Pinion angle, poly bushings, 1 piece DS
 

I read every thread with pinion angle in the title yesterday and did not find anything like my situation so here goes. I really need a driveline guru...

This is a 2010 GT, 300 RWHP, 4.10 gears, GT 500 LCA’s, Lakewood Adj UCA, Poly Differential bushing, automatic trans.
I had the 4.10 gears and a one piece aluminum DS installed at the same time by a reputable shop in GA. I reached back out to them stating I have DS issues but have not received any reply.

Here’s what I have happening after having the 4.10's and driveshaft installed. Above 60 mph I have a DS vibration that gets worse as I drive faster. At 80 mph the rear of the car begins to vibrate enough to cause a hum in the car, at 100 MPH the hum is loud and makes the whole car shake. This is not a tire out of balance shake, this is a driveline vibration. In addition, at 45+ MPH if I get up to speed, then coast and find the spot in the throttle between acceleration and deceleration (point where I am basically matching RPM’s to speed) I have a rumbling sound in the back of the car, almost like what you would hear with a bad u-joint. At 60 MPH it’s loud enough for a passenger to hear it and the rumbling also causes a vibration you can feel in the rear chassis of the car. At 80 mph it sounds like something is about to self-destruct under the backseat.

So far here’s what I have done to try to correct this. Saturday I went to friend’s house, he is an auto / aircraft mechanic and a fabricator. At his home shop he has a mill so we used the table on the mill to index a digital Wixey angle gauge, we had two digital gauges, the Wixey was within .1 so we went with it. When then put the car on 4 equal height ramps. Measuring the transmission flange angle I obtained a consistent -2.6 (towards the ground). I then went to the pinion end of the DS and it measured -.50. To be sure the DS was mounted straight into the pinion flange I had my friend spin a tire and checked depth runout where the DS flange is inserted into the pinion flange. Using a straightedge we did not find any difference in depth. The bolts are all tight, I did not re-torque them because they were tight and I assumed the shop that installed the DS torqued them correctly since there was no runout.

I grabbed the DS trying to see if there was play in any joints, I could not get them to budge at all.

While on the ramps, car in neutral, brake off I placed a floor jack under the flat spot on the nose of the differential with a small block of 2x4 on it. My intention here was to see what kind of bushing flex I have to work with. As we slowly jacked upward, the tires began to lift off the ramps. We raised the chassis using this point on the differential about 2” before stopping, the tires were still in contact with the ramps but we had lifted the chassis 2’’. We figured if the bushing didn’t flex with the weight of the car on it, it was not going to flex much more from driveshaft torque. I measured the angle and it changed +0.2 degrees, yes 0.2 not 2.0. Because of the poly bushings I have +0.2 bushing flex.

So we began trying different angles, the initial measurement was -.50. This resulted in the vibrations and rumbling mentioned above.
Wanting the get the angles within 1 degree of each other with 0.2 bushing flex we began raising the pinion angel. We went to +.50 on the pinion, this didn’t really change anything. Next we set it to +1.0 at the pinion, this made the vibrations less at 60+ MPH but the rumbling was still there. Next we set it to + 2.0, I have a little less vibration at 70 mph than I did at +1.0 but the rumbling is still present. It was getting dark so we stopped here.

With basically no bushing flex could that be causing my issues? Should I buy the tool and reinstall (press fit) the softer factory Ford bushing into the differential? I have read numerous posts where guys have the same poly bushing in their differential and have been successful in installing a 1 piece DS so it does appear possible.

At this point I don’t know what to do other than to lower the pinion and have the stock DS re-installed but given there is no flex where should I set the pinion?

I have volumes of how to set pinion angle and driveshaft vs Trans vs pinion measuring and I believe I understand it. My issue seems to be that I have no bushing flex so typical setting of 0 at the pinion leaving +2 to +3 degrees flex to run parallel with a trans flange angle of -2.6 does seem to work here. I had hoped running the pinion at + 2.0 would solve this but it didn't and here I am....

I really need some advice here, any suggestions?

Wrong thread

I have a question for you did you put new bearings in the rear ( I assume you did)? A hum in the rear most times means bad bearing or not so true pinon / gear. I would lean more on a bearing then pinon, bearings in rears have two parts to them as you know the race could be bad or the pinon races could have been set wrong. I have seen were people install a race crocked at first and then just bang it in till it sits in grove, that will cause a race to distort and cause all kinds of issues. If the gears were installed while on the car and not on bench that could be the case. You have mentioned that you have been playing with angles and have been getting different results that also says another indicator of something going on in rear. I have seen some amazing things happen with bad installation from good shops. I myself have screwed up more then once not by choice just because S#%t happens. You didn't have the problem before the gear change? It also could be a bad driveshaft I would have that checked before I go into rear. I have a 2007 GT with all poly bushings, one peace driveshaft and lowered with Eibachi Sportline springs. My driveshaft is set at 1.7 degrees negative, all bolts are tight and I have no noise from bushings. I had a real bad vibration coming from stick at 70 mph that was insane found bad carrier bearing in trans along with bad pinon bearing change all bearings in rear with new 3:73's runs smooth as glass.

I bought a 33 Vicky with a TCI Chassis and a triangulated 4 link car had a bad vibration at 65 mph. Removed drive shaft took to a shop to balance it was off but very very little they corrected it. Reinstalled no difference. So after re-balancing all the tires and replacing the tires and checking all the mounts and bolts, no change. So pinion Angle was next. I had it on lift and decided to change the angle and man everything was tight! Very tight this is a20 year old hot rod build and after I got the rods to turn and set everything for my first try I was surprised! 85mph plus smooth as butter! So, this car had always had this issue! Pinion Angle was the problem for sure on this little 33 Vicky.

have you found the problem and corrected it? We would like to know, Thanks