Modifying the CMC designed suspension to correct bump steer


Last update: 7/5/08

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All information contained in this article is for instructional purposes only. If you aren't sure of what you are doing when working on the suspension of your car please get help from someone that is knowledgeable in this field.

In other words, if you copy what I've done here and hurt yourself, I take no responsibility for YOUR actions.


Background

CMC used a 1983 RX-7 GSL as the donor for my Locost 7. They converted the front spindles from McPherson to double A arm suspension by cutting the spindle tube to be ~5-1/2" long from the lower ball joint to point where the tube was cut. A 1" long machined "cap" was inserted and welded into the top of the tube to allow the use of a Heim rod end as the upper ball joint..

Apparently CMC modified various front McPherson spindles as a "one size fits all" since another CMC7 in town uses Toyota spindles and the dimensions are very similar to my RX-7 based unit. Both our front ends have extreme bump steer. Our toe in changed ~5/8" when the chassis was moved 5/8" up or down!

The first generation RX-7 used a removable steering arm that also has a tapered hole for the lower ball joint shaft to be mounted to. Even if your suspension doesn't have a removable steering arm, the ideas presented in this article will still apply. The theory behind what I have done is the same for all cars.

I have documented how I corrected the camber change in bounce and dive in another article, "Correcting CMC designed steering geometry, camber". These two articles  are a description of all the modifications I made to correct my suspension geometry. This article is concerned with how I modified my steering arm to correct the Ackermann angle and partly bump steer.



CMC suspension design  problems

My stock CMC suspension design had a combination of problems. These problems were caused by using the stock RX-7 steering arms with a different track and wheelbase and the placement of the rack & pinion. This resulted in incorrect Ackermann affect and contributed to bad bump steer!

The stock RX-7 steering arms are fairly long. The rack & pinion used on CMC cars  has 3-1/8 turns lock to lock. The rack moves 1.48" per turn of the steering wheel. The small rack movement per turn of the steering wheel in conjunction with the long steering arms resulted in a rather large turning radius and very slow steering ratio.

On a brighter note, the RX-7 1st generation spindle with its removable steering arm allows you to try various ideas  to correct the steering geometry (in my case, changing of the tie rod outer end mounting point). The RX-7 steering arm is fastened to the spindle with two 14 mm diameter bolts.



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


Cause of steering problems
Heim close up

This picture shows the cause of the bump steer. Notice that the steering tie rod is aligned upward in the vertical plane in relation to the lower A arm.

Also notice that the upper "A" arm is angled downward towards the left which caused the convergence point of the upper and lower "A" arms to be ~14' to the left of the tire.  The lack of convergence of the the three extension lines was the root cause of bump steer and incorrect chamber change.  The steering tie rod extension line was nowhere near the convergence point of the control arms before the steering arm was also modified.

After the change the convergence point is ~14' to the right side of the tire and the virtual extensions of the three lines meet.

Another minor problem is that CMC originally mounted the tie rod swivels UPSIDE DOWN in the tapered holes in the steering arms!  This could have led to failure of the swivels.
stock top

This picture shows the aluminum top cap I made to correct the suspension but I want to show you how much the tie rod is angled towards the rear of the car at the rod end before my changes to the steering arm.

This is NOT the way a well designed steering system should look. The link angle causes additional Ackermann angle error in turns.

It was not possible to move the rack to the rear as it is up against the vertical frame tubes in the area. If the rack was moved to the rear of the vertical tubes, the rack ends would have an interference problem with the lower rear A arm mounting brackets.

Note:  This picture happened to be composed such that the camera angle is looking directly down the kingpin angle. The suspension actually has ~5.2 of caster.

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How I redesigned my suspension geometry (and things that needed to be taken into consideration)


 A.  Suspension design program used
I used the suspension program "Wishbone.exe" and a demo program of "Suspension_Analyzer v20_w98.exe" to redesign my suspension geometry. I basically redesigned the front suspension in Wishbone and then tweaked and finalized the design with Suspension Analyzer before the demo period ran out. Once you learn to use Wishbone it is easy to transition over to the superior Suspension Analyzer program.

I was able to devise a fairly simple method to correct the Ackermann angle, speed up my steering ratio, reduce the minimum turning radius and correct the bump steer problem by changing the steering arm.

B.   Speed up steering ratio
I wanted to speed up the steering ratio so 2 turns of the steering wheel angled the tires slightly more than the old 3-1/8 turns lock to lock. To do this I had to move the mounting point of the tie rod outer end ~1" closer to the "kingpin".

I also was also able to change the Ackermann angle from 14.7 to 13.9 at the same time. Not much of a change, but it was needed as the Ackermann was definitely wrong with the stock RX-7 steering arms because of the different wheelbase and track.

C.  Steering limiter
Since the steering wheel is still able to be turned 3-1/8 turns, the mounting bolt for the new tie rod end on the steering arm can contact the lower A arm a little over 1 turn of the steering wheel in either direction. This will require that steering limiters be incorporated into the system so 2 turns lock to lock is the maximum the steering wheel can be turned.

This can be done easily at the rack itself where the rack shaft exits the rack housing. I can do this by slipping short metal spacers over each side of the rack shaft. I will do this when I do the final assembly.

D.  Bump steer
The really bad bump steer presented a greater problem because I had to change the vertical angle of the tie rod so the extension line of the rod was correct in relation to the control arms. The extended lines of the top control arm, the lower control arm and the tie rod have to meet at a common point in virtual space with the car driving in a straight line.
If I were still in the building stage I would normally raise or lower the steering rack & pinion until the bump steer was correct. My car was already built as a roller, the rack was already mounted and couldn't be raised due to other considerations. I took the "think out of the box" method of design and lowered the outer end of the tie rod mounting point instead of raising the rack & pinion.
The correct angle for my car is to move the outer end of the link downwards ~1" from the original position. The tie rod now angles down towards the ground ~2 at the normal ride height. This new location is well below the original steering arm and would be located in mid air compared to where was originally located.

E.  Hint
If your rack is already the correct width for your chassis and you want to speed up the steering, "just" change the mounting point of the tie rod end while keeping the instantaneous center of the rod and control arms correct. The Stalker car is a notable example of this modification. You may have to move the fore/aft mounting point of the rack & pinion at the same time.

 F.  Other considerations
1. Rack width
My steering rack is too wide for the position it is mounted at. Normally this would cause bump steer even if the rest of the design was correct. By using the computer design programs I was able to find a solution that tailored the remaining bump steer to my advantage.

The general consensus when racing is that having a small amount of toe OUT helps the car "turn into" a turn. At the same time when taking a corner at speed it is normal for the chassis to roll toward the outside of the turn. And there might also be some be "dive" due to the brakes being applied.

By taking all these things into consideration I was able to have the suspension program optimize the design so that under normal straight ahead driving the car will have a small amount of toe IN (good for straight line tracking) and when cornering the suspension would assume a small amount of toe OUT (a less stable tracking condition but toe out induces better turn in). This portion of the design took me quite a bit of time to work out. At either 1.5" of bounce or dive the program predicts that the toe will change to 0.160" toe OUT. If I align the car with an initial small amount of toe IN I can then have toe IN change to some toe OUT in turns.

2.  Heim placement below new steering arm
The output dimensions that the suspension programs produce are related to the ground and the center axis of the chassis. If the Heim mounting bolt for the steering link is perpendicular to the ground at the end of the steering arm there is no particular problem in setting the height of the Heim from the ground.

But as most things in life are comprises, so is Heim placement in the real world. The placement of the Heim center gets complicated because of the kingpin angle (12) and the caster angle (~5.2) which causes the steering arm to be at a combination of those angles to the ground..

I induced another angle because wanted the new tie rod Heim  to be centered in it's angular movement range at normal ride height. This required that the new steering arm (in affect) be bent upwards so the Heim mounting bolt would be perpendicular to the tie rod at normal ride height.

Once the steering arm was (virtually) bent, the Heim mounting bolt hole had to be offset so the center of the Heim would be correctly located in space with the mounting bolt NOT perpendicular to the ground.  Oi vay, more Autocad time.

3.  Rod end and steering arm
The original tie rod end could not be used as it is so bulky that the if the rod end was mounted upside down the new steering arm would hit the lower A arm. If the rod end was mounted right side up, the rod end would hit the lower A arm.  (Scratch the original rod end.)

By using Autocad, I found that a 1/2" Heim rod end would work fine. Again, by using Autocad, I came up with a rather simple test steering arm made from 1/8" thick steel plate.
In retrospect, the 1/8" thick plate wasn't thick enough and it had a slight amount of flex while turning the steering wheel if the car wasn't moving. For testing I temporarily made 2/10" thick brackets that fit between the Heim mounting bolt and the original RX-7 tie rod end hole to brace the 1/8" plates.

These temporary brackets stopped the flexing of the 1/8" steering arms.
I've been asked why I didn't just make the steering arms out of 1/4" or thicker material. I wanted to cold bend the test arms to keep the material strength. I felt if I used thicker material that I wouldn't be able to make the tight radius bends down the length of the new arms without heating them.

I decided what I really needed for the final design was a bushing of some sort to be welded to the RX-7 steering arm under the new steering arm. This would provide a solid anchor for the Heim end of the steering arm.

After digging around in my "spare parts", I found a steel washing machine pulley that had an extended mounting boss on one side. The boss was 1-1/16" OD with a 1/2" diameter hole bored through the center.

I cut off two 1/2" thick pieces of the boss from the pulley to use as the support bushings. The bushings are mounted under the 1/8" test steering arms and were later welded to the original RX-7 steering arms. In affect the test steering arm became a fixture to position the support bushings for welding.

I have a friend who is a certified welder do the welding. He said right away that an ordinary welding rod would not do. He had some NI-99 rods that he said were the ones to use. If you decide to do/have any welding done on the steering arms etc talk to the welder and make sure that the proper rod is used.

After the bushings were welded, the 1/8" thick steering arm could be eliminated but I decided to use both the plates and the bushings. I like the idea of having a "belt AND suspenders" at this critical area of the steering.



Drawing of new steering arm.

Please note that this arm was designed for my particular suspension. Do NOT make an arm from this drawing for YOUR suspension unless you check out the design first. Most likely it will not be correct for other cars without some changes.


steering arm dwg

This drawing is presented to show what I had to do to correct MY Ackermann angle.

The upper 1/2" diameter holes are to allow the original mounting bolts to pass through the adapter to clamp it between the original steering arm and the spindle. The large hole centered between the mounting bolts is to clear the lower ball joint nut and carter key that holds  the ball joint to the RX-7 steering arm.

The  Ackermann angle (13.9) can be seen starting at the center of the ball joint nut (the kingpin angle passes through the center of the nut) and extends downward on the drawing 3.625" to where the tie rod Heim center should be. The 3.625" length is what determines the steering ratio (the original RX-7 length was 5.687").

If the plate were parallel to the ground when installed, the Heim center would normally be on a line extending perpendicular to the drawing located at the crossing point of the Ackermann line and the 3.625" radius line.

The steering arm plate was bent up 14 (12 kingpin inclination + ~2 tie rod angle) in the area where the Heim mounting bolt will be located. I did this so the Heim will be centered in its angular movement range at the normal ride height.

The small hole drawn 5/8" perpendicular to the straight ahead center line and on the circle radius, 3.625" from the center of the large hole is the correct place for the Heim mounting bolt with the plate bent upward. This offset allowed the center of the Heim to be correctly positioned in space by the bent plate.  Autocad is great for doing virtual things like this.  You have to trust the measurements you input to the drawing and the result that Autocad produces.




Making and installing the new test steering arms
arm templates

This is the progression of templates I made while checking the placement and fit of the test steering arm.

Left to right; cardboard, thin sheet metal and the first 1/8" plate steel arm.

making arms

This is how I held the two steering arms in alignment so they would be identical when they were finished.

When I was ready to do drilling or smoothing the edges I either bolted them together or used a grip pliers to hold them in position.
arm right

This is the partially finished right arm with the Heim bolt area bent up 14.

The small hole is one of the reference holes and was drilled out when the correct Heim mounting bolt offset position was drilled.
edge view

Here is a side view of the same arm.





arm end

End view of the bent new steering arm.

You can see that the Heim bolt mounting area is bent upwards and twisted to compensate for the three angles involved.

stock arm

This is the stock RX-7 steering arm before I "improved" it..

The lower ball joints are from a  VW Golf Mk1.


New-old steering arms

I've separated the ball joint from the RX-7 steering arm. I've also started grinding the RX-7 arm for Heim clearance and the arm support bushing (described later).
Finished assembly

This shows a test fit of the new arm in position on the RX-7 arm without the support bushing..



arm edge

End view of arm with axis of ball joint at the 12 kingpin angle and the slight upward angle of the new arm to center the Heim joint in its angular movement.
arm assembled

Top view of assembled arm. The large hole had to be made big enough to clear the nut and the carter key for assembly.

ruler

In this picture I have placed a ruler on the original Ackermann angle line to show the difference between it and the new angle.

The original RX-7 arm was designed for a different wheelbase and track. Therefore it was necessary to relocate the attachment point for the new Heim rod end.





link adapter

A tie rod adapter is needed to couple each 1/2" x 20 male thread of the Heim joints to the 14 x 1.5 mm male thread of the tie rods.

This is what's left of a 1991 Nissan GS? tie rod end after I sawed the 14 x 1.5 threaded portion off of it.

I centered and drilled a 29/64" diameter by 1-1/2" deep hole in the unthreaded end and threaded it for 1/2" x 20. Both threads are right hand.

These were the longest 14 mm threaded rod ends I could find at the junk yard (4-1/2").

spacer-pulley

This is the 2" pulley that I cut the two 1/2" thick support bushings from.

These bushings are used to create the mounting holes on the new steering arms for the 1/2" Heims at the ends of the steering links. Each bushing will be positioned by the test steering arm and then welded to the RX-7 steering arm.

spacer-end

The bushings are 1-1/16" OD x 1/2" ID x 1/2" thick. The right most piece is the remaining piece of the pulley.

The set screw hole won't interfere with the use of the bushing.




spacer on arm

This picture shows the bushing being held in the proper position just prior to welding to the RX-7 steering arm.

spacer arm end

Only the bushing itself will be welded to the RX-7 steering arm.  The set screw hole is next to the test steering arm.


arms -welded

The bushings are now welded to the original steering arms. I'm leaving the original steering arm end on the arm as a place to mount the wing stays if I decide to mount the wings ("fenders" to us colonialists).



arms finished

This picture shows how the Heim mounting bolt is placed in relation to the bushing and new steering arm.

I am going to use the new 1/8" thick steering arm AND the bushing. I like redundancy where it counts.

cones

You can get a few more degrees of Heim angular movement if you use what are called "cones" between each side of the Heim ball and the mounting bolt.

 I have modified the bolt on the left to act as a cone.

cone side view

This view shows the lip I machined on the 1/2" diameter grade 8 bolt to adapt it as a cone. The lip is 0.050" high and 0.620" in diameter.

I didn't change the small chamfered surface where the head of the bolt meets the shank.



cone upper close

This is the upper spacer/cone I made from a piece of thick 1/2" water pipe.

The length of the cone sets the vertical position of the Heim in space.

It's sure nice to have a lathe.

Heim parts

This is an exploded view of the parts that mount the Heim rod end.

I used a rather long bolt so the various parts could seat on the smooth shank.

The threaded portion of the Heim was shortened ~1/2" after this picture was taken.

The next step is to make the steering stops. I can use spacers on the actual rack shaft to act as limiters. The spacers will be inside the gaiters right where the shaft emerges from the rack body.

Basically I will use short tubes (or if you like the sound of it, thick washers) that will fit over the rack shaft to limit the maximum left-right movement of the rack.

See the article "Rack and pinion steering limiter" for details of that modification.
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Pictures of front suspension after the changes.
front view

Notice that the tie rod angle is now approximately parallel to the ground and that the extension lines from the upper & lower "A" arms and the steering tie rod now converge to the right of the picture..

The new steering arm is sandwiched between the lower ball joint adapter and the lower end of the spindle. It is the shiny flat area about 1" above the lower ball joint.

top view

You can clearly see how much the rod end mounting point has been moved from the original location in this picture. The original mount is the open hole on the end of the RX-7 steering arm.

While the tie rod still has more rearward angle than I would like, it is much better than the original RX-7 angle.




     So how does the car handle with all these wonderful changes?

Well for starters, bump steer affects when hitting my favorite manhole cover are gone.

Now when I bounce the front end up and down and I can see NO bump steer change with "stringing the car".

I can only move the front end a total of ~5/8" up/down by hand and at that distance the suspension program indicates that the toe change should only be 0.030". That's a GREAT improvement over the 5/8" measured toe change I had before.

Before when I entered a turn at a pretty good clip the car felt "heavy" and I had to keep a good bit of pressure on the steering wheel to maintain the turn. Now as you start the turn the car seems to turn in by itself. The pressure to maintain the turn is also lighter. As good as the car felt before, it is much better now.

The steering is definitely quicker! As a result the steering is a little heavier than before (this was expected). I ended up with the designed two turns lock to lock of the steering wheel. And I don't have to wind my arms up to turn corners anymore. A 90 street corner that used to take a 3/4 turn of the steering wheel now takes 1/2 turn.

The turning radius has decreased (also expected). Before when pulling out of my driveway and turning right onto the street, the car would end up in the far left lane of a suburban two lane street and I would then continue the turn until the car was in the right lane. Now I make the turn directly onto the right lane.

The affect of any Ackermann angle change is not as easy to detect. The main thing I noticed before was that when hand pushing the car in a tight turn that it was -very- hard for one person to move the car (this might also be an affect of the LSD rear end). It seems that the car is easier to push now (but it may just be wishful thinking).  My riding lawn mower doesn't push the front end after I corrected it's Ackermann years ago.

I will check the Ackermann as soon as I find a lightly sand covered area. An old steering geometry test a real old timer taught me along with how to "feel the tires" to check toe in. Thanks Paul, wherever you are.

One of my lower ball joints is very tight (both joints are new) and I think this is the main reason why my car has no real self centering when exiting corners. With the camber and bump steer changes I can feel a very slight hint of self centering. I think once the parts wear in more that I will have some self centering affect.

All in all, I'd say the suspension changes were a great success!



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Other articles in this series,
Go to article, "Rack and pinion steering limiter"

Go to article, "Correcting CMC designed steering geometry, camber"