Rack and pinion steering limiter


Last update: 7/5/08
Return to home page links



Background
Many production vehicles have mechanical stops mounted on the suspension that limit the spindle rotation. If a limiter wasn't provided it might be possible for the steering parts of the suspension or the tires to contact other parts of the vehicle.

After making changes to the steering system or suspension geometry while building or modifying your suspension, it is possible that some part of the wheel assembly etc might then interfere with other parts of the suspension or chassis.



Details of why steering limiting might be required
I realize that limiting the steering the way I did it is nothing new and has been done many times before. But I want to present the details and the precautions that need to be taken for other builders who might not have seen how it's done.

Some (most?) rack & pinions can move the rack shaft as far as the teeth on the rack will allow it to move. The system depends upon an outside device to limit the maximum amount of wheel angle in a turn.  Often when people build their own cars this limiter device is left off or is in the wrong place because of the changes made to the suspension.

I modified my steering so that two turns lock to lock of the steering wheel turned the front wheels the same amount as the old three turns lock to lock did. The rack and pinion itself could still be turned three turns lock to lock though. This caused my Heim rod end mounting bolts to hit the lower control arms when the steering wheel was turned more than one turn off center. (The bolts are 1" lower than the original hardware.)  Obviously the number of turns the steering wheel could be turned needed to be restricted.



What's involved in doing this change?
Basically all you need to do is remove the flexible gaiter from each end of your rack pinion, measure a few things on the rack and make up some spacers to limit the amount of movement of the rack shaft. You then slip the spacers onto the rack shaft at each end and put everything back together.

Sounds easy enough, but you do have to make your measurements accurate so the steering wheel turns the same amount to the lock position left and right and check that the suspension doesn't bind or cause the steering arms etc to interfere with other parts of the car. And you want your steering wheel to be centered when the car is driving straight down the road.



Why worry about having the steering wheel turn off center the same amount?
The reason is so that the car will have the same turning radius turns left or right. Otherwise you may get into trouble later thinking the car can turn tighter in one direction when it is really the OTHER direction that has the smaller turning radius. Also it is possible that one tire might drag on the chassis and the other does not.



 Considerations and precautions
Some racks may have the bearings for the rack shaft at the end of the rack housing. In this case the spacer can just be centered on the rack shaft diameter and it will limit the amount of travel of the rack shaft. I'd taper the hole ends of the spacer so it doesn't catch on the rack teeth.

Other racks, like mine, have recessed outer bearings for the rack shaft. i.e. the bearing is not at the very end of the housing but is recessed a short distance inside housing. In this case the spacer needs to be made so that it will be a light press fit INTO the end of the housing (or the inner diameter of the spacer should be only a slightly larger than the rack shaft diameter to center the spacer on the shaft to clear the housing).

The reason for this is, if the spacer has a larger diameter hole than the rack shaft and were allowed to lay on the rack shaft it could catch on the end of the housing in a turn and change the amount of lock in that direction. This is not safe!!

So plan on doing a little machining to get the spacers to fit properly on each end of the shaft or into the housing.



Determining the size of the spacers
First thing is to support both lower control arms so the tires just clear the ground. You need to do this so that the wheels can be turned through their full range of movement.

Next remove the gaiters from the rack. If you have to take the tie rod off the shaft to do this and then put the link back on hand tight once the gaiter has been removed. The outer end of the steering links should be left connected to the steering arms.

Measure the maximum diameter of the rack shaft and write it down.

A.  If you know how many turns lock to lock you want your steering to have do this,
Center the steering wheel. Now turn the steering wheel to one half the number of turns you want the steering wheel  to be in either direction. I wanted two turns (total) lock to lock, so I turned the wheel one full turn; first in one direction from center and then the other direction.  Check the various steering parts around the steering arm, links etc on both sides of the car for clearance with the steering one turn off center in each direction.

On my setup, one turn in either direction allowed the suspension parts to have the interference clearance I wanted so I was OK to continue.

Again, do the following in each direction.  Measure the distance from the rack bearing to the inside edge of the mechanism that holds the tie rod to the rack with the steering wheel turned in the direction you are going to measure.  

Or to put it another way, turn the steering in one direction and measure the side of the rack that is sticking out of the rack housing the least.  You want to measure the distance from the rack bearing in the housing to the outer part of the rack that the spacer is going to finally touch to stop the rack movement.

Turn the wheel to the opposite side lock position and take the same measurement on the other side of the car.

Write the numbers down as they will more than likely be different from side to side. These measurements are the length of the two spacers you need to fabricate.

There is a picture below that illustrates where the measurement is taken on my rack.

B.  If you want use the maximum number of turns lock to lock do this,
Center the steering wheel and then while counting the turns of the steering wheel, rotate it in one direction until the suspension or tire has interference with some object. Take note of the number of turns it took to create the interference.  Take into account that with suspension movement up and down the interference might occur sooner than at ride height.

Re-center the steering wheel and then turn the wheel in the other direction to see which direction gives the minimum number of turns to contact something. Use the minimum number of steering wheel turns as your new lock to lock MINUS a small fraction of a steering wheel turn to allow for some clearance between the interfering parts. Don't forget to consider that the interference measurement may change as the suspension moves up and down.

With the steering wheel at your new off center lock turns, measure the distance from the bearing to the inside edge of the mechanism that the spacer is going to finally touch to stop the rack movement.

Now turn the wheel to the opposite side new off center lock turns and take the same measurement. Write the numbers down as they will more than likely be different from side to side. These measurements are the length of the two spacers that you need to fabricate.

I must say though; why you would want the maximum number of turns on a light sports car is beyond me.



Limiter spacer

To see an enlarged view of most pictures, left click on a picture or right click and select "View Image"

sleeve tube

This is what one of my spacers looks like. One of these sleeves is placed on each end of the rack shaft. The length shown here more than likely will NOT be the same that your rack needs.

Also, the two sleeves will probably NOT be the same length or even the same outer diameter and that may determine how you want to make the spacers. The bearing on the pinion side of my rack had a tapered hole beyond the recessed bearing. The other end of the rack had a straight sided tube to the recessed bearing.

I decided to make up spacers that would be a press fit into both ends of the rack rather than "floaters".  I did this because I had a piece of thick walled aluminum pipe that the outer diameter of the pipe could be machined down to fit both ends of the rack housing.  The inner diameter of the holes at the end of the rack housing are much larger than my rack shaft diameter.


sleeve on rack

This picture shows one of the spacers laying on the rack shaft before installation. The spacer outer diameter was machined so it is a light press fit into the rack housing (towards the right of the spacer).

The recessed bearing is positioned about 1/2" inside the end of the rack housing.

sleeve at lock

I first screwed on the tie rod end inner retainer (the large, domed piece of metal in the middle of the picture).

The spacer was started into the housing by hand. I then turned the steering wheel so the tie rod inner retainer pressed the spacer into the rack housing as shown here.  Quick and simple.
After both spacers were installed, I tightened the tie rod retainers and replaced the gaiters.




limiter clearance

This shows how much clearance (~1/4") I allowed between my Heim rod end mounting bolt head and the lower control arm after I had modified the suspension to correct bump steer and to give proper Ackermann.. The other side of the car had the same clearance.

You can see a small chip of paint missing where the bolt head had hit the lower control arm in setting up the suspension.

--