tech4

At my last motocross race outing I noticed that my riding had become a
little more aggressive and my Rickman 500 Triumph twin was bottoming
pretty hard on the worst jumps at the Carlsbad’s Vintage track. We had
suspected from the beginning two years ago that the stock Montesa
Rickman Betor fork springs might be too soft for the heavier Triumph
and the heavier rider, me. So heavier, higher spring rate springs
seemed in order. Since there is no “local” dealer to get the next
heavier spring I called Matt Hilgenberg at Speed and Sport, who
imports many Rickman kits every year. Matt sent me a different spring
but when it arrived it definitely was not a drop-in. This is of course
typical when dealing with these vintage racers. In comparing the two
springs I found that the new spring was about two inches longer, was
made from wire about 2.5 % thicker (not enough of an increase in
spring rate for my purposes), had the same number of coils (56), and
would not fit inside the fork leg.

Now, for a little review of how a spring works. A fork spring is a
long slender beam wrapped in a helical coil with no two coils touching
each other. So we can make an analogy to a beam to understand how to
make that spring have a higher spring rate (become “heavier”). If the
beam is made of thicker material it will have a higher rate. In this
case the wire thickness increase is 2.5% (3.9mm originally to 4.0mm in
the new spring). If we make the beam 10% shorter it becomes 10%
stiffer with a 10% increase in spring constant. This shortening of the
beam (spring) is the approach I chose since the spring was all ready
too long for the forks tubes.

But we have to be a bit careful because if we take too much away from
the overall length, the spring could coil bind (completely collapse
with all coils touching each other) and limit travel and ruin the
spring. Have you ever taken a spring out of an old set of forks and it
looks like a cork screw? Well that is what happens when you collapse
the spring too much and it take a set (yields under the load). In fact
a prominent spring manufacturer says you should not use up all the
spring available travel but leave at least one inch of travel at full
compression. My original spring free length was 16.625 inches long.
The fully collapsed length calculates to 56 coils of 0.153 inch wire
for 8.578 inches. That means the original travel was about 16.625
minus 8.568 inches or 8.047 inches, but I had installed a pre-load
spacer (more about how to determine those pre-load spacers later) of
0.938 inches. This left 7.109 inches of travel, almost EXACTLY the
travel of the fork! This is not good as per the spring manufacturers
recommendation of an extra inch of travel after full compression.
Well, that set up is history so lets look at the new proposed setup.

To fit the new spring inside the fork leg and reduce the length about
10% (and up the spring constant about 10%) I cut the spring to 50
coils. So my new free length is 16.625 inches (was 18.563 inches). The
new fully collapsed length calculates to 50 coils of 0.158 inch wire
for 7.900 inches. That means the original travel was about 16.625
minus 7.900 inches or 8.725 inches, but I installed the same pre load
spacer of 0.938 inches. This left 7.787 inches of travel, which is
more than 3/4 inch more than fully collapsed. Not an ideal design but
not nearly as bad as the original spring setup.

So with the spring length determined and the cuts made, it was time to
put in the fork oil. I like to use real fork oil as it has additives
that keep the fork seal soft and the leg’s outer surfaces oil free
longer. I had originally used a quantity of oil that put the oil 6
inches from the top of the tube when the fork tubes were completely
collapsed with the springs out (the old stand by, one size fits all
measure). There is a handy commercial tool made to help draw out the
excess oil at the specific level you desire. But I noticed that the
dampers were sucking air at the end of the extension stroke. That
means that the top of the dampening rods are not covered in oil at
full extension. The dampening rod’s dampening needs only oil and not
oil and air to work correctly. So I incrementally increased the oil
level until the dampening rods no longer sucked air when the legs were
fully extended. That worked out to 5 1/4 inches from the top of the
oil to the top of the fork tube at full compression of the fork legs
(with no spring installed). Don’t overdo this as you can overfill the
tubes and the seals will blow out on the first bottoming of the fork.
By the way, most vintage forks work best with 20 wt fork oil.

The way to determine how much of a pre-load spacer is required is to
do a laden and unladen sag test. What these tests do is help you setup
the ride height for your weight and determine if there is any hope of
this particular setup working on the track. For short travel vintage
bike (7 inches of fork travel is short compared to modern bikes 12
inches of travel) you would like the ride height with your full weight
on the bike to be about 25% of the total fork travel or in my case
about 1 3/4 inches plus or minus 1/2 inch. The exact height can only
be determined by riding the bike on the track but the nominal 25%
number is probably fine for normal riders like me. You’ll need help to
balance the bike while you bounce up and down on the suspension to get
the forks to find their natural position. The stiction of the tubes
and the seals is such that the forks may want to stop in any number of
spots in the travel so you kind of have to average it out. Measure how
much the bike settles (look for the 25% number). Now get off the bike
and see how much it sags under its’ own weight. If it doesn’t settle a
small amount, like 1/4 to 1/2 inch, of its’ own weight and is topped
out hard, you have added so much pre-load to get to the 25% laden sag
that you should go up to a 10% harder spring rate spring. Fortunately
the original pre-load spacers worked out fine for me for both
conditions.

For fine tuning at the track you can use pieces of PVC pipe and thin
washers as pre-load spacers. Test ride it and limit the number of fork
bottomings to no more than a couple of places on the track. If the
forks never bottom you may be giving up riding comfort but if you are
a hard charger you may prefer the extra security of harder forks. In
my case the theoretical spring rate increase was 10% for the shortened
spring and 2.5% for the thicker wire for a total of 12%. That is about
the same as one step size increase when you get modern springs for
modern bikes.

To prove this change I need to take the bike to the track but I know
that the rate is definitely higher and that can only help in my case.