[Keith Seymore]

Originally Posted by Cammer-6

GM made an entire manual on vibrations
My manuals are somewhere else,
I ll ask for someone to look for it. Keep in mind this was written for engineers not backyard mechanics.

I wrote that.

Last version I saw was in the S/T truck service manual.

I do not have a copy of it electronically.

K

[Keith Seymore]

Originally Posted by Cammer-6

IIRC you want some angle that matches on each end one up one down same amount but less than 3 deg.
Ive heard it said that a straight line is best but I think thats in error.

Straight line is ideal - for a little while.

The problem becomes that the needles brinnel themselves into the bearing caps because the caps are not rotating, causing a disturbance over time.

As a result a "small" angle (ie, 1 to 3 degrees), equal and opposite in direction, is best in real life.

The reality is that it is not going to stay straight, anyways, because of suspension movement and rear axle windup.

K

[JLMounce]

Good information Keith. Do you have anything to add re leaf spring axle wrap? If my engine is down 3* and the pinion moves upwards 4-5 degrees, pointing it down in it's static position by 2* should get me somewhere between 3-4* up opposing the output shaft I would assume?

[JLMounce]

I want to revisit this a bit. I do have a set of 2* pinion shims that I am going to get installed next week. However, I keep coming back to the fact that this new issue cropped up due to a change in transmission, not a change in the rear axle. The car sits the same, the axle wasn't moved in any way etc.

I don't have angle measurements of the previous engine, but I'm fairly certain it was at a shallower angle, simply because I lost ground clearance at the header collector, which is noticeable now that I scrape that coming on and off my lift, where before I didn't.

I know the engine is down slightly over 3* and the driveshaft is up 2.5*. My understanding is that because these are opposing angles, instead of similar angles, I would add those together to get the working angle at the slip joint. That gives me a working angle of 5.6* as the car sits. That is before the pinion climbs the ring gear which would exacerbate that angle further.

I am thinking now, that this may be an "and" solution. Not only do I need to point the pinion down a couple degrees (which will reduce the angle at the slip yoke slightly) but I believe I also need to bring the tailshaft up slightly, further reducing that angle.

I plan on doing the pinion shims first and seeing where I land, but as I think about this more, it may be necessary to make changes to the transmission mount as well.

[johnta1]

You need the rearend angle?

The engine down 3º and driveshaft should be down (from front to rear is direction used) 2.5º ?

Is the new tranny longer than the stock tranny?

[JLMounce]

The car is lowered significantly. About 2" in the front and 3" in the rear. The rear drop alone accounts for much of the wonky driveshaft angle. As the car lowers in the rear, the pinion rises in relation to the transmission output shaft.

The new transmission is longer than the factory TH400 that the car was born with. It's shorter however than the long tail TH400 that was in the car when I purchased it and was just replaced.

So yes, trans output shaft/engine is down 3.1*. The driveshaft is up 2.5*. Because the angles oppose, my research indicates the slip yoke operating angle would be 5.6* at static. As the pinion climbs the ring gear, that angle would actually worsen if I'm thinking about this properly. I'd be well outside the 1-3* operating angle recommended for the slip yoke.

Making an assumption that on the leaf spring rear, the pinion would rise 5*, that would also put that angle out of whack as it would oppose the driveshaft angle under load.

Which it's not a wonder I'm getting a shudder in the configuration.

That's why I think ultimately I'll need both. Get the pinion down a couple degrees to help that joint and the tail shaft up maybe a degree to degree and a half to help that joint.

[johnta1]

Your driveshaft goes up to the rear pinion from the tranny?

[JLMounce]

Yup, it sure does. Consequences of how low I have the car. As the rear comes down, the pinion moves upward in relation to the transmission, causing the driveshaft to travel upwards.

I have some things working in my favor to fix the issues though. I don't run a carburetor, so my engine inclination doesn't need to care about float levels. I also run electric fans, so I don't need to worry about the fan coming in contact with a shroud. Between moving the pinion down a couple degrees and moving the output shaft up a degree or two, I should be able to get the driveshaft in a more favorable orientation, primarily to the slip yoke. I believe that will return the car to shudder free orientation. I may have to play with it a bit though.

I can't really measure it well with the new crossmember in the car, but I suspect the tail shaft sits 1/4 to 1/2" lower in it's current configuration. The factory TH400 crossmember appears to sit higher than the g-force crossmember and there was an added 1/2" bar stock on top of that to extend out the trans mount to support the long tail TH400.

[Cammer-6]

Originally Posted by Keith Seymore

I wrote that.

Last version I saw was in the S/T truck service manual.

I do not have a copy of it electronically.

K

I will PM you with some questions about this

[JLMounce]

A quick update on this. I have installed a 2* pinion shim in both directions. This moved the pinion down about 1.5* and up about 2.5, not quite a full 2* in each direction. No change in vibration was noted with either direction.

I would actually say the vibration is getting worse and it has changed where it occurs from slightly off idle, to a now rpm dependent range of around 2800-3800 rpm before it then clears up.

Of note, because the engine angle is steeper, my header collectors have been scraping pretty badly getting the car on and off the lift. This has created a number of exhaust leaks in the system and moved things in such a manner that there are witness marks on the passenger side pipe from hitting the cross-member, as well as the passenger side collector flange which is now hitting the transmission pan. I know that exhaust hitting floorboard and such can cause some weird vibrations, but this is extremely noticeable in the car.

I have a new set of headers going on the car next week that should fit better than what's on there. They'll re-align the exhaust as well and get it off of the cross-member. I guess we'll see if that has any effect.

I have also ordered some transmission mount shims to try and raise that angle slightly as well. I know its working angle is still beyond what is considered "good."

[JLMounce]

I raised the transmission output by a 1/4" tonight which brought the engine to 2.8* down and the driveshaft to 1.4* up. A much better working angle at 4.2*, but still a bit outside of spec.

I haven't test driven the car yet as it's got a nasty exhaust leak that will be corrected later this week. Hopeful that the change here along with getting the exhaust in a better location will cure this vibration.

[AG]

Have you taken the driveshaft out and measured the transmission angle and then the pinion angle? The angle of the driveshaft isn't relevant.

[AG]

If your trans is down 3 degrees you want your pinion to be up 3 degrees to have a net zero driveline angle. If you have 4 degrees up rotation of your pinion from axle wrap then you would want to set your pinion 1 degree down to get to a zero driveline angle while driving. If you want a different angle than zero you can adjust as necessary. I run Caltracs with split mono leafs on my race car and Caltrac suggested a 2 degree pinion adjustment since there isn't alot of axle wrap with their set-up and I have zero vibration even at 134 mph.

[JLMounce]

I'm using the harmonic balancer to get the engine/trans angle and I'm easily able to get the pinion angle with a straight edge on the pinion flange.

Right now with the engine down about 2.8* and the pinion up at about 1.5*, that should give me a pretty even 2.8/2.5 equal but opposing spread under load. With the driveshaft up towards the pinion, the working angles are bigger than I'd want, but they are now better at around 4* at the u-joints.

[JLMounce]

Well, the search for this mystery vibration continues. The vibration is present with the pinion at it's factory setting, up and down 2* and I have additionally added another 1/4" of spacer to raise the output shaft on the trans.

I just got the car back from the exhaust shop and that is no longer hitting anywhere on the car, so that is also not the culprit.

I guess my next areas to look at are probably the torque converter and possibly the transmission itself.

[Scarebird]

Something to look at:

https://forums.maxperformanceinc.com...&postcount=151

[JLMounce]

That's one of the things I'm going to check.

However, I took the car out for a bit this afternoon and found some new information. This appears to be RPM dependent, not speed dependent. Putting the trans in low gears I can replicate the vibration at low and moderate loads starting at about 3000rpm. I then checked the car sitting still with the transmission in park. Although it isn't as bad, you can feel the vibration start at that same rpm and continue with increasing rpm. It is at its worse between 3000-3500 rpm and does get better as rpm increases, but doesn't go away above that rpm.

This vibration was not there previous to the transmission swap. Looking first at what changed, I need to check the torque converter and the transmission itself.

I'm going to be pretty upset if this 1000 torque converter is somehow out of balance. The TSP unit I had in my TH400 was perfect.

[RocktimusPryme]

I dont think there is a high chance this is your problem given the tests you have done, but to give everyone food for thought. Im fighting a similar issue with vibration. I read a review the other day for the M/T drag radials that I have on my car that says "They balance poorly and rattle your teeth out"

Then it occurred to me that I have only had those drag radials on the back since I started messing with my pinion angle. Before I lowered the car I mostly had the Rally II wheels with generic street tires on it. So Im going to swap back to street tires on the rear and see if it changes or eliminates my shake. Maybe my 60 MPH shake has been tire balance the whole time.

[JLMounce]

Originally Posted by RocktimusPryme

I dont think there is a high chance this is your problem given the tests you have done, but to give everyone food for thought. Im fighting a similar issue with vibration. I read a review the other day for the M/T drag radials that I have on my car that says "They balance poorly and rattle your teeth out"

Then it occurred to me that I have only had those drag radials on the back since I started messing with my pinion angle. Before I lowered the car I mostly had the Rally II wheels with generic street tires on it. So Im going to swap back to street tires on the rear and see if it changes or eliminates my shake. Maybe my 60 MPH shake has been tire balance the whole time.

Good information for sure. I have actually heard this about drag radials in general.

I can create my vibration sitting on the driveway in park, so it's definitely not the tires.

[Keith Seymore]

Originally Posted by RocktimusPryme

I dont think there is a high chance this is your problem given the tests you have done, but to give everyone food for thought. Im fighting a similar issue with vibration. I read a review the other day for the M/T drag radials that I have on my car that says "They balance poorly and rattle your teeth out"

Then it occurred to me that I have only had those drag radials on the back since I started messing with my pinion angle. Before I lowered the car I mostly had the Rally II wheels with generic street tires on it. So Im going to swap back to street tires on the rear and see if it changes or eliminates my shake. Maybe my 60 MPH shake has been tire balance the whole time.

Happens to the best of us.

I chased a vibration for about half a season; had my new Hoosiers balanced at least twice.

Turns out I had flat spotted the front tires at some point. New front tires fixed it.

K