To establish the numerator...
To establish the numerator in Chassisworks' spring rate formula, the difference between the spring's free length (F) and loaded length (L) must be multiplied by the baseline spring rate (R). Most manufacturers print the rate of a spring right onto the coils.
With all the foreplay out of the way, it's time to finally get into some number crunching. Calculating the optimal spring rate for any car can be expressed in the following equation, where "F" is the free length of the spring, "L" is the loaded length of the spring, "R" is the baseline spring rate, and "T" is how much the shock collapses at ride height.
| (F - L) R |
= |
Spring Rate |
| T |
|
|
For the sake of illustration, let's presume that a 500 lb/in spring (R) measuring 8.94 inches (F) compresses down to 6.50 inches (L) at ride height. The difference between the two spring height figures is 2.44 inches, which yields a product of 1,220 when multiplied by the spring rate of 500 lb/in. When matched with a shock featuring 4.25 inches of travel, setting it up for a 50/50 cornering application requires the shock to collapse 2.13 inches (T) at ride height. Finally, dividing 1,220 by 2.13 results in a quotient of 572.77, which is the ideal spring rate for this application.
The ideal amount a shock should...
The ideal amount a shock should collapse at ride height is contingent upon the total travel of the shock used and its intended application. For example, let's presume you're running shocks with 4 inches of travel in a street/strip setup (40/60) where more rebound is desirable. In this case, biasing 60 percent of the total shock travel involves collapsing the shock 2.4 inches at ride height. This results in 60 percent of travel (2.4 inches) reserved for rebound and 40 percent (1.6 inches) of travel reserved for compression. Consequently, 2.4 should be used as the "T" value in the spring rate formula.
In this example, the baseline spring rate of 500 lb/in was too soft, which resulted in a shock that collapsed more than the desired amount at ride height (2.44 inches). Not only would this spring move the shock outside its sweet spot, it would also bias its travel too far toward rebound, not leaving enough travel for optimum compression valving and potentially degrading ride quality. Although it may seem trivial, the extra 73 lb/in as calculated in Chassisworks' formula would effectively decrease the amount the shock collapses at ride height to hit the target sweet spot of 2.13 inches. That yields an equal amount of shock travel in each direction for compression and rebound (50/50), which is perfect for many road course or autocross setups.
| SHOCK TRAVEL USED AT RIDE HEIGHT (COILOVERS) |
| Coilover |
Street |
Handling |
Drag |
Spring |
Street |
Handling |
Drag |
| Shock |
60/40 |
50/50 |
40/60 |
Free |
Max. |
Center |
Min. |
| Travel |
(T) Spring Travel Used At Ride Height |
Length |
Installed Spring Length At Ride Height |
| 2.80 |
1.12 |
1.40 |
1.68 |
7 |
5.88 |
5.60 |
5.32 |
| 3.50 |
1.40 |
1.75 |
2.10 |
7 |
5.60 |
5.25 |
4.90 |
| 4.25 |
1.70 |
2.13 |
2.55 |
9 |
7.30 |
6.88 |
6.45 |
| 5.15 |
2.06 |
2.58 |
3.09 |
12 |
9.94 |
9.43 |
8.91 |
| 6.15 |
2.46 |
3.08 |
3.69 |
12 |
9.54 |
8.93 |
8.31 |
| 7.15 |
2.86 |
3.58 |
4.29 |
14 |
11.14 |
10.43 |
9.71 |
| Use above values for "T" in calculation table. |
Compressed spring length will be close to above values when using the correct baseline spring rate. |
Many hot rodders use adjustable...
Many hot rodders use adjustable coilovers as a means of changing ride height, but Chassisworks advises against it. "The threads on coilover shocks are for positioning the springs correctly in relation to the sweet spot of the shock, not for changing ride height. You only have about 10 percent leeway in either direction for ride height changes. For instance, if your shocks have 3 inches of travel, you can only change ride height 0.300-inch in either direction before you move the shocks out of their ideal operating range. Unless you have progressive-rate springs, the spring rate won't change even if you crank in preload on the coilovers. If you do that, the reason the car will ride stiffer is because you've moved the shock travel out of their sweet spot, not because you stiffened up the spring rate," says Chris.
Since ride height can only...
Since ride height can only be adjusted marginally without ruining ride quality, the preferred method is changing the mounting points of the shocks. This rear coilover conversion kit from Chassisworks will install on virtually any solid-axle rear suspension, and makes adjustments easy.
Unlike a solid-axle rear suspension,...
Unlike a solid-axle rear suspension, changing the shock mounting points on an A-arm front suspension is nearly impossible unless it's completely re-engineered. Fortunately, drop spindles can easily lower most cars up to 2 inches. If a car needs to be raised for any reason, Chassisworks offers upper shock eye extensions that screw onto the company's extensive catalog of shocks.