Where to put the balast

[bobadame]

I was just reading Sum's account of how weight was added to Hooleys Stude and how it helped with traction and stability.  The weight was placed in front of the rear axle and not directly over or behind the rear axle. and so the placement of the balast was a compromise between tractiion and stability. I wonder if it might be a better compromise to split the weight in two and place the ballast as far forward and as far rearword as possible. My thinking is that this would make the car easier to drive should the tires start to spin. With the mass more centered the car acts like a top, it spins. With the mass at the ends of the car it takes more energy to get it (the car not the tires)to spin and it spins slower.  I don't know, what do you think?

[bobadame]

Had some more thoughts on this. Take a couple of bricks and duct tape them into the center of a shoe box. Spin the box. Now tape each brick into each end of the box and spin it. In each case the total weight is the same and the weight bias is the same , 50%, 50%. I  haven't done this but I think the bricks in the center would spin easier.

[Elmo Rodge]

They would start to spin easier but would stop easier also. With the bricks in the end of the box, once it starts spinning it continues. Wayno

[Sumner]

They would start to spin easier but would stop easier also. With the bricks in the end of the box, once it starts spinning it continues. Wayno

I learned this lesson the hard way.  I built a "pump" trailer once for a firm that did water quality samples for perspective coal mines.  They had to get water from numerous wells in the field that were anywhere from 50 feet to over 600 feet deep.  

I used a flatbed trailer and mounted most of the running gear from a Ford Bronco on it.  The 289 motor was in the front with the rear axle under it and I moved the front axle (this was from a 4X4) to the rear.  There was a big drum I made in the middle that had over 400 feet of 2 inch hose on it.  You could put different size submersible pumps on the hose and lower them into the well off the trailer.  I drove the drum with a chain from one side of the front axle (welded the spider gears to lock that axle and also the other one).  In the rear I drove a large Onan generator from one of the hubs.  It could power a 5 hp submersible pump down in a well.  From the other hub I powered a winch I made that I could run a bailing bucket down a 200-400 ft. well. In and out in less than a minute and I could bail about 3 gallons of water a minute using a 12 ft. long piece of PVC pipe with a trailer ball for a check valve in the bottom of it).

To power the drum and lower the pump into the well you put it in 4 wheel drive and un-locked the hubs in the rear and then only the front (use to be the rear axle) would turn and in 4 wheel low you could run the big drum real slow.

To power the generator you would put it in 2 wheel high and just lock the hub on the generator side.  To run the winch you would do the same, but just lock the hub on the winch side.  There was a big boom on top of the trailer that you raise and used with the bailing bucket.

Well anyway all of this machinery was mostly located at the ends of the trailer.  If the trailer started to fishtail on the highway it was very hard to stop.  Most of the towing was done under 60 mph.

I later made a second pump trailer and on this one I used a slant 6 motor and I made the trailer from scratch and all the components were near the center and ahead of the axle and this one did every thing the other one did, but had none of the bad behaviors the other one did on the highway.

c ya,

Sum

[Richard Thomason]

I believe that the measurment of this factor is called the polar moment of inertia. This is why builders went to mid-engined race cars for road races.

[Pat Kinne / Salt201]

Sum - Your answers are spot on - The use of personal expierence and the result - good or bad - are worth reading by both new guys and "veterans" alike.  With your trailer fun - polar moment of inertia has a whole new feeling to it - something about the tail wagging the dog. Not good in your race car and really not good towing either!! Been there and done both. - Pat Kinne

[bobadame]

I agree that in road racing you want a to build a car with low polar moment and you do this so the car will snap in and out of corners. I know nothing about running on the salt but I think it would be a lot like driving on ice. I can see some testing coming up involving an icy road, the old Volvo station wagon, bald tires, and a bunch of sandbags.

[Elmo Rodge]

Might I suggest a wide icy road? Wayno

[bobadame]

I think I'm starting to get a handle on this. My original concern was to build a car that is easy to drive. This because I'm generally a pretty crappy driver. So I want the car to be inclined to go in a straight line with or without my interference. There are lots of factors involved but some generalities begin to appear.

 Once it's in motion, a body with 99% of its weight near its front is more stable than a body with 99% of its weight near the rear. For example, a dart will right itself even if it's thrown backwards or sideways. This is good. However a rear wheel driven car with this kind of front weight bias will loose traction almost immediately and begin to rotate around its center of mass. It will spin out sideways. This is bad. So a compromise has to be made between best traction and best stability.

 Going back to the dart example, it rights itself because of aerodynamic forces. There is a term called center of pressure. The way I understand it's similar to center of gravity. It's the aerodynamic balance point on the side of a shape. For example, the pivot point on a weather vane is ahead of its center of pressure. That's what keeps it pointed into the wind. Aha!

 In the case of the car, Its pivot point is its center of gravity, And so if its center of mass is ahead of its center of pressure it will tend to go straight ahead.

 As far as polar moment is concerned, that seems less important now, whether this flywheel effect is an asset or a liability. My guess is that either extreme, high or low polar moment would be best or worst depending on the dynamic situation at the time. So I'm thinking compromise again and keep it in the medium range or just inbord of the axles. Sum, I think you were right about this.

 Thanks all for your thoughts on this.

[WZ JUNK]

I vote for weight low and near the center of the car.  I think if you imagine the situation in an extreme, with weight way out in front and way out in back of the car, you are able to visulize what the effect will be.  I can see that there will be more discussion on this topic.  

John