Modifying the CMC designed suspension to correct camber change

Last update:  7/5/08

Other articles in this series,
Go to article, "Rack and pinion steering limiter"
Go to article, "Correcting CMC designed steering geometry, bump steer"

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Legal notice: (The SMA stuff.)

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 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 car 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 the point where the tube was cut. A 1" long machined "cap" was inserted into the top of the tube. The cap has a 1/8" thick flange which rests on the top of the cut off spindle. The flange is 2" in diameter. The lower edge of the flange is welded to the spindle tube. The center of the cap is threaded to accept the  5/8"-18 thread for the studded Heim rod end which was used 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!

I have documented how I corrected bump steer in another article, "Correcting CMC designed steering geometry, bump steer". Another article documents how I reduced the steering wheel lock to lock turns from 3 turns to two turns, "Rack and pinion steering limiter".

These three articles are a complete description of the modifications I made to correct my suspension geometry.



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

Heim close up

Notice that much of the Heim rod end movement is already used before the changes. After the changes the Heim is not angled as much.


Problem

Unfortunately the mounting points of the upper and lower ball joints in conjunction with the inner mounting points of the upper and lower "A" arms resulted in badly designed front suspension for camber correction in dive and bounce.

An even worse problem is that by using the stock RX-7 steering arms, with a different track and wheelbase and the placement and width of the rack & pinion resulted in VERY bad bump steer and incorrect Ackermann affect.

I have a fairly simple method to correct the bump steer, Ackermann and to also speed up my steering ratio. I have documented those changes in another article "Modifying the CMC designed suspension to correct bump steer".



Do you really need to have the suspension geometry correct?

The front suspension design is probably the most important and complicated part of building your own car. In affect you ought to design the front suspension first and build the rest of the car around it.  There are several methods you can use to have a correct suspension layout. Here are a few suggestions;

1. Buy a chassis that is known to be correct. Not low cost though.
2. Pick a donor car with truly good suspension and build your frame around the parts.
3. Design your own suspension using computer CAD software etc.
4. Copy another Locost that has correct suspension geometry.

Unless you use the exact same components, dimensions, layout and design of another car that has correct geometry you are just taking a crap shot at having your suspension ending up being truly "correct".

Truthfully, even though I could see that my bump steer and camber change was all wrong, I really didn't notice much adverse affect when driving the car around town.  If I was racing the car it would have NOT have been a well handling car.  I pushed it one time to about 8/10s around a corner and it felt like the outside front tire was folding under.

The only indication that bump steer was a problem in town was while cornering hard and hitting a bump. The car would dart slightly left or right as you hit the bump. Of course this is one classic results of bump steer. A hidden problem is that tire wear can be excessive with an incorrectly designed suspension.

You just can't/shouldn't push a Locost anywhere near it's limit on normal city streets. The more you push the limit, the more troublesome bump steer and handling in general will become.

I decided that as long as I have put all this effort into building the car I wanted my suspension to be correct.



So how do you correct the suspension geometry?

I used the suspension program "Wishbone.exe" and a demo program called "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.

At first I took the normal line of thinking and found that I needed to lower the upper A arm chassis mounting points to induce negative camber change during dive (when the brakes would be applied on entering a turn) and to slightly raise the roll center. I found that if I lowered the inner mounting points ~1" that I could obtain the desired camber and roll center change.

After looking at my chassis and considering the work involved in moving the upper A arm mounting brackets etc I went back to the suspension programs and tried raising the upper ball joint instead of lowering the inner A arm mounts. The result was the same AND it didn't involve rebuilding the mounts! I liked that idea. Maximum affect with minimum change.



Before and after pictures of front suspension change.
Camber arms

This is the stock CMC front suspension using a studded Heim rod end for the upper ball joint. The jack is not supporting the car in this picture. Note that the lower arm is angled downward ~5 and the upper A arm is also angled downward at ~7.

The front swing arm crossing point of the A arms was ~14' towards the left of the tire. The roll center was ~1/4" below the ground.

Notice that the steering tie rod angle does NOT intersect the instantaneous A arm crossing point. This causes the extreme bump steer.

To say the least; this suspension was not thought out completely.   It was a "that's good enough".

Camber finished

This picture is of the front suspension after the modification to raise the new Heim joint.

Note that the lower A arm is still angled downward ~5 and that the upper A arm is now angled upward ~4-1/2 from the center plane of the car.

The front swing arm crossing point of the A arms is now ~14' towards the right of the tire. The roll center is now ~2-1/4" above the ground.

This exercise illustrates why it is best to do the suspension design before you modify your donor car front suspension or finalize the front of the car.  It's much easier to make it right to start with.

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Adapter parts to correct camber compensation

By using the suspension CAD program I found that I needed to raise the center of the pivot point of the upper Heim rod end 1" from the stock position (a total of 2" above the spindle cap). Here is the drawing I made for the spindle adapter to raise the "A" arm Heim rod end.

This drawing is presented to show what I had to do to correct MY suspension geometry.  

PLEASE don't copy it thinking it will correct your suspension unless you have a CMC built roller.  And even then it might not help.  :-(




Adapters top-bottom

These are the two adapters I machined from aluminum. The 6" ruler doesn't show up to well in this image. Click on the picture for a full size image.

I made the adapters from a large sail boat boom end. Do you know why they call that sailboat item a "boom"? That's the noise you'll hear when it hits your head if you don't duck while tacking.


Camber adapter top close

This view shows the top of the adapter with the 0.030" deep x 1" diameter recess to hold the Heim cone fitting in place.

Camber adapter bottom

This view shows the bottom of the adapter with the 0.125" deep x 2.015" diameter recess to position the adapter on top of the spindle flange.

Camber parts

These are the new parts that replace the studded Heim rod end.

The hardened bolt and the Heim are 5/8" x 18 threaded, the two cones are used to allow the Heim to have more angular movement. The Lock washer is used to retain the assembly.
Camber parts assembled

Here is what all the parts look like as they will be assembled to the top of the spindle.

I may re-machine the adapter later so that it is tapered towards the top to lighten it.

Use a Grade 8 bolt.
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How to assemble the new parts to raise the upper Heim joint.
Camber swivel unscrewed

The first step is to block the rear tires and jack up the lower A arm so the tire just clears the floor. Unscrew the Heim joint from the spindle.

Warning: Make -sure- the tire is off the floor so the upper A arm has no suspension forces acting on it. Occasionally check that the tire is still clear of the floor.

Camber stock CMC swivel

This is what the studded Heim joint looks like when it is removed from the upper A arm.

The cap is through threaded internally (about 1" deep) to accept the 5/8" x 18 stud..




Adapter w/spindle top

This view shows the top of the spindle flange and the adapter just before it was placed on top of the flange. I scraped all the paint etc off the top of the flange.

I spread a thin layer of marine grease on the adapter mating surface to inhibit corrosion.


Finished assembly

The lock nut was threaded onto the Heim rod end and the Heim was threaded into the A arm approximately where it should be for 0 camber.

The rest of the parts were assembled onto the 5/8" bolt and the bolt was started into the spindle cap only a few turns. You will have to move the spindle around as you try to thread the bolt into the cap.
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How I set the camber.
Place a jack under the lower "A" arm shock plate as close as you can get it to the tire. Raise the tire so it is just clear of the floor. You don't want the tire to be lifted off the floor more than 1/4". This is so the lower A arm is at the approximate ride height angle. My suspension is stiff enough that raising the tire off the floor didn't change the lower A arm angle.

1. Once you have the bolt started into the spindle cap, push down on the A arm so the adapter parts are all in contact with each other.

2. Measure the camber. If it is not what you want, unscrew the 5/8" bolt, lift up on the A arm and screw the Heim into or out of the A arm to correct the camber.

3. Reassemble the parts onto the 5/8" bolt and screw the bolt a few turns into the cap.

4. Repeat steps 1 through 4 until camber reading is what you want.

5. Tighten the Heim rod end lock nut and torque the 5/8" bolt to ~70 ft. lbs.

I then lowered the tire to the floor, roll the car back and forth a few times to settle the suspension and rechecked the camber setting.

I set my camber to 0 to -1/2.  I  used a Harbor Freight "Angle Finder" to set the camber.