Springs

By James Siddall

Sag

Here is the deal with Springs and Spring Force:

Springs are rated in one of three different numbering conventions.
Either lbs per inch, Kilograms per millimeter or Newtons per millimeter.

KTM springs are all rated in Newtons.

The Standard KTM spring rates are 4.8 Newtons per mm in front, and 150 Newtons per mm (STD) or 140 Newtons per mm(S).

This is why your spring is marked 140-225 or 150-225, these numbers are the rate (in Newtons) and the length (225mm).

Now onto the good stuff:

When you compress a linear spring 1mm, you will store force in the spring equal to it's rate. This applies to any millimeter of compression, whether it comes from preload, a riders weight, or a bump in the road. The spring doesn't care, it's a spring, it's got bigger problems.

So knowing this, lets do a bit of analysis on our 950. The Stock Spring is a 4.8N/mm spring. The stock Preload is 17mm assembled. Why did I say assembled you ask? Good Question. When fiddling with Preload adjusters on the outside of a fork, it is important to remember that when the fork was assembled, in most cases it was put together with some preload already on the spring, often in the form of spacers. When you turn the external adjuster, you are simply adding to this total. So to do the math , we need to know the total preload, not just the external.

So, we have 17 times 4.8 = 81.6N of force stored before we take the bike off the stand.

Now we drop the weight of the bike on top of this poor spring, and as you probably guessed, 81.6Newtons of force is not going to hold our 950 all the way up, so our spring will sag some.The sag of the bike under it's own weight is the Static sag. This sag compresses the spring further. Let's say in stock trim the bike sags 50mm under it's own weight. Add 50mm of spring compression at 4.8N per mm = another 240N of force stored. For a total 321.6N of force. Now knowing the spring rate, the preload and the sag, we can change any of these variables and calculate the others with the following formula:

Spring Rate*(Preload+Sag) = Load (as we just explained above)

4.8*(17+50)=321.6

Then to calculate the required change in preload should you change the spring:
(Load/new spring rate)-sag = new preload

Or the change in sag if you change the preload:
(Load/new spring rate)-new preload = new sag

Keep in mind that you can use this calculation for Static sag or for Total(commonly called Race) sag. But you cannot mix and match. One or the other only please.

Sag diatribe authored by Burnt Guy James Siddall @ www.superplushsuspension.com