Hey Guys, I'm new to the suspension world and looking for some information. My ultimate goal is to determine "spring rate" for the rear of my XJ6. I want to do a custom setup and trying to pick my strut/spring. The calculator I'm using asks for rear corner weight, for sprung weight. It asks for corner unsprung weight. Motion ratio and spring angle. Any information would be great! Cheers

 

There will be a small/large difference in weight between any two cars, so you will have to do your own measuring. Your car needs to be loaded/unloaded and in the "as driven" condition to be accurate. That includes gas in the tanks(how much), spare tire(or not), passengers(or not, how much will they weigh), suit cases or bicycle in the boot(or not, how much will they weigh), etc. How much do you weigh?

When the car is sitting on a hard level surface in this condition, you need to have something that can weigh about 1300 to 1500lbs accurately. Weight pads are ideal, but not many people own them. I have used a 300lb bathroom scale and fulcrum to get estimated weights. Example: say one corner (tire) is going to exert 1100lbs force and you set the center of the 300lb scale 4ft from the center of the tire. Use a 5+ft fulcrum (metal square/rectangular tubing is good) and bridge it between the scale and another point 5' to the other end. These two bridge points should rest on "knife edges", like pieces of angle iron, to be most accurate. Set the tire down at the 4' mark on the fulcrum and do a little math. Tire to scale is 20% of the distance between bridge points, so the reading on the scale is 20% of the weight being exerted by that tire. 257lbs on scale=1285lbs at the tire. When you do something like this, it's very important to put ALL FOUR tires on something the same height, the tires have to be on the same plane. If you don't, the cross-corner weights will increase and the alternate corners will decrease, giving you unrealistic numbers.

Sprung Weight/Unsprung Weight. As it says, only the weight that is being supported by the springs. Short answer, all of the car/load and half of the suspension. The other half of the suspension is "unsprung weight" This is not exactly accurate because the individual weights of the suspension vary widely. For example, a solid axle is unsprung weight and is very heavy compared to the two much lighter leaf springs that attach it to the car, half of each leaf spring would also be unsprung weight. The IRS in the XJ6 is "sprung weight" because it is attached to the car only, whereas half of the half-shafts and all of the outer hub/tire is unsprung weight. You would have to guess at these weights unless you have the complete suspension stripped out of the car and can weight it.

I can only guess what Motion Ratio would be: maybe the difference between "steady state" stance and maximum amount of lean (angle) that you want. How stiff do you want the suspension? Spring Angle can be measured pretty easily by setting your ride height and then screw a couple of pieces of lath together on one end and use them as a big protractor to match the angle of the mounted springs.

I hope this helps and good luck.

Dave

 

Jaguar have lost the factory spring rates for the XJ saloons up to and including the Series III. There is a business in the UK who have this data, and they may well be prepared to give it to you. Contact details here;
PETER PEARCE

Coil Springs (1989) Ltd
94 ATTERCLIFFE ROAD.
Sheffield S4 7WW
Tel from the USA:- 0044 114 275 8573
E:- coilsprings.sheffield@gmail.com

It sounds from your post that you are not going to use the OEM rear suspension, if you are not, then obviously the OEM spring rates will not help you much as the leverage/fulcrum/hub positions etc etc will be different.