Spring Rate 101by Roland Graef
Suspension customers shopping for aftermarket springs will sometimes resort to shopping for springs by spring rate. While the rate of the spring is a measure of spring performance, it is not the only influence of vehicle performance and handling, especially when the chassis is lowered. There are many factors that come into play when it comes to suspension tuning.
When does a higher rate spring feel softer than a lower rate spring and a lower rate feel harder than higher rate spring? These differences occur when you lower the chassis and the bumpstop is trimmed or not trimmed. The bumpstop acts like a small progressive spring (see Micro Cellular Jounce Bumper). A lower rate spring that lowers the ride height of the chassis with no bumpstop trimming, making the bumpstop more active, would feel about as stiff as a higher rate lowering spring with the bumpstop trimmed, making the bumpstop less active. Also, the amount of ride height lowering also affects suspension rate.
Since there is no standard for quoting spring rates, most manufacturers just quote numbers without any regard for spring function and spring rate ramping—spring rate ramping is the difference in ride feel between springs of different shapes with the same spring rates under suspension compression. The only way to truly compare spring rates is by using working spring rate numbers.
When a manufacturer quotes a progressive spring rate such as 80#, 150#, 225#, and the stock rate is a linear 135#, the new spring looks super progressive. Visually it seems to start off softer than stock and gets progressively stiffer as needed. But what these rates don’t tell you is that the chassis is already sitting at the 170# rate at loaded height. This means the “working spring rate” is actually 170# to 225#. The lower spring rate range below 170# is the dead or inactive spring coils which do nothing but give the spring tension at full rebound. Note that this does not take into consideration the bump stop engagement and their effect on chassis ride height with the installed lowering springs. This is only one example of why using the “working spring rate” is more accurate when making comparisons. It is also important to have already driven on a different rate than stock to feel the difference before making any comparisons based on the quoted rates.
There are many other factors that influence suspension rate that include, but are not limited to: Shock dampening values, tires, bushings, and of course the most important “personal driving style”.
Keep in mind that if you’re shopping for springs based only on spring rates, then you are not taking into account a number of other variables that affect ride height, performance, and comfort.