Center of Gravity

[randywebb]

Anybody know where on the z-axis the Center of Gravity is on a 911?

[island911]

Good question.

IMO CG, of various vehicles, should be published along side of 0-60 times.

[masraum]

It seems I've seen the number 18" posted here by one of the senior members. I believe he did some calculations to get that. It may have been Bill V, but I'm not certain.

[jluetjen]

My guesstimate is 21 inches. I've tried to measure it explicitly, but I just don't have the ground clearance to do it -- yet. So I made some estimates of the engine and transmission alone (crankshaft height and somewhere between the two transaxle shafts respectively), and then the balance of the chassis.

Note that it depends a lot on the car's ride height since I would expect that lowering the car by one inch will lower the CG by almost one inch. Duh!

[Wil Ferch]

My foggy brain recalls a number like 16" or 18" above ground at normal ride height.....don't hold me to that, though...

Wil

[Grady Clay]

In the early ‘70s Porsche built a lightweight 911 test mule. I think it was a full racer made even lighter. They repositioned everything from in the tunnel and made a track system that extended from just inside the roof down through the tunnel and out the bottom of the 911 at the intersection of the X and Y CG axis. There was a weight that could be dynamically repositioned along the track – as I recall it was about one cubic foot of lead.

The result was a 911 with a variable vertical CG. As I recall a 2” difference in CG had the same performance change as street tires to Dunlop race tires of the day – a huge difference. This was on the newly constructed Weissach skid pad.

In F1 they build the cars way underweight at enormous cost and then add a huge Tungsten plate under the chassis to bring it to weight spec.

Best,
Grady

[randywebb]

Bill posted a diagram of a 996 showing the CG on the length axis... if he posted on the Z axis, I don't recall it.

Well, let's see if anyone has ever seen a citation....

This can be measured directly on a modified rotisserie... Race Car Vehicle Dynamics describes it a bit... not something I wanna do tho.

But it is the least well known figure of merit on these cars, and an important one.

[Superman]

Interesting. I've mentioned before my curiosity regarding what happens when a vehicle's COG is below the axles. Surely this is possible, and my guess is that the COG of an F1 car is exactly where they want it. Because, it can be below the axles. And if it were, then wouldn't you be placing more weight on inside tires than outside tires, in a turn?

[randywebb]

I think the position of the CG rel. to the roll center would be important(?)

[jluetjen]

and my guess is that the COG of an F1 car is exactly where they want it.

Not quite true. The CG of an F1 is as low as they can make it without compromising the aerodynamics, HP or chassis strength. Last years Renault probably had a lower CG then most of the rest of the field as a result of their wide angle V10. Never the less, they reverted to the standard ~72 degree V10 this year (and the higher CG) because of the improvement in engine performance and packaging of the total car. 1986 Gordon Murry designed the somewhat star crossed BT55 to try to build the lowest CG possible with the help of BMW's lay-down turbo 4.



Well, the results were not steller. But the low CG approach came good a couple of years later with the MP4/4 of 1988 which won 15 of the 16 races entered and led 97% of all of the laps.

Because, it can be below the axles. And if it were, then wouldn't you be placing more weight on inside tires than outside tires, in a turn?

No, because the axle level does not represent the point around which the chassis rolls. That is the roll center which is defined by the suspension geometry. If the designer puts the roll center below the tires' contact patch (the other point that matters rather then the axle height), then yes the car will tend to roll load onto the inside tire rather then the outside tire. Putting the roll center below ground level is almost trivial design-wise. Having it be the right answer to the design problem is something else again.

[Bill Verburg]

If I knew once it is long forgotten

CoM is CoM, it is where it is. the thing to remember is that you want it as low s possible , but there is little that you can do to achieve it.

• no sun roof, c/f replacement parts
• no headliner
• low profile spoilers
• oil tank location
• seats
• driver
• door trim
• glove box, radio, heat control etc
• dash
• glass

[VaSteve]

Originally posted by Bill Verburg
If I knew once it is long forgotten

• no sun roof, c/f replacement parts
• no headliner
• low profile spoilers
• oil tank location
• seats
• driver
• door trim
• glove box, radio, heat control etc
• dash
• glass

• Short, headless driver with big feet.

[randywebb]

OK, I guess it is time to post my fat ass comment...

And, of course, the worst is a driver with a fat head.

[patkeefe]

FWIW, I just spent a few days considering the effects of corner balancing on Center of Gravity (Center of Mass). I figured out how to build a jig to put the car on to determine the true center in the x-y horizontal plane. I have yet to figure out how to hang the car on it's sides for the rest of the calculations.

As Bill says, it is where it is.
Pat

[jluetjen]

Apparently the standard method to calculate the Z location of the CG is to put either the front or the rear of the car up on a measured set of blocks, about 2 feet higher then the other end. Then redo the corner weight measurements and calculate the shift in the x-y location of the CG.

[69911e]

Originally posted by Superman
Interesting. I've mentioned before my curiosity regarding what happens when a vehicle's COG is below the axles. Surely this is possible, and my guess is that the COG of an F1 car is exactly where they want it. Because, it can be below the axles. And if it were, then wouldn't you be placing more weight on inside tires than outside tires, in a turn?

I think you may be asking what happens when the CG is below the roll center (very easy to do). The car will lean in the wrong direction.
If the CG is at the roll center there will be no roll of the body. Just remember, changing body roll has almost no effect on weight transfer (only an almost trivial amount caused by a small lateral shift in the CG).

John is correct about easy way to measure the Z location of the CG(I have done it this way before). Just remember the scales need to be very precise; the change in the two measurements will be small.