If you lower them using the standard methods like dropped spindles, springs, or air bags w/o changing the pivot points, I could see that.

The front control arm mounting points for this project were raised 3" w/a R&P set-up to ensure correct steering geometry. I modifed the rear suspension & moved the location/mounting points of it up 3" to match the front & keep the consistency. I wanted to keep the drivetrain as low as possible within the frame so it's the only thing not raised the entire 3" like the suspension points.

The Truck
 

Hello,

Greg Frick from IEDLS here again.
Scott invited me to join in this discussion so here I am with some questions.

What kind of truck is this?

What is the distance in inches from the end of the transmission output shaft to the centerline of the pinion u-joint?

Will the shaft head down towards the pinion. be level of rise toward the pinion?

What is in the way of the driveshaft?

What will the engine/transmission bump into if you have to start juggling components to achieve angle changes?

Will the truck always be at the same ride height or does it have hydraulics or air bags ?

With this information in hand I might be able to suggest something to try.

Greg

Awesome.... I was going to contact you next week, you beat me to the punch. Answers above in RED.

A Suggestion
 

Good Morning,

What follows is unconventional but is less work than cutting up the tunnel and it MIGHT work for you. It has worked in some street rods with similar shaft lengths.

You could get complete u-joint angle cancellation by dropping the pinion to 3 degrees down. In this case you will have both mounts pointing down. If the angle made by the transmission/drive shaft is angle "A" and that made by the shaft/pinion is angle "B", then A - B = 0 with the working angles both on the same side of the shaft.

Why isn't this done all the time? I avoid going into this subject because it gets into very unfamiliar territory but you need to know the answer. Driveshafts are subject to forces called secondary couple loads. These loads operate at 90 degrees to the shaft. With equal and opposite angles these loads cancel out and are not an issue. When the angles are equal, but both on the same side of the drive shaft (the top in your case), they add up. The result of additive secondary couple loads is a shaking of the mounts: the transmission and the rear end. This shaking goes everywhere in the car and is very aggravating.

An odd feature of the secondary couple load problem is that the farther they are apart the less of a problem they seem to be. Unfortunately there is no rule on how far is far enough or how close is too close. What works in a '53 Studebaker will not work in a bucket T.

I suggest you raise the transmission end as much as is convenient and match what you get at the pinion end using shims. As someone mentioned earlier, less angle is always better. You can also leave the transmission alone and match the 3 degrees at the pinion end. Allow for revision later so you can "tune" the set up if it shakes.

The net result of this is the drive shaft being dropped somewhat and allowing additional clearance.
Good luck and let us know how it works out,

Greg

Thanks for this info. I'm currently waiting on some shims to dial in less change @ the pinion (I only had 2.5 & 3° sets). I do feel that if I match the motor/trans 3° angle @ the pinion, the shaft will be slightly angled down if not level. I'm assuming 3-3.5° will be acceptable as it cancels out w/o exceeding the >4° rule (since the truck arms are essenitially 51" ladder bars). I'm not sure how exact I'll be able to get it.

I've found that you must move the pinion around twice what the driveshaft angle will move. If you can get working angles between 3-4 degrees that cancel eachother out, that will likely make for a smooth ride. Less than 3 is ideal.

I'm a litte confused by what you are saying Greg. The way I read it, you are saying that he could drop the pinion down to where his working angles are both on the same side of the shaft and not equal and opposite or try it with equal and opposite closer to 3-4 degrees of working angle??? I'd certainly try the equal and opposite first in this situation. Not ideal but not huge angles either.

If I did the equal & opposite angles, I'd be back w/the d.shaft sloping up toward the pinion from the trans which is what I understand to be the greater of my 2 evils.

Clarification
 

You have it right.

Equal and opposite, less than 3 degree working angles is the perfect set up.
In some cases, especially with finished cars, it is worth the effort to try equal but not not opposite, small working angles as an alternative to major surgery. Sometimes it works if the u-joints are far enough apart.

You can see why I avoid this set up. It comes with built-in problems but sometimes works. If it doesn't work you have to start over trying to get equal and opposite. It is easier to get it right when in the building stage.

If at all possible, stick to equal, opposite small angles.

Greg

ive never heard it explained like that before. the program i used to set up my driveline lists that combo as a perfectly acceptable arrangement.

Scott, if you can achieve 3 degrees of driveline angle and say 2.5 degrees of pinion with a driveshaft less than 1 degree sloping up, your working angles would be between 3-4 degrees. Under acceleration and suspension compression, your angles would get larger. You could simulate that without springs and shocks. Raising the tailshaft another degree would probably get you in the money(2-3 degrees equal and opposite). Really depends on if you want to do surgery. If I was still in build stage, I'd have cut mine for small equal and opposites. Do you have a carrier bearing mid shaft? That could complicate things here. I'm so used to everyone having 1st gens around here.:D

The alternative is what I've done in my car. Lower the pinion angle below level to run the smallest working angles on both end of the shaft(2.5-3). That puts the working angle on the same side of the drivshaft as Greg said. Small angles but not canceling. My driveline angle is 4 degrees(Stock). My driveshaft would rise 2-3 degrees. That makes for 6-7 degrees with equal and opposite angles which is way to much for high speed.