DSMs center of gravity and center of pressure

[jabinya]

How do I determine the center of gravity and center of pressure on my vehicle?

I just watched the movie "The Fastest Indian" where they mention changing the position of the center of pressure in relation to the center of gravity to change handling characteristics. This got me to thinking about my car and what changes I could make to it and what effects it would have.

Here is a link to the definitions of each for anybody who is interested;

Center of Gravity

Center of Pressure

 

[SILV BLT]

I think thats going a little too far into it. Since you're probably not doing high speed runs and lots of road racing, judging by your profile, this kinda stuff isn't really going to help you much. You can mess with your center of gravity a bit to help handling and this has been discussed extensively. Things like battery relocation, parts removal, etc. You can't do much to alter the inherent nose-heavy effects of FWD, but little things help. If you're really concerned with the aerodynamics of your car, start looking at the bottom side. all the little nooks and crannies cause turbulence in the air as it passes under your car. Thats why F1 cars and exotics have under body diffusers to channel the passing air.

 

[WatchItExplode]

At least you're lucky enough not to have a sunroof! (by the looks of your pictures anyway). If I ever repaint my car, I'm welding a piece of sheet metal in place of my sunroof to help get rid of that weight up high.

I have no idea what a "center of pressure" is, but I think SILV might have a point. By the sounds of it, it doesn't seem like something worth messing with unless you're building a balls-out race car. But I think it's definately a good idea to find ways to lower your center of gravity other than just lowering your ride height. Have you considered a carbon fiber hatch with a lexan window?

 

[Defiant]

So long as your Center of Pressure is behind the Center of Gravity, the car will be stable- just like an arrow and its feathers. The factory made sure of all this, very early on.
As for the weight of the sunroof assembly affecting the height of the Center of Gravity, it won't make ¼" worth of difference.

 

[jabinya]

I saw a video explaining the process of making a aluminum roof for the new EVOs to drop 4 kilos of the top of the car. They took that EVO and another (with a steel roof) and timed them around a autocross track. The aluminum roof EVO got .1 or .2 seconds faster around the track.

If you're really concerned with the aerodynamics of your car, start looking at the bottom side. all the little nooks and crannies cause turbulence in the air as it passes under your car. Thats why F1 cars and exotics have under body diffusers to channel the passing air.

That brings to mind what I saw on the 2G tech manual. On the rear lower control arm there was a air diffuser that bolts on the front of the arm. I suppose this would create less drag. My car doesn't have this diffuser. I suppose I could fab one.

 

[bjones18]

For the most part, you get what you got. Before you "piece-meal" a vehicle design... figure out what your end goal is: autocross, drag, daily driver, pick-up-chicks, look cool, bar-stool racing... etc.

As stated previously

CG for-aft: battery in rear.
CG lower: lower car 1-1.5" with supporting wheel alignment.

Aero: lower car. Changing the center of pressure of the car would involve either chaning windshield angle or adding a rear wing. Not sure how much down force you need at the rear of a FWD car.

Smart guys know how to improve times with fewer modifications, at low cost.

Your best way to shave off lap times is to take a drivers training course, Bondurant, Skip Barber or the like. Everyone is a great driver, even a sixteen year old just out of drivers training. The reality is somewhat different. How well do you understand weight transfer and slip angles? (rhetorical question... search and you will find).

 

[Defiant]

Changing the center of pressure of the car would involve either chaning windshield angle or adding a rear wing. Not sure how much down force you need at the rear of a FWD car.

The Aerodynamic Center of Pressure in this case refers to the center of pressure from the side, not downforce on the front or rear. That is, when struck by a sidewind, will the car try to counteract it by steering into it, or will it be blown in the direction of wind by it. With the CoP behind the CoG, it will steer toward the sidewind and effectively cancel out its effects, while the CoP being ahead of the CoG will make a body very unstable and "darty".

 

[Defiant]

If you're really concerned with the aerodynamics of your car, start looking at the bottom side. all the little nooks and crannies cause turbulence in the air as it passes under your car. Thats why F1 cars and exotics have under body diffusers to channel the passing air.

Much of the underside of F1 cars involves creating low-pressure to assist in downforce and thereby high-speed stability and better road holding. And the designers get very, very antsy if you're caught trying to get a look at what's under there.

 

[mavisky]

Center of gravity can be calculated with a couple easy measurements like wheel base and trac. Then you just need a 4 post scale to figure out how the weight sits on those 4 extreme corners and you can calculate the center of gravity from that information.

No clue on the other, and I question its relevance in this particular application.

 

[bjones18]

The Aerodynamic Center of Pressure in this case refers to the center of pressure from the side.

Wow, we must be talking about a full sized van, or a 200+ mph car.

 

[jtmcinder]

You need to do a lot more than corner-weigh the car to get the GC location. You must re-weigh the car with one end higher than the other, and then re-weigh again with one side higher than the other. This is not a simple process, although spreadsheets exist to do the math for you once you have the three sets of corner-weights.

- Jtoby

 

[Defiant]

Wow, we must be talking about a full sized van, or a 200+ mph car.

Oh, a drive down 780 by Cordelia in an air-cooled VW microbus would pretty-much explain it all to you.
Wind starts to matter to cars around 35mph. And yes, it does matter more with more speed. Moving the Aerodynamic Center of Pressure rearward is why such cars as the racing Jaguars and Lotus 11 had a fin on the back. It still goes on today, but the wind tunnel studies allow designers to make the adjustments much more subtley.

 

[4pistons]

The Aerodynamic Center of Pressure in this case refers to the center of pressure from the side, not downforce on the front or rear. That is, when struck by a sidewind, will the car try to counteract it by steering into it, or will it be blown in the direction of wind by it. With the CoP behind the CoG, it will steer toward the sidewind and effectively cancel out its effects, while the CoP being ahead of the CoG will make a body very unstable and "darty".

That sounds like Center of Moment. The point around which the car will rotate. Sort of like the Center of Gravity but in a horizontal plane. If you spun your car around on a smooth sheet of ice it would be the center point of the spin. You can balance it by moving weight around fore and aft.

Our Mitsus have a that moment pretty far forward because the engine (a big chunk of mass) is hanging out over the front axles. If you move some weight to the rear you still get a high moment because the weight (front and rear) is still far from the center of rotation. To have a short center of moment you'd need a mid engine car with transaxle to keep the weight towards the center.

In theory it would seem a short moment would be good for Autocross and Road Racing, easier to get the car to turn, but for high speed, like Bonneville, you'd want a high moment to resist the car from spinning.

Rick - '91 GSX

 

[FlumpDSM]

That sounds like Center of Moment. The point around which the car will rotate. Sort of like the Center of Gravity but in a horizontal plane. If you spun your car around on a smooth sheet of ice it would be the center point of the spin. You can balance it by moving weight around fore and aft.

The center of gravity in a horizontal plane is still the center of gravity. If you spun a car on ice assuming no friction it will spin about the CG. If you throw any object in the air it will also start spinning about the CG just in a 3-D spin instead of 2-D.

 

[jtmcinder]

CoP refers to the center of the side of the car, since it is related to the effects of wind from the side. This is a point in two-space. (There is also the frontal CoP, which is another point in two-space. One does not combine the two, since the height of the lateral CoP is usually different from the height of the frontal CoP and keeping track of these separately is useful.) The CoG (or CoM) is the center of the mass of the car. It's a point in three-space and plays a huge role in weight transfer. The moment of inertia is not a point in space. If you know statistics and think of the the CoG as the mean, then the moment is akin to the variance. It plays a role in preventing a spin and, then, once you have spun, preventing you from catching the car.

Cliff notes: I agree with Defiant.

- Jtoby

 

[Defiant]

That sounds like Center of Moment.

That's different from the Center of Pressure. Moment's the car's distribution of mass, Pressure's just the way the wind hits the profile from the side. If you take an anvil and put a three-foot aluminum vane on it, the Center of Pressure will be somewhere along that vane. The Center of Moment will mostly be the anvil or very close to it, depending on just how huge you make the vane.

It was interesting in the seventies when Porsche brought over the VW 914, and touted what a marvelous, responsive chassis it was with the fuel tank and engine being within the wheelbase and have such a low polar moment. A couple of years later, they brought out the 928, and were explaining how great it would handle because the engine was out at one end and the transaxle was out at the other, and its high polar moment was a good thing.
Coming back from Tahoe last week in a sizeable snowstorm, the only moment I was concerned with was getting back some control on an icy, snowy downhill and hoping my sphincters wouldn't choose that moment to fail and shit my pants.
From what little Driving Too Fast I've done in the Talon, it kind of resembles throwing a sledge hammer.

 

[jtmcinder]

The attraction of high vs low moment depends on the use of the car. If you want a tossable car, which means a car that you can catch, then you want a low moment. If you want a stable car, then you want a higher moment. 1G MR2s and 914s are tossable. A DSM isn't. I've only ever driven one car that somehow was both: stable at speed and tossable when autocrossing. Not surprisingly, it was a 95 M3.

- Jtoby

 

[jabinya]

Wow, I've learned alot from this thread. Thanks for the intelligent discussion guys!

There has been an idea running across my mind for some time now and wanted to see what you guys thought of it. Granted my knowledge on this subject is severely lacking compared to yours.

Take a industrial gyroscope; say one that was designed for torpedo steering, a sensor that registers direction change and run it from a program on a PC to combat cornering forces.

 

[jtmcinder]

The amount of mass you would need to move to significantly alter the moment of a 3000# car are too great. Plus, I don't see how you would ever reduce it on the fly, given that the engine isn't going anywhere.

On the positive side, yaw is used by the Evo's drivetrain computer, so you're not totally off-base. Yaw is used mostly to control the AYC rear. It's an amazing piece of engineering and a blast to drive.

There are also some cars that include yaw in the list of inputs that control dynamic suspension damping.

- Jtoby