (and my car has a good jacking point for one of my floor jacks )

Dick
85 VD with cast iron head (even with me out of the car . . . )

[Purple Frog]

OBTW, i left out one instruction in the "procedure" to find CG height.

You need to put "dummy rods" in place of the shocks and springs. And set them to hold the car at race ride height. Then lift one side of the car. Otherwise you get a lower than real measurement, because the suspension sort of unloads when you lift the car on it's side.*

Damn, I wish i would have made a YouTube video the time we did it. It was entertaining.

*OBTW... when the car is on its side as described, it is possible to replace the floor.
Now, how would i know that? (Steve told me )

[provamo]

since the car is raced with a driver and fluids i guess one might want to add balast to approximate those weights (and their locations) ......as well as account for the now "missing" springs and dampers?

[Purple Frog]

Like i said above... you should do it one time. Highly entertaining.

If not readily apparent what the result is. But, all witnesses were rolling in laughter.

[R. Pare]

I would think a FB would be lower than say a FC (especially a pinto). FCs have a lot of iron in the engine higher than 11".

I haven't done the comparison, but I'd bet that you are correct.

I always liked the Pare/Lathrop method of measuring. Put a bunch of air in the tires and lift one side of the car until it is balanced. Then imagine a vertical line from the tire patch on the floor going up through the car. Where it intersects the centerline of the car is close to the CG height.

Definitely the most accurate way to do it - BUT you have to be very careful not to tip the car on to its top!

[starkejt]

So how are you measuring the intersection? Seems like it would be easy to mis-estimate in a few places and have the error stack up to a couple inches on the cg height.

[provamo]

Paul Van Valkenburg's "Race Car Engineering and Mechanics" book has a decent diagram and text and equation

[Daryl DeArman]

As to where it is, I haven't a clue. However, if you aren't in the design theory stage and actually want to measure it. I'll attempt to explain:

On a set of scales first locate the horizontal CG location using F:R ratio and wheelbase.

Lock shocks so that ride height stays constant.

Raise one end of the car a certain height (your choice), use pythagorean theorem to get a new hypothetical wheelbase.

Use the new weights to determine the new horizontal CG location.

Draw both scenarios to scale on the same piece of paper locating the end that stayed at ground level traced over each other. Now extend a vertical line through the original horizontal location. Then draw another line through the one end raised horizontal CG location perpendicular to the wheelbase. Where those two lines intersect is your CG in two axis.

I would imagine you could nail down the CG location by repeating the above side to side intersecting all three lines.

I don't have the above mentioned book, but I am pretty confident it does a better job of explaining what I just tried to

[Purple Frog]

Daryl, i think your way makes a lot of sense, and may be easier.

But, not as entertaining for a bunch of bench racers hanging around the shop.
Especially if adult beverages are part of the equation.

[mousecatcher]

too much error when lifting at low heights (jack heights). you have to get the car balanced, and that's pretty much impossible to do F/R. i have the math somewhere but i just moved so gonna be awhile before i can dig it out, but even the first google hit (http://www.longacreracing.com/articles/art.asp?ARTID=22) says raise the car 20"

[Jnovak]

1. The more you raise the car the better the accuracy.
2. Make sure that you pump the tires up to a high PSI.
3. Fill the fuel tank or make sure it is empty.
4. Do not forget to use rods in place of the shocks.
5. Put the driver in the seat. In some cars the driver is the biggest mass especially if you include his head.
6. Do it more than once so you can see the range of the results, they do vary.

This methodology gives reasonable results.

Thanks ... Jay Novak

[Purple Frog]

Keeping the driver in the seat was the biggest challenge when we tried the "side balance" method.

[Jnovak]

Keeping the driver in the seat was the biggest challenge when we tried the "side balance" method.

Use a "dummy" and cinch the belts down real tight.

Thanks ... Jay Novak

[Marty Nygard]

Put the driver in the seat. In some cars the driver is the biggest mass especially if you include his head.
Are you trying to say something about driver's egos?
Marty

[racerdad2]

If the car does topple over, its a good way to verify roll-over protection, with the driver strapped in. Also, a good time to see what fluids will be leaking in the event you should land wheels up... Be sure to have video.... We will want to watch, for educational purposes....

[Daryl DeArman]

Here you go:

http://www.youtube.com/watch?v=aLTWhYvnmQw

[starkejt]

I like how the guy was going to run around and catch it.

[racerdad2]

Thx ! You Apex guys rock !!!

[racerdad2]

Safety straps are a great idea ! Just might want to check the load rating... First

[brownslane]

Here you go:

http://www.youtube.com/watch?v=aLTWhYvnmQw

This is what we call in our shop an ID-ten-T......any guesses?