Stupid question of the week

[Swampdog]

Polar Moment of Inertia. The attached graph shows the polar moment. It looks like the farther you move from the axis of rotation (rear axel) the more total moment you get from a specific mass. Hence the invention of the funny car. The NHRA had limits on high and how far aft you cold mount ballast when I built dragsters.

I would think that it would have an effect but not all that much, like a battery relocation. But with precise weights and locations you could figure it out.

 

[65 3 times]

I'm ecstatic of the fact that I am not alone in this life. Here I stand with two 5 gallon bucket handles in my hands. (One in each of course) Inquisitive minds want to know !! Thanks All Dave

 

[HT413]

The spring is causing the rotational force and transferring it to the car. Example: with no spring it takes 20 pounds of force to lift lid above the latch. With spring it takes only 1 pound of force to lift the lid. That means the spring is putting 19 pounds of upward force on the lid. Equally, it is putting 19 pounds of downward force on the latch. When you remove the spring you feel the extra weight of the lid but are forgetting to subtract the loss of down force on the latch that the spring was creating. You can feel the force of the lid against gravity but can not feel the down force so it gets forgotten. But it is still there. "For every action there is an opposite and equal reaction". You are weighing the lid. We are not concerned with the weight of the lid, but the transfer of the weight of the lid. The transfer of the weight is at its center regardless of its mounting point.

Here's how I would do the analysis.

I) here we have no spring, and an equally distributed 40lbs. So I think we can all agree that the hinge and latch each take 20lbs, but this analysis shows how it could be.

II) here I added a spring that exactly counteracts the deck lid (just for simplicity). This shows the latch having no force, but also that the spring is counteracting the rotational force (moment) of the deck lid. also, if the latch sees no force, newtons 3rd law states that the force has to be counteracted by something, which has the be the only other contact point - the hinges.

 

[bobtile]

You guys are blowing my mind.
Lmao
As Mike said,
You are all right.
Depending on who posts last of course.

 

[dla4567]

Here's how I would do the analysis.

I) here we have no spring, and an equally distributed 40lbs. So I think we can all agree that the hinge and latch each take 20lbs, but this analysis shows how it could be.

II) here I added a spring that exactly counteracts the deck lid (just for simplicity). This shows the latch having no force, but also that the spring is counteracting the rotational force (moment) of the deck lid. also, if the latch sees no force, newtons 3rd law states that the force has to be counteracted by something, which has the be the only other contact point - the hinges.

View attachment 354156

Now you're making my head hurt. The lid has no rotational force, just weight (gravity). The spring is using its moment to lift the weight of the lid and is counter acted by the added down force on the body of the car. As the spring pushes the lid up, it also pushes the car down. The further from the pivot point (or axis), the less pressure, but still equal up and down. The force of gravity increases weight. The force of the spring does not because it must provide an equal and opposite force. Pushes up as much as it pushes down. When you put your scale between the trunk lid and the car you are adding in the up force of the spring but not the down force because the down force is in the body of the car below the scale and therefor taken out of the equation. In order for down force to be read as weight, it must pass through the scale. Now I find myself rambling.

 

[dla4567]

On a side note: I reversed the spring on my trunk lid so it actually pushes the lid closed. Took 2 seconds off my quarter mile times and added 38 mph. (But I can no longer do burnouts).

 

[bobtile]

 

[dla4567]

Spot on!

 

[Cranky]

Just remove the damn springs lol.

It was mentioned but maybe not in the right context about rotation....pitch rotation. let's say you have 200 lbs you can move around. Where would you put it? In a lot of cases, it would depend on how the suspension is set up but I would try and put the majority of it in the center or as close as possible. Yes, sticking weight out on the bumper does lever the car more than if you placed it in the middle or over the rear but it also slows down the speed of the pitch rotation when it's out past the axle. The more weight a car has out near the ends, the worst it handles too. It's the bar bell effect. Move the weights out to the ends of the bar and then try to twirl it quickly. Not gonna happen unless you're King Kong. Bring the same weights in towards the center and you can twirl it much easier. Now all you have to do is find a happy medium with the weight that is movable. Engine set backs anyone? How about moving the axle forward an inch or more? Every inch counts!

 

[HT413]

.

 

[HT413]

Cranky, I couldn't agree more! I just considered the decklid in a static situation.

So in effect, imho, no springs = more weight of over the bumper as the car sits on the line. However, as soon as the car accelerates, there is an opposite effect, since the weight is farther from the point of rotation, weight transfer is slightly slowed / lessened.

Swampdog, this is where your polar moment of inertia comes into play.

All this to say that the only way to know how it really affects the launch... is to launch.

 

[TetanusShot]

I have to take back what I said, I'm not so sure about this anymore. Last night my brother and I talked it over for a long time and dug out the old statics book to calculate it out.

 

[Budnicks]

On a side note: I reversed the spring on my trunk lid so it actually pushes the lid closed. Took 2 seconds off my quarter mile times and added 38 mph. (But I can no longer do burnouts).

.

 

[HT413]

Cranky, I couldn't agree more! I just considered the decklid in a static situation.

So in effect, imho, no springs = more weight of over the bumper as the car sits on the line. However, as soon as the car accelerates, there is an opposite effect, since the weight is farther from the point of rotation, weight transfer is slightly slowed / lessened.

Swampdog, this is where your polar moment of inertia comes into play.

All this to say that the only way to know how it really affects the launch... is to launch.

Wait, I have to rethink this last part lol.

 

[65 3 times]

YES----- We have lift off. Everything is perfectly clear now.

 

[bobtile]

It was mentioned but maybe not in the right context about rotation....pitch rotation. let's say you have 200 lbs you can move around. Where would you put it?




I put it in the drivers seat.

 

[dla4567]

I had thoughts of piecing this together with an actual trunk lid, springs, and a steel base. But after 5 min. in the garage, the humidity was so bad I had to wring out my shirt. So I slapped this together using the crap that was on my garage floor. First I laid a 2x4 across 2 scales (car body).Then another (trunk lid) I mounted up on a couple blocks (springs). I even added a cross block to simulate excess weight at rear of lid. I then weighed it at both ends, removed screws to detach from blocks (springs), then rechecked weight. Yes, it was the same exactly (scale reads to the 10th of an ounce. I know this is crude, but I believe effective. Hope this helps.

 

[65 3 times]

My garage in Iowa was just as bad this week. THANK YOU for your proof. It is a very difficult concept for my feeble mind but your mock up was perfect. Thanks again... Dave

 

[HT413]

Dla, I love it! Now that's commitment!

I was planning on doing something similar in my garage and when planning it, I realized I was dead wrong.

Well fellas, it's been a fun discussion but I'm ready to put up the white flag here.

 

[65 3 times]

DONE & FUN...... THANKS