I was reading and need a little iinterpretation

[eldabo]

What I was reading is Dave Morgan door slammer. I now have my fuel cell battery and n2o bottle as far back as possable ( its an el camino and has a good bit of overhange). Would I be better off with the weight of all that placed over the rear axel or leave it were its at?

[Mike Peters]

It depends on what you're trying to acheive. If you're looking for the most leverage for weight transfer then, you have the weight in the correct location. Your current weight placement has more effect on the leverage to lift the front of the car than if it was placed over the rear end housing. Think of the car as a see-saw (or teeter-totter). The rear end housing would be the fulcrum on which the see-saw (car) pivots. Now think,..... is the see-saw (car) going to pivot easier if the weight is near the end of the lever or over the fulcrum?

[eldabo]

I agree but I think the book is saying that if its closer to the point of rotation that it will rotate quicker. More like a weight thats closer to the center of a wheel will be much easer to rotate than one thats out side . I was wondering if moving it would help the hook and my 60 foot time.

[mytmouz]

Look at your vehicle from the side. Try to visualize how much weight you have behind the center of the rearend, vs in front of it. Moving your weight from behind the rear to over the top of it will make more of the weight ratio be on the front side...

[eldabo]

Thanks thats why I put it there in the first place,but In the book he say thats where the 200 pound rear bumper came from and that worked good in the old days and that having the the front of the car jacked up in the air did to. But with better tecnologie shocks springs and so on that the weight could be better used if placed differently in the car .

[Dave Morgan]

Howdy Guys,
You know I HAVE to weigh into this conversation!
The topic relates to an effect known as the Polar Moment of Inertia. Typically, drag cars have high Polar Moment when compared to corner-turning cars. This means that we tend to locate our weight away from the center of the car, which is what we are doing when we place weight at the extreme rear of the car.
Placing weight at the very rear of the car gives us more rear weight bias but it also makes the car sluggish to wheelstand. At the time I wrote the ChassisBook, there was a new trend to position ballast near the center of the car to help it pitch rotate (wheelstand) more easily. This is exactly what we want in a Pro-Tree combination, pitch-rotate up and get a quick reaction time.
While placing weight at the extreme rear of the car will slow down the pitch rotation, we can also see an advantage because it will also make the car sluggish to rotate in Yaw (side-to-side when viewed from the top). This helps the car go straight in a drag racing environment. The corner-turning guys don’t like high Polar Moment for this reason. While it helps improve a drag car’s stability, it hurts a car’s ability to transition into a corner.
So, if you’re going to race on the Pro Tree, place the weight more towards the middle of the car. If your Full Tree racing, take the extra rear weight bias and the stability and put the weight as far back as possible.
I hope this helps.
Dave

[eldabo]

I think I will buid a few different mounting points this winter and play with it next summer.By the way Dave I run on a track that is 60 feet short of an 1/8th mile,so what would you do if it were you?


Thanks


David

[Mike Peters]

Howdy Guys,
You know I HAVE to weigh into this conversation!
The topic relates to an effect known as the Polar Moment of Inertia. Typically, drag cars have high Polar Moment when compared to corner-turning cars. This means that we tend to locate our weight away from the center of the car, which is what we are doing when we place weight at the extreme rear of the car.
Placing weight at the very rear of the car gives us more rear weight bias but it also makes the car sluggish to wheelstand. At the time I wrote the ChassisBook, there was a new trend to position ballast near the center of the car to help it pitch rotate (wheelstand) more easily. This is exactly what we want in a Pro-Tree combination, pitch-rotate up and get a quick reaction time.
While placing weight at the extreme rear of the car will slow down the pitch rotation, we can also see an advantage because it will also make the car sluggish to rotate in Yaw (side-to-side when viewed from the top). This helps the car go straight in a drag racing environment. The corner-turning guys don’t like high Polar Moment for this reason. While it helps improve a drag car’s stability, it hurts a car’s ability to transition into a corner.
So, if you’re going to race on the Pro Tree, place the weight more towards the middle of the car. If your Full Tree racing, take the extra rear weight bias and the stability and put the weight as far back as possible.
I hope this helps.
Dave

Good to hear from ya Dave. It's been a while. I hope things are going well for you. I've got a question now........I understand the Polar MOI with the weight being placed close to the axis of rotation but, unless the described car can place the majority of total car weight close to the axis of rotation, wouldn't a unbalanced front/rear weight bias offset any possibility of increasing the POI by moving the weight closer to the axis of rotation? For example, if he's got 55% front weight bias at one end of the see-saw, I would think he needs to offset that bias by having more leverage applied further out at the other end of the see-saw. Now, if he has a weight ratio closer to 50/50, I agree that the weight being closer to the axis of rotation will allow the car to pitch rotate quicker. I never did see any mention of front/rear weight ratios on this car. For a description of polar moment of inertia for those who for some reason haven't ordered Dave's "Doorslammers" bible, think of it as having two dumbells of equal weight but, different lengths. Grab both dumbells in the middle and try to turn your wrist. The shorter dumbell with the weight closer to the middle turns easier, doesn't it? This is the differences between high and low POI.

[eldabo]

It was i think 55/45 weight split its been a long time ago.

[Mike Peters]

It was i think 55/45 weight split its been a long time ago.

OK, now we've got some more info to go on. Now, if you think about the example in my previous post of the dumbells (which is the object example used in Dave's "Doorslammers" book) and how much harder it is to quickly twist the longer one because the weight is further away from the axis of rotation, also remember the weight at each end of the dumbell is equal while the weight at each end of your car is not. Think about a dumbell that didn't have equal weights at each end and think about if you grabbed it closer to the end with the light side. This would cause one side of the dumbell to be longer and even heavier due to the weight being further away having an even larger leverage effect which, looking at your car and where these points are in relation to the lop-sided dumbell as being the same. Yes, I agree that the closer the weight is to the axis of rotation, the easier the object rotates but, I also think the weight needs to be moved to provide the highest amount of leverage until that balance of weight ratios is achieved. To add enough weight while adding it close to the axis of rotation (rear end housing) to achieve a better weight ratio would require more weight to be added than if the weight was added further away or, more rearward in your case.

[eldabo]

It may be too much time waiting on my motor to get back to me .You know the devils work shop and all that.I still would like to have multi mouting points to play with.

David