Pinion angle for lowered SWB Square?

[RaysnCayne]

Hey guys,

I feel like I see a lot of talk and info about pinion angles for lifted rigs, but precious little for lowered ones.

My SWB '87 R10 (currently undergoing a mild "hot rodding" makeover with a freshly-installed 3.5"/4.5" drop) is up on stands with full weight on it. I'd like to see if I can dial in a decent pinion angle before taking it off the stands and hitting the road.
I'm currently looking 6.8º down on the trans tailshaft face and 7º up on the differential yoke. That's pretty darn close to parallel. But this is my first rodeo lowering a C10 and working with pinion angles on a leaf spring vehicle. This will be a very tame cruiser (still has the OG 305). So I wasn't anticipating a whole lot of "spring wrap" and thus not planning on the differential angle to change much. But a friend said I should try to bring the rear angle down about 2º (shoot for ~5º up instead of 7º) because even light cruising will cause the axle to wrap.

I've also been told by a guy that does a lot of 4/6 drops that he's never touched the pinion angle and his C10s ride great.

I'd love to hear some insight from those more experienced in this area. Am I good as-is, or should I do some shimming to the rear angle? Or am I way off on all accounts?

 

[ReefkoiC10]

Yeah it makes a big difference when your truck makes enough power to spin the tires on dry pavement. If the angle isnt close it will have bad wheel hop which is super violent if you’ve Ever dealt with it.
That said I have corrected mine on my 5-7 drop and the best way to do it is to go over to quickperformance dot come they have a nice chart and diagram that explains how to measure. i can tell you that the stock leaf spring perch mounts on my 7” drop put my pinion angle 12 degrees up! It was terrible! I got it just in spec at 5 degrees negative and no hop at all ever. You can go 7 degrees negative that’s supposedly the upper limit

 

[RaysnCayne]

As in, you currently have your diff yoke pointed 5º down, towards the ground?? Does your trans (engine) point up rather than down? I thought you wanted the two to be opposite and parallel?
So if it was 12º up and you now have 5º down, did you cut the perches and re-weld them to get that big of an angle shift? I was hoping to get away with a 2º shim or two under the springs on the perch.

 

[Keith Seymore]

First thing: the u joint working angles are with respect to each other (ie, shaft to pinion, or shaft to trans). They don't care what the relationship is to the vehicle, or the ground. Your truck could be upside down and the u joint working angles would be the same (assuming - of course - the suspension is compressed the same).

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Said differently - the guy in the video could lay his demonstration table over on its side and the results would be the same.

Secondly: depending on your rear spring rate you'd be surprised how much that pinion nose moves up and down during acceleration and braking. I drove my truck for a week with no pickup box and, even with a little V6, you could see that pinion nose moving all over the place.

Lastly - I've lowered about a dozen square bodies and I never did make any adjustments with the pinion angle.

K

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[RaysnCayne]

Yep. I'm pretty pretty familiar with working angles (that video you shared is always the first one that pops up when searching "pinion angle" on YT and I've seen it many times) and how they don't care what plane they're working in - just that they need to be the same (or as close to it as possible).

I've never messed with lowering a leaf spring vehicle. I've lowered my 68 Biscayne (rear coil springs) and had to play with pinion adjustment a fair bit. But that tends to stay more static than a leaf spring rear. So I'm trying to factor in the dynamic working angle (of the rear) side of things. But I guess at the end of the day. I'll just try it where she lands and see what happens.

On those dozen you lowered, Keith, I'm guessing if they never gave you trouble you didn't, but did you ever verify their working angles were the same? I'm also now guessing they would've "talked to you" if they weren't.

 

[ReefkoiC10]

So Keith you are saying you never had tire shake with those 12 trucks so this guy shouldn’t check his pinion angle either since you didnt Have to? Did you verify the angles on those 12 jobs or did it just drive smooth so no worry? Im not being smart just wondering since you seem to have the job there at GM but man I tell ya a positive or even slightly negative pinion angle ruined my truck damn near until I corrected it.

 

[Keith Seymore]

Yes - correct. No issues, so it's not something I checked or worried about.

Intiuitively, it should be better, since lowering the truck (ie, raising the rear suspension up into the frame) lessens the angles on both ends of the shaft. It's probably more important for the working angles to be equal and opposite on the ends of the shaft, so that they cancel each other out, than to worry about the absolute value of the pinion working angle alone.

Since the red truck is still in that configuration it is something I could still check the next time I have it up in the air.

It was a daily for roughly 30 years and 200,000 miles in Michigan, Montana, Arizona and back to Michigan.

K

 

[RaysnCayne]

I had a 4-door, pea green 74 Malibu as a snot-nosed 17y/o, Keith. So when I went to check out what sounds like a cool '74 Chevelle of yours, I was bummed to find the article is not at that url in your profile. I'd love to learn more about it if you have a fresh link.

 

[Keith Seymore]

Hmm -

This is from 13 years ago -

Keith Seymore's 1974 Chevy Chevelle - Two Tons Of Fun

When you spend 31 years racing the same car, you amass a knowledge base on that car like no one else. You know every nut and bolt-which ones will strip if you t

www.motortrend.com

And this one is from this spring:

Keith Seymore is committed to honing his skills and his combo for multiple drag racing classes

www.nmcadigital.com

K

 

[ReefkoiC10]

As in, you currently have your diff yoke pointed 5º down, towards the ground?? Does your trans (engine) point up rather than down? I thought you wanted the two to be opposite and parallel?
So if it was 12º up and you now have 5º down, did you cut the perches and re-weld them to get that big of an angle shift? I was hoping to get away with a 2º shim or two under the springs on the perch.

Sorry I didnt reply earlier I missed this reply somehow. It’s hard to explain but with a leaf spring car/truck essentially you want 5-7 degrees negative pinion angle. With ladder bar/4 link only 1-2 degrees! Imagine if the driveshaft formed a perfectly straight line to the rear axle this would be 0 degrees. Most vehicle have the trans output shaft usually higher than the rear end pinion so ithe driveshaft angles negative but in the case of my 7” dropped C-10 Im almost a straight level with the dshaft if you put a level on it. So as the pinion climbs the ring gear the angle of the pinion goes positive that makes things shake violently under power and if you havent ever had this happen then thank the engineer that got the angle right for you. Now this isnt in relation to the ground or frame or springs or anything its the difference between the trans output shaft and pinion yoke essentially. When you accelerate hard with decent horsepower the pinion wants to climb up the ring gear And so you want it nose down— pinion angle to combat the rising of the pinion angle as the accelerate.

Nothing to do with how the truck “drives” just how it accelerates Under hard throttle with decent torque numbers.

 

[Turbo4whl]

Hmm -

Keith Seymore is committed to honing his skills and his combo for multiple drag racing classes

www.nmcadigital.com

K

Very nice!

 

[RaysnCayne]

Wow Keith, that Chevelle of yours is awesome!
Far cry from the wheezy 307-equipped one I had in high school. Didn't know you could spec them out in police form with the 'vette Rallys and such. Too cool! Love how - for better or worse - you've stuck with a lot of original, factory (looking anyway) parts and still manage solid 9s. Really impressive!
I've been fiddling with my 1968 Biscayne for nearly 25 years. Put it on a bit of a diet but she's still a 3940lb behemoth with me in it. But I'm happy with a very streetable 406 SBC that'll run 11.70s right off the street.

 

[Keith Seymore]

I (re)discovered this while I was looking for something else;

I wrote these for inclusion in the S/T Truck Service manual.

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[RaysnCayne]

Great info, Keith. Thanks!

I was in great haste the last month trying to get the truck together for a show about 4 hours away. So, since SOOO many other guys have lowered their SWB squares with zero pinion adjustment, I figured I'd cross my fingers and hope for the best.
I made the show this past weekend, but boy my driveline was NOT happy. I'm attributing it to a bad pinion angle as I had the tires balanced just before I left and I believe the u-joints to be in passable shape.
I finally got under there and measured the angles last night. (attached)
As you can see, my trans angle is quite steep. The problem is (as I see it); I can't push the trans tailshaft up any more. It's already real close to the tunnel. So I'm stuck as to what I can do to bring that 6º operating angle at the trans closer to the more ideal 3º. If I tip the pinion up with wedges, that'll make it worse, right? And if I tip the pinion down, that'll really take the angles out of whack and turn the driveshaft into a jump rope, y/n?

 

[Keith Seymore]

Agree - not good. Your best hope is to put some clearance in the floor and see if you can get the trans up. Moving the pinion around is not really going to help.

One other option would be to add a CV (constant Velocity) joint on the front, but now you are into additional engineering and expense.

You don't need to worry about jumping rope. The jump rope effect would be caused by spinning the shaft faster than it's critical speed (allowing it to bend) or pinion flange run out (swinging the end around in a circle).

The jump rope effect - and imbalance - are both "first order" of shaft rotation disturbances (ie, one "kick" per rev).

Driveline working angle issues are "second order" of shaft rotation (ie, two "kicks" per rev).

They are unrelated even though they both exist on the same part, and are controlled independently.

K