Pontiac GTO Forum banner

1 - 10 of 10 Posts

·
Registered
Joined
·
59 Posts
Discussion Starter #1
Hi,

I am trying to adjust my pinion angle and have not done this before. I bought adjustable rear upper control arms from Spohn for my 1967 GTO because I am getting wheel hop after installing new rear springs (Eaton) which raised the rear 2 inches.

I have the instructions from Spohn and watched some YouTube videos. I raised the car and placed a concrete block under each wheel to simulate actual load.

Finding the angle of the driveshaft is pretty straight forward (see attached image called driveshaft) and I believe I am getting +3.5. (Spohn instructions state anything to the left of 0 on the driveshaft is positive when viewed from the drivers side)

For getting the pinion angle, I am not sure I am placing the angle finder in the right location. I placed it under a flat horizontal groove on the underside of the pumpkin (see attached image called pumpkin). When doing this, it reads -4 (Spohn instructions state that on the rear end anything to the right of 0 is negative)

I figure I am at approximately -.5 , does that sound right?
 

Attachments

·
Registered
Joined
·
59 Posts
Discussion Starter #3 (Edited)
Thanks Alky, that was a helpful article. I placed my car on blocks again and measured all the components one more time. This time I removed the drive shaft and held the angle finder to the vertical face of both the tail shaft and rear end. I didn't know I had to measure the trans tailshaft as well (this was not in the instructions Spohn provided)

This is what I found with my latest measurements:

Drive Shaft = +4.5
Pinion = -3
Tail = +4.5

Thoughts?
 

·
Registered
Joined
·
59 Posts
Discussion Starter #4
I called Spohn tech support and confirmed that I need to take my pinion down to -6 which which will give a -1.5 pinion angle (-6 pinion) minus (+4.5 drive shaft) .

I will adjust later tonight and post on the results.
 

·
Registered
Joined
·
59 Posts
Discussion Starter #5
I adjusted the pinion angle and am right around -2. I went out for a road test and noticed a few things that concerned me:

1. The car cruises fine up to 45MPH, anything over this and I get vibration in the steering wheel. The vibration persists even if I am accelerating, it feels like it is coming from the rear.

2. When I punch it from a dead stop it lays a solid patch of rubber with no hopping (maybe a 15 foot patch), then as I reach 5000 and get ready to shift into 2nd the wheel hop begins.

Thoughts out there on what could cause this? I thought adjustable upper control arms were the answer to my wheel hop problem :banghead:
 

·
Registered
Joined
·
417 Posts
No-hop bars are worth their weight in gold, especially with modified (i.e. raised) suspensions. As mentioned earlier, i could still not get hop out of mine with adjustable uppers, 1,000 different pinion angles, etc. no-hops were the magic bullet, and are pretty easy to install.
 

·
Registered
Joined
·
322 Posts
Pinion angle is relative to output shaft angle, you need both. Each should be equal opposites and the working angle no greater than 7 degrees for 1310 style u-joints. That means if you have a transmission output or axle pinion measuring + or - 9 degrees you're gonna need a CV style 3-joint drive shaft. Even raising the ass end of the car 2" shouldn't even be close to this. Those extreme angles are usually only found on big lift straight axle trucks and Jeeps. Anyway..

An example would be your magnetic angle finder reads -4 on the output shaft and +4 on the pinion, thats perfect cause they are equal opposites.

Say we got -5 on the trans output shaft and +3 on the pinion then id raise the pinion 2 degrees to make both ends -/+ 5. Just remember equal opposites. I have a TKO600 in my '66 with a longer tail shaft than the TH350 that was in it so my drive shaft needed cut down 1.75" to make it work. With my UMI adjustable uppers I dialed in equal opposites at -/+4 and the car drives great. No low speed or high speed drive line vibrations whatsoever and i make 112 mph passes in it.

Now when its track day the recommendation Spohn made to have some negative pinion angle is advantageous. When you slam the gas and hold it open the pinion wants to lift nose up and stay there till you let off. The idea is by setting up the pinion to be down say 1.5-2 degrees at rest for drag racing you now have equal and opposite angles going down the track at WOT by compensating for that pinion lift under that type of extreme load and abuse. For daily driving though make them equal opposites on flat ground and your vibes will be gone assuming you have good joints in the shaft and no other issues like unbalanced tires.

Your lift springs, are they wagon cargo coils or what kind? Might be too dang stiff..
 

·
Registered
Joined
·
322 Posts
This is what I found with my latest measurements:

Drive Shaft = +4.5
Pinion = -3
Tail = +4.5

Thoughts?
I called Spohn tech support and confirmed that I need to take my pinion down to -6 which which will give a -1.5 pinion angle (-6 pinion) minus (+4.5 drive shaft) .

I will adjust later tonight and post on the results.
This is a good high performance or drag race angle but for daily use try -4.5 on the pinion to match your +4.5 trans and go drive it.
 

·
Registered
Joined
·
59 Posts
Discussion Starter #9
OK, I will give it another shot and make sure the transmission tail shaft and the pinion angle cancel each other out at zero. I am hoping this cures the vibration and the wheel hop. If it doesn't, I will drop some coin on the traction bars that leeklm suggested.

I am also open to getting my drive shaft balanced and replacing the u-joint that connects to the transmission tail shaft (rear one was replaced already) - although everything worked fine until I raised the rear 2 inches with new Eaton springs....
 

·
Registered
Joined
·
33 Posts
Tonysgoat, this thread is helpful as I'm going to replace the air shocks and stock springs with Eaton 2" taller springs too and kyb gas adjust shocks. After the adjustable arms, were you able to get the pinion angle set? No-hop bars needed? Can you throw a pic up as well of how yours sits. I've got 275 50s on the car, came with them.
 
1 - 10 of 10 Posts
Top