Front Suspension

The geometry of the E.R.A. front suspension gives good anti-dive, excellent camber control, and near-zero bump steer.  The roll center height is designed to complement the rear suspension, giving handling characterists that are predictable and safe, or as safe as any car with this much power and a 90" wheelbase!

"Bump steer" is the toe-in change that the front or rear tire takes as it travels up or down. With poor bump-steer characteristics, a car will dart from side to side over bumpy roads and have potentially dangerous transient reactions.

Through careful design of the E.R.A. 427SC geometry, bump-steer is near 0 over the full range of the wheel travel.  (More details on the Longacre web site) 

fsusp

And - our wheel travel is extremely generous::  More than 3" in both jounce and rebound directions so that you can lower or raise the car somewhat without worrying about running out of travel.

The car goes where you point it, even under poor road conditions.

The modified Flaming River rack and pinion steering gear has 3.1 turns lock-to-lock, with a 34 foot turning circle.. Effort, even without power assist is fairly light, and the steering has plenty of feel.  Custom fabricated tie rods and lower steering column are also supplied with the kit.  The optional E.R.A. steering column and wood steering wheel are visual duplicates of the original parts. E.R.A. makes the upper and lower control arms, and we supply a lightweight steering knuckle for quicker response.

Real front suspension

Modified GM uprights, new upper control arm cross-shafts, ball-joints, bushings and fasteners are all included in the standard kit, ready to accept very large vented brakes on stock or optional pin drive hubs.  A large anti-sway bar is optional.

E.R.A. makes the upper and lower control arms, and modifies the included GM steering knuckle for quicker response.

Here you can also see the optional anti-sway bar and optional double-adjustable coil-over-damper.  Yes, even damping can be changed with a screwdriver without removing the unit.

We have duplicated the original car's "X" member and coil-over damper system to complete the correct "look".  Front coil-over shocks are mounted to the X just like the original Cobra. We recommend our custom made Spax units with external adjustments for both damping and height, but you can use any brand that meets the proper specifications.


A Little Independent Front Suspension History

At the time of the "birth" of independent front suspension, it was conventional wisdom that the contact patch must travel in a straight vertical line as the car encountered bumps.  It made some sense back then, when it was not unusual to drive on very uneven dirt roads.

That movement constraint required that the roll center had to be at ground level. The shorter upper arm derives from the time when control arms were almost always parallel to the ground. With narrow tires and high center of gravity, the angle of the contact patch wasn't that important with narrow tires anyway.

Front suspension

Classic SLA design

Both the upper and lower arms are horizontal with the car at normal ride height.

Modern unequal-length, non-parallel
arm design

The arms may not be horizontal at ride height.

So... the short/long control arm design was created to keep the position of the contact patch in a straight vertical line under bump and rebound. The tire tilts in going up and also tilts in going down.

However!  If you want to see some "interesting" contact patch movement, analyze a McPherson/Chapman strut.  Some excellent cars use struts.  The best designs compensate by using very long control arms to control roll center movement.


At any rate, the (optimal) object is to keep the roll center at a constant height, the tire vertical under bump and vertical when the car rolls.  That Can't Happen in the real world.


Smooth roads and wide tires have now changed what we look for in suspension geometry.  We are more concerned with keeping the tires vertical under roll.  Consequently, the upper and lower control arms have become non-parallel to keep the inside tire rotating (in front view) as it goes down.  That's how we ended up with the second design.

 2011, Era Replica Automobiles