RC & WT for my car

So for the past couple weeks I have been reverse engineering the suspension of my car and I thought I would share the information I came up with. A friend of mine who is very knowledgeable about suspension set-ups has been helping me get my suspension straightened out. While I’ll admit I don’t understand everything he is telling me, yet, I thought I’d share what I know and add to it as time goes on. Most of this information is going to apply to a track only car because it doesn’t do anything for a street car. Finding your RC will help with a dual purpose car, especially if you care about “function over form”.

One program you will want to download is the Roll Center calculator program from Performance Trends. You can get a demo version for free from the URL below. You put in some measurements of your suspension and it returns your Roll Center height. There are 2 files with the dimensions of the front and rear suspension of my car that you can use to calculate your own roll center that I have linked below. Just copy the files into the directory named RCFILES in “Program Files” then copy whichever one you want to look at to a file named “MUSTANG”. Run the calculator and open that file. You will need to use a text editor to edit the file for changes because the demo version does not allow you to edit values in the program. Just match up the values in the file to the values displayed in the program. Ideally, you want your RC to be above ground. I had my car so low in the front the RC was belong ground. The RC should be above ground, so if I raised my car 1 3/8”, or with the front control arms parallel to the ground, my RC is now about .5” above ground. Google Roll Center to find out what it means. To get the RC for your car you would need to take all the measurements in the “Static Layout Dimensions” section, which are dependent on ride height. If your front control arms are flat, i.e. the ball joint and inner pivot are the same height, you can use my measurements. You can also enter your “Other Specs” to get your wheel rate, camber changes, etc. The program will also let you see what your car looks like in corners which is cool. You can make changes in the program and see how they would affect your car, like changing the ride height.

Linked below is also an Excel spreadsheet that shows weight transfers and other information about a car based on a bunch of measurements. You can change the information to fit your car and see just how bad the weight distribution is for our cars. Keep in mind that my front end sprung weight is probably higher than most because of my motor/tranny. You will need the RC value from the above program in this spreadsheet.

The results:
Ideally, you want the “magic number” to be +5%. With the way my car is set up right now, if I disconnected my rear sway bar, my magic number is -12.88%. If I change to 900 lb springs in the front, 275 lb springs in the rear with no rear sway bar and added 75 lbs of weight to the rear, the car will be right where it should be. Or a 1.5” front bar and 350 lb rear springs and no rear bar… The cool thing about the spreadsheet is that you can change dimensions “on paper” and see how they look. These can be used to aid in setting up your car but you will still need lap times to tell if your car is set up correctly.

A friend of mine with will be coming over soon to get an accurate weight of my car and will be adjusting the corner weights. I’ll update the spreadsheet once I get the weights.

PT RCC URL: http://www.performancetrends.com/download.htm#rcc

URL to files: http://www.filefactory.com/file/a002dhb/n/RCFILES_zip

 

No problem. Hopefully this info. will help some one else. I only wish I had done this about 4 years ago.

So my friend came over yesterday with the scales and I got my car weighed. Before we started, I had the ride height set so the car was level side to side with the weight of the driver in the car.

The car, by itself with just a couple gallons of gas weighs 2452, 1602 (65.4%) front and 849 (34.6%) rear. The cross weight is 48.9% rf-lr, so 51.1% lf-rr.

With me in the car (~155 lbs) and about 40 lbs added for fuel, the car weighs 2658, 1693 (63.7%) front and 964 (36.3%) rear. The cross weight is 49% rf-lr, so 51% lf-rr.

I raised the right front 1 1/6th turn of the lower spring mount and that put the cross weight at 49.9% rf-lr.

It seems the placement of the driver is pretty good considering adding the driver and about 5 gallons of gas only changed the cross weight .1%

I changed the weights in the spreadsheet and it helped, but not much.

Now I need to start looking for shocks. I’ll probably go with single adjustable Koni’s. It was recommended to stay away from anything Japanese.

 

I can't seem to find the thread about your car. I remember you gutted your car, but does it have a cage? I think your car with the same amount of gas as mine would weigh the same as mine. I have a cage and probably has been lighened more than yours.

I would love to see the weights of a stock 4th gen; the weights of each corner to use for comparison. Besides replacing or removing glass and removing the under-coating, I can't get mine much ligther. I might remove the door glass in the dry months.

 

Interior wise, my car only has door panels and the dash, everything else is gone including heater & A/C stuff. I also removed the crash-bars from the doors and the metal from both bumpers. I realized I can remove another 20 lbs or so by making some aluminum brackets for the seats, but need to find a save way to mount the brackets. I don't want to just bolt them to the sheet metal floor.

 

I would agree. I removed my seat bracket because my personal dimensions don't change and once I have my seat position set, it doesn't change. This also allowed me to move my seat slightly inboard so my helmet isn't touching the upper cage bar. Unfortunately, removing these brackets also moved my COG slightly higher since the weight came off one of the lowest parts of the car.