The effect of progressive springs is too much for use on light weight dirt bikes. On adventure bikes progressive ones are an excellent investment and can greatly improve the versatility of your bike. At the same time they can improve your handling and ride. On a long tour or Trans Africa trip they increase chassis and suspension reliability. Let’s see why…
Linear springs have a constant rate throughout their compressible range. They are often referred to as constant rate springs.
Progressive ones change spring rate as they compress. You can easily distinguish progressive from linear springs: when you look at them from the side the linear springs have evenly spaced coils. Progressive ones have coils spaced closer together on one side of the spring than the other.
When you compress a spring to the point that the coils are touching each other they become “inactive”. Active coils are those that are not touching each other. This is important because the spring’s stiffness is determined by the diameter of the wire combine with the number of ACTIVE coils. If two springs that share the same diameter but differ in the number of active coils, the one with the fewer active coils will be harder/stiffer.
So as the progressive spring compresses the coils that are closer together touch each other and become inactive. In other words a portion of the spring becomes coil bound, while the rest remains active. This means the spring has fewer active coils and becomes stiffer.
Dirt bikes have adequate progression built into the suspension linkage systems or into the shock system such as KTM PDS shocks. These systems are designed to work best with the correct linear springs for your weight. The same holds true for dirt bike forks due to the air spring compression.
A progressive spring in this case is an overkill and makes the suspension too stiff deeper in the stroke. Dirt bikes are always ridden solo, and your weight and the weight of the bike remain constant (for some riders more than for others ;-). This means that once you have the correct linear spring rates the bike should always work perfectly from ride to ride.
Adventure bikes differ greatly from dirt bikes. One day you ride solo, the next you lift a passenger. You add bags, lots of extra gas or extra gas tanks etc. In other words there can be big weight variables from one ride to the next. Theoretically you need one set of shock and fork springs for solo riding and another for riding with a passenger or laden bike. You would have to change springs each time you load up or lift.
Obviously it helps to balance the bike in these situations by adding and removing preload but this is only a quick fix. The problem with a quick fix is that the soft springs still remain too soft for lifting or loading. You will still experience handling problems.
On long trips this is bad as it puts great strain on chassis and suspension and reduces long term reliability. Therefore the progressive version is a great help and can offer the best of both worlds.
The progressive spring functions in such a way that the beginning third of the suspension travel remains soft and offers a comfortable ride acceptable for solo riding. Thereafter some of the spring coils start to close up and become inactive. They create a substantially harder spring for the last two thirds of the suspension travel. This is very good for hard solo riding and for situations where you load the bike or carry a passenger.
It’s almost like cheating and having two springs in one on your motorbike!
The Dirt Bike Garage Manual
HOW TO GUIDE for "do it yourself" riders