Hillbilly physics question. Cantilevered weight.

[Kenny]

Like the title says, hillbilly physics question;



When it comes to cantilevered weight, does light weight ever trump low weight? My definition of cantilevered being any weight that is outside (especially below in this case) the roll center, and weight that is ahead of the front or behind the rear axle centerlines. The question is specific to my Blazer project which has a 50" or so body height. It looks as though there is a lot of room to relocate weight to the rear in a very low position ( close to or below the roll center).

The question concerns whether there should be concentration on lightness in the components when place this low, and critical placements. Should lead acid batteries be used to manipulate CG, or should I concentrate on some LiFEpo4 batteries that weigh less than 10 lbs? Subwoofer boxes,amps,ABS, diff and trans coolers? There will be the obvious concentration on keeping everything including fuel load between the axles. The next question up would be lateral placement, and the cantilever effects by having an outboard (from chassis centerline) mounting scheme?

Just another thing to obsess over instead of completing a project?

[Norm Peterson]

This sounds like this is in the realm of of mass moments of inertia, which are tied to how rapidly the car will yaw (steer, turn in), pitch, and roll. "Polar Moment of Inertia", as applied to yaw is perhaps the most widely known of the three.

Relative to the CG (wherever that might happen to be), the MOI contribution from any given component is roughly a function of its mass "m" and a squared function of the distance "r" between that mass's own CG and the car CG. If you're talking about a rather heavy component vs a much lighter one, keep in mind that the car CG's wouldn't be in quite the same place for both cases. The mr^2 math would use slightly different values for r even if the two components were located in the same place in the car.

BTW, "r" is not purely a fore/aft, lateral, or vertical distance unless the CG of the component in question sits in purely those directions from the car CG in 3-D space. You have to measure r to where your individual mass CG is projected in the plane perpendicular to the axis of vehicle rotation that you want to evaluate it for.


Chances are, you won't be able to lower the "PMOI" of a 100" WB 60" track 3000+ lb car too far. But some cars have intentionally utilized a higher PMOI in order to enhance stability or to reduce the demands on the driver.


Norm

[sik68]

If you're talking about actually using the blazer on the track, then one of the first items on the agenda is usually to eliminate understeer by balancing the front-to-rear weight distribution. Secondarily it is a good idea to keep inertia to a minimum, but it's my perception that weight distribution is more perceptible to a driver than yaw rate. At least on a road course, cars don't actually yaw that quickly...turn-in, apex, turn-out is ideally supposed to be a minimal yaw rate maneuver. Autox is different though...I could see a stronger consideration for inertia...but still maybe not at the expense of overall neutral weight balance.

[Kenny]

This sounds like this is in the realm of of mass moments of inertia, which are tied to how rapidly the car will yaw (steer, turn in), pitch, and roll. "Polar Moment of Inertia", as applied to yaw is perhaps the most widely known of the three.

Relative to the CG (wherever that might happen to be), the MOI contribution from any given component is roughly a function of its mass "m" and a squared function of the distance "r" between that mass's own CG and the car CG. If you're talking about a rather heavy component vs a much lighter one, keep in mind that the car CG's wouldn't be in quite the same place for both cases. The mr^2 math would use slightly different values for r even if the two components were located in the same place in the car.

BTW, "r" is not purely a fore/aft, lateral, or vertical distance unless the CG of the component in question sits in purely those directions from the car CG in 3-D space. You have to measure r to where your individual mass CG is projected in the plane perpendicular to the axis of vehicle rotation that you want to evaluate it for.


Chances are, you won't be able to lower the "PMOI" of a 100" WB 60" track 3000+ lb car too far. But some cars have intentionally utilized a higher PMOI in order to enhance stability or to reduce the demands on the driver.


Norm

If you're talking about actually using the blazer on the track, then one of the first items on the agenda is usually to eliminate understeer by balancing the front-to-rear weight distribution. Secondarily it is a good idea to keep inertia to a minimum, but it's my perception that weight distribution is more perceptible to a driver than yaw rate. At least on a road course, cars don't actually yaw that quickly...turn-in, apex, turn-out is ideally supposed to be a minimal yaw rate maneuver. Autox is different though...I could see a stronger consideration for inertia...but still maybe not at the expense of overall neutral weight balance.

Okay, you guys are right on what I'm after!! Now let me cut this into digestible non-techie hillbilly gibberish........ Sorry, it's all I speak.

I have no problems getting to a 50/50 weight distribution. The quantity in raw measurements is a piece of cake, now to work on the "quality" of that distribution. It is simple physics that any weight located in front of the front axle or behind the rear axle centerlines behaves like it is more than the weight itself, we all know that a battery in front of the front axle actually creates lift on the rear because it is cantilevered. I know you guys already know this, so bear with me. In a dynamic situation it actually hurts if you had added weight at the extreme end to get 50/50. You already know this too....... What I want to do is apply this same principle to weight that is above the roll center by using cantilevered weight below the roll center.


This is a street-able vehicle first and foremost, so there will be factory glass etc. The intended improvements will be geared toward slalom/auto-x dynamics because of safety. You just never know when a child or dog will run out in the road or someone pull out in front of you. The goal is to maximize the effects of moving components, if it is worthwhile at all.

[exwestracer]

In terms of vertical weight location, lower is almost always better. You have a pretty tough hill to climb in that the Blazer carries a (relatively) LOT of weight up high.

Here's the problem with your basic theory... Moving a significant amount of mass down to roll center height isn't going to be easy or cheap. For instance, you'll want a front roll center height of about 4-6" for proper handling. How do you get a significant amount of mass below that height without scraping it on the ground? Granted, the rear RCH is usually a little higher but the same basic problem remains.

I've spent a number of years playing with race cars that have almost no roll moment (RCH = CGH). On banked tracks, we can actually get the cars to "Reverse roll"; that is, lean to the inside in a corner. Keep in mind, this is with a 2" ride height and engine laid over 40deg to the side. It would be nearly impossible to get anywhere near this with a street car unless it was an EV with the entire floor made up of heavy batteries and a very light, low body shape.

[nekkidhillbilly]

this aint hillbilly physics............that requires duct tape..........i know im a expert

[exwestracer]

this aint hillbilly physics............that requires duct tape..........i know im a expert

Hey, duct tape got the Callaway Sledgehammer over 250mph! And those guys were "real" engineers...

[Kenny]

I'll tell you guys, I'm just not as young or willing to fight the laws of physics as I once was........ I am seriously thinking of foregoing the Blazer project for a stepside standard cab S-10. That will ease the problem of rear cg height and give me plenty of space under the bed for component relocation. It's just not as suited for a family road trip. As has been pointed out, there just isn't a good way for me to get weight below roll center height, especially since I will be using the Pontiac Solstice IRS instead of a leaf sprung solid axle.

The cool thing is that the 4x4/ AWD suspension setup in a S-10 lends itself to an easy 120+ lb front weight loss with aftermarket components.

So now the question is if there is an advantage available by manipulating lateral position of weight located near the RC height? Is it possible for a heavier weight that lowers the CG to work better than light weight with a higher CG?

[exwestracer]

So now the question is if there is an advantage available by manipulating lateral position of weight located near the RC height? Is it possible for a heavier weight that lowers the CG to work better than light weight with a higher CG?

Flywheel effect works the same for body roll as it does in the engine. The closer the mass is located to the roll center, the less effect it has on chassis roll. You'd have to provide some hard numbers on mass and moment arm length to get any sort of accurate comparison. Guesses are useless in this case...