Lee_in_KC
Active Member
I thought I'd start a new thread instead of continuing this one, since I'm going to be covering several topics beyond "king pin inclination." If you didn't see the first thread, I'd recommend you go back and read it so you have the history of the discussion.
First, the wobble/shimmy is cured, I hope. I have been unable to get it to happen under any of the previous conditions. Seems very stable now. What did I do? Well first, I re-measured the king pin inclination properly... AT the king pins!! Turns out the steering arms weren't a very reliable proxy for the real thing. Since I wanted to check the king pin bushings, I went ahead and pulled the spindles off. I set my angle finder on the front side of the king pin bosses and found that I actually have 6.5 degrees on both sides!! To double check, I set the angle finder on the top of the boss, aligned fore and aft, and again got 6.5 degrees on both sides. To triple check, I clamped a straightedge to the INSIDE of the king pin bushings (which were in great shape, by the way) and measured the angle of the actual king pin hole... 6.5 degrees again. So I now know that my king pin holes were drilled square to the bosses and my steering arms are not exactly 90 degrees to the king pins. From what most people have been saying, 6.5 degrees is dang-near perfect! I re-assembled the spindles on the axle and put the wheels back on.
Next, using Toolman's method, I rechecked the squareness and alignment of my chassis. Dead nuts on, as it had been the last time I checked it a year ago. I highly recommend you try Tim's method... very accurate if done properly.
Part of Tim's process includes precisely checking toe. I had about 1/16" toe in at that point. I drove the car through the area that I almost always got the wobble started, and sure enough, I still had it. Of course, other than disassembling and re-assembling the spindles, I hadn't changed anything since last time I got the wobble! Drove back to the garage and cranked in another 1/16" of toe in, and test drove. Wobble occurred at about 20 mph instead of the usual 25 mph and was not as violent. Went back and cranked in another 1/16" and drove again. Wobble is almost gone. I reduced tire pressure from 32# to 24# and viola!... wobble is gone! Woohoo!
Next I resolved to confirm my suspicions that my big fat rear tires are the cause of a fairly violent rear shake in the 55-60 mph range. I still have the wheels and tires that came off of the Jag rearend I bought three years ago. They are brand new 205/75R15 Tiger Paws on stock Jag wheels and had been balanced when they were on the Jag ('77 XJ-6). I pulled off the 31x18.50x15 Hoosiers and Center Lines, put on the Tiger Paws and went for a spin on the freeway. Dang it, smooth as glass!! The Hoosiers and/or Center Lines are obviously the culprit, as I and others who had ridden with me to diagnose the problem, had suspected.
As a side lesson I guess, I learned that I'd like to go to a 3.8 or so rear gear next opportunity I have to get into the differential. How did I learn this? Well the Tiger Paws off the Jag are 15% smaller around than the Hoosiers, so I expected some change in the way the car drove and the speedometer reading. Wow, the change was really noticeable. The car is noticeably quicker off the line and is more responsive to throttle at lower RPM. I did some calculations and found that going to a tire that is only 85% as big around is the equivalent of going from the 3.31 gears I have now to about a 3.8 set. I really like the difference in performance.
So now my challenge is to chose and new tire/wheel combo for the rear while not dramatically changing the looks of my T, which I really, really like!! I'll report back after I do some research and get y'all's thoughts on what I think I want to do.
Hope my experience will be helpful for others out there!! Don't hesitate to ask if you want more details about anything!
First, the wobble/shimmy is cured, I hope. I have been unable to get it to happen under any of the previous conditions. Seems very stable now. What did I do? Well first, I re-measured the king pin inclination properly... AT the king pins!! Turns out the steering arms weren't a very reliable proxy for the real thing. Since I wanted to check the king pin bushings, I went ahead and pulled the spindles off. I set my angle finder on the front side of the king pin bosses and found that I actually have 6.5 degrees on both sides!! To double check, I set the angle finder on the top of the boss, aligned fore and aft, and again got 6.5 degrees on both sides. To triple check, I clamped a straightedge to the INSIDE of the king pin bushings (which were in great shape, by the way) and measured the angle of the actual king pin hole... 6.5 degrees again. So I now know that my king pin holes were drilled square to the bosses and my steering arms are not exactly 90 degrees to the king pins. From what most people have been saying, 6.5 degrees is dang-near perfect! I re-assembled the spindles on the axle and put the wheels back on.
Next, using Toolman's method, I rechecked the squareness and alignment of my chassis. Dead nuts on, as it had been the last time I checked it a year ago. I highly recommend you try Tim's method... very accurate if done properly.
Part of Tim's process includes precisely checking toe. I had about 1/16" toe in at that point. I drove the car through the area that I almost always got the wobble started, and sure enough, I still had it. Of course, other than disassembling and re-assembling the spindles, I hadn't changed anything since last time I got the wobble! Drove back to the garage and cranked in another 1/16" of toe in, and test drove. Wobble occurred at about 20 mph instead of the usual 25 mph and was not as violent. Went back and cranked in another 1/16" and drove again. Wobble is almost gone. I reduced tire pressure from 32# to 24# and viola!... wobble is gone! Woohoo!
Next I resolved to confirm my suspicions that my big fat rear tires are the cause of a fairly violent rear shake in the 55-60 mph range. I still have the wheels and tires that came off of the Jag rearend I bought three years ago. They are brand new 205/75R15 Tiger Paws on stock Jag wheels and had been balanced when they were on the Jag ('77 XJ-6). I pulled off the 31x18.50x15 Hoosiers and Center Lines, put on the Tiger Paws and went for a spin on the freeway. Dang it, smooth as glass!! The Hoosiers and/or Center Lines are obviously the culprit, as I and others who had ridden with me to diagnose the problem, had suspected.
As a side lesson I guess, I learned that I'd like to go to a 3.8 or so rear gear next opportunity I have to get into the differential. How did I learn this? Well the Tiger Paws off the Jag are 15% smaller around than the Hoosiers, so I expected some change in the way the car drove and the speedometer reading. Wow, the change was really noticeable. The car is noticeably quicker off the line and is more responsive to throttle at lower RPM. I did some calculations and found that going to a tire that is only 85% as big around is the equivalent of going from the 3.31 gears I have now to about a 3.8 set. I really like the difference in performance.
So now my challenge is to chose and new tire/wheel combo for the rear while not dramatically changing the looks of my T, which I really, really like!! I'll report back after I do some research and get y'all's thoughts on what I think I want to do.
Hope my experience will be helpful for others out there!! Don't hesitate to ask if you want more details about anything!
