The MGA With An Attitude
SWIVEL PINS & LINKS, Threads and Vertical Motion -- FS-140
On January 17, 2020, Jim Juhas in Cheshire, CT, USA wrote:
"On my '58 Coupe I discovered that a previous owner installed left-side kingpins & links on the right side, and right-side kingpins and swivels on the left. Because of the threaded ends, this means that the outside wheel spindle while in a turn will lower slightly (push down toward the road) instead of lifting up. Since this is only the effect of perhaps 40 degrees of rotation on the Acme thread I expect the effect to be minimal, but I wonder if in fact there is some impact on handling".
This can be a bit confusing, because it is not the spindle rising or falling, but is the suspension, chassis and body of the car rising and falling (because the wheel and tire sit on the fixed road surface).
There is no change of length of the swivel pin, as one link screws on as the other screws off. However, there is up/down motion as the thread is rotating. LH threads on left side of car, and RH threads on right side.
The swivel links are fixed to the suspension arms and do not rotate. The swivel pins do rotate with steering motion. Think in terms of screwing the pin into or out of the links, like you can screw a bolt into or out of a fixed nut. Consider the ground to be a flat reference surface, and the tires, wheels, bearings, knuckles and swivel pins do not move vertically.
Consider the right side first, as it is easier to think about right hand threads. When you turn to right, the right side swivel pin is bring screwed into the lower swivel link and out of the upper swivel link (RH threads). This causes the RH links to rise slightly, which in turn causes the right side of the car to rise slightly (because the swivel links are attached to the suspension arms). Meanwhile the LH side is doing the opposite (LH threads there), so LH side of the car is dropping slightly with right hand turn. Overall result is that the car frame and body will lean slightly away from direction of the turn. That may be counter-intuitive, as you might prefer the chassis to lean into direction of the turn.
In reality vertical motion of the swivel links is very small (do the math). If the thread pitch was 10 threads per inch, and the steering knuckle turned 45 degrees (1/8 turn), the vertical motion of the swivel link would be 1/80th of an inch (0.0125"). The opposite side moves roughly same distance in opposite direction, so the differential motion is 0.025". That's like running one tire over 8 sheets of paper, or inserting 4 sheets of paper between a coil spring and spring pan. I would call that insignificant, as it will not materially affect handling of the car.
The short of it is, having the swivel pins and links on wrong side of the car is not detrimental. It may be a little confusing to the next mechanic to look at it, which is why we are now in this discussion. The only detriment is how many man hours of time is spent posing and answering the question (and everyone here reading it). Of course if you order a replacement swivel link, and it cannot be installed due to opposite thread direction, then someone will be getting cursed out for being the DPM. That's the real reason for getting it right, so you don't waste someone else's time in the future.
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