goron59 wrote: ↑Wed Jan 23, 2019 4:17 pm
I suspect it's just a packaging thing so not really 20cm forward. And the 40:60 split only tells part of the story. That's the axle weight distribution, but it doesn't tell you where the weight actually is. A proper mid-engined car will have its weight between the axles, whereas the 911 typically has it beyond, so you get this horrible double pendulum effect. Generations of enthusiasts have been conditioned to think this is a good thing though
My experience of "messing" around with *polar moment of inertia in F1 and BTCC tells me that it's actually a very overstated parameter. The fore/aft position of the CofG (which obviously determines the axle weight distribution) and it's height off the ground are all that really matters when it comes to fundamental handling performance (in terms of the mass and its distribution that is, obviously there are other parameters that significantly affect handling), although you can change the "feel" a little by increasing or decreasing the polar moment i.e. wider/narrower distribution of that mass. Any feeling you experience in a 911 is largely down to the 40/60 weight distribution rather than it's slightly higher polar moment of inertia compared to a mid-engined car. Even a couple of percent change in fore-aft weight distribution is often quite noticeable in handling and traction etc. In F1 we often moved the CofG 2-3% forward or backward to suit different circuits or bias toward traction off the line e.g. Monaco where you can't afford to lose places off the grid.
What you do inherently get with a 40/60 weight distribution is superb straight line traction and braking performance and relatively light steering at the expense of ultimate lateral grip potential. I've always thought that was a better compromise for a road car, despite all the hype over mid-engined cars. For pure track racing then a mid-engined layout is optimum, but the 911 is hardly uncompetitive on a track day!
* Actually more accurately it should be termed "mass moment of inertia" so as not to be confused with torsional deformation, but the automotive industry tends to use the former term.
Last edited by Deleted User 1874 on Thu Jan 24, 2019 1:42 pm, edited 1 time in total.
goron59 wrote: ↑Wed Jan 23, 2019 4:17 pm
I suspect it's just a packaging thing so not really 20cm forward. And the 40:60 split only tells part of the story. That's the axle weight distribution, but it doesn't tell you where the weight actually is. A proper mid-engined car will have its weight between the axles, whereas the 911 typically has it beyond, so you get this horrible double pendulum effect. Generations of enthusiasts have been conditioned to think this is a good thing though
My experience of "messing" around with *polar moment of inertia in F1 and BTCC tells me that it's actually a very overstated parameter. The fore/aft position of the CofG (which obviously determines the axle weight distribution) and it's height off the ground are all that really matters when it comes to fundamental handling performance (in terms of the mass and its distribution that is, obviously there are other parameters that significantly affect handling), although you can change the "feel" a little by increasing or decreasing the polar moment i.e. wider/narrower distribution of that mass. Any feeling you experience in a 911 is largely down to the 40/60 weight distribution rather than it's slightly higher polar moment of inertia compared to a mid-engined car. Even a couple of percent change in fore-aft weight distribution is often quite noticeable in handling and traction etc. In F1 we often moved the CofG 2-3% forward or backward to suit different circuits or bias toward traction off the line e.g. Monaco where you can't afford to lose places off the grid.
What you do inherently get with a 40/60 weight distribution is superb straight line traction and braking performance and relatively light steering at the expense of ultimate lateral grip potential. I've always thought that was a better compromise for a road car, despite all the hype over mid-engined cars. For pure track racing then a mid-engined layout is optimum, but the 911 is hardly uncompetitive on a track day!
Actually more accurately it should be termed "mass moment of inertia" so as not to be confused with torsional deformation, but the automotive industry tends to use the former term.
Not really the same thing , mass moment of inertia is the measure of an object's resistance against the angular acceleration ie the energy required to turn a flywheel...On the other hand, Polar Moment of Inertia is a measure of a body's ability to resistance against the torsion when torque is being applied ie the stiffness of the shaft attached to the flywheel.
goron59 wrote: ↑Wed Jan 23, 2019 4:17 pm
I suspect it's just a packaging thing so not really 20cm forward. And the 40:60 split only tells part of the story. That's the axle weight distribution, but it doesn't tell you where the weight actually is. A proper mid-engined car will have its weight between the axles, whereas the 911 typically has it beyond, so you get this horrible double pendulum effect. Generations of enthusiasts have been conditioned to think this is a good thing though
My experience of "messing" around with *polar moment of inertia in F1 and BTCC tells me that it's actually a very overstated parameter. The fore/aft position of the CofG (which obviously determines the axle weight distribution) and it's height off the ground are all that really matters when it comes to fundamental handling performance (in terms of the mass and its distribution that is, obviously there are other parameters that significantly affect handling), although you can change the "feel" a little by increasing or decreasing the polar moment i.e. wider/narrower distribution of that mass. Any feeling you experience in a 911 is largely down to the 40/60 weight distribution rather than it's slightly higher polar moment of inertia compared to a mid-engined car. Even a couple of percent change in fore-aft weight distribution is often quite noticeable in handling and traction etc. In F1 we often moved the CofG 2-3% forward or backward to suit different circuits or bias toward traction off the line e.g. Monaco where you can't afford to lose places off the grid.
What you do inherently get with a 40/60 weight distribution is superb straight line traction and braking performance and relatively light steering at the expense of ultimate lateral grip potential. I've always thought that was a better compromise for a road car, despite all the hype over mid-engined cars. For pure track racing then a mid-engined layout is optimum, but the 911 is hardly uncompetitive on a track day!
Actually more accurately it should be termed "mass moment of inertia" so as not to be confused with torsional deformation, but the automotive industry tends to use the former term.
Not really the same thing , mass moment of inertia is the measure of an object's resistance against the angular acceleration ie the energy required to turn a flywheel...On the other hand, Polar Moment of Inertia is a measure of a body's ability to resistance against the torsion when torque is being applied ie the stiffness of the shaft attached to the flywheel.
I guess you didn't read my note
But just to clarify, in this automotive sense it means the mass resistance to spinning around its CofG i.e. how far away the mass is distributed around its CofG i.e. mass moment of intertia. Certainly not the torsional definition of polar moments valid in other spheres of engineering - hence my note.
goron59 wrote: ↑Thu Jan 24, 2019 1:30 pm
It is a marvel that the current 911 doesn’t swap ends at the slightest opportunity like it did 3 or 4 generations ago.
My limited experience in 997.x and 991.1 variants shows it’s still quite keen to do so vs something inherently more balanced though.
Perhaps it’s more to do with the axle ratio than how far away the mass is, as said above!
Well I've driven front engined cars that have far more inherent oversteer than a 911 of say 80's vintage and even by the early 70s they were pretty well sorted unless you seriously provoked them. A lot of bollox is talked about 911 snap oversteer, although they do demand a slightly different driving style to get the most out of them. I remember discussing all this with John Cleland back in the 90s when going from front wheel drive Touring Cars to 911s. Hardly surprising that!
Of course the 40/60 weight distribution requires wider rear tyres to balance out, but not much else. There's also a bit more mass moment of inertia than in a similar mid-engined car, but the main effect of that is to make it feel a little slower in response to a steering input and actually more stable and easier to counteract. Cars with extremely low mass moments of inertia (like F1 or Karts for example) are very quick to get away from you when you reach the limits and you need cat-like reactions to control. It's a bit like balancing a broom handle on the palm of your hand. It's quite easy and slow to react, but make that stick short like a pencil and it suddenly becomes a lot more difficult.
