Setting up an IRS Question

Ok Max, I don't know if this has been covered yet or not so I will ask. Let me give you my complete lowdown...
 
Right now I am overseas doing contract work and will not be back to complete my project for 1-2 years but I want to get my stuff together so I know what I am in for. Here is what I have planned. I have a 1983 Mazda B2000. I plan on installing a 4.3L out of an S10 Extreme. I already have my rearend sitting in my garage. It is a 2nd gen rx7 irs with a lsd and disc brakes, etc. I also plan on building the rear half of the frame to be used as my air tank. My question is this, when I set all this up there will be no movement in the driveshaft due to everything being hard mounted to the frame, so all i really need to know is what angles do i setup up for? How do I know how high or low and where left to right do I mount the rear? I bought Herb Adams book Chassis Engineering before I left but I forgot to bring out with me to study, so I have to wait until my wife sends it out to me.
 
I hope this isn't a retarded question but this will be my first build and I want it to be right. Thanks boss.

The pinion angle is of little concern since the thirdmember doesn't move, but setting the initial angle IS important. You would use the same practice for setting up the pinion as you would on a non IRS system, the only real difference is that you would only want 1o to 2o initial angle. As for how to deal with lateral placement, don't even worry about it. Without going into detail, the angles will be right as long as the motor and tranny and the thirdmember are installed straight in the vehicle. The height placement is dependent on your wheel/tire combo and the suspension setup. I suggest mounting the thirdmember so that the center of the CV is at aproximately half the overall height of your wheel/tire. That will help keep balljoint bind to a minimum.

I have little experience with IRS but I learn more everytime I come on here. I have a couple of questions regarding setup that I was hoping for help with as well.  Is it a major deal to have 2 different length CV shafts in an IRS setup? It doesn't seem like it would be but maybe there is something I'm not thinking of. Providing that different shaft lengths are okay the arms lengths and mounting points would be kept the same length though , correct? If you were starting from scratch on an IRS would you try and keep the camber curve the same as the front end? Would duplicating the control arm lengths and mounting points used in the front end be a good way to go? Is there an IC rule for IRS?  The rear knuckle would be custom so there would be freedom to change mounting points of the control arms to suit the desired geometry. Thanks for you valuable input in advance, it is apreciated.

I haven't forgotten about you, I just haven't had a chance to sit down to this one. It is going to be a long one. I will try to answer it tonight.

No problem Max, whenever you have a minute is good. I appreciate you taking the time to help out, thanx.

me to max, it is much appreciatated boss. and thanks maz duh for the added questions, i don't know what to ask and this sounds like it could get good. haha. thanks guys.

Alright, let's see if I can make some sense out of this...   It is not a major issue to have 2 different length axles and yes you would still keep the a-arms the same length from side to side. The issue would be if you were squeezing every last ounce of travel out of the system, you would want to relate the suspension to the axles and that would be an issue with one side having longer arms than the other.   More important than the camber curve being the same from front to rear would be to keep the roll-center as "attached" to the chassis similiarly. This is more advanced than anything that I have ever touched on here on SSM, but for as much performance as possible, the rollcenter is probably more important than the camber. This is arguable from designer to designer tho. For most of the minitruck world, having similar camber curves would be quite tolerable, but I would always design the suspension so that the rear has slightly less camber for a couple of reasons.   Copying the front suspension would probably work, but you'd have to try it to see if it would do what you needed.   IC isn't used in A-arm suspension design the same way that it is in 4-link design. The IC of an A-arm suspension is really only used for calculating than it is for set-up.   On an IRS the knuckle has far more freedom than a front knuckle that would be steering the vehicle has. The fact that there will be no steering of the rear wheels makes the knuckle very "dumb." There is no castor, Ackermann, spindle inclination or scrub radious to worry about, only the axles and camber are of any concern (and of course everything associated with camber, but that's not a major concern here)   I hope this helps you guys out some.

I'm on board with what you are saying here. The differance in axle length I mentioned is about 1 1/4" inch due to having the pinion flange of the rear end mounted along the center line of the frame. I will try and keep my arms on the longer side of things to help reduce the plunge during travel of the rear suspension. I don't see it being a problem to scale the rear camber curve back a couple of degrees in comparrrison to the front end for what I'm thinking, its good to know that I can do so. I am going to have to break out with the paper dolls and do some seroius calculating here I guess.  I'm looking forward to it, this will be my first IRS setup. You help gives me the confidence to give it a whirl, thanks Max. I'll probably be back at you with more questions before I'm done, I'll post some pix as I go.

Keep in mind that the A-arms should be around the same length as the axle not just "on the longer side of things." You would actually address the axle similarly to how you would calculate the tie rod on an IFS. But being that the axles are different lengths, I would figure them out based on the shorter axle.

also, as far as arm to knuckle mounting. would you want them to go into a point with a balljoint/uniball on top and bottom and have a dummy tie rod or would it be okay to have it like a triangulated 4 link setup to control front to back motion that way? and if so, can tops and bottoms share one mounting bolt/pin for pivoting or would seperate ones be better? and can arms be too wide if the axles are similar? like for a fullsize, can the inner pivots be right at the cv making arms that are very long? i plan to IRS my ramcharger to keep the diff mounted low so i can still have a backseat.