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Pinion angle, poly bushings, 1 piece DS

Old 09-08-2015, 10:32 AM
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Default 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?
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Old 09-10-2015, 09:43 PM
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Wrong thread

Last edited by CPTCO; 09-10-2015 at 09:46 PM.
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