But would it be as stiff as a 620lb spring rate? Or kind of an in between stock and stiff?

 

The lightly modified Mustang suspension was good enough for Shelby to run full race coarses hard and often and it will be/is more then enough then what the average person here will ever put it through.
Lynn

 

I would do the "Arning" drop (he invented it, not Shelby) and stock-height GT spring, which should set you about 5/8" lower than factory. Any lower than that, and you better be driving on really smooth roads. Around here, forget it.

Arning/Shelby Suspension Drop

 

What do you do for that drop??
Lynn

 

The Arning or Shelby drop is where you drill upper control arm mounting holes 1" lower(you can go more than 1 if you re align the ball joints with wedge shims, but I've heard that's a real pia) to change the suspension geometry. Without changing anything else at all, the Shelby/Arning drop will have the single biggest improvement in handling performance, Fords factory geometry sucked.

And yes, most people will never push their car hard enough often enough to be able to justify the cost of converting the suspension to tubular. Boxing the arms is quite inexpensive though, and very effective.

Cutting coils will increase the spring rate, but can also decrease the springs life.

And sway bar changes have more effect than just controlling body roll. A larger sway bar decreases body roll on the end it's on(front or rear) and also acts to effectively increase spring rate under cornering to the outside wheel, since the compressive force is transferred through the sway bar to the other side and attempts to compress the inboard spring as well, in effect "sharing" some of the spring rate from the inside wheel. Sway bar sizing from front to rear also effects the suspension balance front to rear. A larger sway bar on one end relative to the other effectively decreases traction on that end(relative to the other)....in other words a larger sway bar decreases traction relative to the other end and a smaller sway bar increases traction. For example, adding a larger sway bar to the front causes the rear to "gain traction"(or the front to lose traction, whichever way you want to look at it), and adding a larger sway bar to the rear causes the front to "gain traction"(or the rear to lose traction). Basically, a simple way to think of it is that a smaller sway bar makes one end "softer," and the softer end will "dig in" to the road better(a VAST over simplification, but it helps to illustrate the effect).

As a general rule of thumb...

A larger sway bar in the front will help to correct an oversteer situation, from anything such as rear ballast, or a heavier fuel load with a rear fuel tank, or anything else that may imbalance the suspension or weight transfer causing oversteer. A smaller front sway bar reduces understeer situations, such as from a heavier front engine(big block cars are especially notorious for this). A larger rear sway bar corrects an understeer situation, and a smaller rear sway bar corrects and oversteer situation.

The actual physics involved deals with changing the proportion(front to rear) of total roll stiffness of the vehicle, which alters the slip angle of the front and rear outside wheels, affecting how the car wants to steer....in case you're curious.

Most of the time with Classic Mustang suspension that's stiffer than factory, it's balanced such that a single decent size front sway bar is used(7/8" or 1"), as the rear is already pretty stiff and light as it is. But, as you get into a more aggressive front end from even stiffer shocks and/or springs, or a larger sway bar, you need to start adding rear sway bar to balance the rear to the front(or reduce the size of the front sway bar). Sway bar sizing front vs. rear alters understeer or oversteer, the actual size of the bar itself alters roll stiffness(or body roll as most people understand it).

Keep in mind that the larger the sway bars, the less suspension independence and the rougher the ride.

 

OK, I didn't realize the Arning drop was the same as the Shelby drop, I never heard it called that before.
Thanks for the sway bar info.

Lynn

 

Klaus Arning designed an independent rear suspension as a performance option for the Mustang. To match the operation of the IRS, he modified the front by relocating the upper arms 1" lower. The IRS proved too costly to be a viable option, and the idea was dropped. The front end modification proved to be a big part of the improvement, and had been observed by Shelby American, who incorporated it in all 65 GT350's and those 66's that were built from leftover 65's.

Frankly I think Ford screwed up big by not simply incorporating the change in all Mustangs, this one simple change would have greatly enhanced the car's performance reputation.

Arning/Shelby Suspension Drop

 

Absolutely. Instead it ended up getting a reputation as one of the worst handling American cars of the mid-late 60's. And it would have been a nearly costless change, since they still had to drill 2 holes, just 1" lower. Maybe some equipment modification, or new jigs. Fords crappy geometry has been the culprit behind many needlessly destroyed front tires as well.