rotational noise at 70+ mph

[70chevellemalibu]

hey guys my 70 chevelle is making a high speed rotational noise heres the specs on my car and the story:

It has full UMI suspension, 4 wheel discs, 350CI, 200r4, 12 bolt 373s and posi.

Transmission and 12 bolt is freshly rebuilt, front wheel bearings are also fresh, drive shaft balanced, all 4 wheels balanced.



At city speed i do not notice the rotational noise (maybe the exhaust tone is masking it?) but i don't think so. Once i reach 70+ MPH i get a waaa quiet waaa quiet waaa quiet waaa sound no vibrations seems as smooth as it can be for a 50 year old car but its not as smooth as my newer sierra. This noise is not a constant waaaaaaaaaa sound its rotational.

Anyways i don't remember the noise being there last year and now its bugging me as i think there is something wrong with the diff.

Could the noise be tire noise or air chopping through the wheel wells or something? has anyone else experienced this?

Can anyone suggest any tips? i had the car looked at by the shop who rebuilt the trans and diff and they said they couldn't replicate the noise and everything seems tight and fine with the car but i want the car to be silent and perfect.....

[GoodysGotaCuda]

What do the driveline angles look like? Binding/Unbinding u-joints can make a sound like that.

[70chevellemalibu]

thanks for the tip i will look at the angles this week.

[70chevellemalibu]

okay i have done alot of reading to make sure i understood what i have:

Here are my results using a digital angle finder.

1. put car on jackstands decently level.
2. place laser on belhousing and "zero".
3. place laser on bottom of the driveshaft: 85.8 degrees up with the laser upside down (magnet to the driveshaft). 85.8 - 90 = -5.8 or +5.8 i am assuming +5.8 due to drive shaft pointing up from the transmission to the diff.
4. "zero" laser on the driveshaft
5. pinion 88.7 degrees down: 88.7 - 90 = -1.3 degrees
6. trans 85.6 degrees down: 85.6 - 90 = - 4.4 degrees

Check math (Operating angles)

Transmission angle minus Driveline angle: -4.4 + 5.8 = 1.4 degrees (.1 degree off)
Transmission angle minus diff angle: -1.3 + 5.8 = 4.4 degrees (okay)

So if my optimal working angles are going to be 2 degrees i need to move the transmission up approximately 1/2" to obtain 2 degrees down and i need to get adjustable upper control arms to rotate the pinion back 1/2" to get the pinion 2 degrees up. (1/2" dimension based on using my snowblower chute to measure angles and distance).

does the above sound right?

thanks

[andrewb70]

I can't really follow your math, because some of the things don't make sense. I can tell you that when A-bodies are lowered in the rear, it is impossible to get the drive-in angles set-up properly. In a stock configuration, when the frame is level, the engine points down to the rear about 3-4 degrees, the driveshaft points down to the rear 4-5 degrees, and the pinion points up a degree or two. So with stock rubber suspension bushings in the rear, under power the pinion rises and the front and rear operating angles essentially zero out, making things run smoothly.

When you lower the rear, the front operating angle forms a shallow V, instead of the shallow inverted V. As the pinion rises under power, the front operating angle increases, instead of decreasing as it would in a stock set-up.

I (and many others since) solved this in my 70 GTO by using a driveshaft with a CV joint in the front and a u-joint in the rear. These shafts are available from The Driveshaft Shop.

Your other solution is to try to raise the back of the transmission as high as possible in order to reduce the front operating angle.

The general rule is that, under load, the front and rear operating angles need to be equal, but opposite and as small as possible (under 3 degrees) but not zero.

Andrew

[joeko23]

It doesn’t make sense that your driveshaft is pointing up from tranny to rear. It should be pointing down. Unless ofcourse your rear is super low (above the tranny). First recheck that your driveshaft is actually pointing up when going from tranny to rear. Ideally you should see tranny pointing down about 3 degrees and rear pinion pointing up 3 degrees. That’s a 0 driveline angle which is ideal.

Also you just put the car on jackstands supported by the frame? If so this is wrong as well, besides putting the car on jack stands you have to support the rear by putting the tires on blocks or something to get it to ride height. When trying to find pinion angles the car must be at ride height.

[andrewb70]

When A-bodies are lowered to what we like for ride height, the rear axle ends up being above the trans. My front operating angle in my 1970 GTO is 6 degrees, forming a shallow V.

Andrew

[70chevellemalibu]

Andrew what part doesn’t make sense?

This is the tutorial I used:

https://shiftsst.com/blog/post/driveline-angles.html

Some of your PDFs posted from spicer.

Also you are correct based on the angles i found yesterday I have a shallow V. I have about 1” available to move up before I hit the tunnel.

[andrewb70]

I've said all I have to say.

[70chevellemalibu]

Lol really? I’d like to know what I did wrong if anything. If I’m measuring wrong to begin with based on your comment you can’t follow my math, then your comment of you’ve said all you’ve had to say is leading me to fail.....

Some clarification would be nice because the way I understand all the posts and PDFs I’ve done it right...

[andrewb70]

Here is how I would measure. I like to keep the angle finder orientation the same, because it eliminates having to do extra math that has to be done if you start orienting the angle finder 90 degrees out.

As noted before, the rear axle should be supported so that the weight of the car is putting the rear axle at ride height. The front wheels don't have to be loaded, but it helps, because of the slight change in weight distribution if you were to put the front jack stands under the frame. Ideally, make some wheel stands from 2x4s and put them under the front and rear tires. This makes the car very stable and basically mimics the position as it would be on the ground. The frame does not have to be level.

I would then find a horizontal plane on the block that is parallel to the crank (by definition this will also be parallel to the transmission output shaft). If you have a billet slip yoke, it will have such a location, if not, then there are various surfaces on the block that can be used.

Take the measurement. If the frame happens to be close to level, that measurement will likely be 3-4 degrees, pointing down to the rear. If the frame is not level due to stance, etc. that's OK, just be aware of the direction of the measurement. If you have a lot of rake, the measurement could be zero or even pointing up to the rear. Write this number down.

Then measure the driveshaft angle in the front and in the rear. If the front angle and the rear angle don't match...that's very bad :-) Again, take careful note of the direction. This can get tricky if the driveshaft is close to level (which it might be!). Write this number down.

Those two numbers are all that you need to figure out the front operating angle. For instance:

If the engine is 3 degrees pointing down to the rear and the driveshaft is zero, then your front operating angle is 3 degrees, forming a shallow V. As I said before, in a stock car, this angle would be an inverted V.

Now, measure the pinion angle. Again, you want to find a surface that is horizontal, which avoids confusion. If you have a billet rear yoke, it will have a suitable surface to measure. If not, you may have to resort to measuring vertically off the u-joint caps, but take extra time to note the direction, etc...write this number down.

You now have all the information you need. To figure out the rear operating angle, use the angle of the pinion and the angle of the driveshaft.

Since the angle of the driveshaft is the same front and rear, we can do our calculation. Using the previous example where the driveshaft was level, and the pinion gear pointing down toward the front at 2 degrees. Your rear operating angle is 2 degrees, also forming a shallow V.

The numbers in the above example are not uncommon and present a problem. Remember what I said about the end goal:

"The general rule is that, under load, the front and rear operating angles need to be equal, but opposite and as small as possible (under 3 degrees) but not zero."

I don't agree with the link you posted, because they state that at 3500RPM driveshaft speed, the max angle can be 5 degrees. While that might be the MAX, it will not give you smooth operation, or at least that has been my experience.

You said you have 3.73 gears, you did not state tire height, but assuming a 275/40-17 tire, which is 25.66" tall, at 70MPH the driveshaft is spinning 3420RPM!

Let's go back to our hypothetical example. Front operating angle is 3 degrees, forming a shallow V and rear is at 2 degrees, also forming a shallow V. Keep in mind, these are static numbers, without load! So what happens when power is applied? The pinion wants to rise up. How much or how little greatly depends on your rear suspension bushings. Soft rubber bushings will deflect A LOT! You can find plenty of videos on YouTube of that. Even poly bushings deflect quite a bit, and frankly you should not be using poly bushings at all 8 points in the rear suspension, but that's another discussion.

So, let's assume that under a light load, on the highway, you have 2 degree of pinion pinion rise. What does that do for our operating angles?

We started with 3 front, shallow V and 2 rear, also shallow V, but now the pinion is further up by 2 degrees and the rear operating angle is at zero degrees and the front is more than 3 degrees. You can use some geometry and trig to figure out the exact angle, but we don't need to know exactly, all we need to know is that our angles are out of whack, under load:

Front is < 3 and rear is zero. This does not follow the required guideline, so what do we do to fix it?

You can shorten the upper rear control arms and tilt the pinion down to the front a little more. This will change both the front and rear operating angles, because the driveshaft is attached to the pinion gear, so you will have to rinse and repeat your measurements and reevaluate. You can also raise the back of the transmission some, but you can only go so far before the front slip yoke hits the floor or the accessories start crashing into things.

I have gone over this many times in my 70 GTO and ultimately went with the CV driveshaft, and many have done so as well. In my Cougar I actually have a CV joint at the front and at the pinion. I have 4.33 gears and 25.5" tires and routinely spin the driveshaft to 5000RPM on the highway (about 80mph). It's as smooth as a new car.

I hope this helps.

Andrew

[70chevellemalibu]

thanks for the in depth description ill get back under the car this week.

Currently the jack stands are on the rear diff and the front of the car is on ramp therefore the car is resting and ride height but it may not be 100% level.

Do you use the digital levels or one like this:

https://static.grainger.com/rp/s/is/..._AS01?$zmmain$

[andrewb70]

thanks for the in depth description ill get back under the car this week.

Currently the jack stands are on the rear diff and the front of the car is on ramp therefore the car is resting and ride height but it may not be 100% level.

Do you use the digital levels or one like this:

https://static.grainger.com/rp/s/is/..._AS01?$zmmain$

I use a digital angle finder I got at Home Depot. I think it is their Husky brand.

Andrew

[70chevellemalibu]

Okay see the attached files for my results.

No where close to being equal, I’ll order up some UMI rear adjustable and play with shining the transmission.

FYI I have 6 poly bushings in the back and 2 rubber.

Thanks Andrew

[andrewb70]

Good job.

Unfortunately, if you get adjustable rear upper control arms and make them shorter, the front angle will get much worse. As you see, your number are very close to my hypothetical numbers, which is not by accident. I've dealt with this before.

Andrew

[70chevellemalibu]

Right I would have to make the control arms longer, should i not bother with buying the arms and just try to raise the trans And ultimately buy a cv joint shaft? Or do the arms have enough adjustment to get me to at least pointing up? I just had my driveshaft cut and rebalanced for my new transmission and 12 bolt. I’d hate to have to buy another shaft.

Thanks

[andrewb70]

Right I would have to make the control arms longer, should i not bother with buying the arms and just try to raise the trans And ultimately buy a cv joint shaft? Or do the arms have enough adjustment to get me to at least pointing up? I just had my driveshaft cut and rebalanced for my new transmission and 12 bolt. I’d hate to have to buy another shaft.

Thanks

You're right. It was late when I responded. Yes, uppers would need to be longer, but that will only make the front operating angle worse. Mine is at 6 degrees in the front and 1 degree down to the front in the rear. Mine is worse than yours because lsswap, etc...

[Tsaints1115]

Isn't these angles not as sensitive if one runs cv joints on the drive shaft as you've mentioned?

[70chevellemalibu]

So if im visualizing this right i need to buy the adjustable rear arms and make them longer so i can move the pinion up by doing this it also changes the angle of the joint at the transmission and also moves it up but at a different rate than the diff which is why you recommend shimming up the transmission.

Im going to have to buy the arms and shim the transmission and hopefully find a sweet spot without needing a $1000 USD shaft unless i can modify my shaft to have a CV joint in the front with a regular u joint in the rear like yours?

Like you said my angles are not as aggressive as yours so i might be able to make small movements to get me dam close.

sound good to you?

[andrewb70]

Before spending any money, see how far you can raise the trans. I suspect, maybe 1" at most. Remeasure. The second step would be to temporarily adjust rear ride height and see if the vibrations improve.

Chasing these issues is very frustrating and the last thing you want to start doing is throwing parts at it.

Another thing you need to do is check the run-out of the driveshaft, front and rear.

Andrew