Most people respond with a look of horror when I talk about 800 pound or 1100 pound springs on the front of an FFR Cobra. "But that will make it too stiff to drive on the street, won't it?"
Not at all. What affects your car's ride and handling is the spring rate at the wheel and tire, as well as the rate of damping from the shock absorbers and the overall amount of roll resistance. All of these are impacted by something called the motion ratio.
The spring rate at the wheel and tire is called the wheel rate. The wheel rate is the product of the rate of the spring and the motion ratio:
Wheel Rate = Spring Rate x Motion Ratio
Motion ratio is a way of referring to the leverage the spring has when installed in a particular suspension geometry.Motion ratio affects both spring and shock rates, as well as the effectiveness of the anti-roll bar. The lower the motion ratio that a given spring is working with, the lower the wheel rate will be.
In the case of the FFR Cobra replicas, the stock springs supplied with the kit, installed in the FFR suspension geometry, give a front wheel rate that is much too low. Going to 800 lb springs just brings the front wheel rate into line with the rear wheel rate (if you leave the stock FFR springs in the rear).
How can this be? Read on, dear reader!
The motion ratio is used to calculate the wheel rate given the rate of the spring and given the suspension geometry of that particular installation.
There are two ways to measure the motion ratio: at the ball joint and at the tire contact patch. It's much easier to measure it at the ball joint, because you just measure the lower control arm and the location of the spring/shock mount relative to the length of the arm.
However, with this method there is considerable opportunity for error, especially if the geometry happens to yield significant camber gain.
Doug's method is to measure the motion ratio at the tire contact patch. This is much more accurate. In the case of the FFR Cobra, it's critical, because the front suspension gives large camber change and the rear suspension relatively little.
Measuring the motion ratio at the ball joint on our FFR Cobra, which has stock '91 Mustang spindles, suggests that the front and rear springs (and shocks) should be about the same rate, which is what FFR supplies (450 lb front and rear).
However, measuring the motion ratio at the contact patch - which is what the tire actually sees - shows a wheel rate of about 140 lb for the front and for the rear, using the stock 450 lb springs. This is quite soft for a race car, and relatively soft even for a sporting street car.
After some research, we realized that - for the street - the front springs could be as much as 800 lb. This gives a wheel rate of 250 at the front, which is just about equal to the rear wheel rate in the live axle version of the FFR Cobra with stock 250 lb springs.
Based on our original motion ratio measurements, we ran 800 pound springs on the front and 500 pound springs on the rear for much of the summer.
After watching the video of the car on the track in October at Loudon, we felt the rear seemed to be too soft relative to the front, so we decided to re-measure the motion ratio at the rear. This time we got a number equal to what we got for the front.
After that event, we took the 800 lb springs out of the front and put them in the rear, and put 1000 lb springs in the front. This made the car much better balanced, with less understeer, and also helped control roll and pitch better.
However, with the stiffer rear springs, ride is somewhat compromised. It's probably a bit firmer now than most people would want for driving on the street. For a street-only car, I'd probably try around 700-800 in the front and 550-650 in the rear. (I'd keep our anti-roll bars at 320 front and 110 rear.)
Since our car has become primarily a track car, we may eventually go to 1100 lb on the front and 900 on the rear, to get the wheel rates Doug initially calculated we should be using.
Note: I've updated the Spring and Shock Rate Spreadsheet to reflect the new rear motion ratio measurement.
With 800 or 1000 lb springs, you need shocks that are over double the stiffness of the Pro Shocks that come with the kit. According to their Web site, Pro Shocks' small body shocks can't be valved anywhere near that stiff. So at least for the front, something with more oomph is needed.
Gary Cheney is trying a set of 2" body Pro Shocks, valved as stiff as Pro Shocks will make them. See his site for his thoughts on these once he gets his Daytona Coupe on the road.
Ok, so you still don't believe me. Do the numbers yourself.
Read my Measuring the Motion Ratio page, download my Spring and Shock Rate Spreadsheet, get out your jack, your shock wrench, and your tape measure, and get busy!