8 3/4 pinion angle

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MOhorsePAR

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Well the shop is narrowing an 8 3/4 to fit under my '73 Dart. The next step is welding on the spring perches. What pinion angle do i want? I read some where that it should be around 6 to 8 degrees. Where is that measured from, above the centerline or below and what is the optimum angle?
Thanks for your help guys!

Steve / MOhorsePAR
 
7 degrees nose up. If they don't know what that means, they don't have any business moving spring perches.
 
grumpuscreature said:
7 degrees nose up. If they don't know what that means, they don't have any business moving spring perches.
Click to expand...

Hate to disagree with you David, but shouldn't that be nose down to compensate for spring windup under acceleration. I have mine set at 5 nose down and have no issues.

This is how you measure it.
Make sure the car has all the weight on the wheels.
Using a magnetic based angle finder.
Attach it to the tranny on a flat surface and determine the angle that the tranny is at.
We will assume it is at zero to make things easy.
When you have your rearend in place sitting on the perches and the u-bolts semi tight, attach the angle finder to a flat surface on the pinion housing.
This will give you another angle.
I used a small hydraulic jack to raise the nose of the pinion until I had the proper angle.
I then tack welded the perch to the rearend housing.
After that, I removed the rear from the springs and finished welding the perches to the housing.

Using grumpuscreature's 7 degree's, if you found the tranny was at +3 degrees, you would set the rear at -4 degrees or 4 degrees nose down.

It is very imprortant to use surfaces that are parallel to the drive line.

Somebody please correct me if I am wrong. But this worked for me.

Jack
 
It will be "down" relative to the driveshaft, to some degree. How much depends on the DS angle from your trans output, what type rear suspension you are running and how much torque you are getting to the gears. The ideal pinion angle would yield equal and opposite u-joint angles at both ends of the DS under max load (launch). You want an additional number of degrees down to account for spring wrap under load. How many depends on the strength of your springs and whether or not you employ things like a snubber, Caltracs, etc. As little as 2 degrees additional down angle on the pinion with Caltracs and SS springs is doable. That's how I have mine set up.
 
Disagree all you want Jack, it's cool. The pinion angle depends on the transmission output shaft angle. They have to be parallel. For example, if the transmission is 3 degrees down the pinion should be 3 degrees up. Fortunately, a few degrees either way won't make that much of a difference.
 
I just checked a 68-70 B body rear I have. With the perches level, the mating surface of the pumpkin is 5* UP.

As far as setting it up in the car, you need to account for axle wind. After applying torque you want the angles the same. If you put the rear in the car with the static angles providing the desired 0* difference, it will not be right under power. BTDT... My car vibrated like a cheap motel massage bed.

Different rear axle traction set ups require different pinion angles. All are pinion DOWN in relation to the trans output shaft.

Stock daily driver 0 to -2
SS spring -7 to -10
Caltracks ~-3
Ladder bars ~-1
4 link ~-2*

Every car is a little different and a 2* change can make a world of difference on a leaf spring drag car.

Since the OP is asking about perch orientation, the rest of the deal after the pumpkin mount surface is 5* down is moot.
 
This is always a confusing topic. The funny part is, most of the time, everybody is saying the same thing a different way.

Jack
 
As I recall it is relative to the drive shaft, up or down.
(But that's on memory):-?
I swapped a 8 1/4 Diplomat into a Duster body. (It solved the puckered up rear end syndrome.) This is how I measured. That's what you asked.
Probably won't help you though since you are having someone else do it and not in the car.
I have the whole thing with pictures and explained if anyone want to try it.
I used the car for a fixture.
I checked the pinon angle of 71-4 once it was on stands. Put same angle on 8 1-4. That way it's apples to apples. (Springs are hanging down like this so it isn't a "real" number. In other words the weight of the car isn't on the tires.)

mesure pinion (Small).jpg


img291 you'll need to weld in fender washer cause the hole is bigger. (Small).jpg


img296 get it centered compare both sides of car (Small).jpg


img300 pull it back out and do a good weld. (Small).jpg
 
It's fairly easy to mock it up in the car and get it pretty close with literally no special tools at all. Get it a few feet up in the air with the weight on the suspension and the perches in place, but not welded yet.

Then find a couple broomsticks or similar sized lengths of pipe or whatever, it doesn't matter. sleeve them a little if necessary to fit snugly in the trans and pinion yokes where the U-Joint is normally installed.

Let them hang straight down, walk away from it about twenty feet or so and see where they point. Parallel is where it should be under load. Get it there, then move the pinion down however many more degrees you want, depending on your suspension setup. Tack the perches, take everything apart, finish welding and you're done with your pinion angle setup - If you got it right..
 
I was a little cornfused until my guy at coast driveline made it super easy. Forget about the drive shaft angles. I did mine a number of months ago. my car has a tko-500 so the tranny output angle was first. I think it was 1* up. then all I did was set the differential at 1* down. tacked the perches and done. then I measured for my driveshaft. done. seems to be working good so far.

I think this about the same as what everyone else is saying.
 
OOPS. I see the Fishy's link now. Never mind.

Things to keep in mind. I think I remember this from studying the subject.
For a driver keep angles small. But not 0. Surprisingly enough U-joints need a little angle to work right. Ideally the angle on the front and rear should be the same to “cancel” out each other. The drive shaft angles down from the transmission and the pinion angles up from the drive shaft. (But that is still “nose” down).
As I said it is relative.
The assembly would look like a zee if it were exaggerated.
So if you put a protractor on the drive shaft (like my picture) and it measured 3 degrees and you then put it on the pinion you would do the math for the “difference” for a total angle.
Now if you are building a drag car the angle on the pinion is more because it winds up harder and tends to straighten things out for about a quarter mile.
That’s the difference in angles for different cars.
 
: perch welding From: DoctorDiff
I assume this is for an A-body?

If everything is stock, you can weld them out of the car:
43" on center, perches level, pinion 5 degrees up.

If you changed a bunch of things, you should mock the housing into the car and set your pinion angle based on the transmission centerline. For most street/strip cars make the pinion centerline parallel to the transmission centerline, then roll the pinion downward 3 degrees.

this is right from dr. diff
 
That's good advice for a stock engine with a stock suspension. Or a higher-powered engine with a beefy rear suspension of some sort. The problems arise when you've got more power and the springs aren't up to it for whatever reason. If that's the case. it probably doesn't have driveshaft loop(s) either and gets real ugly.
 
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