Please note, I originally wrote these posts for the blog at Cardomain.com. I just noticed today that the blog is long gone. Luckily, I found a partner website that still had them so I’m archiving them here.
Last week I showed the design and build of my subframe. This week I’m going to show the design and build of my control arms. This is without a doubt, the sexiest part of any suspension. It’s also the most difficult to get right. This is the difference between a fun car to drive and something more violent.
Before we get started, I want to encourage you again to read Chassis Engineering by Herb Adams. I’m going to be referring to it off and on in this post. There are terms that I’m going to use that are defined in that book. If you don’t have the book, Google is your friend. If you have questions, go ahead and put them in the comments section and I’ll try to answer them.
First, let’s talk about the design of the lower control arms (LCA). The LCAs will locate where the wheels are and the track width. They will also support the entire weight of the car, plus any shock loading. Braking forces are also taken up in the lower arms. They need to be strong. I started the design of the LCAs with .75″ rod ends. I chose this size at the recommendation of a friend. I figured if the stock leaf springs used 9/16″ bolts to support the car, .75″ rod ends should be more than enough to support the car. I bought mine through McMaster-Carr. These can get very pricey, and when you’re buying six, they add up quickly.
Next, I knew I needed a way to align the suspension once it was installed. I needed some sort of adjusting nut with right and left hand threads. I found these from QA1, part number: ADJ12-12. They were a little pricey and their chrome flaked off easily, so there might be something better. Next, I knew that I needed a threaded bung to weld into the tubing of the arm. Most of these were in the $12 range for one. Multiply that by six and they add up fast. Enter A&A Manufacturing. They sold their 3/4″ bungs for about $5. This was the best deal I could find, and they were good pieces too. These bungs dictated that I use tubing with an inner diameter of 1.00″. I did some basic bending stress calculations, and found that .25 wall tubing provided a safety factor of seven. (This didn’t take into account shock loading, so I picked the high safety factor to be, well, safe.) I’m being a bit vague here I know. But it’s good to verify the strength of your materials so things don’t break. Get someone who knows what they’re doing (ie: mechanical engineer) to check for you. So I picked 1.00″ ID, .25″ wall seamless tubing. The last bit I needed for the LCAs was brackets to attach to the Thunderbird knuckles. I used .25 thick steel and had a local shop bend them up for me. These are seen below.
After I picked my material and hardware, it was time to layout my LCAs. Remember when I marked the center of the wheel on the quarter panel? This is where it comes into play. I used that to locate the hub/knuckle assembly. This told me where the brackets for the LCAs needed to be. I laid these out on the computer in Solidworks. Take note that I designed the LCAs to be level with the ground to simplify the design. Once I had these laid out, it allowed me to build a jig. The jig I built is shown below. It allowed me to control the important dimensions of the arms (brackets and threaded bungs).
Experienced fabricators will probably laugh at this jig. That’s okay, I learned my lesson. I used the jig to tack everything together, but once I welded it up solid, it warped some. Next time I do this, I’ll make a jig that solidly clamps everything down tight! The warpage wasn’t enough to worry about, however.
A tool that I invested in to make the tubing fitment easier was this tubing notcher. It was just a cheap one off ebay, but it worked well for what I was doing.
Here’s a picture of the LCA attached to the knuckle. Can you see the problem? I didn’t for a while. I’ll give you a clue: look where the halfshaft will mesh with the hub.
That’s right! The CV joint interfered with my tubing! So, off came those brackets. I designed another pair that angled up, to move the arm down and away from the CV joint. After I got these back from the fab shop, I learned another lesson. Stress concentrations are a pain in the butt! I’d designed the brackets with a sharp corner, and the material cracked at that point when it was bent. See below.
Next time, I’ll include a radius in those corners. I ground down and opened up the cracks on all the brackets, then welded them back up.
The picture below shows the new brackets on the arm. Take note the plate I welded on top of the tubing. This is for the spring mount. I knew that it would end up somewhere in that area, I just didn’t know where.
And now to the last piece of the puzzle: the spring mount. I finally decided to attach it right up against the bracket. This reinforced the bracket in a critical area. The weight of the car is on this point. Also, keeping it as far outboard as possible makes the springs more effective and takes stress off the LCA. See the picture below.
Now, on to the upper control arm (UCA). I admit the UCA is not as cool looking. But its function is vital to handling. The UCA helps determine the roll center height, swing arm length and camber gain. This is an unequal length suspension design. This means that as the suspension compresses, the top of the tire is pulled in, making negative camber. Here’s why this is important: tires are most effective when they’re perpendicular to the ground. In a corner, the car’s body will roll, making the tire tilt outwards. This loses cornering power. The suspension needs to compensate and keep the tire perpendicular. Thus it needs to gain negative camber as it compresses. (This is very simplified. If you haven’t got the clue yet, read the book!)
The basic design of the UCA is just a bracket with tubing and a threaded bung. I added a slight bend to it to help clear the saddle above it. See the picture below.
Here’s how I figured my geometry. Herb Adams recommends a UCA length between 50% to 80% of the lower arm. That meant that my UCAs needed to be between 11.00″ and 7.00″ long. The next piece of the puzzle is the angle that the UCA made with the horizontal. This is where I got all geek. I drew a simple diagram of the links, with the angle as a variable. I used this to write an equation that solved for swing arm length based on that angle. Adams recommends swing arm lengths between 100″ and 150″. Using my equation, I solved for the angles in that range. Going in one degree increments, 5, 4 and 3 degrees to the horizontal satisfied the swing arm length requirement.
So, this is quite a spread. What exact length and angle should I choose? The last piece of this equation is the roll center. Adams recommends a roll center of 1.00″ below the ground to 3.00″ above the ground. I put all the possibilities in a spread sheet and started solving for roll center. The winner would have a roll center in this range and the roll center wouldn’t move around much throughout the travel.
I decided on a UCA length of 11.00″ and to attach it at 3 degrees to the horizontal. This gave a roll center of 1.74″ above the ground, camber gain at full bump of -2.61 degrees and a swing arm length of 148.88″. If none of this made sense to you, read the book!
Below are a couple pictures of the suspension arms installed on the car.
To conclude this post, I’d like to talk about what I learned and what I’d do differently. First of all, like I talked about in the subframe post, I’d like to model this entire system in the computer. That would allow me to check for full suspension travel, geometry and interferences. Second, I’d do my LCA jig differently with plenty of clamps. Third, I’d add a radius to my brackets to get rid of that stress concentration.
I think I nailed the geometry, but this is my first time doing this. I’m sure that more experienced racers and builders have their own ideas on geometry and it also varies from car to car. I’m open to suggestions. If you have any questions, leave it in the comment section. But read the book first! (but don’t take my word for it!)
Join me next time for fun with halfshafts and all the other annoying bits that make a car go and stop.
Mechanical Engineer, Mopar guy, reluctant defender of the universe.
David Belau 
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