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All About Preload
I am back to discuss the preload on a spring, and
what effect it has on your suspension. As I stated in my last article,
a spring is simply a coiled wire with a very good memory. In the last
article I used 5.0 kg/sq cm and 8.8 kg/sq cm springs as examples. I
will continue using these two sizes in this column.
First, we need to understand that a spring that is not
compressed (i.e., is at its free length) will have only potential energy.
Only when compressed will the spring have stored energy. The preload on a
spring is the compressing of that spring a certain amount so that the spring
is storing a specific amount of energy. The reason a spring is preloaded is
to achieve the proper race sag, which is a secondary preload that occurs
when you sit on the bike. The spring has a memory. It wants to be
lazy, and it is always pushing back, trying to get to its free length.
That 5.0 kg/sq cm (280 lb/sq in) spring, if preloaded 1 inch, would be
storing 280 pounds of energy, trying to push back to its free length.
Likewise, that 8.8 kg/sq cm (492 lb/sq in) spring, with the same 1 inch
preload, would have 492 pounds of stored energy.
In a link type system, the ideal preload is l/2 inch,
the maximum preload is 3/4 inch, and the minimum preload is 1/4 inch.
If a bike equipped with a 280 lb/sq inch spring has 3/4 inch preload, you
will have the equivalent of 210 pounds stored energy. If it has 1/2
inch preload, you will have the equivalent of 140 pounds stored energy, and
if it has only 1/4 inch preload, you will have the equivalent of 70 pounds
of stored energy.
In a PDS (KTM) system, the ideal preload is 1/4 inch,
the maximum preload is 3/8 inch, and the minimum preload is 1/8 inch.
If a bike equipped with a 492 lb/sq inch spring has 1/4 inch preload, you
will have the equivalent of 123 pounds of stored energy. If it has 3/8
inch preload, you will have the equivalent of 185 pounds of stored energy,
and if it has 1/8 inch preload, you will have the equivalent of 62 pounds of
stored energy. As you can see, as the preload is increased, the stored
energy increases also.
What does all this mean? Lets say your bike (link
system, not PDS) is equipped with a 280 lb/sq inch spring and you had to
turn the preload up to 3/4 inch to achieve proper race sag, you would now
have 210 pounds of stored energy. If you would install instead a 290 lb/sq
inch spring and were able to drop the preload to 1/2 inch, you would now
have145 pounds of stored energy. That 290 lb/sq inch spring, preloaded
to 1/2 inch, would require less force to begin movement of the rear of the
bike (i.e., on a smaller bump), than would the 280 lb/sq inch spring
preloaded to 3/4 inch. That 290 lb/sq inch spring would require 870
pounds of force to bottom (3 inch shock travel) compared to 840 pounds of
force for the 280 lb/sq inch spring. So the heavier spring (the correct
spring in this example because it achieves the proper race sag) is more
progressive than the spring that was only one step too light. The
correct spring will move over the small bumps better and resist bottoming
better. If you require more than one step difference in spring rate,
the results be much more dramatic.
The PDS (KTM) springs become more critical since they
have heavier spring rates than suspensions with link systems. Lets say
you should be using an 8.8 kg/sq cm (492 lb/sq inch) spring but you are
using an 8.4 kg/sq cm (470 lb/sq inch) spring (PDS springs normally come in
.4 kg/sq cm or 22 lb/sq inch increments). If you had to preload the
8.4 kg/sq cm (470 lb/sq inch) spring1/2 inch (235 pounds stored energy) in
order to achieve the proper race sag, but the correct spring 8.8 kg/sq cm
(492 lb/sq inch) would have required a preload of 1/4 inch (123 pounds
stored energy), the 492 lb/sq inch spring would make the shock much more
efficient on small bumps. It would also resist bottoming much better,
since the 492 lb/sq inch spring would require 1476 pounds to bottom while
the 470 lb/sq inch spring would bottom with only 1410 pounds.
Now lets examine the influence of race sag (the
secondary preload). Most race sags should be 4 inches (+ or – 1/4
inch). This will add 1 inch additional preload on the spring before
you hit the first bump. Add this additional stored energy to the
examples above and you will see the large increase in overall stored energy.
By adding another 280 pounds of stored energy (created by the one inch
additional preload due to the race sag) to the 280 lb/sq inch spring that is
preloaded 3/4 inch (210 pounds stored energy) you would have a total of 490
pounds of stored energy. By adding another 290 pounds of stored energy
to the 290 lb/sq inch spring that is preloaded 1/2 inch (145 pounds stored
energy) you would have 435 pounds of stored energy. By adding another
470 pounds of stored energy to the 470 lb/sq inch PDS spring that is
preloaded 1/2 inch (235 pounds stored energy), you would have a total of 705
pounds of stored energy. By adding another 492 pounds of stored energy
to the 492 lb/sq inch spring that is preloaded 1/4 inch (123 pounds stored
energy), you would have a total of 615 pounds of stored energy.
Remember, you must have a force at the rear wheel greater than the total
stored energy of the preload and the secondary preload to just make the
shock move at all, even as little as 1/16 of an inch.
OK. So lets suppose you decide that you won’t
spend your money on a new spring, and will just run more race sag and less
preload to make the suspension work easier. WRONG. The amount of
stored energy you are going to reduce by backing off the preload will be
more than offset by the additional secondary stored energy that will be
added by increasing race sag. So, if you run too much race sag, the
secondary preload goes way up and the suspension will become harsher on the
small bumps. In addition, the rising rate linkage that the shock is
hooked to (or the tapered needle inside on a PDS shock) will be in the
harder part of its travel. I hope I have convinced you to use the
right spring, which will guarantee a smoother ride. I have used the
rear spring in this article, but the front suspension works the same way,
only using lighter springs. |
