What a difference 4 degrees makes!
Thread Starter
Joined: Dec 2004
Posts: 1,458
Likes: 6
From: Olivehurst, CA
What a difference 4 degrees makes! Even though I have been quite careful to get my pinion angle correct in relationship to my output shaft, I still had a vibration under acceleration. I figured out that under power the rear axle was rotating due to the leaf springs wrapping up or flexing. This lifted the pinon angle and put it out of whack. The 50 year old springs, designed for a 90 hp flathead six, just can't handle the LS1 torque! I know that the ultimate solution will be to add some sort of traction bar, I will also change all the spring bushings and possibly upgrade the springs themselves. But as a bandaid or interim solution, I had some 4 degree shims made that brought my pinion down. The vibration is completely gone! This tells me that I must have had it right on the very edge of tolerance and even the slightest rotation put it our of synch. I know the springs are still wrapping up, but now the angles are staying within a range that the U Joints can handle. The car is just a joy to drive and I am no longer reluctant to punch it! 
This week-end I am heading out to a friends shop to see what sort of traction bars we can design or adapt that will be the ultimate solution.
This sure is a lot of fun!
Pat
This week-end I am heading out to a friends shop to see what sort of traction bars we can design or adapt that will be the ultimate solution.This sure is a lot of fun!
Pat
TECH Resident
Joined: Dec 2004
Posts: 841
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From: Baltimore, MD.
Pinion angle is a much bigger deal than most people think.
I worked on a box truck that was on its 7th Allision transmission in 32,000 miles because the pinion angle wasn't right. It had 10 degrees more pinion angle than it should have. You'd have thought that it would have broken u-joints or vibrated like a paint shaker on steroids. But it didn't, it just killed transmissions.
The reason for this is that u-joints only make constant velocity when the input and output shafts are parallel (in both axes). When they are not parallel, you get a velocity ripple at speed (during every revolution, the shaft tries to speed up and slow down twice).
Glad to hear you chased down your vibration. You teeth, transmission, and wallet will appreciate it.
'JustDreamin'
I worked on a box truck that was on its 7th Allision transmission in 32,000 miles because the pinion angle wasn't right. It had 10 degrees more pinion angle than it should have. You'd have thought that it would have broken u-joints or vibrated like a paint shaker on steroids. But it didn't, it just killed transmissions.
The reason for this is that u-joints only make constant velocity when the input and output shafts are parallel (in both axes). When they are not parallel, you get a velocity ripple at speed (during every revolution, the shaft tries to speed up and slow down twice).
Glad to hear you chased down your vibration. You teeth, transmission, and wallet will appreciate it.
'JustDreamin'
looking at you project notes, looks like you orignally had the trans tail at 4.5 degrees....if you raised it up another 4 degrees, does that mean it is at .5 degrees down?
I am asking because I just got done doing my tranny crossmember. The tail is sitting at 4 degrees down. This is the lowest the tranny will ever sit....that way if I ever need to change the angle I can shim it.
A vibrating car is a huge pet peeve of mine so I am doing as much as I can to ensure this thing will not vibrate.
Also where did you get the shims at?
I am asking because I just got done doing my tranny crossmember. The tail is sitting at 4 degrees down. This is the lowest the tranny will ever sit....that way if I ever need to change the angle I can shim it.
A vibrating car is a huge pet peeve of mine so I am doing as much as I can to ensure this thing will not vibrate.
Also where did you get the shims at?
Thread Starter
Joined: Dec 2004
Posts: 1,458
Likes: 6
From: Olivehurst, CA
I did not change the trans tail shaft. I had the pinion and the tail shaft pretty much parallel when the car was static and level. I put a 4 degree wedge between the leaf spring and the spring perch to bring the pinion down. This makes up for the pinion raising under acceleration when the rear end rotates due to torque and the leaf spring wrapping up. As I mentioned I view this as a band aid until I can figure out something to control the spring wrap. You can get the shims from most speed shops including Summit and Jegs, they come 2 and 4 degrees. I actually had mine cut at the machine shop from an old set of lowering blocks I had laying around. This gave me a combination lowering block and angled shim.
If you are planning on keeping your leaf spring suspension, you may want to take alook at the CalTracs system from www.calvertracing.com. Their system is widely used by stock class drag racers that have to keep their stock suspension. They have a street version that I am considering. They claim they improve many handling aspects and don't impact ride to badly.
Pat
If you are planning on keeping your leaf spring suspension, you may want to take alook at the CalTracs system from www.calvertracing.com. Their system is widely used by stock class drag racers that have to keep their stock suspension. They have a street version that I am considering. They claim they improve many handling aspects and don't impact ride to badly.
Pat
