The Stovebolt Site Forums The Shop Area Driveline Pinion angle

Check my logic here. Context is a 1951 Chevy with a later open driveline.

Is the following statement true or false?….

If the rear axle is moved BACK (by using an axle perch hole that is farther forward)….the front of pinion flange will move DOWN (relative to the ground).

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That’s how my brain pictures it working.

If there's any angle change at all, it will be minuscule. We used to do something similar on the dirt track cars, but in the other direction. Track rules required a certain minimum wheelbase, but shorter wheelbase cars go around turns better. A wheelbase check was included in the tech inspection at the beginning of the racing season. Toward the end of the season anybody who wanted to stay competitive was running a few inches shorter- - -helped out by several spring center bolt holes in the mounting pads, and a shorter driveshaft. Pinion angles didn't change enough to matter, and the change was not obvious on the open wheel cars we ran.
Jerry

Cool. That’s what I needed to know.

Of course, applying racing anecdotes to street machines can lead one down all sorts of rabbit holes. We put a very radical down angle on the pinion flange, to minimize the U joint angle under hard acceleration. Spring windup under full throttle with a 400+ HP 327 Chevy engine, running in second gear of a Muncie 4 speed transmission against a 4.56 gear was considerable. Doing that full throttle acceleration out of the turns twice a lap put some incredible stresses on everything in the powertrain. My motto was "Build it three times as strong as you think it needs to be- - - - -and don't be too surprised when it breaks!"
Jerry

The “rule of thumb” guidance that it won’t change much, is plenty good enough for me. I’d like to make some changes to my rear axle. The current perches are too close together. I’d like to move the wheel slightly back in the wheel well.

The rear end in the truck is pretty worn out. I have a replacement rear in the shed that would need new perches. I think I can get this all pretty close to correct on the first try….
based on the current angle (which isn’t ideal, either) and the measured relationship between the pinion and perches.

Usually the pinion shaft should be parallel to the transmission output shaft with at least 1 degree between them and the driveshaft. That would make the two u-joint operating angles the same. There are resources on the web that can help.

Just remember to add 1 degree down to the differential before you weld the perches to the tubes and you'll be fine. With a leaf spring scheme, as power is applied to your rear end, your differential will try to roll clockwise as you look at it from the driver's side, lifting the front of the differential upward. Put another way, your top leaf spring will "wrap up" (deform) allowing this rotation. The extra 1 degree will keep things correct because when wrap-up happens that 1 degree will disappear. Wrap up will be higher when your truck is loaded and lower when it isn't but even with no load at all, it will occur. And also if you're moving your differential backwards, please be mindful of how your springs were designed and don't move it too far back. With leaf springs you have what is known as bias. The front part of the leaf spring assembly generally just locates the differential. The rear part of the spring is what determines your ride quality. Less bias in the rear and you'll ride softer and quieter. A little more rear bias can help with cornering but will stiffen the ride. Too much rear bias and you'll ride very harshly. More bias in the front and you'll have fewer problems with wrap up and vice-versa. It is an important balance. Moving your differential forward or rearward on the spring can mean you upset the bias balance designed into your springs and an inch or two can make a difference.

The original torque tube would have placed the axle 2-ish inches (going from memory) back in the wheel opening.

I was thinking of moving an inch or so. Currently, the spring center bolt is centered on the axle tube.

The axle is 2 inches forward of center.

I think it is worth remembering the whole rear mount scheme was different in the AD and when you change to a open driveshaft that scheme gets changed. Not a monumental change but nonetheless a functional change. When I installed the new differential, I moved the hole in the spring perch mount frontward by as much as seemed safe (meaning it is about 1 inch and an eighth forward from the center hole that came in the mount). That puts the rear end a bit closer to the center of the wheel well but not exactly. Putting it exactly in the center of the wheel well would have meant I needed another 1 inch at the front of the perch. I could have welded a plate to the bottom of the perch and accomplished that but I wanted to keep the spring bias as close to original as possible. Plus I wanted to use the original shock mounting hardware, so that meant another little dance of sorts. I haven't had a single person notice the rear wheel needs to be moved back an inch to be exactly positioned

One other thing that should be mentioned. The GM engineers had less concern about bias in the AD springs than they had in the later applications because with the torque tube, differential rotation couldn't happen. Their concern about spring bias would have been limited to (a) ride quality (b) cornering and safety concerns.

My point in mentioning this: installing an open driveshaft rear end will relocate and alter things. The original springs were designed with an offset center pin and a different system of downward weight distribution. To me it comes down to common sense...the further back you slide the rear end the more you alter how the springs are going to work...especially the way they're pinned together. Further back...stiffer. Hope this makes sense.

I believe the spring "center" bolt isn't actually centered on the leaves. I've heard rumors of people reversing the springs on a torque tube setup when converting to an open driveline to move the tie bolt toward the rear a little. Never done it myself, so a "cut and try" might be in order.
Jerry

That's right, Jerry. The center pin is offset (maybe 2 inches). You'll remember the old torque tube type had the spring mount welded to the front side of the tube so that downward pressure went to that offset pin. I did try reversing the springs but as I recall there were challenges there.

That’s precisely what I planned on.

JW51, what gearing does this other rear end have? I know you have looked into adding your T5.

It’s your standard 3.90 rear from a 55-62 truck. Same thing that’s in the truck now. Works surprisingly well at 65+ with the SM420. If/when I install the T5….OD will be plenty usable.

Would love to find something in the 3.4-3.5 range. But the price on this rear was about right (free).

With the 3.90 rear you will at least have the bottom end torque when you need it. It should work well with the OD in a T5.

I found weld on spring perches at TSC. Re drilled the center hole forward to bring the rear end back.

Our "go to" source for spring perches on the round track cars was plain old channel iron, available at any steel supply business. Pick a width that's close to the size spring leaf that's being used, and hole saw a radius to match the axle housing diameter. The perch can be made any length that's appropriate for the application. Ours were about 6 inches long, with 4 or 5 center bolt holes drilled into them. It made adjusting wheelbase a very simple process.
Jerry