quick questions about this car in snow

[Ultramaroon]

Originally Posted by bluesman

Simple - putting those sandbags all the way at the back of the car increases its polar moment of inertia, which means it's less sensitive to steering input, it's less prone to spin, and spins will be slower. But if the rear does break loose, it'll be harder to stop the skid until the rear is in front. So when the rear breaks loose, the rear will end up leading the car (the backwards dart analogy) unless stopped by a snowbank or guardrail in mid-skid. It's the skater with his arms held wide apart.

If you put those sandbags as far forward as you can (e.g. against the back of the raised rear seat), you reduce the car's polar moment of inertia. This makes it more sensitive to steering input, more prone to spin, more likely to keep spinning, and more likely to spin faster. It's the skater with her arms tucked in at her sides.

Shorter wheelbases make PMI a more critical factor in handling. So shorter lighter cars like ours can be affected by even 50 pounds of sand put in the wrong place during winter driving.

Weight at ends of car + loose rear end = slower spin to 180 or less. Weight toward center of car + loose rear end = fast 360s.

Oh, I had hoped I misunderstood you. I am disagreeing with your argument of "more prone to spin" and especially "more likely to keep spinning"

For the sake of simplifying the model, let's assume identical corner weights so in both cases center of mass is right smack in the middle.

Case 1 - mass is also concentrated near center mass. (a low polar inertial moment)

Case 2 - mass is distributed out near the wheels.

In Case 1 less force is required to accelerate the system about that center of mass. The opposite is also true. Less force is required to stop a rotation.

If one wishes to lessen the tendency of a vehicle to spin, decrease the polar inertial moment.

This argument is "kinda sorta" similar to the argument that decreasing unsprung mass improves suspension responsiveness and overall stability.

Ok, have at me. Pile on.

[Tcoat]

Originally Posted by Ultramaroon

Oh, I had hoped I misunderstood you. I am disagreeing with your argument of "more prone to spin" and especially "more likely to keep spinning"

For the sake of simplifying the model, let's assume identical corner weights so in both cases center of mass is right smack in the middle.

Case 1 - mass is also concentrated near center mass. (a low polar inertial moment)

Case 2 - mass is distributed out near the wheels.

In Case 1 less force is required to accelerate the system about that center of mass. The opposite is also true. Less force is required to stop a rotation.

If one wishes to lessen the tendency of a vehicle to spin, decrease the polar inertial moment.

This argument is "kinda sorta" similar to the argument that decreasing unsprung mass improves suspension responsiveness and overall stability.

Ok, have at me. Pile on.

Oh GOD Ultra has slipped over to the dark side of the Technical Debate Group! Come back to us Ultra before it is to late!

[FRSBRZGT86FAN]

I wanna see one of the triplets with snow chains

[Tcoat]

Originally Posted by FRSBRZGT86FAN

I wanna see one of the triplets with snow chains

Me too! Those big cleated ones we used to put on our trucks in the army.

[Ultramaroon]

Originally Posted by FRSBRZGT86FAN

I wanna see one of the triplets with snow chains

no way, man. fuck up my plasti-dip. yo trippin.

[Ultramaroon]

Originally Posted by Tcoat

Oh GOD Ultra has slipped over to the dark side of the Technical Debate Group! Come back to us Ultra before it is to late!

[Tcoat]

Originally Posted by Ultramaroon

MUCH BETTER!
Though we lost you for a minute there.
Have to do my science shit all day so like to get away some times!

[bluesman]

Originally Posted by Ultramaroon

For the sake of simplifying the model, let's assume identical corner weights so in both cases center of mass is right smack in the middle.

Case 1 - mass is also concentrated near center mass. (a low polar inertial moment)

Case 2 - mass is distributed out near the wheels.

In Case 1 less force is required to accelerate the system about that center of mass. The opposite is also true. Less force is required to stop a rotation.

If one wishes to lessen the tendency of a vehicle to spin, decrease the polar inertial moment.

Ok, have at me. Pile on.

Let's start with the impossibility of maintaining identical corner weights after throwing 50 pound bags of sand in the car......

All I can do is quote a few of the many experts on this. From a pistonheads.com moderator:

"Mid-engined cars tend to have a lower moment of inertia. This means they have more tendency to rotate... whether you want them to or not. It makes them quicker to turn when you want them to, but also quicker to rotate when you don't, making them harder to hold at a given angle past the limits of grip."

From Car & Driver in a comparison of the Cayman and the 911 on the skidpad. The cars were set up to achieve essentially equal staedy state handling, i.e. "[t]he 911 GT3’s 1.01-g roadholding topped that of the Cayman R by a slim 0.01-g margin. Both demonstrated minimal understeer.

Under steady throttle, the Cayman finally spun at 75 mph. Backtracking a bit, we found that it became nervous at 65 mph with a tendency to drift sideways when full throttle was applied at that speed. When the throttle was abruptly lifted following 90 degrees of steering at 65 mph, the response was a gentle spin.

The 911 confidently stepped up the speed ladder to 80 mph before things got dicey. Adding throttle at that speed was no problem. But dropping the throttle at 80 mph made this car extremely loose. At an entry speed of 85 mph, the 911 spun every time. Why? Because the combination of lateral (cornering) and longitudinal (propulsion) loading and the 911’s large polar moment of inertia finally overwhelmed the rear tires."

Higher PMI = higher cornering speed before spinning. Lower PMI means easier spins at lower speeds (although it also means faster response to steering input short of that point).

[Ultramaroon]

I was looking forward to a rational discussion based on some solid physics but I guess that's not going to happen. No worries.

[Tcoat]

Originally Posted by Ultramaroon

I was looking forward to a rational discussion based on some solid physics but I guess that's not going to happen. No worries.

But dude he quoted the moderator from pistonheads you do not get more authoritative than that.

[bluesman]

Originally Posted by Ultramaroon

I was looking forward to a rational discussion based on some solid physics but I guess that's not going to happen. No worries.

Sorry about that - I lost the desire to take it further when confronted with the concept of shifting weight bias while maintaining equal corner weights.

I'm not sure I have the deep technical knowledge to discuss, let alone calculate, angular velocity and complex trajectories of skidding vehicles at the level to which you clearly wish to take this. Those quotes were simply provided as quickly accessed examples of the generally accepted truth that a low polar moment of inertia means less resistance to steering input short of the limit and less resistance to spinning when the limit is reached. No mathematical explanation is necessary - it's the common observation of many many experts (of which C&D is, I assume, an acceptable example to you).

I started off trying to make the point that you should be careful when and where you put sandbags in your car when trying to improve winter driving, because they could materially and adversely affect handling in unexpected ways. I don't think a 50 pound bag of sand is going to improve traction in a 2500+ pound car - but it could contribute to a skid if put in exactly the wrong place.

There's no disrespect intended to your intellect or knowledge - I simply think you're wrong on this issue while recognizing that it could be me instead. I can live with it either way.

Best regards -

David

[Ultramaroon]

Originally Posted by bluesman

Sorry about that - I lost the desire to take it further when confronted with the concept of shifting weight bias while maintaining equal corner weights.

I'm not sure I have the deep technical knowledge to discuss, let alone calculate, angular velocity and complex trajectories of skidding vehicles at the level to which you clearly wish to take this. Those quotes were simply provided as quickly accessed examples of the generally accepted truth that a low polar moment of inertia means less resistance to steering input short of the limit and less resistance to spinning when the limit is reached. No mathematical explanation is necessary - it's the common observation of many many experts (of which C&D is, I assume, an acceptable example to you).

I started off trying to make the point that you should be careful when and where you put sandbags in your car when trying to improve winter driving, because they could materially and adversely affect handling in unexpected ways. I don't think a 50 pound bag of sand is going to improve traction in a 2500+ pound car - but it could contribute to a skid if put in exactly the wrong place.

There's no disrespect intended to your intellect or knowledge - I simply think you're wrong on this issue while recognizing that it could be me instead. I can live with it either way.

Best regards -

David

Thanks. Very much appreciated. Zero dis/offense taken. FWIW, I agree with just about everything you're saying except the point of less resistance to spinning once it starts.

Also equally prepared for being wrong - actually looking forward to it. Means I learned something.

In the big picture, I think we can all agree upon:
1. Not driving like an asshat.
2. Being very careful about any changes we make to the vehicle's configuration.
3. Once in a snowbank, transfer some sand from the bags to under the tyres.

[86-tundra]

this forum is pretty funny during the winter.

I mean, it's pretty funny for me -

I would avoid this car at all costs if there were snow on the ground. but I live in texas

[Pat]

Originally Posted by wheelhaus

Isn't there something to be said for the weight on the rear axle applying more pressure to the tire's contact patch? If there's more weight, it effectively acts as a larger fin on a dart.

Interesting analogy. While I understand what you mean in terms of rear tire grip, also consider it can mean the opposite once traction is lost. The more weight there is at the front of the car, the more likely it is to track straight, much like a dart.
Of course, when there is a lot of weight at the rear (ala Porsche), the car inherently wants to rotate once traction is lost.