Hot! Ferrari F12 Berlinetta vs. Lamborghini Aventador LP700-4

[martinbo]

I believe that applies to mid-front engine since the distribution has only slight rear bias (something like 48%/52% or 47%/53%) so the moments of inertia are based right in the center with the driver.

In the case of a rear mid-engine, generally the rear bias could be around 58% or higher making the polar moments of inertia skew to the rear substantially, which results in a steady-state oversteer/understeer behavior with a substantial rear bias since in corner entry there will be more weight transfer back to front resulting in understeer and while going through the turn, the rear will have a tendency to swing out more due to more inertia being exerted on the rear.

In other words, if latter half of the car is heavier than the former half then there will be more forces of inertia exerted on the rear half and vice versa. Hence more centrifugal force on the rear pushing it outwards.

In a straight line, no doubt the rear mid-engine configuration has the upper hand due to more front to back weight transfer resulting in more weight over the rear wheels giving more traction off the line by pressing the tires harder against the pavement.

Nope, your explanation doesn't impart an understanding of polar moment of inertia - from the above post you mistake weight distribution for polar moment of inertia. Simple example of a weightlifter's adjustable dumbbell:

A. 20 cm shaft; 4 kg weight at one end; 6 kg weight at the other. Weight distribution: 40% - 60%. Higher polar moment of inertia.
B. 20 cm shaft; 4 kg weight close to centre; 6kg also close to centre. Weight distribution: Still 40% - 60%! Lower polar moment of inertia.

Clearly, in example A. we see a greater propensity for rotational inertia around the centre axis because the mass is located further from the centre. In example B. we see a lesser propensity for rotational inertia around the centre axis because the mass is located closer to the centre.

In the example of a front mid-engined car, clearly the mass of the transaxle and engine is further apart by virtue of being separated by the passenger compartment whereas in a rear mid-engined car all of the mechanical mass of engine and transmission is located much closer to the rotational centre of the car - just behind the driver. This is the fundamental tenet of the rear mid-engined concept. i.e why Formula 1, Le Mans sports prototypes and true hypercars all exhibit this layout.

It's by no means perfect; having less mass over the front axle means less grip at the front. This results in initial understeer that can quickly transition to oversteer when rear axle slip angles suddenly overtake those at the front wheels. This is a common ailment of such cars. The benefit? Much greater transitional reactivity and agility because the mass is located more toward the centre of the vehicle.

As a side note, whilst the term weight transfer is ubiquitously used, I much prefer to use the words load transfer as this more accurately describes the forces at work. It's not like Sammy the St. Bernard suddenly jumped from the back seat into the front. Rather, it's the accelerative and decelarative forces that increase the load on the axles relative to where Sammy is located thus resulting in the phenomenon of greater & lesser grip as you describe above. And this has everything to do with weight distribution.

 

[Sunny]

^Martin's "physics for dummies" to rescue.

 

[Beemer B773ER]

Sammy agrees!

 

[Beemer B773ER]

Trust Sensai Martin to only intervene when he sees fit.

 

[Levi68]

There is none, people want the 911 to stay that way so Porsche just gives the people what they want.

Lucky them. If BMW gave what people wanted.... Looks like poeple want a FWD, and not a CSL.




And back to off-topic, another proof to ditch the 911 and give the Cayman the engine it deserves.

 

[Deckhook]

Lucky them. If BMW gave what people wanted.... Looks like poeple want a FWD, and not a CSL.

Did Porsche fans want the Cayenne..... NO, did they want the development a saloon..... NO.

Looks like even Porsche have to bow to market pressures.

 

[Levi68]

Did Porsche fans want the Cayenne..... NO, did they want to the development a saloon..... NO.

Looks like even Porsche have to bow to market pressures.

Sure, but they always have the 911 GT3, even if PDK. There is even possibility of a more hard-core version of Cayman. At least Porsche pleases both sides, and uses a one part of the profit for the other side. BMW bows only to one side. Well, there is the M235i/M2 coming. Thank God.

 

[330CIZHP]

I guess, we can agree to disagree.

By the way, I know polar moments of inertia very well as I am an engineer who studied university level physics. My usage of weight (or as you say, load) distribution was in no way interchangeably being used with polar moments of inertia.

The difference is, in the assumptions we are making. For example, using the example of a dumbbell would liken it to a car that has its engine pinned the front radiator and the transmission pinned to the rear bumper.

My assumption according to the architecture I have seen for a front mid-engine car is that the engine is mounted completely behind the front axle, close to the center of the car and the two seats cabin is pushed back over the rear axle just aft of the center point of the car while the transmission is mounted over the rear axle.

Therefore, according to the architecture diagrams I have seen of front mid-engine is that the mass is still concentrated near the center of the car with slightly more mass just aft of the center closer to the rear axle (where the transmission is). So the rotational axis is still as close to the center of the car as possible.

Using my assumptions above, I fully believe the polar moments of inertia in a front mid-engine car would be completely apples to oranges with your metaphorical example of the dumbbells.

Theoratically, I believe a rear mid-mounted engine is not at the center of the car. It is actually mounted in the rear half of the car so the rotation axis in my personal opinion is actually near the rear

One example is, call me crazy, but this Carrera GT diagram where the engine and transmission is well aft of the center point of the car. The cabin is right at the center point. Therefore, the polar moments of inertia is near the rear than the center.

Nope, your explanation doesn't impart an understanding of polar moment of inertia - from the above post you mistake weight distribution for polar moment of inertia. Simple example of a weightlifter's adjustable dumbbell:

A. 20 cm shaft; 4 kg weight at one end; 6 kg weight at the other. Weight distribution: 40% - 60%. Higher polar moment of inertia.
B. 20 cm shaft; 4 kg weight close to centre; 6kg also close to centre. Weight distribution: Still 40% - 60%! Lower polar moment of inertia.

Clearly, in example A. we see a greater propensity for rotational inertia around the centre axis because the mass is located further from the centre. In example B. we see a lesser propensity for rotational inertia around the centre axis because the mass is located closer to the centre.

In the example of a front mid-engined car, clearly the mass of the transaxle and engine is further apart by virtue of being separated by the passenger compartment whereas in a rear mid-engined car all of the mechanical mass of engine and transmission is located much closer to the rotational centre of the car - just behind the driver. This is the fundamental tenet of the rear mid-engined concept. i.e why Formula 1, Le Mans sports prototypes and true hypercars all exhibit this layout.

It's by no means perfect; having less mass over the front axle means less grip at the front. This results in initial understeer that can quickly transition to oversteer when rear axle slip angles suddenly overtake those at the front wheels. This is a common ailment of such cars. The benefit? Much greater transitional reactivity and agility because the mass is located more toward the centre of the vehicle.

As a side note, whilst the term weight transfer is ubiquitously used, I much prefer to use the words load transfer as this more accurately describes the forces at work. It's not like Sammy the St. Bernard suddenly jumped from the back seat into the front. Rather, it's the accelerative and decelarative forces that increase the load on the axles relative to where Sammy is located thus resulting in the phenomenon of greater & lesser grip as you describe above. And this has everything to do with weight distribution.

 

[Gianclaudio]

Are you taking in count the driver's weight/mass?

That helps balance load distribution......

 

[330CIZHP]

Are you taking in count the driver's weight/mass?

That helps balance load distribution......

I agree. It does. I am only arguing both have their goods and bads.

 

[Gianclaudio]

One point that also wanted to discuss, is when you say having the engine on front and conecting by a shaft to diferential/transaxle means more parts and having to engineer a transmission tunnel, bear in mind that tightly packaging all these units in a compact one must be just or more expensive to design and build. After all, going ultra compact also has their unique requirements

 

[330CIZHP]

One point that also wanted to discuss, is when you say having the engine on front and conecting by a shaft to diferential/transaxle means more parts and having to engineer a transmission tunnel, bear in mind that tightly packaging all these units in a compact one must be just or more expensive to design and build. After all, going ultra compact also has their unique requirements

That is true. But, I think you can extend the back out more than you can extend the nose out. No?? I don't know if this is a good example, but have you seen the Lamborghini V10s and V12s with their respective transmissions?? They are absolutely gargantuan. The first time I saw it, I was surprised at how they fit all of that in there.

 

[Sunny]

One example is, call me crazy, but this Carrera GT diagram where the engine and transmission is well aft of the center point of the car. The cabin is right at the center point. Therefore, the polar moments of inertia is near the rear than the center.

But the primary aim (to get a car to turn as quickly as possible) is to reduce the polar moment of inertia, not move it to the physical center of the car. And the former requires concentrating mass as close to the COG of the car, not the physical center of the car. The physcial center of the car has very little bearing on the car's moment of inertia (look at Delta wing for very extreme example of this). Which is why mid engine cars have the advantage, you can concentrate the heaviest masses in a car (engine and transmission) together around the COG reducing the polar moment of inertia.

This is the main principal behind the delta wing - it tries to concentrate all the mass around the COG which is towards the rear and make everything else as light as possible.

 

[Gianclaudio]

That is true. But, I think you can extend the back out more than you can extend the nose out. No?? I don't know if this is a good example, but have you seen the Lamborghini V10s and V12s with their respective transmissions?? They are absolutely gargantuan. The first time I saw it, I was surprised at how they fit all of that in there.

I've never seen a Lambo engine outside the car, in person. Saw a couple Mercedes V12 and mother of God they were massive. Still nothing compared when I used to visit the local railroad shops.......

 

[330CIZHP]

I agree. My reply was just in context of the analogy of how dumbbells have weight at the opposing ends and how the rotational axis will change once you bring the weight near the center.

But the primary aim (to get a car to turn as quickly as possible) is to reduce the polar moment of inertia, not move it to the physical center of the car. And the former requires concentrating mass as close to the COG of the car, not the physical center of the car. The physcial center of the car has very little bearing on the car's moment of inertia (look at Delta wing for very extreme example of this). Which is why mid engine cars have the advantage, you can concentrate the heaviest masses in a car (engine and transmission) together around the COG reducing the polar moment of inertia.

 

[Gianclaudio]

By the way guys, you may find this book interesting:

http://www.amazon.com/H-Point-The-Fundamentals-Design-Packaging/dp/1933492376/ref=sr_1_1?ie=UTF8&qid=1369179505&sr=8-1&keywords=h point

While is not the uttmost technical book, it gives you the overall idea about packaging in cars. I like it

 

[Giannis]

But the primary aim (to get a car to turn as quickly as possible) is to reduce the polar moment of inertia, not move it to the physical center of the car.

In engineering (or just basic physics) language:

Rotational forces (=torque =force times times arm) = polar moment of inertia times angular acceleration

Given the forces, the only way to increase acceleration is by lowering the polar moment of inertia. To achieve that, you either decrease mass or decrease distance from the point of reference, or both.

 

[Soup]

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