Posted by Clint Dixon on Friday, February 23, 2007 at 15:50:53 :
In Reply to: Re: Clint and Paul, repost from last forum posted by Paul (in NY) on Thursday, February 22, 2007 at 22:51:45 :
Since I agree with Paul 99%, and he beat me to it, I will not repeat everything. ;^)
The points I would like to make are:
1) You should start at one extreme end and work your way to the other. Paul's method of starting at the back is just fine. In some cases, starting at the front and working back may be better, especially with stock engines. Here is my reasoning: OEM engines were made to work with a particular vehicle, or rather, the vehicles body was designed around an existing engine sitting in an existing chassis. This means that clearances between bellhousing and center hump, fan and radiator shroud, or radiator and radiator cowl, can be very close. If you need to angle the engine another 3 degrees from where it sits now, this can sometimes be very hard to do. Think about an engine that saddle mounts at the timing cover and mounts in the rear at the bellhousing, transmission, or even married transfer case. The distance from front (or side) mount to the rear mount can span a long distance. Changing the overall angle from horizontal by 3 degrees could mean jacking the front or rear up or down by an inch or more. Sometimes there just is not enough room to do this. Now in a repower, you have a little more leeway as you are probably designing a lot of stuff, angles, brackets, mounting points from scratch anyway. Go ahead and change away. Whereas, with the rear axle housing, it is a little simpler to change the angle. You are dealing with a much shorter distance, the distance from the center of the u-joint elliptical orbit to the axis of the axle shafts. Maybe about 12 inches compared to 36 inches between the front and rear engine mounts for example. You only have to rotate the axle housing 3 degrees to get 3 degrees. As long as they are not more than a few degrees, shims are okay. They should not be so thick that the head of the spring bolt no longer enters the hole in the spring perch. Also rotating spring perches accurately is something any competent driveline shop should be able to do.
2) I would not be surprised to find an axle housing rotated a little from what it was when new. Just as springs sag over time, my belief is that they can take a memory. If the truck has done a lot of hard pulling all of its life, while at work, the rear spring pack has been in constant twist in one direction from the torque of the axle housing wanting to twist in the opposite direction from the rotation of the wheels.
3) I have never owned a vehicle, either new or old, that had perfectly vertical shackles while sitting on level ground. I have been told that if you arch or build up the spring packs enough that the shackles are vertical, that this is a definite no-no. Under certain radical circumstances, vertical shackles can lever or rotate the “wrong way”, that is fold under as the spring pack compresses. I’ll admit that you would have to be doing some really out of the ordinary crossed-up wheeling with probably a lot of other factors like stumps and rocks in the mix to get this to happen, but that is the theory anyway.
4) You will probably never get a vehicle set up where the front driveline angles cancel each other out. Basically to do so, all axis would have to be perfectly horizontal. About the best you can hope for is to tune in the rear (engine through rear pinion) and then get the angles at the front and rear of the front driveshaft equal but opposing. This would be true if everything from the engine to the rear angled down at 6 degrees and the front pinion angled up at 6 degrees. You would have a 6-degree angle at both ends of the front driveshaft, opposing each other, but at least they are equal. I will not get into what it takes to rotate a front axle housing and the castor changes necessary.
5) Number four above is why you see so many constant velocity (double cardan) u-joints used in front drivelines.
Junior
Follow Ups: