To
show as an example, I've made up this imaginary shock chart below.
Some shock manufacturers offer shocks by size, or print the information
to double check fitment. By using their charts, you can easily
choose a shock you need.
For
simplicity, we will use shocks that have an eye at each end for
a bolt.
We
took the old shock off our vehicle and we know it is 16 inches
collapsed, and 19 extended. The travel is around 7 inches. We
know the bolt hole sizes, the top is 9/16, the bottom is 1/2 inch.
We
want to get a shock that's a little longer, because we added a
slight lift to our suspension (around 1 1/2 let's say) with a
leaf spring modification, and need a longer shock.
By
looking at the chart below, the PX2 would be our best choice.
It's 22 inches long. Though it's possible we could use a shock
that is 21 or 23 inches, depending on the modification. The travel
is close, we are getting one more inch of travel, but we could
also use a shock with up to 2 inches of travel if needed.
The
goal is to have a shock long enough to that we continue to have
around the 7 inches of travel (the suspension travel on the vehicle
would be less than that), but still have a shock long enough to
it can do it's job, dampen the suspension.
If
a shock is to short, and the travel is only around 2-3 inches,
that may not be enough for the piston and valves inside to function
to their best.
In
an extreme case, the inside piston will be pounding into the end
of the body of the shock. You can kiss that shock good buy after
a beating like that. It might last a few days.
Use
a shock that's too long, and the piston will try and pull itself
out of the shock body. Again, that will only happen once, and
the shock will be destroyed.
In
the end, you want to take the time to get all your measurements
before choosing a custom length. But you can see that once you
know the dimensions, it's a snap.
It's
also not a perfect science, meaning, an inch here or there is
plenty of leeway. It just depends how much travel (articulation)
the suspension has, and what the modifications call for.
On
a stock vehicle, the shock never collapses all the way (there
is always something on the suspension, such as a bump stop / rubber
stop on the frame), that prevents a shock from doing that.
For
the most part, vehicles are rarely airborne, so it's pretty hard
for a suspension to hang just by the shocks (though on 4x4 trucks,
limiting straps are often installed to prevent this issue when
they are airborne, such as in racing, to protect shocks).
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