Driveshaft angles....

So, my transmission is at 89 degrees when I put my angle finder on it.

Rearend is 86 deg.

Am I asking for a major vibration?

Neither trans nor rearend is easy to move. Like at all.

What says the hive?

What is the angle of the driveshaft?

Relative angles are IMPORTANT, more important than absolute angles. You say that your trans is 1 degree off of horizontal and the rearend is 4 degrees off of horizontal. If the driveshaft is 2-3 degrees off of horizontal, then both U-joints are working at a 1-2 degree angle and they will cancel each other out, all is well. If you have one U-joint at zero and the other at three, or worse 1 the other way and 6 at the rear, your life will suck.

That reminds me, I really don't like to see U-joint angles of over 4 degrees under any circumstances. Which is why sometimes you need to point the pinion down instead of up, so the U joint angles are complementary even though the absolute angles are screwy.

The key is, you want both U-joints to be working at close to the same angle as each other.

Agreed: Equal and opposite. Absolute angles of the trans and rear should be equal. That will guarantee that the driveshaft angle will be equal where it exits the trans and enters the rear.

Then set the angles so that there is no less than about 1/2 - 1 degree deflection, and no more than 4-5 degrees.

Like this for example. The angle of the transmission output should be parallel to the pinion shaft. That way the angles of the two U-joints will always be equal and opposite. But the most important part is the actual angles that are seen at the U-joints. Looking at this picture, imagine shortening the wheelbase. The actual angle of the U-joints would increase with the shorter shaft. In that case, if the angle of deflection exceeds 4 degrees, you might consider tilting the trans down one degree and the pinion up one degree to lessen the angle at the U-joint.

But for most typical RWD cars, as long as your output angle and pinion angle are parallel, things will be good.

There are many times were absolute angles can not be equal since it would force U-joint angles to go extreme.

I corrected a Studebaker's driveshaft by pointing the pinion down. Someone put a SBC in it and due to a combination of low floor and remarkably space-inefficient steering linkage, the drivetrain was in the car at about a 7 degree angle. If you put the pinion 7 degrees up, the U-joints would have been running at some really extreme angle like 10-12 degrees. This in itself will cause a vibration as higher U-joint angles will require a greater speed oscillation twice per revolution, and speeding up and slowing down the mass of the driveshaft that much is gonna shake like a wet dog. I ended up dropping the nose of the pinion a few degrees, which also had the benefit of making the driveshaft's angle different so it lessened the front U joint angle and therefore the angle I had to shoot for at the pinion. Worked great.

1st-gen RX-7 guys have known about this for a long time. RX-7 drivetrains sit in at a 4 degree angle, and when lowered for road racing you are not going to get that much pinion angle without floor surgery, so you point the nose down instead of up. When you have 5.12 gears and hit 150 on the straights, driveshaft vibration becomes high on the list of things on your mind

To clarify, the trans and rearend are not parallel. They are off by 3 degrees.

I assume this is bad. I can't really fix it without major surgery. How bad is it?

If you can't fix the angles, what's the possibility of getting the driveshaft modified to have a double-cardan joint at one or both ends. That should make it much more tolerant of bad angles without vibration.

Ideally, you'd want the DC end to take most of the angle and the single u-joint end as straight as possible if you only put a DC on one end.

To put it another way, the front of the diff would need to move down 3 degree OR the trans output needs moved down 3 degree.

Anybody else wanna chime in?

If it matters, the driveshaft is 28 inches between u joints and will turn 5k at most.

If anybody knows how to change the diff angle in a 2nd gen rx7, I'm all ears!

wvumtnbkr wrote: To clarify, the trans and rearend are not parallel. They are off by 3 degrees. I assume this is bad. I can't really fix it without major surgery. How bad is it?

What is the driveshaft angle. Full stop.

What matters is not the angle of the trans and the diff. What matters is the angle that the U-joints operate at. To know that, you must know the driveshaft's angle. You have the tool, stick it on the driveshaft and find out.

If you don't actually have a driveshaft yet, get one made, then measure. It's not like you won't need a driveshaft anyway.

I can do that. The angles Will be minimal....

The trans points right at the diff.

I wanted to know if I should get a cv driveshaft right away. That is what was holding me up from getting a driveshaft made.

In reply to wvumtnbkr:

You won't know until you have something in there to measure. This is one of those "need to break eggs to make an omelette" situations.

1 degree and 4 degrees might be perfect or it might be awful. I had a Corvette that had angles similar to that but the driveshaft was perfectly in-line with the trans so the effective U-joint angles were 0 degrees and 5 degrees. It smoothed out under cruise (load does tend to kill oscillations) but under neutral throttle the U-joint oscillation was so bad it made the ring and pinion rattle. And if you thought your FC had no good way to fix angles, you ain't seen an early C3 yet. Since the front U-joint was zero, we had the driveshaft shop put a double Cardan in place of the rear U-joint and it came out perfect.

The guy who owned that car (a T56 swap, the reason for the issue) also had a manual transmission swapped Impala SS, which looked kinda neat.

If the trans points right at the diff, then yes, go with a CV driveshaft. Single u-joint at the trans, double cardan at the diff should do the trick.

I'll tell you what I did, I got the free App from Tremec for my iPhone and used it to measure the angles and it told me what was good and what wasn't

This thread is very relevant to my interests

wvumtnbkr wrote: If anybody knows how to change the diff angle in a 2nd gen rx7, I'm all ears!

You'll need some compliance in the rear bushings, but you should be able to change the angle by remaking the front diff mount or the bracket it attaches to. I'm pretty sure with worn stock bushings the pinion angle moves more than 3 degrees on its' own

You can shim the trans mount a bit. Get longer bolts and some washers.

I'm living this right now on my latest build. Here's the best I was able to get with the limitations I face. My transmission is parallel to the ground. The central axis of my differential has a slight negative slope (1.3 degrees relative to the ground) and my drive shaft is also on a negative slope (3.5 degrees relative to the ground). I think, but I'm not sure, that means my transmission-side operating angle is 3.5, and my differential side operating angle is 1.95 the other way, for a difference of 1.3 degrees.

I had the car on the road for the first time on Saturday and things seemed fine. I was testing it at sustained periods of higher engine RPMs and driveshaft revolutions (as in, five minutes at 45 mph in second gear, 65 mph in third gear) and it didn't seem to vibrate.

It will be two weeks before I have it out on track and really start spinning it all the way out to the redline ceiling. Since it's a track car, and one of the U-joints is a conversion joint to boot, I have a stout drive-shaft loop, and I plan to change the U-joints every 75 hours anyway. I don't know whether that's the right schedule, but the job is not that hard.

In reply to JBasham:

As a note for testing, engine RPM won't matter for driveline vibes. It's just driveline load and rpm that matters (and that's basically just closed throttle / neutral throttle / open throttle and road speed).

Thanks. So I'm assuming that when I was at the higher end of the possible speed range for a given gear, I was rotating my driveshaft "fast." Sound about right?

I follow you about the load.

Driveshaft speed is related only to road speed. Not trans gear, engine speed or anything else.

For a given rear end gear, the driveshaft will always turn the same rpm at a given road speed. Using my Jeep as an example with 3.73 gears and 29" tall tires, at 60 mph, the driveshafts are turning 2593 rpm. I could have the trans in 2nd (1.45:1), 3rd (1:1) or 4th (0.69:1) at that speed giving 3760, 2593 or 1789 engine rpm. But no matter what gear I'm in, the driveshafts will spin at 2593 rpm only changing with a change in road speed.

Remember, the trans is between the engine and driveshaft. So you're not changing the gearing between the driveshaft and the wheels.

Keeping driveshaft RPM down is why some Euro cars have historically gone for a 1:1 top gear or a very mild overdrive and taller rear end gears instead of low rear end gears and a deep overdrive. The taller rear end gears lead to a lower driveshaft speed for a given road speed.

Over the length of a driveshaft, the error in the angles that you get by estimating it with a broomstick and duct tape is minimal. Give it a shot and see what the driveshaft's angle is.