|The MGA With An Attitude
FUNCTION of the STEERING RACK -- SR-101
SR-101 explains the function of the steering rack and its internal components.
At 10:05 AM 1/4/03 -0500, Steven R Schultz wrote:
>"I am writing an article on the MGA steering rack. In the article I would like to include information on how the steering rack works to reduce the amount of front end shimmering.
>1.) What does it do or how does it work? I assume the oil moves from side to side like a shock and that the pistons under the springs help absorb the shock. Is this correct?"
The steering rack is not a shock absorber. The best you can do to reduce shimmy is to remove most or all of the clearance from the moving parts so there is no free play between the road wheels and the steering wheel. This will not eliminate shimmy completely, but it will certainly reduce the magnitude of the problem.
>"2.) Why is there a primary and a secondary spring of different sizes"?
The damper spring over rack above the pinion gear serves to force the rack into intimate contact with the pinion gear to eliminate any free play in the gear teeth. As the gear teeth have very little friction, this spring force can be substantial without causing much drag. And in fact the spring force here must be fairly large to keep the gear teeth close, as the mating angle on the teeth is about 20 degrees, which tends to force the teeth apart with high loading.
The damper spring at the other end of the rack forces the moving rack against the inside bore of the housing to eliminate side play in the rack. This side force is not as great as the side force caused by the pinion gear, as the side force here comes only from the push of the tie rod when it's not perfectly aligned with the rack. And in this configuration the rack is dragging directly against the bore of the housing, so it is desirable to limit the side loading from the damper to a minimum.
In the case that you might hit a pot hole and encounter a sudden high shock load at the tie rod, the rack will be dislodged radially out of the desired position momentarily, but this motion is small and not repetitive, so it does not contribute noticeably to shimmy.
>"3.) Why does the rack use oil instead of grease? If my assumption in question #1 is correct grease wouldn't follow. What weight oil is best?"
When the rack moves the gaiters at either end expand and contract moving air through the rack from one side to the other. There is also more oil in the rack (or at least there should be), more than will fit and remain in the main rack housing. So as the rack moves to full lock at end of travel some oil should be displaced in and/or out of the gaiters. This is how the oil gets to the inner tire rod end to lubricate that joint. So the oil has to be able to move freely through the end bushing area of the rack housing, and grease does not flow easily enough to do that.
With the tires off the ground you can grip one wheel and move the steering quickly from side to side to full lock. When you have the correct amount of oil in the steering rack you can hear it gurgle slightly through the gaiters as the rack approaches end of travel.
Proper periodic service calls for a shot or two of grease in the nipple on the input shaft bushing. So the proper procedure is for gear oil in the Zerk on the rack tube, and grease in the Zerk on the input housing.
>"4.) Do you know why the factory used grease nipples (Zerk fittings?) instead of the old fashion tilt top oil caps? I know they had an oil filler gun but I would suspect that the choice for of the grease nipples came first."
I think on the production line and in a service shop it is easier and quicker to force oil through a Zerk fitting than it is to squirt it through a small open hole with only gravity assist. For a quick fill you need either a very large pouring hole or a pressure tight joint on a small pressure feed hole. Lift caps are fine if you only need a few drops at a time. But the steering rack wants about 6 ounces of oil for initial filling, or any time the boots have failed and are replaced.
>"5.) What do the shims do? Do the shims change the tension on the springs or the travel of the pistons? In either case it wouldn't appear to be enough distance to make much of the difference."
You're right, it doesn't make much of a difference in the spring force. But if you remove all of the shims, then you tighten down the cover nut, you should be forcing the plunger solid against the rack like a set screw so the rack would bind and would not move. The idea is to install enough shims under the cover nut to keep the plunger from binding, but to maintain the smallest practical space for motion of the plunger. That way when you hit hard bumps with the tires there's very little wiggle room for the rack to move radially, which will minimize rattle and shimmy. The workshop manual tells how to set the damper shims, and calls for .002 to .005 inch end clearance for the damper plunger.
Now for the next part of your report you should also reveal how to adjust the inner tie rod joints for minimal clearance (shims), and how to adjust the pinion shaft for minimal end float (also shims). You might also describe the configuration and function of the seal on the pinion shaft (a little chuckle here when you figure it out). And of course the outer tie rod ball end joints are not adjustable for internal clearance and must be replaced when worn, and ditto for the input steering shaft flex coupling (u-joint) and the bushings in the steering column.
[Side note] You can now tour the Steering Rack Tech Day at SR-201.htm
I can tell you right out that the factory specified tools for disassembling the inner ball joint will not work on a 40 year old steering rack. The pin drive holes on the inner side of the joint are too small, so even hardened steel dowel pins will sheer off before the joint will unscrew. About the only solution here is to take the tie rod assembly to the work bench, clamp the flat side of the joint in a vise, apply lots of heat with the blue wrench, and unscrew the large acorn nut with a pipe wrench. When you do that you will most likely destroy the aluminum shims (yup, aluminum, not brass).
If you have a large assortment of (expensive and rare) shims handy you might reassemble it properly with minimal end play. Otherwise I have my own devious method of adjusting the clearance in this joint with a flat face grinder wheel. If the joint is loose you can grind a bit off of the front end of the acorn nut so it screws on farther, and you can make a large adjustment in this direction. If the joint is tight you can grind a bit off of the back of the button seat inside to give it a little more clearance, but it takes a lot of grinding to make a small adjustment in this direction. This is more work than adjusting shims by the book, but it might be worth the fiddling if you don't have the right shims in hand when needed.
When you come to the realization that reducing shimmy requires reducing clearance of all moving parts to a minimum, then you start thinking about all the places in the front suspension that can develop slop and wobble. Then you can catch a case of the might-as-well and end up rebuilding the whole front end of the car.