Rear suspension design

[70Longhorn]

All the good work around here has got me fired up to get going on this Longhorn project. I thank you all for that!

I'm starting my rear suspension design and am looking for some tribal knowledge. Does anyone know the height of a C-20's center of gravity? Ok, I know that's like asking the size of a gray suit, but there ought to be a general rule of thumb out there. Maybe it's about the height of the crankshaft at OEM ride height? Maybe it's lower than that? Higher?

I just want to get into the ballpark to get a good baseline for anti-squat/anti-dive geometry, and roll center calcs.

CG in X and Y are easy. Anyone know a good Z number?
Thanks!

[leftcoast66]

I htink your thinking a bit deeper into lowering than most people do. Not sure any answers for you, sorry. I would talk to the people at airride and see if they know any of the answers to the questions you have as a suggestion.

[craig@ridetech.com]

Ok, I hope this helps.

To find your CG lengthways (front to back) divide the weight on the rear wheels by the overall vehicle weight and then multiply the result by the wheelbase:

here is the formula
rear wheel weight
CG behind front wheels = ----------------- x wheelbase
overall weight

To find you CG sideways (left to right) you need to start off with your track width or the distance between the centers of the treads of the tires on either side. Typically it is described in terms of how far it is off-center towards the heavier side. To find that , divide the weight on the lighter side by the overall weight and multiply the result by the track, then subtract that answer from 1/2 the track. If there is a significant difference between the front and rear use the average of the two.
Here is the formula
track weight on light side
CG off-center on = _____________ - ( ________________ X track)
heavy side 2 overall weight

Ok now for the Vertical position. On a car with a front mount V-8 and rear wheel drive the CG will usually be 14-22 inches above the ground or about the height of the camshaft. To find that height just simply take a yardstick or tape measure and measure from the ground up to the centerline of the camshaft on the front of the engine. If you want to get more precise read on:

Start again by weighing the truck at all 4 wheels. Easiest to do with 4 separate scales, one under each wheel. Record the overall weight and the front and rear wheel weights. Then raise one end up to 24inches if possible with the wheels at the other end still on the scales and note how much weight is transferred to that end. It doesn't matter which end you raise up. (If you are getting really technical- at the end of the truck which remains on the scales, the added weight from the lifted end could deflect the suspension and tires and throw off the readings or the tires on the lifted end could drop enough to provide an inaccurate measurement of how far that end was lifted. In other words if you want it to be really precise deactivate the suspension with steel rods so that there can't be any compression or rebound.) Once you know the weight of the truck and the amount of weight transfered when one end is raised there are three dimensions in inches you need. 1. the wheelbase with truck level 2. the wheelbase at ground level with one end raised 3. the distance that the one end was raised. Now that you have all that data acquired to find the CG vertically multiply the wheelbase with the truck level by the wheelbase with one end raised at least 24inches by the added weight shown when with the one end raised. Then divide the answer of that calculation by the distance the one end was raised multiplied by the overall truck weight. Here is the formula:

level wheelbase X raised wheelbase X added weight on scales
CG Height = ____________________________________________________
distance raised X overall weight


I hope this will help you out. Good Luck.

[craig@ridetech.com]

sorry but the formulas were lined up when I was typing them. for some reason after I posted this they all went off kilter.

[68C15]

LSC71, may I ask how you came about this knowledge? this is cool stuff.

[craig@ridetech.com]

I have a really cool book called Automotive Math that tells you all about this kind of stuff and lots of other information on Displacements, final drive ratios, g forces and just about any kind of racing math you want to know. Its a really good read. Plus I work with this kind of stuff pretty much everyday.

[70Longhorn]

[QUOTE=LSC71;1889259]Ok, I hope this helps. [END QUOTE]

Yup, that's good stuff.

I can't seem to find any scales around here, so I'm going to use my camshaft height (about 24") to start with. I needed to build the suspension with adjustment anyway because I won't know how much anti-squat/dive to go for until I can drive it.

So does anyone have overall weight and front/rear bias info for a 3/4-ton Longhorn with a 350 and automatic? Ballpark is fine for what I'm doing on this first build.

Thanks!