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blocks vs angled blocks


mikec

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It's all about the drive shaft angles. A U joint transmits power through a bend but is physically joined to both sides at the same time. Sometime take a driveshaft and form a bend at the U joint. Now turn the driveshaft as smoothly as you can. You'll find that the driveshaft tends to turn in a jerky motion. This is because the U joint is turning in an ellipse or oval shape and has to speed up and slow down causing a vibration. The tighter the bend the worse the jerking motion. To offset this, drive shaft U joints are installed so that when one is accelerating the other is decelerating and they tend to mostly cancel out each other. Now for this to work best, the two angles must be close to the same. The angle out of the transmission through the front U joint to the driveshaft must equal the drive shaft angle through the rear U joint to the differential.

 

Lowering (or raising) the ride height will upset this angle balance. I lowered my 710 with 2" blocks and there was a vibration or more like a low frequency HUMMMMM when driving so I investigated. Naturally you can't exactly jack the car up to measure as this will upset the measurements. You want the car sitting naturally. Well I found the front angle was larger than the rear one. Not much you can do at the transmission end but at the back I could tilt the differential down at the front by placing an 1/8" shim between the leaf spring and the spring perch at the rear. Luckily the first try was perfect and the angles were so close there was no difference, but the ride was totally quiet now.

 

Here are the two possible ways to get the same angles so the vibration cancels...

transdriveline.png

 

 

You could measure the angles and see if one is different, they may be fine, in which case adding a shim will not be needed. I can't say what will work on yours, shim forward or shim back. I suppose you could try both and see which is quieter but by measuring you can see if even more angle is needed than what you have to get it exact.

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