on April 17, 2010, 5:02 pm Daze wrote:

Jaguar IRS pinion angle debate (keep in mind I have already decided on what I am doing)

This is a repost from the Mustang forum I frequent to get the ball rolling here:

Ever go to do research on something and find your self, smack dab in the middle of a huge debate??  I recently did.  I was trying to decide what I wanted to do about the pinion angle on my Jaguar IRS install.  There is a 5-degree slope milled in to the top of the differential that could easily be used to set the pinion at a 5 degree up angle. (perfect since my T5 n my Mustang hangs down 5 degrees)  The cage on a Jaguar IRS unit has the opposite 5 degree slope in the differential mounting section which cancels out the 5 degree slope on the differential and sets the differential up with the pinion flange perpendicular to the ground.  I completely understand the importance of setting up the correct pinion angle but Jaguar had set the system up with 0 pinion angle when there transmission angle is around 3 degrees. This made me wonder what others had done, mount it with a matching pinion angle or mount it with the pinion flange perpendicular to the ground. 
 

 
As I began to read up on the subject I found several long debate threads on several forums.  the debate was as heated as the one we in the Mustang community see between modified and original.  One thing I learned is that Jaguar rear ends have been set up both ways and in both situations seam to be working just fine.  Argument for setting up a matching pinion angle on any rear end 
 
the following info was posted by a 34 chip on the NSRA forum 
 
first and foremost a universal joint is NOT a constant velocity joint.(this is where the concern lies). this means if its driving a shaft at any angle other than zero, although the half of the joint that is connected to the transmission is rotating at a constant velocity the other half of that same joint is NOT rotating at a constant velocity it is actually constantly accelerating and decelerating on each revolution as the joint is working itself around the angles .at one point it has further to travel because of the angle and so speeds up and at another point it has less distance to travel and so slows down(relative to the trans speed).this means the prop shaft is rotating in a 'jerky 'movement constantly slowing down and speeding up even though the trans is at constant speed .now down at the diff end this 'jerky ' rotation will be transferred to the diff u joint which will be transferred to the wheels so what we would have is the trans trying to drive the wheels at a smooth constant speed but the u joint is trying to slow down and speed up the wheels. this means they are in conflict together causing stress and vibration. the more the angle the worse this 'jerky movement becomes. now to get over this if the diff pinion is set parallel with the trans the half of the u joint at the diff end connected to the diff pinion is also rotating with this 'jerky' movement, slowing down and speeding up each revolution but its equal and opposite to the trans u joint so they cancel each other out and the result is the diff pinion and trans are rotating together at constant speed however the prop shaft is still slowing down and speeding up. twice per rev in fact .if the diff pinion is not set parallel with the trans then this canceling effect does not take place and so the diff will not be rotating at a constant speed in relation to the trans. it is for this reason front wheel drive cars use constant velocity (CV)joints on their driveshafts not u-joints because if you imagine a front wheel on full steering lock the angle of the wheel compared to the drive shaft would be so great this slowing down and speeding up of the wheel joint would be so severe while the drive shaft is trying to go at a constant speed it would shake the whole thing apart instantly. saying all this because the angles we're dealing with are relatively small compared to the front wheel drive scenario if the diff pinion isn’t parallel with the trans then maybe its not noticeable from a driving point of view but this jerky movement is still going on and can cause premature wearing of the joints but as many rods aren’t driven as much as a daily probably not noticeable.
 
This info is WHY we need to set up the correct pinion angle in any car, but when you point this out to half the individuals who have put an IRS in their car , truck, or hot rod you get one or all of the following replies, with the first one being the most common: 
 
"It doesn't apply because the Jaguar differential is hard mounted where as a live axle differential moves up and down with suspension travel" 
 
"Jaguar didn't do it that way" 
 
"The  lower control arms (wishbones) on a Jaguar IRS are  perpendicular to the pinion flange so mounting the diff at 0 degrees pinion angle allows the suspension to move perpendicular to the road.  If you set the pinion at an angle the wishbones will also be at an angle and move backwards as the suspension compresses." 
 
In my opinion the first response is nothing more than something that "sounds good" to try and justify doing it, but really has no bearing on the issue.  As the pinion on a live axle moves up and down the angle (in theory) is not changing and the pinion is remaining parallel to the transmission, further more if the road is smooth and the live axle springs are stiff enough the amount of differential movement will be minimal, but you still want the correct pinion angle to prolong u-joint life. 
 
The second response, I believe, is the main reason this debate has two divided camps.  “Jaguar did it that way so it must be right."  To a point I tend to agree with this sort of thinking.  Having no formal training in engineering I must trust the designs of those that do, however  this argument is in my opinion only completely valid IF AND ONLY IF you set the system up completely as Jaguar designed it, with a cage, with Jaguar cage mounts that have a specific amount of flex designed in to them, with Jaguar trailing arms, and with the transmission angle at 3 degrees down.  The more of these thins you change, the argument “that’s the way Jaguar did it"  becomes less and less valid as you are not setting it up the Jaguar did.  Coupled in with this, as hot rodders copied the way Jaguar did it, and then helped others do the same thing they said “this is how I did it and it works just fine" 
 
The third argument seams to me to have the most validity, and I speculate is the reason that Jaguar did things the way they did.  If the pinion is perpendicular to the ground the wheels will stay centered in the wheel well, and all forces will be applied vertically.  All that said I do not see , with only being 5 degrees off of perpendicular, that there would be that big of an issue.  I did a little math and with a 5-degree angle on the wishbones 3"  of suspension travel would only result in .262"  of backward movement at the wheel.  A leaf spring set up moves the wheel backwards at least that much as the spring compresses so I fail to see the issue.   
 
As I said in the title I already know what I am going to do.  As you probably guess from what I have written I am going to set the correct pinion angle to match my transmission, but I am curious what others of you think of all this.