Driveline angle question. Experts inside please....

[Joe 1320]

Just a little background info first. Full BMR adjustable rear suspension with Intrax 2" drop springs.....

After doing the drop, everything has been fine. I am more than pleased. Yesterday I actually had some play time with the car, performance was just great. I realized also that I had yet to fill the tank to more than half full. I went ahead and filled it up and the rear suspension due to the added weight, dropped a little more. No clearance issues or anything, but..... I took it out and gave it WOT and I felt a driveline vibration. I immediatly backed off and things were smooth. The vibration only occurs at WOT, is most noticeable in 1st gear and subsides somewhat as the rpms climb. Basically I could only feel it around 3K rpm in first, after that the vibration is barely noticeable.

Before I get the car up into the air and start measuring the pinion angle this weekend, does this sound like the drop has altered the pinion angle enough to cause a vibration?

[Kevin - Blown 95 TA]

It could be that you altered the pinion angle and that is inducing a little vibration - easy enough to check, or you coulda lost some straight line accelleration stability cause the lower control arms geometry is off and all that first gear torque is causing the rear end to jump around. Are the lower control arms lowered with the LCA brackets?

[94 formula]

I'm not sure if this would pertain to this or not, but I owned a jeep wrangler and put a set of 30 in BF Goodrich All Terain TA's on it. I picked up a vibration afterwards that ended up being a small weight on the driveshaft that was a balancer that had fallen off. I never noticed it before but with putting on larger tires the angle of the driveshaft moved just enough that it was now noticable at speed. You could check for something small like that first before getting into anything too big.

[Joe 1320]

It could be that you altered the pinion angle and that is inducing a little vibration - easy enough to check, or you coulda lost some straight line accelleration stability cause the lower control arms geometry is off and all that first gear torque is causing the rear end to jump around. Are the lower control arms lowered with the LCA brackets?

I have the lower control arm relocation brackets. I did notice that I lost a little bite, but not much at all. When the car wasn't lowered, I had the relocation brackets but I didn't use the most aggressive, lowermost setting. I had them in the upper hole, it still had plenty of traction and the rear of the chassis was lifting instead of squatting, so they were doing their job. Once I installed the drop springs, I went with the lower attaching point and things seems just fine. The difference was when I added a full tank of fuel instead of merely putting in enough to get back to a half tank. I'll check the pinion angle as see what gives. I might have to check it with a full tank and again at 1/4 tank and see how much of a change occurs. -1º is the preferred setting, no?

[fastTa]

I have the lower control arm relocation brackets. I did notice that I lost a little bite, but not much at all. When the car wasn't lowered, I had the relocation brackets but I didn't use the most aggressive, lowermost setting. I had them in the upper hole, it still had plenty of traction and the rear of the chassis was lifting instead of squatting, so they were doing their job. Once I installed the drop springs, I went with the lower attaching point and things seems just fine. The difference was when I added a full tank of fuel instead of merely putting in enough to get back to a half tank. I'll check the pinion angle as see what gives. I might have to check it with a full tank and again at 1/4 tank and see how much of a change occurs. -1º is the preferred setting, no?

Joe after I did my last rebuild on my 97 WS6, we originally set the pinion angle at 2° down. That made a slight increase in bite out of the hole from stock, but I needed much more. I then set the angle to 4° down and WOW! what a difference. My car hooked very well even on the street after that. I still had good off turn grip but the only thing was that I had to re-learn to gas out of turns because even though we did not technically change the thrust angle, the car just wanted to go straight instead of turn. Once I got used to that I liked it, but my primary focus was straight line acceleration in the first place.

Just for refernce I am running the Eibach Sportline rear springs, stock DeCarbon rear shocks, BMR boxed LCA's, and stock 7.5" 10-bolt w/ 4.10's.

[Joe 1320]

Went home and measured the pinion angle.......0º No wonder. I set it for 2º down and no vibration. That was it. I still could use more bite though.

At least now I can play around with the angles and not worry. I'm going to try 4º, but I'm a little worried about wearing the U joints. Some people have suggested that that much of an angle, when used on a regular daily driver basis, causes premature wear. I could always reset it on the occation that I go to the track. Any thoughts?

[Kevin - Blown 95 TA]

If the goal is to have it straight under accelleration load when you're drag racing (and you have traction), that requires you to set it to 4 deg down or so, but for the street you don't want to drive around with that much angle cause of wear so 2 deg down is about right as a compromise. If the joint has to pivot much at all it will wear funny cause almost all of the load is on just a few of the needle bearings. If you never got on the car hardly at all, you would set it to 0 deg. Depends how and where you drive.

Joe - I started writing this before your last post - but, yeah, you should reset it to 4 at the track. Glad that was the problem.

[Joe 1320]

If you never got on the car hardly at all.... .

Well, you can forget about that. Even though I am anal about maintainance and care, the car is definately driven spiritedly out in these back country roads.

By the way.... Thanks to Kevin² or Kevin X 2 or is it Kevin(s)?

In any case, thanks guys!

[fastTa]

It is generally said that u-joints are constructed in a fashion that they can handle -1° to -2°. Some say up to -3°. For a daily driven ride, I would say 2 to 3 would be the limit without abnormal flexing of the u-joint. 4-5 would be optimal for our chassis at the track however. You want at least -1° on the street though, because you have to account for the pinion pushing up under a hard load. You want the centerline relationship between the crank, DS, and pinion to be as straight as possible under load. (My TA was driven like this 90% of the time )

Here is a pretty good article:

http://www.carcraft.com/howto/91758/

[Injuneer]

When I installed the Spohn TA with poly bushing, I tried -3deg, and it was fine for the street. When Steve Spohn installed an upgraded arm with the rod end, he set it at -1.8deg (using his $200 electronic level). He installed the rear sway bar at the same time. I assume that you have to take the displacement of the front end of the torque arm into consideration, and the rod end would seem to displace less than the poly bushing under load.

When you think about it, the objective of controling the pinion angle is primarily to set the angle of the pinion at the same angle as the tranny output shaft (under load). Only when those two angles are equal, will the two u-joints maintain a constant velocity. But the standard procedure for checking the pinion angle seems to only measure the relationship of the pinion shaft to the driveshaft. That seems to ignore the need for the tranny output shaft and the pinion shaft to be at the same angle, after any displacement due to load on the torque arm.

[BadWS6]

Good Info and maybe this is a stupid question but how do you adjust the pinion angle on a F-Body

[Joe 1320]

Good Info and maybe this is a stupid question but how do you adjust the pinion angle on a F-Body

With aftermarket adjustable lower control arms and/or adjustable torque arm.

[BadWS6]

Thanks for the info

[Joe 1320]

When you think about it, the objective of controling the pinion angle is primarily to set the angle of the pinion at the same angle as the tranny output shaft (under load). Only when those two angles are equal, will the two u-joints maintain a constant velocity. But the standard procedure for checking the pinion angle seems to only measure the relationship of the pinion shaft to the driveshaft. That seems to ignore the need for the tranny output shaft and the pinion shaft to be at the same angle, after any displacement due to load on the torque arm.

I was concerned about that. Especially with the 1 LE trans mount appearing to be slightly taller than the stock unit. I'm going to do some measuring in the morning and see if I can't get things dialed in a little better. All I did was an inital setting to see if the vibrations would go away.

Just great... now there's something else to get anal about.

[Injuneer]

Good Info and maybe this is a stupid question but how do you adjust the pinion angle on a F-Body

Just to clarify... you can only adjust the pinion angle with the torque arm. The adjustable LCA's will not do this. As you lengthen or shorten the LCA, the axle simply moves backward or forward in the wheel opening. The front of the TA will simply move back slightly... in the stock TA this is simply the arm sliding back out of the bushing, in the aftemarket arms its handled with a pivot or a slider. This small change on length of the TA is not enough to appreciably change the pinion angle.

The adjustable TA's include an adjusting nut that pivots the rear mount position with respect to the TA.... its the bright metal nut on the lower tube of the arm, right in front of the lower rod end bolt in the rear mount: