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Discussion Starter · #1 ·
I am looking at new 9" Ford rear end housings. There are centered housings and pinion centered housings. The offset is 2 inches. My drive shaft is 30 ". The vertical u joint working angle is a little less than 3 degrees. On the high end, but within the recommended "no more than 3.5" that I have always lived by. If I offset the pinion 2" in 30 " I get about 4.7 degrees for the sideways angle. I did this by using a 30" straight edge flat on the table, raising one end 2" and measuring the angle with my digital angle finder. (I can't do the math anymore. Been too long.:))
I get 4.7 degrees. I sure would like to have the centered housing, but I don't want to have U-joints wearing out either. 4.7 looks like to much to me. I'm not really sure on how the compound angles figure into this though.
Any of you guys running offset pinions with relatively short driveshafts?
I think I am going to have to bite the bullet and go with the offset housing. It looks funny from the rear.
What do you think?
 

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What are you using it on? How many miles per year? In other words if it's just a weekend around town car I wouldn't worry about it. It's only 1 degree outside the "norm" and new U-joints once every 5 years is still cheap compared to new axles etc.
 

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Discussion Starter · #3 ·
What are you using it on? How many miles per year? In other words if it's just a weekend around town car I wouldn't worry about it. It's only 1 degree outside the "norm" and new U-joints once every 5 years is still cheap compared to new axles etc.
This is for my 26T coupe. I average about 5000 miles/year in it. It is my DD. I currently have an 8" ford in it. The housing is offset and the pinion is centered.
When I first built it, I miscalculated and had a 5 degree WA. I had to replace the rear u-joint at about 3000 miles. I replaced the joint, and corrected the angles to near 3 degrees. No trouble in the next 20K miles. Now, maybe it was a bad joint to start with. Who knows? But It's kind of suspicious. I am a form follows function kind of guy. After sleeping on it, I am going to go with the centered pinion. I don't want to be worrying about u-joints all the time. I'll just have to live with the offset housing.
 

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That 8" will work forever in that car. I am of the opinion that having a 9" is a "keep up with the Jones'" thing. Every car built MUST have the 9" to be "right". Yup they are more common, it is "cool" to say you have one but bottom line, the 8" or even the 8.8" will never break with street tires.
 

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If you have a rear spring set up that allows rotational movement (upward) of the pinion when you apply torque, your u joint can go past where it should be. The rear end is physically lower to the ground than your transmission. What confuses a lot of measuring is that Hot Rods often are built so that the way they sit is not parallel to the ground, and big rear tires raise the pinion to a higher plane from the ground. So the first thing is to check and see that your pinion is actually going to be lower than the transmission tailshaft. (several inches) Then check and see what angle your engine/trans is mounted compared to the ground, not the car. You want your rear end pinion to be on approximately the same angle (slightly upward 3 degrees or so).

Now if you have a spring set up that lets the rear rotate upward as torque is applied, the relation to the transmission will change. If you have the pinion up at 5 degrees to start with, it only gets worse as torque is applied. For that reason its actually better to set the pinion a little less than the engine/trans angle and as torque is applied it rotates up to where you want it. Its better to be a little less angle wise than a little too much, because the assembly goes overcenter and is at a mechanical disadvantage.

If your rear spring set up is something like a 4 link where there is no spring wrap up, then you set the rear at the correct angle as the torque will not cause a change in the pinion angle.


As for the side to side angle. It doesn't hurt if there is some sideways angle because u joints are designed to work and it helps rotate the bearings. In a very short vehicle like a T or A Ford, the offset should be limited or centered because the short driveshaft will accentuate the angle.

My suggestion is to buy a wider 9 inch Ford and it has room for narrowing. Then shorten the ends different lengths so that you end up with a pinion maybe an inch fro center. Get a complete rear end so you don't have to spend a fortune on the backing plates and stuff. Complete rear ends are available for $150 or so if you just don't hurry. I bought 3 of them on one day earlier this year and never gave over $150 for any of them. You also might look at the Speedway disc brake conversion if you have to buy the brake stuff new. Some axles can't be narrowed because they have a taper right where you need to shorten them. I'd just buy some Moser axles with new bearings once you have the housing narrowed. I recommend looking on Amazon and buying a book they have thats tells you all you need to know about Ford 9" and 8.8" rears for about $20 .
617256
617257
 

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Discussion Starter · #6 ·
I didn't do anymore research on compound u-joint angles, but I assume that as long as the WA's don't exceed 3.5 degrees in any plane that it should work just fine. I am very familiar with the math involved in figuring driveline angles. but everything I have ever worked on was straight on the centerline. The compound angles just threw me for a minute.

I did some estimating on the cost of a used 9" vs. a new 9".
Used:
purchase.......$150
sandblast........$50
new rebuild kit with 3.89 gears, Eaton Truetrac, bearings and seals.......$550
set up gears.........$150 (I don't do rear end gear setup)
Shorten housing........$200?
Axles, bearings and seals......$400
Total.....approximately $1500
I spent $1750 for a new bare offset housing (centered pinion) and axles with bearings, and a brand new 9" center section with a Truetrac posi delivered right to my house, ready to install, with a warranty. The extra cost was worth not having to do all the work and my hands never left my computer. Until the parts got here. LOL
And I still have the 8" to sell to recoup some of the cost. I think new was the way to go for me.
There was a real nice benefit to changing the 8" out for the 9". The pinion is about an 1" to an 1-1/4" lower in the 9". This amount made my driveshaft go from slightly uphill to slightly down hill which made my u-joint working angles much better, down from 3.2 degrees to 1.75 degrees.
The rear end project is complete and successful. Not yet completely broken in, but the gears are very quiet so far. Another good thing is I didn't have to change the driveshaft. The length was still good.
I'm running out of things to do to my model T. In the last 8 months I have rebuilt the engine, had the trans gone through, and now a new rear end. The drivetrain ought to out last me now.
 

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If you have a rear spring set up that allows rotational movement (upward) of the pinion when you apply torque, your u joint can go past where it should be. The rear end is physically lower to the ground than your transmission. What confuses a lot of measuring is that Hot Rods often are built so that the way they sit is not parallel to the ground, and big rear tires raise the pinion to a higher plane from the ground. So the first thing is to check and see that your pinion is actually going to be lower than the transmission tailshaft. (several inches) Then check and see what angle your engine/trans is mounted compared to the ground, not the car. You want your rear end pinion to be on approximately the same angle (slightly upward 3 degrees or so).

Now if you have a spring set up that lets the rear rotate upward as torque is applied, the relation to the transmission will change. If you have the pinion up at 5 degrees to start with, it only gets worse as torque is applied. For that reason its actually better to set the pinion a little less than the engine/trans angle and as torque is applied it rotates up to where you want it. Its better to be a little less angle wise than a little too much, because the assembly goes overcenter and is at a mechanical disadvantage.

If your rear spring set up is something like a 4 link where there is no spring wrap up, then you set the rear at the correct angle as the torque will not cause a change in the pinion angle.


As for the side to side angle. It doesn't hurt if there is some sideways angle because u joints are designed to work and it helps rotate the bearings. In a very short vehicle like a T or A Ford, the offset should be limited or centered because the short driveshaft will accentuate the angle.

My suggestion is to buy a wider 9 inch Ford and it has room for narrowing. Then shorten the ends different lengths so that you end up with a pinion maybe an inch fro center. Get a complete rear end so you don't have to spend a fortune on the backing plates and stuff. Complete rear ends are available for $150 or so if you just don't hurry. I bought 3 of them on one day earlier this year and never gave over $150 for any of them. You also might look at the Speedway disc brake conversion if you have to buy the brake stuff new. Some axles can't be narrowed because they have a taper right where you need to shorten them. I'd just buy some Moser axles with new bearings once you have the housing narrowed. I recommend looking on Amazon and buying a book they have thats tells you all you need to know about Ford 9" and 8.8" rears for about $20 . View attachment 617256 View attachment 617257
Why worry about how it sits in relationship to the ground? That has absoloutely zero to do with the angle of the driveshaft from the tranny to the pinion. Pinion angle will change while driving from 3 degrees to 0, to minus 3 depending... With the pinion parallel to the frame, and tailshaft some inches above, pinion angle (at rest) will be the same, regardless of rake of the car. You can put 40" tires on the back, and 2" tires on the front. Pinion angle will not change.
 
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