CofG in car design

[raymondu999]

This is a point of question I've been wondering for a while. When designers design a car, do they design it to have a low CofG, or do they attempt to design the "base," unballasted design as light as possible to then put the ballast down low?

[silente]

in my experience, you normally try to do both.

But if i had to chose between light or low, i would choose light.

[raymondu999]

Ah, but if you were designing to a minimum weight, a la F1?

[silente]

again, if you are light and you have room to put ballast in different zones of the car, you could use weight distribution as a tuning parameter.

And of course, in a similar situation you would probably at least try to put the ballast as low as possible.

[olefud]

again, if you are light and you have room to put ballast in different zones of the car, you could use weight distribution as a tuning parameter.

And of course, in a similar situation you would probably at least try to put the ballast as low as possible.

A low CoG is a basic parameter that, with track width, determines lateral weight transfer during cornering. Ballast, IMO, a less significant tuning aid dealing with factors that often have other fixes. So, as a generality, I would opt for low CoG.

[Jersey Tom]

Always, always, always light weight. Almost more so in a series with a minimum weight than a truly open class (not that there are many of those).

Ballast, IMO, a less significant tuning aid dealing with factors that often have other fixes..

Not sure I'd agree with that. CG location is pretty critical, an important element in handling. Springs and bars and kinematics and such can only at best be crutch solutions to poor mass distribution. For that reason I think it's super critical to get lightest weight possible, so you can use the most ballast and put it where it needs to be.

[olefud]

. For that reason I think it's super critical to get lightest weight possible, so you can use the most ballast and put it where it needs to be.

If the ballast can and may be located to gain back the optimum CoG, that's a winner all around.

[riff_raff]

This is a point of question I've been wondering for a while. When designers design a car, do they design it to have a low CofG, or do they attempt to design the "base," unballasted design as light as possible to then put the ballast down low?

I would say that designing the chassis for minimum polar moments in pitch, roll and yaw are just as important. Another consideration is designing the chassis such that there are minimal changes in chassis dynamics as the fuel load changes. This would mean locating the mean fuel mass CofG as close to the chassis CofG as possible. Lastly, there is also the aerodynamic CofP to consider.

[Lycoming]

In most cases I suspect you could drop vehicle CoG closer to the ground by minimizing weight rather than designing for low CoG of a certain part. Moreover, designing for low weight potentially allows you to reduce yaw and other inertias, though the relative value of those may or may not be anywhere near as important as CG height. Also, if you're dealing with parts that contribute to unsprung and/or rolling mass, it becomes even less of a question.

Not sure I'd agree with that. CG location is pretty critical, an important element in handling. Springs and bars and kinematics and such can only at best be crutch solutions to poor mass distribution.

Agreed. If you make a high powered rear wheel drive racecar with less than 50% of the weight on the rear wheels, for example, there's some issues you just won't be able to tune out with other methods.

[DaveW]

Good stuff, everybody.

I would say that designing the chassis for minimum polar moments in pitch, roll and yaw are just as important.

I can't disagree with that statement, but would suggest that in low grip conditions a higher yaw inertia can make a car easier to drive at the limit. Putting it another way, "rain masters" are super-sensitive to yaw acceleration, and evidence suggests that more normal drivers can be helped by slowing down the yaw response of a vehicle.

I don't think anybody has mentioned lateral position of the c.g. In an otherwise symmetric vehicle, an offset c.g. causes a coupling between lateral and vertical responses (the vehicle will "cross-weight" when subjected to pure vertical inputs), and this can cause the car to be "unsettled" in a corner. More generally, for example in the kind of vehicle JT plays with, set-ups that minimize dynamic cross weight induced by symmetrical inputs are usually well-received.

[olefud]

Not only are the low position CoG and modest polar moment of concern, but the concurrent longitudinal position of the of the CoG is important. For instance, the cantilevered engine in the Porsche 911 is often criticized relative to mid-engined designs. But the rearward Cog with a light front weight maintains an advantageous weight distribution under braking with a pronounced such bias when putting down the power. More rear rubber accommodates lateral needs. With a light front end, this is accomplished with a reasonable polar moment of inertia.

[silente]

I am with Dave in thinking that a low polar moment of inertia is not always something you would want to aim for.

I honestly believe than in several situations some more polar inertia could help, making the car more stable.

I didn´t play much with that in race cars i have worked on, but i believe in certain series, the tendency to try to increase front weight static distribution is helping performance also by changing polar moment of inertia and namely making it bigger.

It of course depends also on the kind of tracks you are racing and on how big it is, but a very low polar moment of inertia (keeping wheelbase the same, in this simplified example), would in general lead to less understeer in corner entry and, potentially, to some unstable behaviour in transients.

[turbof1]

I was also wondering; do they design parts (save for ballast) close or on the floor with heavier materials on purpose?

[Tommy Cookers]

Not only are the low position CoG and modest polar moment of concern, but the concurrent longitudinal position of the of the CoG is important. For instance, the cantilevered engine in the Porsche 911 is often criticized relative to mid-engined designs. But the rearward Cog with a light front weight
this is accomplished with a reasonable polar moment of inertia.

under acceleration of yaw the inertial effect 'wants' rotation around a point about which the rotational inertia is minimal

interestingly,the 911, (Alpine-Renault) etc, configuration has on this basis less M of I than any other
(this was also found with Fiats, the rear-engined cars had less than the mid-engined X1/9)

the same must apply with pitch acceleration, but this is disadvantageous to ride ?
rear-engined cars visibly pitch (ie amplitudes are quite large) about a conspicuously rearward point ?
similarly with spinning

[olefud]

Not only are the low position CoG and modest polar moment of concern, but the concurrent longitudinal position of the of the CoG is important. For instance, the cantilevered engine in the Porsche 911 is often criticized relative to mid-engined designs. But the rearward Cog with a light front weight
this is accomplished with a reasonable polar moment of inertia.

under acceleration of yaw the inertial effect 'wants' rotation around a point about which the rotational inertia is minimal

interestingly,the 911, (Alpine-Renault) etc, configuration has on this basis less M of I than any other
(this was also found with Fiats, the rear-engined cars had less than the mid-engined X1/9)

the same must apply with pitch acceleration, but this is disadvantageous to ride ?
rear-engined cars visibly pitch (ie amplitudes are quite large) about a conspicuously rearward point ?
similarly with spinning

Perhaps there are those concerned with ride.
I suspect there is some issue with the “natural” center of rotation as a function of the tire forces and substantially rearward CoG that would affect turn in.