Motorcycle Fork Oil Change
I had some problems with
my Triumph Thunderbird Sport motorcycle front suspension. These
problems are described in "Part I" of this saga:
Tire Change - Triumph Thunderbird Sport".
recap, I noticed that the front end of the motorcycle had a repetitive
bouncing motion that I just couldn't get rid of. No
combination of rebound, damping or compression adjustments
seemed to help. I thought perhaps it was the oversize tube
that I installed with the recent front tire change, so I found
the correct size tube and checked the balance several times, but
nothing seemed to help. This is all described in detail in
So the next logical step was to remove the forks and take a look
inside. I had never taken a pair of forks apart before, so
I was a bit wary of what I might be getting in to. But the
Haynes manual is a gem, and I had no significant problems with
this maintenance task. I'm glad I did it, because I think
I've solved the problem.
I had planned on changing the fork oil anyway, since the bike is
nearing 10,000 miles, so the work wouldn't be in vain, even if
it hadn't solved the bouncing problem. The bottom line is
that I found that the forks had only about 1/2 of the amount of
This could certainly cause the problems I
had encountered. I'm not sure why the motorcycle had only 1/2
the amount of fork oil -- the previous owner mentioned that he
hadn't done anything to the forks, so I'll have to assume that
the bike came from the factory this way. So if you're
having some problems with your Triumph motorcycle, and the front end feels
like it's bouncing around and you can't get it damped, this may
be the problem.
As I mentioned, the Haynes manual is fairly clear on the
procedure for removing and disassembling the forks. You
don't actually have to completely disassemble the forks, but it
is necessary to remove the top nut that holds the rebound and
preload adjusters, and to remove the spring, so that the correct
amount of oil can be installed.
Here are some tips based on my experience:
The Haynes manual suggests loosening the 27mm top nut that holds
the rebound/preload adjusters before you remove the forks.
I didn't do that, and ended up having to re-insert the forks in
the triple clamp to remove the nut.
BUT, Gary G. wrote to
let visitors know that a drawing in the Triumph manual shows
that the correct way to hold one complete fork leg in a vice is
by clamping the outer fork tube (lower) in the vice with soft
jaws by the lowest brake caliper mounting boss. This
sounds like a good way to do this also.
The problem is that the top nut (white arrow, photo 1) is held
on with 23NM of torque, and you don't want to hold the fork tube
in a vise while you remove it, because you don't want to risk
crushing the tube.
The triple clamp offers
a good, strong hold on the tubes and allows you to loosen the nut,
so you're better off doing this while the fork tubes are still on
I was worried that the spring would come popping out of the top of
the fork tube if I loosened the nut, but if you back off the
preload and rebound adjusters all the way (blue arrow, 2), you shouldn't have a problem.
the forks from the bike is a straightforward process, and the
Haynes manual clearly describes the procedure.
Remove the brake
calipers, the speedometer cable and the front tire and axle, per
the wBW article "Front
Tire Change - Triumph Thunderbird Sport".
The front fender must be removed; it's held on by 4 hex
bolts and is easy to remove.
Then loosen, but do not remove,
the four (2 on each side) lower fork tube clamp bolts and the 2 (1 on
each side) upper fork tube bolts. The forks should slide out
with little effort.
Since the top nut has already been loosened, it
should be fairly easy to unscrew the entire top assembly that
houses the preload and rebound adjusters.
By the way,
I had a flat stamped metal 27mm open-end wrench that fit the top
nut, but I found that a 1-1/16" open end wrench will also
work, although it is a bit loose; if you use one, be careful that
you don't round the corners of the nut.
The top assembly (pink arrow, photo 2), includes the
preload/rebound adjuster (white arrow) and unscrews from the
rebound rod that runs down through the center of the spring.
There is a U-shaped washer (blue arrow, photo 2) that sits
between the top of the spring and the top assembly; it slides out
to reveal a locknut that tightens up against the top assembly to
The spring must be removed to drain and refill the fork.
Once the locknut is loose, the top assembly can be unscrewed via
the 17mm preload adjuster and
remove the washer and the spring.
Pump the forks up and down several times while holding them over a
container for the oil, and also pump the preload rod in and
out. I left the forks upside-down in an oil drain pan
overnight to make sure they were fully drained, and still had to
pump them some more the next day to get the last little bit out.
As long as
I was changing the oil, I figured I'd experiment a bit with the
fork oil weight. I used BMW 7.5 weight fork oil, which is a
bit heavier than the 5 weight called for by Triumph.
Here's the surprise: each fork had about 240cc's of fork oil when
I drained them, and I was careful to drain them dry. But
when I followed the manual's directions to fill the fork to 145 mm
from the top of the compressed fork leg, I found that this took
about 470cc's in each fork!
The original owner told me he never changed the fork oil, so the
only conclusion is that the factory under-filled the forks by
almost half. I can't believe he drove it for 9,500 miles
without noticing it -- I noticed something was wrong right away.
As you fill the forks, be careful to expand and compress the forks
several times; cycle the preload rod up and down, etc. to make
sure that the fresh oil is completely distributed throughout.
manual advises using a special tool that you can set for 145mm and
place over the top of the fork tube to measure the correct
height. The tool will suck out any oil over 145mm in
But it's just as easy to hold a scale (ruler) down
in the tube and fill it up until the height of the oil column is
correct as shown in this photo.
I poured in about 425cc, and worked it up from there, expanding
and compressing the fork tube and the preload rod each time, until
I finally hit the 145mm mark.
re-assemble the fork, first insert the spring, with the coils that
are closest together located towards the top.
So if this piece is held with the 27mm
nut, you may end up with an incorrect location for the entire
assembly because it will slide over the preload threads, rather
than tighten against the locknut.
Then locate the top of the locknut 11mm from the end of the threaded rebound
rod, and screw on the top assembly. Tighten the top assembly
against the locknut, and try to keep the locknut from moving up or
down along the threaded rebound rod.
photo shows the 27mm flat open-ended wrench holding the
top nut, but I found that you're better off using a 17mm wrench on
the preload adjuster to tighten it, because the 27mm top nut turns
the entire outer assembly, which fits over the preload adjuster and
screws up and down.
Installation is the reverse of the removal procedure; just
remember that you'll need to tighten the 27mm top nut after you
have the forks secured in the clamps. It's supposed to be
torqued to 23NM, but there's no way you'll be able to find a
socket deep enough to fit over the entire preload/rebound assembly
to tighten the nut, so I just gave it a good tug with the 27mm
open-end wrench. Alternatively, Gary G. also suggested using
a "Crow's foot" attachment for a torque wrench to
achieve the correct torque.
Also, don't forget to correctly
torque the socket head cap screws on the triple clamp to their
correct torque after you re-install the fork tubes!
So far, the bike's handling and feeling has improved dramatically
-- like two different bikes. The adjusters now actually seem
to do something! So if you're experiencing front end
problems, you may want to try changing the fork oil.