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Street Race Truck More... The Toolbox

Understanding Shelby/Arning Drop Changes

12/6/2017

Every car enthusiast I’ve ever met can agree on one thing. Old cars don’t handle very well. It’s pretty easy to see from the small hollow sway bars to the odd steering / suspension geometry. Now for the time this was quite advanced but let’s be honest, the technology at the time was limited. Now prior to the late 60s the auto manufacturers were all focused on one thing when it came to suspension, comfort. That was the selling point that most car makers tried to push, that their ride was the best ride. Then along came the great muscle car era, the late 60s. Auto manufacturers could tell that the consumers wanted something different then your cozy little Sunday cruiser they wanted power and good looks too. So, designs changed the cars got faster and much better looking, according to my standards, but a lot of these vehicles still handled like a 30-year-old bus. Then along came a man whose name quickly became associated with fast fords, Carroll Shelby. Carroll saw that handling was lacking and this wouldn’t do. So, he turned to one of the engineers at Ford who had come up with an idea on how to help this issue, Klaus Arning. Klaus’s worked with Carroll on the GT350’s and GT500’s specifically on the suspension, he should Carroll his idea the so-called Shelby / Arning drop. This idea was simple yet effective. Lower the upper control arm and bring it rearward. This novel idea did three things, it lowered the vehicles center of gravity, it reduced body roll by nearly 10% and it greatly improved the camber curve of the vehicles. Now that this brief little history lesson is over let’s get into the theory behind this modification.

How the Shelby / Arning drop changes depends on the year of vehicle, 64-66 Mustangs, (this also would cover 60-66 Falcons and Comets), lowered the mounting hole for the control arm 1” in a perpendicular line to the original holes and moved them rearward by 1/8” on that same line. Word of caution make sure that the holes are always moved 1/8” rearward

The 67-70 Mustang, (and 67-70 Falcons, Comets, and Cougars), are only lowering the control arm mounting holes by 1”.

In both cases you will want to drill the new mounting holes out to 17/32nds. The area around where the mounting hole sit are some the strongest, and thickest of the metal in the vehicle. Having said that it is suggested that you start small and work your way up to the 17/32nds in 1/32nds increments if possible. There are several places to find templates for doing this process. There are lots of online companies and individuals that offer metal templates that bolt in location of the control arm and give you pilot holes to start drilling the new mounting holes. You can also search online to find paper templates that can be printed off and taped on the shock tower. I personally suggest getting a metal template they’re often very cheap and it takes away the guess work.

Now let’s talk about what this process is actually doing. Firstly, the center of gravity is lowered, since you are lowering the upper control arm by 1” the lower spring perch is also being lowered by 1” and thus your front end is being lowered by roughly 1”. Considering that almost 1/3 of the vehicles weight rests purely in the engine bay then this is going to effectively lower the center of gravity make the car more stable, especially around turns.

Secondly, reducing body roll, anyone who has driven an older car has experienced body roll and often time it’s enough to slide you across the seat, (if you have a bench seat that is), now some of this on the mustangs was caused by their unibody constructions where the body also acts as the frame, this design usually allows more twist and flex than your normal body on frame construction. Now by lowering the control arms your reducing the amount of leverage the upper control arms have on the shock towers which reduces the flex of the body lessening the amount of body roll.

Lastly, and most importantly, the improved camber curve. For those not in the know, camber is the inward or outward tilt of the top of the tire when looking at the face of the wheel. Moving the top of the tire in towards the car is called negative camber and moving it outward away from the car is called positive camber. The more negative camber the more tire that is actually making contact with the road. In a similar fashion during normal suspension travel a vehicle can have the top of the tire move either away from the car, (positive camber curve), or towards the car (negative camber curve). Stock the mustangs and their likewise based suspension brethren have a positive camber curve. So, when you hit a bump or the suspension moves upward the control arms actually moves the tire outwards reducing negative camber and causing less contact with the road, reducing handling. After doing the drop the camber curve has changed now the upper control arm brings the tire inward increasing negative camber keeping more tire on the road and increasing handling. Another added benefit of the drop is the flattening of the arc for the camber curve changing the camber less throughout the suspension travel keeping it closer to the specification it was aligned to.

Word to the wise, while this modification does have many positives one should be aware that it also has some pitfalls. Biggest one being that by lowering the control arm you change the angle that the ball joint normally sits at, this usually isn’t too much of an issue but during hard turning it is possible to “bottom out” the travel of the joint and breaking the stud is possible. It should also be noted that the control arm now runs a higher risk of making contact with the bump stop for the front suspension. It seems counter intuitive that lowering the control arms would make it move closer to the bump stop but it’s all due to the angle the upper control arm sits at under new “normal operation”. In this figure the black and red represent how the control arm normally sits and where it moves to under normal travel black being static and red being the top of travel. While red and blue represent the control arm after the drop with red being static and blue being the top of travel. This change should be understood when performing this modification as it could potentially cause issues.

You may be asking yourself does this change my alignment? Yes, it most definitely does. There are new specifications suggested when doing this swap.

These handy little cards are perfect for taking to your alignment shop and given them all the specs that you need to make sure your vehicle is aligned to give the optimum handling.

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